The eruption of Volcán de Pacaya on May - June, 2010. Report in progress.

Rüdiger Escobar Wolf Michigan Technological University, July 2010. rpescoba@mtu.edu

Volcán de Pacaya erupted explosively on May 27, 2010, destroying or damaging nearly 800 houses in nearby communities, forcing the evacuation of nearly 2000 people, injuring 59 people and killing one news reporter. The most severe and damaging impact to people and property was inflicted to nearby villages at 2.5 to 3.5 km to the north of the vent, caused by ballistic projectiles and to a lesser extent by the accumulation of several centimeters of tephra. A wider area including virtually the whole extent of the Guatemala City conurbation was affected by the deposition of a thinner tephra blanket, which most significantly shut down the international La Aurora airport for five days. A large lava flow was erupted over the following days from a vent formed on the southeast flank, becoming the second largest flow since 1961. The eruption formed a very conspicuous trough on the northwest flank of the MacKenney cone, aligned with the vent of the new lava flow and the central MacKenney cone vent. Considering the volume of erupted tephra and lava, this eruption can be considered the second largest from the current eruptive episode, starting in 1961. From the collection and integration of data from several sources, a general picture of the eruption and surrounding events is presented, with special emphasis on the main eruptive products and associated processes. In light of the previous activity at Pacaya and considering the size and characteristics of the recent eruption a brief assessment of the hazards and potential implications of future eruption is discussed. All the field data were provided by the various collaborators cited in the text. Chronology and description of events that culminated in the May 27th eruption January 2010: Effusive activity had been increasing over the previous weeks (G. Chigna personal communication), feeding several lava flows from vents on the flanks of the main cone. This increased activity followed a long period of mainly lower effusive activity with occasional breaks, which started in 2004. February 5, 2010: Several lava flows are simultaneously active, producing hot rock falls on the E and SE flanks towards Los Llanos (CONRED Boletin informativo No. 564, dated on Feb 10, 2010 at 9:54). This is the first explicit record in a CONRED bulletin of an increase in the activity at Pacaya. April 18, 2010: A Venezuelan tourist and a Guatemalan guide die on Pacaya, apparently when they got hit by a rock avalanche caused by a small (?) explosion (see http://tinyurl.com/2vvaj4j). May 17, 2010: Abundant lava effusion on the SE flank is observed and CONRED recommended to the Pacaya National Park authority to restrict the visitors access to the lava flows (CONRED Boletin informativo No. 708, dated on May 17, 2010 at 16:35). The activity has produced a mound at the source vent, feeding lava flows that reach 1.5 km in length. This activity is considered to be relatively high, but no explicit mention to a possible crisis is made at this point. According to the press the access to the volcano is being restricted following the recommendation by CONRED (see http://tinyurl.com/2wb8p9t)

May 26, 2010: Eruptive activity increased markedly during the day, released seismic energy has and an eruptive plumes reaching 1 km above the vent has formed, dispersing fine tephra particles on the villages around the volcano (CONRED Boletin informativo No. 726, dated on May 26, 2010 at 17:17). CONRED recommends closing the access to the Park altogether and warns the air traffic authorities about possible ash in the air near the volcano. No mention has been made at this point about a possible evacuation of the nearby villages. May 27, 2010: The activity increased during the day (see http://tinyurl.com/246hohe) producing more intense explosive activity at around 15:00 hrs. Around this time CONRED started to mobilize personal to the villages near to the volcano, to discuss and assist in a preemptive evacuation. Explosions were happening at a frequency of one every second, projecting material up to 500 m above the crater. The ash plume reached 1.5 km over the crater and dispersed fine tephra over the nearby villages. The recommendation to evacuate has been met with resistance by some of the population in the villages surrounding the volcano, nevertheless seven shelters are being prepared in San Vicente Pacaya to accommodate the refugees (CONRED Boletin informativo No. 729, dated on May 27, 2010 at 17:14). After 18:20 the activity reached a climax during a brief burst of intense fire fountaining and vigorous tephra and ballistics ejection (Hetland personal communication and CONRED Boletin informativo No. 856, dated on June 16, 2010 at 10:00). The most intense fire fountaining lasted as little as 15 min (see http://tinyurl.com/3x654qc and http://tinyurl.com/36pqglj). A tephra shower covered the country to the north of the volcano up to several tens of kilometers in distance. The evacuation of villages to the west (El Rodeo and El Patrocinio) was already underway when this intense phase started. However, due to the direction in which the ballistics and tephra was dispersed, the most affected towns were El Cedro, San Francisco de Sales and Calderas. These towns are located closest to the volcano in the north direction at distances between ~ 2.5 and 3.5 km, and they received the impact of ballistic up to 80 x 50 cm (Hetland personal communication), but most within the range of 20 cm or smaller (see figure 1a). Hot ballistics pierced through tin and fiber-cement roofs, and caused fires in houses (see figure 1b), destroying among other public properties, the roofs of public school, churches and the Park visitors center. The tephra accumulation reached ~ 8 cm in this area.

Figure 1a. Ballistic fragment ~ 20 cm in diameter that fell in San Francisco de Sales. Photo: B. Hetland.

Figure 1b. House that burned down after being impacted by a hot ballistic fragment. Photo: B. Hetland.

During the following hours more than 2100 people were evacuated from these and other villages, to the town of San Vicente Pacaya. A group of news reporters from a national television station (Notisiete) were caught by a shower of ballistics during the most explosive phase of the eruption, nearby Cerro Chino at

probably less than 1 km from the vent. Several of them were able to find cover but one of the reporters was killed, apparently by a direct impact of a large ballistic, being the only officially confirmed death caused by the eruption (see http://tinyurl.com/363y5gz). A strong wind blowing towards the north dispersed the tephra over the Guatemala City conurbation and beyond (see figure 2). The tephra fell mixed with rain, and thickness reports vary from 10 cm in the southern parts of the City (e. g. the north shore of Amatitlan Lake) to 0.5 cm in the central part of the City. However, thicknesses in excess of a few centimeters at those distances should be taken with skepticism, as will be discussed later.

Besides one person killed and three missing, the eruption had injured 59 people and prompted the evacuation of nearly 2000 (CONRED Boletin informativo No. 734, dated on May 28, 2010 at 13:49). The tephra accumulation disrupted traffic and shut down La Aurora international airport for several day. Classes in public schools in three departamentos (provinces) were canceled. The intensity of the eruption decreased during the following hours, none the less remaining at relatively elevated activity levels. May 28, 2010: The volcanic activity increased again during the early afternoon, producing eruptive plumes that reached elevations of 1.5 km above the crater and dispersing tephra as far

as Guatemala City(CONRED Boletin informativo No. 735, dated on May 28, 2010 at 14:24), but in much smaller quantities than during the previous day. At this time 1924 people had been evacuated, 1865 of which remained in public shelters. May 29, 2010 onwards: The volcanic activity continued decreasing, with only a relatively small eruptive plume that occasionally produced minor tephra fall in the communities surrounding the volcano (CONRED Boletin informativo No. 742, dated on May 29, 2010 at 11:01). This activity continued for the following days (see http://tinyurl.com/28x9z69) and transformed into a predominantly effusive eruption from a flank vent (see figure xx), which produced a large lava flow that at the time of this writing (June 20th) has already become the second largest flow since the current activity cycle began in 1961. The lava flow has reached relatively low slope grassy terrain (see http://tinyurl.com/23a4tfl and http://tinyurl.com/248fefq) and was originally moving at a very high speed (~ 100 m/hr) and threatening to destroy populated areas to the S and SE of the volcano (CONRED Boletin informativo No. 748, dated on May 29, 2010 at 23:16), but has since then slowed. By May 29th a total of 2635 people were in shelters due to the eruption, ~ 400 houses had been slightly damaged and 375 had been severely damaged (CONRED Boletin informativo No. 748, dated on May 29, 2010 at 23:16). In the following days the attention of the emergency shifted from the eruption to the storm Agatha, as both disasters blended in a continuous emergency.

Figure 2. Isopach map of the Pacaya May 27, 2010 eruption, based on tephra thickness reports from SERVIR – CATHALAC.

Ballistic fragments A region to the north of the volcano extending over 3 km from the main source was severely affected by volcanic ballistic fragment impacts (see figure 3). The location of the source of the eruption is not exactly known but it is assumed to be at, or close to the previous central vent at the summit crater. The ballistics are made of juvenile material, and although some broke on impact, the shape of some suggests that they may have still behaved plastically on impact, i. e. as volcanic bombs (Hetland personal communication). The ballistics fell at a very high temperature and most likely were derived from the same magma that fragmented to smaller pieced and formed the tephra that fell over a larger area. As seen in hand samples, the ballistics are vesiculated and have a low density, with bubbles typically larger than 1 mm, and few phenocrysts (also larger than 1 mm) most likely of plagioclase (Hetland personal communication). The ballistics were most likely traveling at terminal velocities and in trajectories with very high angles (nearly vertical). Clasts ejected from the source are influenced by both the initial conditions of ejection and by the transport system in the atmosphere. This “behavioral domain” is a continuum spectrum between two extreme cases: On one extreme there is the ballistic dominated behavior, mainly influenced by the initial kinetic energy and direction of ejection, and less influenced by the wind velocity field. On the other extreme is the subaerial “wind carried” particle sedimentation, more influenced by advection and diffusion transport in a wind velocity field, and less influenced by the initial kinetic energy and direction of ejection. It is not clear what the criterion should be to differentiate between these two regimes in terms of size or terminal fall speed, since the transition will be gradual. However this transition is likely to be close to the transition from clasts that can cause important damage by impact (ballistics) to clasts that cannot (airborne tephra); clasts larger than several centimeters (> 4 – 5 cm) are probably near that transition. The maximum distance at which damage from ballistic impacts was observed extends some 4 km from the inferred vent, beyond that distance there is no important damage associated with ballistics impact. Damage to buildings, other human made artifacts, and vegetation can be used to infer some characteristics of the ballistics. At Cerro Chino (< 1 km from the source), ballistic impacts broke concrete roofs (see picture 4a), destroyed cars (see picture 4b), started fires in the sheds that host equipment for radio transmission, and knocked down radio towers (see figure 4c). Ballistics in excess of 0.5 m (long axis, see picture 4d and 4e) fell in the immediate vicinity (< 1 km) of the vent, but their size diminished with distance. Of the group of news reporters that were in the Cerro Chino area, one apparently died in situ from injuries caused by the direct hit of a large ballistic (see http://tinyurl.com/363y5gz). The other news crew members were able to dodge the ballistics and find temporal shelter, before running away from the area. Given the size of the ballistics and the damage they caused on structures, it seems unlikely that any personal protection gear (e. g. hard hats) would have avoid serious injury from the impact of the largest fragments, but they could have protected against smaller fragments that also caused injuries to the news crew members. At the communities immediately to the north of the volcano ballistics larger than 20 cm pierced through tin and fibro-cement roofs and lit houses on fire (see figures 4f and 4g). At this distance (~ 2.5 and 3.5 km) the damage inflicted on roofs varied widely between concrete slab and corrugated tin roofs, and even among tin roofs. Concrete slab roofs withstood ballistics impacts of fragments in excess of 20 cm (unconfirmed reports of fragments of up to 80 x 50 cm may be referring to the deformed dimensions of a bomb after impact). The quality of the corrugated tin sheets apparently also played an important role in the damage they received (see figures 4h and 4i), with older and more corroded roofs being more

vulnerable and less resistant to the impacts. Damage may have also depended on the angle of impact on the surface (see figure 4h); however given the distance from the source the ballistics would be expected to hit the ground at very high angles (~ 80º, Alatorre-Ibargüengoitia et al. 2006).

Figure 3. Areas affected by ballistic projectiles impacts. From the damage observed on roofs, the density per unit area of impacts with enough energy to pierce through the tin and fiber-cement roofs was of more than 1 m-2. These fragments were traveling with a kinetic energy that is probably above the lowest threshold to cause serious injury and even death, if they hit an unprotected human being (Baxter and Gresham, 1997). Although some communities were partially evacuated before the most intense phase of the eruption, the communities mostly affected by the ballistics shower had not been completely evacuated; several hundred people may have been directly exposed to the ballistics, as it has been stated in the terrifying testimonies of some of them who lived through the ordeal (see http://tinyurl.com/27celhb, http://tinyurl.com/2gxxrqd and http://tinyurl.com/26ux356). It is unknown how many people were directly exposed to the ballistics, i. e. how many people were in El Cedro, San Francisco de Sales and Calderas when the ballistics impacted those towns, but an estimate can be made given the residing population and number of evacuated people. Nearly 2000 people out of a total population of nearly 5000 evacuated from six communities, including the three that were hit by ballistics. From the field reports it seems unlikely that the rest of the population (~ 3000 according to the population projection) didn’t evacuate and stayed in the communities, especially in those impacted by ballistics. This discrepancy could be either to inaccuracies in the population data or in an underestimation

of the total number of people evacuated. If we assume that the majority of the population from the towns impacted by ballistics (~ 2800 habitants) evacuated, that alone could account for more than the 2000 evacuees reported by CONRED. CONRED also reported that 775 houses were damaged, 400 of them severely; this number of houses would roughly corresponds with the total number of houses in the towns impacted by ballistics and matches with a population between 2000 and 3000 people.

Figure 4. Damage produced by ballistic projectiles. Photos a through c show the destruction caused by ballistics that impacted the slopes of Cerro Chino. Photos d and e show some of the ballistics. Photos f through i show the destruction caused by ballistic impacts in San Francisco de Sales. Photos a, b, c and d, by G. Chigna. Photos e, f, g, h and i, by B. Hetland. Taken the number of 59 injured people given by CONRED, and if approximately 2000 people were exposed (present in the towns) to the ballistic impacts, the percentage of injured people is close to 3% of the total population exposed. However the number of injured people may also be underestimated, especially in the case of less severe injuries. CONRED reported that the injured were being treated at the “Centro de Salud” (the local small clinic) and the shelter in San Vicente Pacaya, and some were treated at the Amatitlan hospital. I had no access to information on the type and severity of injuries, but it seems that only a fraction of the people reported as injured needed to be treated at a hospital. It seems therefore than a maximum of only a few percent (even perhaps less than 1%) of the exposed population was injured to the point of needing hospitalization. The low percentage of serious injured people and the fact that there were no deadly casualties reported by people in the towns impacted by ballistics are rather surprising, considering the damage that ballistics

a b c

d e f

g h i

inflicted to buildings, especially corrugated tin and fiber-cement roofs. Testimonies of people taking cover inside houses, even when ballistics pierced through their roofs, show that maybe this behavior significantly reduced the direct exposure, since a fairly large amount of kinetic energy is expected to be dissipated when the ballistic pierces through the roofs. Some people also took refuge in buildings with concrete slab roofs (http://tinyurl.com/2d66up2), and some used whatever hard and resistant objects they could find to protect them from the falling rocks, including hiding under furniture and using pots and pans to protect their heads (see http://tinyurl.com/2bfpt7r and http://tinyurl.com/26ux356 ).

It is unclear how much protection the standard protective gear like hard hats, and which are usually recommended for people exposed to volcanic hazards (Aramaki et al., 1994, Baxter and Gresham, 1997), would have provided, but it would most likely have reduced the severity of injuries. Safety hard hats like those used in construction are usually not built to sustain the impact of very hot objects, and this would make them vulnerable to this kind of ballistics. These kinds of considerations are very important for, among other people, the emergency workers who go into areas that are affected by ballistics (e. g. the search and rescue team that spend several hours in very close proximity to the vent, searching for the missing people and the diseased news reporter). As for the people living in these communities, the quality and type of material of which the roofs were made certainly played a key role in how vulnerable these people were. The spatial distribution of the area affected by ballistic seems to be heavily influenced by the direction of winds, although probably less than for the smaller tephra fragments (see figure 3). There are no records of the size of ballistics that fell towards other directions from the vent, but from previous eruptions of comparable size it has been observed that larger ballistics dispersal tends to be influenced in a significant way by the wind direction, i. e. during the January and February 2000 eruptions a strong wind blowing towards the south carried the tephra and ballistics in that direction, causing only minimal deposition of tephra to the north.

Interpolating wind reanalysis data (Kalnay et al. 1996) for the Pacaya location on May 27th at the time of the eruption, the wind profile over the vent seems to have been characterized by a strong vertical sheering (see figure 5), with wind blowing towards the southeast in the lower levels (< 7 km), but progressively rotating counterclockwise to strong winds blowing towards the north. Therefore, to be transported to the north by that wind pattern the ballistics would have had to be more than 7 km above the vent for a good part of their trajectory, which seems seams a very high value. Perhaps the location of the wind levels is not accurate, as the conversion from pressure levels to elevation levels was done assuming a standard atmosphere. There is also the possibility that the eruption was not vertically “directed” or symmetric around the vent, and that the ballistics left the source with a significant horizontal velocity component in a specific direction. A large trough opened during the eruption, connecting the summit with the base of the Mackenney cone direct to the northwest (see figures 6), at the base of the cone the fissure ends in what looks like a crater. If the ballistics were issued from an elongated or linear source (e. g. the fissure type

Figure 5. Wind velocity vectors interpolated from NCEP- NCAR reanalysis data, for the Pacaya location at 18:00 local time, on May 27th. The numbers correspond to the elevation of each vector in km.

structure) they could have been directed in trajectories parallel to this linear feature (i. e. directed to the northwest).

Figure 6. Trough opened on the northwest flank of the MacKenney cone of the Pacaya volcano. There is an inherent randomness in the variables that determine the destructive power and the distance that ballistics can travel, which makes precise deterministic predictions of the time and locations of ballistic impacts impossible. For individual ballistics, both the destructive power and the distance it can travel are governed by the initial kinetic energy (i. e. velocity and mass), size, density, shape, temperature and ejection angle of the ballistic, and the wind field in which it travels until it impacts the ground. When we consider an eruptive event as a whole, the ballistic impact hazard would be determined by the total size (total volume or mass of ejected ballistics), and the distributions of each of the variables that govern individual ballistics behavior. But before even considering this, the future timing and location of the source of ballistics ejection can also vary. Precise a priori knowledge of all this variables is impossible, however based on experience and observations of previous events, combined with intuition of what the behavior of the volcanic system may be in terms of vent location, energy of eruptions, etc., an idea of a likely range of scenarios can be developed. Two factors that result very important to gain an idea of how the hazard behaves are the relationships of individual ballistics reach and density of impacts with distance. For a ballistic of a given destructive power (e. g. kinetic energy) it can be shown that the likelihood of reaching a given location decreases rapidly with distance (Alatorre-Ibargüengoitia et al. 2006); moreover, for a given volume or mass of ejected ballistics, as the distance from the sources increases the area over which the ballistics will impact the ground also increases in inverse proportion to the distance and the angle of spreading.

These two factors combined result in a rapid decrease of the ballistic impact hazard with distance from the source, which is in agreement with what would be intuitively expected. Two sets of authors have published hazard zoning maps for Pacaya, including hazard zones for ballistic impacts. Banks (1986) considered a very large area (zone A in their map) as subject to both, large tephra accumulations and ballistic impact hazards. This area is elongated in a northeast – southwest direction, apparently reflecting the predominant wind directions (see figure 7) and extends to a maximum of ~ 10 km in the direction of the semi-major axis, and ~ 5 in the direction of the semi-minor axis. JICA et al. 2003 defined a circular area with a radius of 4 km centered on the summit of the MacKenney cone. These are very significant differences in the choice of a hazard zone and probably reflect very different criteria on the scenarios of activity to which they should be associated, but this, as with most other volcanic hazard maps, is something remains unclear at best. For eruptions of the magnitude and intensity of the ones that have happened during the present episode of activity (since 1961), the 4 km radius circle seems to be a reasonable area to be considered as hazardous due to ballistic impacts. However larger and more energetic explosions may eject ballistics at larger distances. Alatorre-Ibargüengoitia et al. 2006 considered distances of up to 12 km as subject to ballistic impact hazard at Volcan de Colima, Mexico. They chose this very long distance based on modeling of ballistic trajectories and a choice of (what appears to be) the worst case scenario for the energy at which ballistics could be ejected, by analogy to the energy with which a ballistic was ejected by a prehistoric eruption from neighboring Popocatepetl volcano (note that the ballistic from Popocatepetl was ejected to a distance of 7.8 km, but by using the model with different ballistic size, density, ejection angle and wind field they obtained a maximum distance of 12 km).

In any of the cases of hazard zoning discussed it becomes clear that towns at a distance of 2.5 to 3.5 km from the vent, and potentially closer if we consider migration and opening of new vents (e. g. in the case of the fissure discussed earlier) are well within the reach of ballistic projectiles. It shouldn’t therefore be too surprising that the ballistics ejected during the May 27th eruption reached these towns causing injuries and large amounts of damages to property. In at least five prior eruptions that occurred in January 21 and 25, 1987, May 20, 1998, January 16, 2000, and February 29, 2000 (GVP 2010) ballistics have reached populated areas (including the same town damaged in the recent eruption) injuring people and damaging property.

If as discussed previously the wind field plays an important role in determining the areas that will be affected by ballistic impacts, it seems that the towns of El Cedro, San Francisco de Sales and Calderas, located to the north of the volcano, are less exposed than the towns of El Patrocinio, El Rodeo and El Caracol, located to the west of the volcano (see figure 3), because the average wind directions at

Figure 7. Areas most severely affected by ballistic impacts and tephra accumulation, compared to previous hazard maps. The area researched by Kitamuara and Matias (1995) is also shown for reference.

different elevations levels are much more likely to transport the ballistics in that direction. Figure 8a shows the rose diagrams for the directions towards which the wind blows obtained from interpolating 60 years of wind reanalysis data (1948 to 2008) for Pacaya location at 1 km elevation intervals. Low level (< 7 km) and high level (> 18 km) winds are more likely to be blowing towards the west, whereas intermediate level (7 – 18 km) winds are more randomly distributed, if we do not consider the seasonal variation. This has been the case of several of the eruptions that have happened since 1961, for instance the eruptions of January and February 2000 ejected ballistics that impacted the town of El Rodeo.

Was then the May 27th eruption a very unlikely occurrence? Although the overall wind directions are overwhelmingly more likely to transport ballistics and tephra in the western direction if the time of occurrence of eruptions is randomly distributed throughout the year, this probability changes given that the eruption happens during a particular season of the year. Figure 8b shows another rose diagram but representing the wind directions for the months of April and May, and it can be seen that between 11 km and 17 km the predominant direction in which the wind blows is north-east, and the likelihood that the wind will be blowing in the north direction is not negligible. Two cases of eruptions happening on January 21st, 1987 and May 20th 1998 show a similar pattern of tephra and bombs dispersal. The January 1987 eruption ejected ballistics that injured 12 people and damaged the roofs of 25 houses in the town of Calderas (GVP 2010), whereas the May 1998 eruption dispersed tephra over Guatemala City, but without reaching any populated area with ballistics (GVP 2010). A note of caution has to be added at this point, because as mentioned earlier, the range of elevations at which the wind would transport ballistics in the north directions seems too high. It is possible that the wind reanalysis model interpolated for this location, inaccurately describes the wind field at different elevation levels. It is also possible that the conversion from pressure levels to elevation levels assuming a standard atmosphere is not valid for this case. These considerations have also important implications for the estimation of the column height derived from the tephra dispersal pattern, as will be discussed later.

Figure 8. Wind directions (towards where it blows) obtained from interpolated NCEP- NCAR reanalysis data for the period from 1948 to 2008 at the Pacaya location, and at 1 km vertical levels. Data are generated for each location every 6 hours. a, wind direction diagrams considering all the seasons during; b, wind direction diagrams considering only data from April and May.

a b

Tephra dispersion and deposition Tephra from the May 27th eruption was dispersed to the north of the volcano over an area in excess of 1,000 km2 (see figure 2). As noted earlier, some of the clasts falling within ~ 4 km of the vent and roughly within the north quadrant can be considered ballistics, and caused serious impact damage, however a larger fraction (by mass or volume) of clasts of smaller size also deposited within this area and beyond. Measured thickness of the tephra deposit range from 47 cm at 1 km from the vent (Hetland personal communication), to a few millimeters at distances of over 70 km. Maximum grain size also decreases with distance, from > 50 cm bombs within 1 km from the source to ash size material at several tens of kilometers.

Figure 9 (R. Cabria personal communication) shows some tephra clasts fallen in Guatemala City, some 22 km to the NNE of the source. The grain size of tephra that fell in Guatemala City ranged from sub-millimeter to centimeter size and the clasts consisted of black to dark brown vitric (crystal poor) scoria. Fine ash (< 0.063 mm) fragments must have only amounted to a minor or insignificant fraction, and suspended particles in the air were not reported during or following the tephra fall. The tephra fell mixed with rain, which complicated estimating the accumulated thickness of the deposit; especially where the deposit is thin (< 1 cm). Thickness reports vary from 10 cm in the southern parts of the City (e. g. the north shore of Amatitlan Lake) to 0.5 cm in the central part of the City (see figure 2).

However, thicknesses in excess of a few cm at those distances should be taken with skepticism; thickness values reported by (apparently) anonymous contributors to the SEVIR – CATHALAC web page (accessible at http://tinyurl.com/2963ohn) vary between 2 mm and 10 cm, and show a very complex spatial pattern over Guatemala City (see the “problematic area” annotated in figure 2), with local maxima and minima in thickness dispersed more or less randomly over the area, instead of a more uniform and smoothly varying thickness surface. Whether this pattern is real or an artifact produced by errors and inaccuracies in the reported thickness and locations of the sampling points, it is not clear. The methods used for measuring the thickness and locating the sampling points are not known either. Unfortunately this is the only detailed dataset on tephra thicknesses available. Official reports for the central and southern parts of the city account for a maximum thickness of 2.5 cm (CONRED Boletin informativo No. 856, dated on June 16, 2010 at 11:00), and a maximum thicknesses of 8.7 cm has been measured at ~2.5 km to the north, at the Park Visitors Centre in San Francisco de Sales (Hetland personal communication). I seems from this that some of the data reported in the SERVIR – CATHALAC dataset may be exaggerated, especially those values ≥ 70 mm in the Guatemala City area. From the thickness data points reported by SERVIR – CATHALAC, an isopach map for three thickness values (1, 10 and 100 mm) has been constructed (figure 2). This map however is inconsistent at several

Figure 9. Tephra fragments fallen in Guatemala City. Photo by R. Cabria.

data points, because of the difficulty of fitting a smooth surface to the data, as noted earlier. By fitting a power law to the relationship between thickness and area enclosed by each isopach, and then integrating this function over the domain of thicknesses (from 1 to 100 mm) a volume of 1.3 x 107 m3 has been calculated (figure 10a). This volume can be compared with the volumes of tephra deposits produced by three other recent eruptions at Pacaya, obtained by the same method applied to isopach maps published by the GVP 2010… (See Table 1 and figure 10).

From this comparison it can be seen that the volume of erupted tephra in the recent eruption was larger than any of the other eruptions considered, but in the same order of magnitude. It has to be noted that these volumes (including the volume of tephra for the recent eruption) may be very inaccurate due to the poor quality of the original data, mainly the poor constrain on the number and location of sample points, and the possibly large errors in reported thicknesses. Large inaccuracies can also arise due to the poor fit of the power law relationship to the isopach thickness and area dataset, as well as the choice of integration limits. No attempt is made to define an uncertainty measure but the real values of tephra volume could be several times higher or lower than the reported values, such that the estimate is probably good to only the order of magnitude estimation. Kitamura and Matias (1995) reconstructed the tephra stratigraphy for eruptions of Pacaya for the last ~ 1,500 years, deposited to the west of the volcano. They reported nearly 500 individual unit thickness measurements for points distributed over an area of ~ 70 km2 (see figure 7). From a total of 19 units and subunits, isopachs for 6 units were used to estimate volumes of tephra deposits. Some of the original isopachs by Kitamura and Matias (1995) had to be “closed” in order to calculate the areas, this was done considering the likely source for the tephra unit as either a point coincident

Figure 10. Linear best fitting of isopach area on thickness data in a logarithmic space (power law fitting) for tephra deposits erupted during the May 27, 2010 (a), July 27 – 31, 1991 (b), November 11, 1996 (c), and May 20, 1998, as reconstructed from the isopachs reported by the Global Volcanism Program (GVP, 2010).

a

b

c

d

with the current central vent (at the summit of the MacKenney cone) or the Cerro Chino vent. The volumes of these units (Pc – Pt 2, 5b, etc.) are also shown in Table 1. Notice that the lower integration limits for these units are 10 or 20 cm, and therefore their total volumes are grossly underestimated, the actual volumes could be several times larger than the volumes presented in Table 1, and therefore these values are strictly minima. Table 1. Tephra volumes.

Tephra Unit Volume (m3 x 106)

Lower integration limit (m)

Source of the Isopachs

May 27th 2010 13 0.001 Derived from thickness data from SERVIR – CATHALAC

May 20th 1998 1.6 0.001 GVP, 2010 November 11th 1996 1.8 0.001 GVP, 2010 July 27 – 31, 1991 7.6 0.001 GVP, 2010 Pc – Pt 12 c 1 0.1 Kitamura and Matias, 1995 Pc – Pt 12 2.7 0.1 Kitamura and Matias, 1995 Pc – Pt 10 a 2.9 0.1 Kitamura and Matias, 1995 Pc – Pt 7 5.3 0.1 Kitamura and Matias, 1995 Pc – Pt 5 b 2.6 0.1 Kitamura and Matias, 1995 Pc – Pt 2 5.8 0.2 Kitamura and Matias, 1995

Figure 11. Thickness vs. distance plot for the data published by Kitamura and Matias (1995), and the data for the May 27, 2010 eruption.

Figure 11 shows a plot of thicknesses vs. distance from the vent for two data points from the recent eruption, and for the Kitamura and Matias (1995) units individual measurements (points), and unit isopachs (lines), showing also the distances to the town closest to the volcano. It can be seen that although at a close distance from the vent (1 km) the thickness of the tephra deposit from the recent eruption is very large, it thins very rapidly, such that if falls below the thicknesses that are commonly associated with static tephra loadings capable of damaging roofs (Spence et al., 2005).

It is interesting to see that in the last ~ 1,500 years, there has been a significant number or eruptions that produced thick (> 10 cm) tephra deposits beyond distances comparable to those at which many towns around Volcán de Pacaya are located. These older deposits are several times thicker than the deposit from the May 27th 2010 eruption, at those distances.

However this dataset comes from an area to the West and South-West of the volcano; from Figure 8a we can see that winds would tend to transport tephra predominantly in that direction. This is the area where the towns of El Patrocinio, El Caracol and Los Pocitos are located. To the north, wind directions are less likely, and therefore the hazard for town in that area would be less, however, as the May 27th and other eruptions show (e. g. May 20th 1998, see figure 12), non predominant wind directions shouldn’t be ignored when considering tephra fall hazards. The tephra fall hazard map presented by Banks (1986), shows two nested oval hazard zones (see figures 7 and 13). The inner zone (zone A in the map) is defined as the high hazard zone, and it is the same as the ballistics hazard zone discussed earlier, i. e. an area elongated in a northeast – southwest direction, apparently reflecting the predominant wind directions (see figure 8a) and extending to a maximum of ~ 10 km in the direction of the semi-major axis, and ~ 5 in the direction of the semi-minor axis. The outer zone (zone B in the map) is defined as lower hazard, and has a similar shape to the high hazard zone but covering a larger area and with an elongation axis slightly counterclockwise rotated with respect to zone A. In the original map available to the author, zone B was not a closed polygon (the boundaries of the zone went outside the maps printed area), therefore, in order to present a coherent picture of the hazard zoning the polygon was closed based on my criterion. For the zone B the semi-major axis has a length of ~ 22 km and the semi-minor axis has a length of ~ 11 km. The elongation of the hazard zones, as noted earlier is probably intended to represent preferential directions in which the tephra would be carried by the wind.

Figure 12. Isopach maps for tephra deposits associated to recent eruptions from Pacaya.

The map presented by JICA et al., (2003) defines two concentric circular areas of radii ~5.5 km and 8 km, for hazards zones where tephra thicknesses in excess of 10 cm and 5 cm are expected, respectively (see figures 7 and 13). The circular geometry of these hazard zones implies that no preferential direction is considered. It becomes clear that both groups of authors considered different criteria to define the tephra fall hazard zones, in terms of eruption size and intensity, as well as wind velocity field influence. These differences

make it difficult to compare both maps and to interpret them in light of the recent eruptions, but more important, it may also make it hard for the final users of the maps (e. g. civil protection officials and civil authorities). The obvious influence of the wind in the tephra dispersal pattern justifies including an analysis of the preferential wind directions in the hazard assessment. To illustrate these effects, two probabilistic tephra thickness exceedance maps (see figure 13) were constructed for eruptions with volumes of 107 m3 and 108 m3, and eruptive column heights of 12 and 19 km, respectively. The maps were generated by a Monte Carlo technique (10,000 runs), recursively running the numerical tephra dispersion model “Ashfall” by Hurst (1994) in Matlab ®, assuming a grain size distribution equal to that of the October 14, 1974 Fuego tephra (Rose et al. 2007), and using interpolated 6 hour wind reanalysis data (Kalnay et al. 1996) from the 1948 to 2008 period. Elevations were derived assuming a standard atmosphere. The resulting expected 10 mm and 100 mm isopachs have a strong westward orientation, more so than the ovals defined by Banks (1986). The probabilistic maps were constructed using wind data from all times during the year, but seasonal maps could also be constructed. As with the case for ballistic impact hazard, the towns to the north of the volcano seem to be less exposed to tephra fall hazards than towns at a similar distance but to the west of the volcano, due to this being the preferential direction in which wind will transport the tephra. The rose diagrams for wind directions derived from wind reanalysis data (Kalnay et al. 1996) discussed earlier (figure 8) lead to an analogous discussion, i. e. low level (< 7 km) and high level (> 18 km) winds are more likely to be blowing towards the west, whereas intermediate level (7 – 18 km) winds are more randomly distributed, if we do not consider the seasonal variation. But if we consider the seasonal variation (e. g. the wind directions for only the months of April and May) the pattern changes in a subtle but significant way, with winds between 11 and 17 km blowing predominantly towards the north-east. The distribution of the few tephra deposits for which there are geographic distribution data seem to agree with this (see figure 12) As discussed earlier for the case of the ballistic impacts geographic distribution, the wind profile during the eruption seems to have carried tephra and even ballistics, at high altitudes in the north-east and north

Figure 13. Tephra fall hazard maps and modeled probabilistic hazard maps.

directions (see figures 2 and 7). If the wind reanalysis model and the interpolation are correct, a significant amount of the tephra in the eruptive column must have reached elevations well above 7 km. Wind directions start to change rapidly between 8 and 14 km becoming erratic, and with speeds becoming smaller (see figure 5), corresponding with the location of the tropopause; this implies that for tephra to be dispersed to the north-east, it would have had to penetrate the tropopause and maybe even the lower stratosphere, above 14 km.

Even for relatively high elevations (8 – 17 km) the main direction of winds would be expected to transport tephra to the north-east, rather than to the north-north-east where most of the tephra seems to have deposited according to the SERVIR – CATHALAC thickness reports (see figures 2 and 7). The wind pattern changed over the following hours, to a more common west trend, as can be seen from the dispersal of an SO2 cloud that most likely derived from this eruption, the cloud was visible on satellite images taken ~ 18 hrs after the eruption, and it was located ~ 100 - 400 km to the west-south-west of the volcano (S. Carn, personal communication). The reanalysis wind profile between 9 and 12 km shows this western wind direction (see figure 14), which further helps to contain the elevation of the cloud. The SERVIR – CATHALAC tephra thicknesses dataset include predominantly locations within the Guatemala City conurbation. The thickness and locations data are based on apparently spontaneous and voluntary reports by users and visitors to the web site. Although the distribution of locations could be representative of the tephra deposit (i. e. reflecting the real spatial distribution of the deposit), it is very likely to be influenced by the geographic distribution of potential users and visitors of the SERVIR – CATHALAC web site. In that case the spatial distribution of points would be an artifact of where most people are concentrated; in other words, even if tephra fell on regions outside the Guatemala City area, less o no reports come from those regions because less o no people were available to record and report those data. Such a region could be located to the east and south-east of the City (i. e. to the north-east and east of the volcano).

Tephra dispersion was modeled using the numerical tephra dispersion code “Ashfall” by Hurst (1994), taking the reanalysis wind data for approximately the time of the eruption (18:00 local time), and using a volume of 107 m3 and heights of 3, 7 and 13 km as inputs for the model (see figure 15). As expected, the overall dispersion pattern shows counterclockwise rotation from east-north-east for low elevation columns (i. e. 3 km) to north-north-east for higher elevation columns (i. e. 13 km). In any case, a significant amount of tephra would have been deposited to the east and south-east of Guatemala City, and therefore the estimate of volume based on the isopachs derived from the SERVIR – CATHALAC dataset would underestimate the total volume or erupted tephra. The SERVIR – CATHALAC ground also analyzed Landsat satellite images acquired shortly after the eruption (see http://tinyurl.com/2963ohn), which however show the area to be directed more towards the north-north-east. Given the uncertainty in the size (i. e. volume) and intensity (i. e. column height) of the eruption, an inversion for these parameters using an advection – diffusion model was attempted. The Asfall code by Hurst (1994) was used recursively in Matlab ®, and the sum of the squares of the differences in tephra thicknesses predicted by the model and those reported in the SERVIR – CATHALAC dataset was minimized, by performing a grid search over the two parameter space of tephra volume and eruption

Figure 14. Wind velocity vectors similar to those shown in figure 5, but for May 28, 12:00 local time.

column height, giving 41 volume values between 106 m3 and 108 m3, and 48 column height values between 1.5 km and 25 km, for a total of 1968 runs (see figure 16).

From figure 16 it can be see that it was not possible to find a well constrained minimum for the explored volumes and column heights parameter spaced. The poor quality of the distribution of data points and reported thicknesses in the SERVIR – CATHALAC discussed previously may be responsible for this, although a poor wind model as derived from the reanalysis dataset may also have contribute to the poor optimization. From the inversion a rather large volume and unreasonably high eruptive column are obtained as the best fit to the actual dispersion data. The lack of data to the north-east and east of the volcano play here a very

important role, since this is likely to be the area where the fit is the poorest (see figure 16). Running the Asfall code with the optimum values obtained from the inversion (a volume of 8 x 107 m3 and column area of Guatemala City would help to better constrain the model and may allow a successful inversion.

Figure 16. Residuals of a simple ash inversion for two model input parameters (volume and column height). Part a shows the 2D plane view of the sum of the squares of model fit residuals, and part b shows the same surface in 3D. Regarding the damage caused by the tephra fall, it is difficult to assess the relative contribution of tephra static load to the damage caused to buildings in the areas where tephra accumulation were significant (> 5 cm), in part because these areas also suffered the damage from ballistic impacts. Some roofs seem to have been weakened by the damage inflicted by ballistic impacts, and subsequently collapsed by the static load of the tephra, but this is only a conjecture. The impact of the tephra blanket on agriculture may also be significant, but no estimates of this were available to the author. The impact of the tephra blanket on the city was overall small, other than the surprise it cause on the population. The only serious

Figure 15.

a b

consequences were the indirect death of one person, who died when the roof he was cleaning the tephra from collapsed (see http://tinyurl.com/2uu6xoo), and the closing of the La Aurora International Airport for five days (see http://tinyurl.com/2d3ahcr). The tephra has been linked to a case of subsidence that happened a few days after the eruption in northern Guatemala City, during storm Agatha (CONRED Boletin informativo No. 779, dated on June 2, 2010 at 7:07, see also http://tinyurl.com/37d4k8t) , but so far no link has been proved, and the proposed link seems very unlikely, via clogging and erosion of the sewer system by the tephra that accumulated in parts of Guatemala City, and was subsequently transported to the sewer system. Lava flows Lava flows at Pacaya have been active almost continuously since 2005, being one of the main attractions for tourist who visited the Park. The increase in the effusive activity since January (see first section) reached a climax around the 27th of March and the following days, when the activity migrated to the south-south-east flank of the volcano (see figure 17). The rate of advance of the flows in the first days after the eruption was very high, on May 29th the flow had destroyed three isolated houses and according to CONRED was moving at a speed of ~ 100 m/hr (CONRED Boletin informativo No. 748, dated on May 29, 2010 at 23:16, see also http://tinyurl.com/2erbrfk). The flow rapidly slowed down over the next few days, to a speed of ~ 15 m/hr by June 6th (see http://tinyurl.com/2a83ab8) and to ~ 1 m/hr by June 8th (http://tinyurl.com/252goob), reflecting a similar decelerating eruption rate. With the slowing down of the flow the threat to the communities to the south also decreased and only a few dispersed houses were within less than 400 m from advancing lava flows (http://tinyurl.com/252goob). The last field mapping of the lava flows by INSIVUMEH was done on June 4th (see http://tinyurl.com/27jwql9). By June 9th the activity at Pacaya had reached a level considered “normal” by CONRED, and no further specific mentions were made about the hazard associated with the advancing lava flows, but a general statement about the hazard for the area is included in the official bulletin of that day (CONRED Boletin informativo No. 832, dated on June 9, 2010 at 12:37). The location of the new vent marks a significant change in the activity at Pacaya, as it is located “outside” the Old Pacaya collapse scar (see figure 17). All the previous vents formed during the present activity episode since 1961 were located “inside” (to the south and west) of the Old Pacaya edifice collapse scar (Matias, 2010) and most of them were clustered near the MacKenney cone central vent (see figure 17). Whether this is due to structural reasons (e. g. the area is an easier path for the magma to go through, because a significant part of the edifice was removed in the collapse) or not clear. The relatively low (~ 1800 masl) and far (~ 1.8 km from the MacKenney cone central vent cluster) location of the new vent is unusual, and has happened only in a few occasions since 1961, most significantly for the early (1960’s – 70’s) and voluminous flows at Pacaya. The largest flow previously erupted since 1961 (the Cachajinas flow, see discussion ahead) had a vent located at a similar elevation and distance from the MacKenney cone central vent cluster. The locations of some vents of older (but perhaps historic in age) flows seem to have been located also “outside” the collapse scar (see flows OL – 1 through 5 in figure 17), to the east of the volcano. Although these flows have been previously mapped as prehistoric, their young appearance on high resolution orthophotos, in comparison with other recent lava flows, make me think that they may actually be younger, i. e. associated to eruptions that happened during the historic period (1565? – 1846?). The new vent location falls on a NNW – SEE linear trend between the historic Cerro Chino (1775?) vent, the central MacKenney cone vent, and older flow vents (flows OL – 1 through 5), a trend that was also

followed by the “fissure like” structure that formed during the most intense phase of the May 27th eruption (see figure 6, and 17). This has some interesting implications on the possibility of a strong structural control on the location of the vent, and maybe on the prospect of the volcano edifice stability, as will be discussed later.

Figure 17. Location of the new features generated by the May 27th 2010 eruption at Pacaya, compared with older geologic elements. The flow (referred in singular from here onwards) reached a total length of ~ 5 km, from the vent to the furthest point along the flow path (according to the INSIVUMEH mapping). The planimetric are of the flow is ~ 1.8 km2, including an area of ~ 0.8 km2 that was not included in the INSIVUMEH map (area delimited by dashed red lines in the map of figure 17) and was inferred as being part of the flow, given the flow extend and the vent location. The flow originated at an elevation of about 1800 masl, and followed down the local gradient with almost south direction (azimuth ~ 190º), down to an elevation of ~ 1400 masl (~ 3 km of horizontal distance), where the local gradient turns to the west along the channel of the Chupadero river, and followed down that direction to an elevation of ~ 1200 masl (~ 2.4 km of horizontal distance). This turn in the trajectory was very fortunate for the communities to the south, some of which were within

500 m of the front of the flow. Older flows originating from vents slightly to the east have reached those populated areas (i. e. OL – 1 in figure 17).

To put the May – June 2010 lava flow event in context a comparison is made with mapped lava flows erupted from 1961 to 2009. The map in figure 17 shows the largest lava flow erupted in the current activity episode since 1961, the Cachajinas lava flow. The Cachajinas flow reached a length slightly over 5 km, barely larger than the flow emplaced since May 27th. Figure 18 shows the lava flow lengths, planimetric areas and volumes, as estimated by Matias (2010) for 248 mapped lava flow units erupted between 1961 and 2009. The recent flow (shown as a red circle) ranks as the second longest, the third most extensive (in planimetric area covered), and the seventh most voluminous in the entire dataset. Some of the older flows that had vents located outside the collapse scar to the east of the volcano (see flows OL – 1 through 5 in figure 17) reached also very long distances and cover even larger areas. This reminds us of the potential that such a larger flow could happen in the future and reach inhabited areas. Before 2006 only a limited number of lava flows had crossed the scarp formed by the most recent edifice collapse (Eggers, 1972). In 2006 the first lava flows erupted in the current activity episode (since 19610 crossed the collapse scarp, opening the possibilities of flows reaching populated areas at ~ 2.5 km to the north (i. e. San Francisco de Sales, Calderas and El

Cedro). Renewed effusive activity at the MacKenney cone has the potential to generate flows that would flow in that direction. However, as for now, the activity remains focused on the new vent. The lava flow hazard map published by Banks (1986) defines three hazard zones, from a high to a low categories, and it includes locations that extend beyond 18 km from the MacKenney cone central vent (see figure 19). The definition of the hazard zones seems to be determined by the likelihood of future vent locations and the topography that would constrain the flow of the lava from those vents. The high hazard zone seems to be defined by considering vents located “inside” the Old Pacaya edifice collapse scar, with

Figure 18. Comparison of lava flow geometric parameters for lava flows erupted since 1961 (as calculated by Matias, 2010), shown as blue dots, and the geometric parameters of the lava flow erupted during the May – June 2010 eruption, shown as the red circle.

flow extending some 12 km from the highest point within that zone (the MacKenney cone central vent). The exception to this seems to be flows that could be issued from a vent near or at Cerro Chino, since the area to the north-west of Cerro Chino is also included in the high hazard zone. For the medium hazard zone the authors seems to have tried to account for vents “outside” the Old Pacaya edifice collapse scar, as well as for longer run out flows “inside” the scar. The low hazard zone accounts for flows with longer run outs to the northwest.

The hazard map by JICA et al. (2003) shows a single area as subject to lava flow hazards (see figure 19), which is much smaller than the areas considered by Banks (1986). The JICA et al. (2003) map also places a large emphasis on the Old Pacaya collapse scarp as a topographic barrier for flows with vents to the west and south of it, apparently without considering the possibility of vent locations to the east.

According to the JICA et al. (2003) map, the only areas considered as subject to lava flow hazard are those that could be reached by lava flowing over the collapse scarp were it has been buried by other deposits, to the east of the MacKenney cone, and where in fact some historic (pre 1961) flows have crossed the scarp. Both hazard maps seem to have underestimated the possibility of what happened during the recent eruption, the formation of a vent “outside” of the collapse scar. This vent location and the possibility that activity continues on that vent, and even that other vents form on that flank of the volcano, put a large area to the east within the reach of potential future lava flows, which changes the hazard exposure situation at the volcano. Before newest vent formed, only four communities to the west and south west may have been considered to be in close range of future lava flows (see figure 19), totaling ~ 2,000 people exposed. Considering the possibility of vents forming on the east flank of the volcano and lava flowing in that direction may expand the number of towns to 19 or so, with a total exposed population of ~ 7,800.

Figure 19. Comparison of the area affected by the new lava flow and previous lava flow hazard maps published for Pacaya.

Changes in the edifice, tectonic - structural implications and relationship with new vents Previous to the May 27th eruption a series of lateral vents feeding lava flows had open on the flanks of the MacKenney cone and had typically been active for periods of days to months. This activity of sporadic flank vent openings and lava flow eruptions had lasted 5 years and had built a series of mounds around the MacKenney cone, mainly on the north and west quadrants. The May 27th eruption significantly altered the morphology of the MacKenney cone, forming a trough extending from the summit to the base of the cone, in the north-norht-west direction (see figure 6). The trough is approximately 100 m wide, 600 m long and 50 – 80 m deep. The lower part at the base of the MacKenney cone is wider, with a slightly circular planar shape, and resembles a crater. The nature and significance of this trough is unclear, but it is likely to be related to the local and regional structure and tectonics, and possibly reflect the state of stresses of the volcanic edifice, at least during the eruption. Different hypothesis could be considered to explain its origin and nature: first, it could be a fissure that opened during the most intense phase of the eruption, and from which the tephra was ejected explosively; second, it could have formed when a crater opened at the base of the MacKenney cone and the flank of the cone upslope from the crater collapsed down-slope in a retrogressive erosion process forming the elongated depression; third, it could be a large fissure that opened by extension as a large sector of the edifice moved to the south-west in the down-slope direction; and fourth, it could be due to subsidence after a large volume of magma withdrew (or was erupted) from underneath the volcano.

The fire fountain observed in some of the video footage (B. Hetland personal communication) appears to show an extended source, or possibly two vents separated by an area of less or non incandescent material ejection. This observation would be consistent with the hypothesis of a fissure as the linear vent for the most intense phase of the eruption; however the video resolution is not good enough to confirm this. The margins of the trough seem to be draped with a smooth surface or lava that could have originated from the fissure (see figure 20), but it is difficult to tell only from pictures.

The hypothesis of the formation of the trough by retrogressive land sliding / erosion caused by the opening of a crater at the base of the cone would require explaining the faith of the eroded material, and the geometry of the trough. If the sliding / erosion happen coevally with the intense pyroclastic ejection, it is conceivably that the material was incorporated in the eruptive column and consequently was dispersed as ballistics projectiles and smaller tephra fragments over a wide area. The geometry of the trough, which from the photographs appears to have a more or less constant width, would depend on the mode of failure of the portion of the edifice that moved down-slope. Considering that

Figure 20. Upper end of the trough, draped in ejecta. Photo by G. Chigna.

the edifice has roughly a conical morphology, and assuming that a failure surface that terminates on a more or less circular bottom crater would have an inverted conical shape (becoming wider upwards), the geometry of the failure surface would be determined by the intersection of these two geometries; i, e, the conical shape of the volcanic edifice with the inverted conical shape of the failure surface. In any case it would be expected that the trough had a narrower base, determined by the crater at the base of the edifice, widening upwards to a maximum and narrowing again at the summit area. This is not the geometry observed for the trough and it seems difficult to explain its rather straight chute morphology by invoking this mechanism. The displacement of a large portion of the edifice as the origin of the trough would imply a large scale deformation over a good portion of the cone. This isn’t apparent from the photographs, but a more detailed examination is required, possibly using GPS or other high precision surveying techniques. A GPS network of benchmarks for high precision surveying has been established by the Instituto Geografico Nacional of Guatemala (IGN), and more recently this network has been expanded by a group of researchers at Michigan Technological University. This offers the possibility to further assess the possibility of a large scale permanent edifice movement. This possibility is especially worrisome for the hazard implications it carries. If the volcanic edifice experienced a large movement, the possibility of a collapse in the direction perpendicular to the trough, to the south-west (see figure 17) would become a major concern in the event of another eruption or any event that would destabilize the part of the edifice. At least one previous edifice collapse (Vallance et al. 1995, Kitamura and Matias, 1995, Siebert et al. 2006) has happened in the last 1500 years, and other structures, like the horse-shoe shaped depression that holds the Calderas lake, could also be related to previous but relatively young collapses.

Regarding the fourth and final hypothesis, the eruption of large volumes of lava during previous eruptions at Pacaya has caused significant subsidence features. At the start of the current cycle of activity in 1961, the longest flow of the cycle so far was erupted (the Cachajinas flow, see figures 17 and 18), the following year (on June 10, 1962) and without any other eruption happening in between, an oval area ~300 x 200 m subsided on the north-west flank of the volcano, very close to the summit (see figure 21). The very elongated shape of the trough that formed in the May 27th eruption however makes this mechanism seem unlikely as a possible explanation for its origin. It is unclear if a very elongated magma body (e. g. a dyke) would cause such a subsidence. In the case of the 1962 subsidence, a link with the evacuation of a large volume of magma at a lower elevation (erupted as the Cachajinas flow) seems to be related to the subsidence. The eruption of a large lava flow during and shortly after the May 27th eruption (see figures 17 and 18) could be the cause of the hypothesized subsidence. A smaller flow is also visible at the foot of the trough, but its volume seems rather small to produce a large subsidence. If subsidence has occurred due to magma eruption, it would imply a very shallow magma storage system, an idea that has been hypothesized for Pacaya before (Eggers, 1983; Matias, 2010).

Figure 21. Collapse crater from 1962 and other structural features aligned in a north-north-west orientation. Photograph IGN 1962.

The trough may be related to the structure and tectonics of the upper crust on which Pacaya is located. Based on the geological interpretation of fault motion, the focal mechanism solutions and coseismic displacement of local earthquakes, and the measured tectonic movement rate by GPS, as reported by several authors, a synthetic picture of the tectonic deformation regime of the crust on which Pacaya is located can be drafted. The southern portion of the Guatemala is located on the Caribbean tectonic plate, and it’s subject to an ~ 8 mm/yr east-west crustal extension (and possible block rotation), which has formed a series of north-south trending grabens (Burkart and Self, 1985; Guzman-Speziale, 2001; Guzman-Speziale et al., 2005; Caceres et al., 2005; Lyon-Caen et al., 2006).

The Caribbean plate in this region is also split by the right-lateral strike-slip Jalpatagua fault zone, which broadly coincides with the northwest-southeast trend of the volcanic arc, and moves at a relative rate of ~ 10 mm/yr (Carr, 1976, White and Harlow, 1993; DeMets, 2001; Guzman-Speziale et al., 2005; Lyon-Caen

Figure 22. General tectonics and regional deformation patterns around Pacaya. Part a shows the tectonic plates and the regional deformation pattern. Part b shows the local pattern of faults and the focal mechanism solutions for earthquakes located at a depth of less than 35 km. The 30 km and 50 km circles show the areas were the orientation of faults was analyzed (see figure 23)

a b

et al., 2006). A set of conjugate northeast-southwest faults exhibiting left-lateral strike-slip movement (Carr, 1976, White and Harlow, 1993) intersect the Jalpatagua fault zone and the volcanic arc at a high angle (see figure 22). The Caribbean plate in this region is also split by the right-lateral strike-slip Jalpatagua fault zone, which broadly coincides with the northwest-southeast trend of the volcanic arc, and moves at a relative rate of ~ 10 mm/yr (Carr, 1976, White and Harlow, 1993; DeMets, 2001; Guzman-Speziale et al., 2005; Lyon-Caen et al., 2006). A set of conjugate northeast-southwest faults exhibiting left-lateral strike-slip movement (Carr, 1976, White and Harlow, 1993) intersect the Jalpatagua fault zone and the volcanic arc at a high angle (see figure 22). Pacaya is situated at or near to the intersection of the Guatemala City Graben extension zone and the Jalpatagua fault zone (see figure 22), coincident also with the southern rim of the Amatitlan Caldera (Wunderman and Rose, 1984). The exact location and width of the Jalpatagua fault zone is not well defined, but Wunderman and Rose (1984) consider that the fault bisects the Amatitlan Caldera some 10km to the north of Pacaya, whereas Conway (1995) considers that the fault intersects the volcano. The available geologic maps for the area (IGN/Eggers 1969; Eggers, 1972; Carr, 1976; IGN/Bonis 1993) show a system of faults that run parallel to the main (and most obvious) topographic expression of the fault trace, and which if projected would intersect Pacaya to the north-west (see figures xx and yy). Conway (1995) argued that this regime of extension and the intersection of the dextral strike-slip fault would facilitate the ascent of magma at Pacaya, and this in turn would explain the monotonicity and unevolved nature of its magma. Cameron et al. (1992) also found evidence that decompression played an important role in the generation of magmas at Pacaya, and proposed that this could be related to the extensional crustal tectonics. Mapped fault orientations within 30 km and 50 km from Pacaya reflect the orientation of the main tectonic features so far described (see figures 23), dominated by north-south oriented faults, probably related to the east-west extensional structures (e. g. the Guatemala City Graben). The northwest-southeast preferential orientation shown in the rose diagrams most likely corresponds to the structures oriented along the Jalpatagua fault zone.

Figure 23. Orientation of mapped faults around Pacaya, as shown in figure 22. Part a shows faults mapped within 30 km of Pacaya, and part b shows the faults mapped within 50 km of Pacaya. The numbers on the concentric circles give the frequencies of faults within each orientation bin.

a b

From this horizontal stress configuration, a likely stress field for Pacaya can be guessed. Assuming that the stress field at Pacaya is influenced by both the stress field present to the north (characterized by east-west extension) and the stress field inferred for the Jalpatagua strike-slip fault zone (characterized by shearinbg), the stress field inferred by superposition for Pacaya would be an intermediate transtension, with the principal horizontal compression axis aligned on an intermediate orientation between the northwest-southeast Jalpatagua fault zone, and the north trending grabens, this would give a north-north-west orientation (see figure xx). Following Nakamura (1977), the orientation of fissures and the distribution of dikes and parasitic vents can be related to the state of the regional stress on which a volcano is emplaced. A system of dikes radiating from a central conduit will tend to “bend” and align parallel with the direction of the principal horizontal (regional) compressive stress (or equivalently, perpendicular to the principal tensional stress), as it propagates away from the conduit and the regional stress becomes dominant over the stress induced by the presence of the central conduit. This will also be reflected in the distribution of vents, if we consider that the vents are points in which dikes intersect the surface.

In the case of Pacaya, the orientation of the trough appears to be in north-north-west direction, as would be expected from the very simplified stress analysis. Moreover, this orientation also coincides with the opening of the new vent that formed on the south-east flank of the volcano, and with older important vents, e. g. Cerro Chino, the vent of the large flow at the base of Cerro Chino, and the vents of flows OL-1 and OL-2 (see figures 17 and 21).

Nakamura (1977) also notices that other morphological cues can suggest the preferential orientation of vents and therefore of the dikes that radiate from a central conduit, such as the growth of the volcanic edifice being elongated in the direction of preferential vent formation; this also applies to Pacaya, as can be easily seen from the shape of the elevation contours, and it coincides again with the north-west or north-north-west orientation noted before. Most significantly, the north-north-west orientation is perpendicular to the direction of the last debris avalanche collapse. Norini and Lagmay (2005) have shown that symmetrical cones traversed by strike-slip faults may undergo deformation and ultimate collapse in an orientation that forms a high angle with the fault, forming a summit subsidence rift and later developing larger sector collapse failures; although it is only a possibility, this could be the case for the deformation and vent pattern observed at Pacaya. Norini and Lagmay (2005) also noticed that even for large degrees of strike-slip deformation, symmetrical cones tend to remind symmetric, “hiding” the internal deformation at even advanced states of faulting; this is aggravated by recurrent eruptive activity that resurfaces the cone and conceals the deformation. Has Pacaya been significantly deformed by the strike-slip displacement of the Jalpatagua fault zone? Has the east-west extension of the Guatemala City graben combined with the strike-slipe displacement of the Jalpatagua fault zone played a role in the distribution of vents, and more recently in the formation of the trough and appearance of the new flank vent during the May 27th eruption? How compromised is the structural integrity of the edifice due to tectonic deformation, and therefore how serious is the hazard of a large collapse? These questions are difficult to address and can’t be answered with the current

Figure 24. Possible stress state at Pacaya, inferred from the superposition of the main regional and local stresses.

information. From a positive perspective, the very young age of the current Pacaya cone (less than 1,500 years) would still require a long period of time to achieve a high degree of internal deformation due to shearing from the Jalpatagua fault, given the fault’s displacement rate of 10 mm/yr, probably on the order of tens of thousands of years. This however has important implications for other volcanoes along the arc and which could also be traversed by this fault system. The hazard, the exposure, the risk prospects, the possibility of an early warning, and other possible strategies to cope with future eruptions. Communities around Pacaya have been evacuated during eruptive crises in at least 8 previous occasions since 1970 (GVP, 2010), and probably a few more times since the current episode of activity began in 1961. During the same period of time at least five eruptions have injured people in the communities, mainly caused by the impact of ballistic projectiles. Tephra dispersal and deposition of prehistoric and possibly early historic eruptions studied by Kitamura and Matias (1995), in the last 1,500 years have produced more than 10 tephra blankets of enough thickness to cause severe structural damage to houses, at distance comparable to the distance at which the current communities are located. Some of which are located at less than 3 km from the currently active vent; in this sense, the impact caused by the May 27th eruption shouldn’t come as a surprise. More importantly, similarly destructive and damaging eruptions should be expected in the future, if no measures are adopted to reduce the potential impact; this of course is not an easy task. From the three volcanoes currently erupting in Guatemala, Pacaya is the one with the largest population at a closer (< 12 km) and longer (> 15 km) ranges (see figure xx), outnumbering Fuego and Santiaguito due to the high concentration of population in cities to the north and west of the volcano. What are the main risks to which this population is exposed? Previous authors have discussed many aspects related to the hazard associated to the eruptions of Pacaya (Banks 1986; JICA, 2003; Vallance et al., 1995), and a detailed discussion of those aspects would be redundant, but a few points can be made in light of the recent events.

From the preceding discussion it should be clear that ballistic projectiles impact and tephra fall are the most likely hazards to reach populated areas near to the volcano. More than ten thousand people are within the reach of ballistic projectiles and in areas where tephra accumulations could be large enough to cause the roof collapse of the most vulnerable structures; on the short run this seems to be the main problem.

Figure 25. Population within a certain distance of currently erupting volcanoes in Guatemala.

Lava flows invading grazing land, and potentially reaching inhabited areas and important infrastructure (e. g. roads and power lines) would also be a concern, especially on the east and southeast flanks, after the appearance of a new vent during the May 27th eruption; however, this hazard by far not as lethal as the other hazards, and it constitutes a hazard mainly to property. In contrast to Fuego and Santiaguito, the record of other more lethal hazards, mainly pyroclastic flows and lahars, is rather scarce at Pacaya, and therefore hasn’t been considered as such an acute threat. As previously discussed, some of the phenomena observed during this and other eruptions, as well as the geologic record at Pacaya, become reasons to be concerned with the hazard associated to large slope failures, and even large debris avalanches. The slope instability and debris avalanche hazard should therefore be considered and assessed in more detail on the long run. The most likely direction in which an avalanche would travel is to the southwest quadrant, such an avalanche could have a mobility of only a few kilometers, if the volume is small, or up to 25 kilometers if the current cone collapsed entirely, and assuming a similar mobility to the last avalanche (Siebert et al., 2006, Vallance et al., 1995). Several of the eruptions that happened since 1961 have been preceded by a clear and noticeable increase in the activity, sometimes at a steady rate, for weeks and maybe even months before the main explosive event (G. Chigna personal communication). The May 27th eruption was also preceded by at least several weeks of increased effusive activity, and at least one day of more intense effusive and explosive activity, which prompted INSIVUMEH and CONRED to recommend a preemptive evacuation before the most intense phase of the eruption started on the 27th. The possibility that future eruptions at Pacaya will be precede by similarly strong precursors opens the possibility to address the risk problem through an early warning system, this was the de facto and spontaneous evolution of the crisis during the May eruption. Improving the organization for such an early warning system could yield better results in terms of the evacuation, e. g. addressing the problem of people not willing to evacuate for fear that their property will be looted. A very important aspect of such a warning system is the strategy to account for false alarms, since their impact could have a devastating effect on the system, via a loss of confidence and credibility. Another caveat to the early warning approach is the fact that only human lives and relatively mobile goods (e. g. livestock, valuable and transportable house objects) can be protected; fixed infrastructure and goods (e. g. buildings, crops, etc.) won’t be spared of damage through and early warning system. To reduce the vulnerability of fixed goods, other strategies will have to be implemented, mainly through the reduction of exposure, avoiding as much as possible the use of land highly exposed to the hazards (e. g. avoiding the development of crop land in very close range to the volcano), perhaps through physical strengthening of some buildings (e. g. concrete slab roofs for critical facilities, like the public schools, and which could become last resort shelters for trapped inhabitants during a crisis), and by strengthening the otherwise vulnerable livelihood of the population around the volcano (e. g. a large number of people depend from the tourism industry sustained by the Pacaya National Park, which remained closed for weeks after the eruption). Adopting a strategy to deal with these and other issues associated with an early warning, or a reduction of vulnerability in general should include both the volcanological technical (e. g. forecasting of eruptions and assessment of possible impacts of the hazards) and socioeconomic aspects that any risk problem demands (Fischhoff et al., 1984, Slovic, 2000). As stated before, these are very challenging problems that won’t be easy to solve, but now is the time to think about a strategy and a plan to face them, and not when the crisis arrives.

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