Mass movements in tropical volcanic terrains: the case of Teziutlán (México)

Mass movements in tropical volcanic terrains:

the case of Teziutlan (Mexico)

L. Capraa,*, J. Lugo-Hubpa, L. Borsellib

a Instituto de Geografıa, Universidad Nacional Autonoma de Mexico, Coyoacan 04510, D.F. Mexicob Istituto per lo Studio degli Ecosistemi (CNR-ISE), Florence, Italy

Received 29 January 2002; accepted 14 January 2003

Abstract

During the last decade, soil degradation coupled with global climate changes has increased hydrogeological hazards in

Mexico. In tropical volcanic terrains, alteration processes have enhanced the formation of clay minerals that promote water

retention and result in soil/rock weakness. Intense seasonal rainfall can trigger the liquefaction and remobilization of these low-

resistance terrains. During the first week of October 1999, heavy rains affected eastern Mexico, including Puebla State. As a

consequence, approximately 3000 mass movements, consisting of rock and soil slides and slips, debris flows and avalanches

were generated in this area. In the town of Teziutlan (Puebla), which is located on volcanic deposits, a single mass-movement

event caused approximately 150 deaths. In the present work we identified two types of mass movements in the Teziutlan area—

Type 1: superficial erosion of an unwelded ignimbritic sequence forming small detrital fans, and Type 2: thin soil slide/debris

flow from the remobilization of a volcanic sequence composed of clay-rich paleosols interbedded with ashfall horizons. The

clay-rich volcanic paleosols favored the formation of perched water tables on a hydraulic aquiclude. Positive pore-water

pressures triggered the failure. Based on these results, the principal human settlement in the Teziutlan area may be threatened by

future debris flows, which could cause serious harm to the dense population and severe damage to its infrastructure. It is

necessary to prevent future deaths and damage by installation of mitigative measures based on detailed studies. Without any

further study, it will not be possible to prevent and mitigate a natural disaster with the same magnitude as the 1999 catastrophic

hydrogeological phenomena.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Mass movement; Soil slide; Debris flow; Tropical volcanic terrain; Mexico

1. Introduction

During the last decade, soil degradation coupled

with global climate changes (responsible for extraor-

dinary rainy seasons) has caused an increase in hydro-

geological hazards in Mexico. These phenomena have

occurred previously in recent geological history, but

today’s population and anthropic activity increase the

increment of susceptibility for such events.

The instability in Mexican tropical volcanic ter-

rains is caused by (1) low physical shear strength of

volcaniclastic deposits, (2) conspicuous rainfall sea-

sons, and (3) weathering and hydrothermal alteration

0013-7952/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0013-7952(03)00071-1

* Corresponding author. Tel.: +52-55-6224335; fax: +52-55-

5502486.

E-mail address: [email protected] (L. Capra).

www.elsevier.com/locate/enggeo

Engineering Geology 69 (2003) 359–379

of incoherent volcaniclastic deposits. In tropical areas,

the alteration processes determine the soil/rock weak-

ness by the formation of clay minerals that promote

water retention (Pla Sentıs, 1997; Terlien, 1997). The

increase of pore-water pressure can trigger the lique-

faction and remobilization of these unstable terrains.

These failures can result in several types of mass

movements, ranging from slides to debris flows and

mud flows (i.e., Iverson et al., 1997).

During the first week of October 1999, heavy

rains associated with the Tropical Depression Num-

ber 11 affected eastern Mexico, including Puebla

Fig. 1. (a) Sketch map of the study area showing its location in respect to the Trans-Mexican Volcanic Belt. Triangles refer to principal Mexican

active volcanoes. Abbreviations are: CVC: Colima Volcanic Complex. Pa: Paricutin. NT: Nevado de Toluca. Jo: Jocotitlan. Iz: Iztaccıhuatl. Po:

Popocatepetl. Pdo: Pico de Orizaba. (b) Map of Puebla State showing the location of the town of Teziutlan and the area affected by intense

rainfall during October 4–5, 1999 (modified from Lugo-Hubp et al., 2001). Dashed border in (b) indicates the limits of the Sierra Norte.

L. Capra et al. / Engineering Geology 69 (2003) 359–379360

https://www.researchgate.net/publication/228558177_The_physics_of_debris_flows_Rev_Geophys?el=1_x_8&enrichId=rgreq-cd8af593-2bd7-45f0-bd21-48d328e07e6c&enrichSource=Y292ZXJQYWdlOzI0ODUwODUxMjtBUzoxMDMyMDcwODY4NTQxNDVAMTQwMTYxNzg5NTAzOA==

https://www.researchgate.net/publication/222757228_Hydrological_landslide_triggering_in_ash-covered_slopes_of_Manizales_Columbia?el=1_x_8&enrichId=rgreq-cd8af593-2bd7-45f0-bd21-48d328e07e6c&enrichSource=Y292ZXJQYWdlOzI0ODUwODUxMjtBUzoxMDMyMDcwODY4NTQxNDVAMTQwMTYxNzg5NTAzOA==

https://www.researchgate.net/publication/223788633_A_soil_water_balance_model_for_monitoring_soil_erosion_processes_and_effects_on_steep_lands_in_the_tropics?el=1_x_8&enrichId=rgreq-cd8af593-2bd7-45f0-bd21-48d328e07e6c&enrichSource=Y292ZXJQYWdlOzI0ODUwODUxMjtBUzoxMDMyMDcwODY4NTQxNDVAMTQwMTYxNzg5NTAzOA==

State (Fig. 1). As a consequence, approximately 3000

mass movements (landslides), ranging from soil

slides to debris flows and avalanches, occurred. On

October 5, in the town of Teziutlan (Puebla State) a

single landslide caused approximately 150 deaths. All

of the area of Teziutlan and some portion of the

northern sierra were affected by several mass move-

ments, differing in magnitude and flow type (Lugo-

Hubp et al., 2001).

In volcanic terrains, such processes are very com-

mon during exceptional rainfall seasons. On May 5,

1998, after 30 h of continuous rainfall, a large area of

tephra-rich colluvial cover, 15 km east of Vesuvius

volcano (Italy) failed, generating a series of debris

flows that killed more than 150 people (Pareschi et

al., 2000). On October 30, 1998, Hurricane Mitch

caused abnormal rainfall that triggered a flank col-

lapse of Casita volcano, Nicaragua, causing the

deaths of 1550 people (Sheridan et al., 1999; Scott

et al., 2003). The case of Teziutlan presented here is

another example of these events and represents the

first detailed study in Mexico, where, during the last

decade, mass movements have been responsible for

thousands of deaths.

Fig. 2. Daily precipitation records of (a) normal rainfall for the month of October (example from October 1997; Quintas, 2000), (b) October 1–

10, 1999 (from Vazquez-Conde et al., 2001), the rainfall peak of October 4 and 5 corresponded with the initiation of mass movements.

L. Capra et al. / Engineering Geology 69 (2003) 359–379 361

This work presents a detailed study of the textural

and sedimentological characteristics of the paleosols

and volcanic sediments involved in this phenomenon

in the Teziutlan area. The main purpose of this study

is to define the relationships between the soil/rock

characteristics, the types of flows derived from the

mass movement, and the hazards related. In addition,

a qualitative estimation of the triggering mechanism is

proposed, despite the fact that no direct geotechnical

parameters are available for this area. Thus, the

proposed model is based only upon indirect evalua-

tion of the geotechnical/geohydraulic properties of the

soils.

2. Meteorological data

The heavy rainfall that was observed during the

first week of October 1999 originated in different

atmospheric systems (Vazquez-Conde et al., 2001).

Tropical Depression Number 11, which was created in

the Gulf of Mexico, was responsible for the abnormal

rainfall of October 4 and 5. At the same time, flows of

humid air from both the Pacific Ocean and the Gulf of

Mexico increased the amount of water vapor in the

atmosphere, which resulted in heavy rain in the states

of Veracruz, Puebla, and Hidalgo. Finally, Tropical

Wave Number 35 also had an effect on October 4.

According to published data (Vazquez-Conde et al.,

2001), during the first 10 days of October 1999,

precipitation reached 1200 mm, approximately five

times higher than the precipitation registered for the

entire month of October (e.g., 243 mm in 1997);

however, approximately 800 mm of this amount was

concentrated between October 4 and 5 (Fig. 2a and b).

Even though approximately 200 mm of rain had fallen

on the area between September 30 and October 4,

likely saturating the terrain, the sudden rainfall

increase of October 4–5 undoubtedly triggered the

mass failures.

3. Morphological features of the Teziutlan region

From the Landsat image of Fig. 3 and the slope

map of Fig. 4, it is possible to make some observa-

tions of the morphological characteristics of the

studied area.

The town of Teziutlan is located on top of a plateau

at an elevation of 1700 m a.s.l. surrounded by relief

that reaches up to 2800 m a.s.l. The main drainage is

oriented N-S, and NE-SW, probably controlled by

tectonic lineaments (Fig. 3). Erosion has produced

deep (as much as 100 m) parallel ravines at distances

of 50–100 m from each other, generally symmetrical

and rectilinear. The bases of these canyons are narrow

(less than 50 m). From the slope map (Fig. 4), the

steepest gradient characterizes the northern sierra,

with slopes >40j, while the principal inhabited areas

are settled on flat zones that normally are limited by

deep and unstable ravines.

During the past 30 years, rapid human develop-

ment has modified the relief. Disorganized urban

growth coupled with deforestation has led to complete

vulnerability of the Teziutlan area to slope mass

movements.

4. Geology of the area

The town of Teziutlan is located in the Sierra

Norte of Puebla State, the geology of which consists

of thick Cretaceous sequences of folded limestones

and terrigenous sedimentary rocks. The peculiarity of

the Teziutlan region is that the basement rocks are

covered by thick (more than 50 m) volcanic deposits

erupted from the Los Humeros caldera, located 40

km to the south (Fig. 3). This caldera activated

approximately 250,000 years ago with the emplace-

ment of thick sequences of ignimbrite deposits

(welded and unwelded pumice-flow deposits) and

ashfall horizons (Ferriz and Mahood, 1984; Ferriz,

1985). Two main caldera collapses were recognized

by these authors (resulting in Xaltipan and Zaragoza

ignimbrites), separated by the extrusion of volumi-

nous basaltic lava flows associated with cinder

cones, and rhyolitic domes along caldera-rim frac-

tures. Fig. 5 presents a simplified geological map of

the study area, representing only the distribution of

pyroclastic deposits. Teziutlan and its neighboring

towns are positioned on top of this volcanic

sequence. In particular, the flat areas that morpho-

logically correspond to basaltic lava flows are cap-

ped by a sequence up to 5 m thick consisting of

clay-rich paleosols (paleo-andosoils, developed in

ash flow deposits) interbedded with several scoria/


pumice fall deposits. This sequence is rare in the

northern portion of the area, where it appears as

small patches in flat areas. All the ashfall units are

massive and grain-supported, varying in thickness

from 30 cm to 1.5 m, and mostly consisting of

pumice or scoria and accidental lithics. The paleosols

that separate the ashfall horizons vary in color from

light to dark brown, with thicknesses ranging from

20 cm to 1.7 m, and contain some dispersed frag-

ments of scoria or pumice (Fig. 6, sections TZ14 and

TZ22).

In contrast, the steeper slopes and deep canyons,

which dominate the northern region, are covered by a

thick (up to 50 m) volcanic sequence constituted at

the base by a 30-m-thick, massive, unwelded pumice-

flow deposit (Xaltipan ignimbrite) (Ferriz and

Mahood, 1984), crowned by a 1.5-m grain-supported

pumice-fall horizon directly associated with a 4-m-

thick pumice-flow layer, which shows lenses of

pumice accumulation (section TZ06, Fig. 6). The

thicker sections are observable along the main can-

yons, whereas on the southern portion its maximum

observable thickness is 4 m, where it is always

capped by the scoria/pumice fall sequence. Fig. 6

shows selected stratigraphic columns of the volcanic

sequence.

5. Type of mass movement and areal distribution

Integrating the field work performed during 2000–

2001, and the aerial photos taken just after the October

5, 1999, catastrophic episode, we determined the

distribution of the different mass-movement events,

Fig. 3. Landsat image (bands 3, 4 and 7) showing the location of Teziutlan with respect to the Los Humeros Caldera. Note the NE-SW drainage

orientation that dominates the Teziutlan area.


their characteristics and extents. These data, where

reported on a digital-elevation model of the area (Fig.

5) and all other information (area, slope, type of scarp

and flow; see Table 1) were compiled on different

maps using a GIS instrument (ILWIS, 2.1). Due to the

small magnitudes of individual events, we traced on

Fig. 5 the overall areas affected by mass movement,

without tracing each single scarp or flow trace. Two

principal types of mass movements were recognized.

� Type 1: Shallow landslides with vertical walls that

form the deepest canyons in the northern region

(Figs. 5 and 7).

This phenomenon affected mostly slopes steeper

than 40j, constituted by the unwelded ignimbrite

deposit (Table 1). The removal of this incoherent

material resulted in only small remaining talus

deposits (Fig. 7). The total area affected by this

Fig. 4. Simplified slope map of the study area and location of the principal sites affected by mass movements, which mostly fall in a zone with

slopes between 20j and 40j. Contour interval: 20 m.


Fig. 5. Simplified geological map showing the distribution of volcanic units and areas affected by mass failure. These areas are numbered and

their characteristics (extension, slope, and type of scarp) are listed in Table 1.


process is about 0.15 km2. Well-defined scarps are

not observable and, for this reason, a total volume

estimation of the removed mass was not calcu-

lated.

� Type 2: Soil slides/debris flows (debris flows

developed from slides of the soil mantle; terminol-

ogy after Campbell, 1975) (Figs. 5 and 8).

These landslides occurred mainly on slopes

between 28j and 40j where they formed well-

defined scarps (Table 1). This slope range coincides

with the most common slope interval reported for

soil slides ranging from 25j to 45j (Campbell,

1975). This type of movement affected only the

sequence composed of the clay-rich paleosols inter-

bedded with the pumice/scoria ashfall, which char-

acterizes the southern portion of the study area,

including the urban zone of Teziutlan. In fact, this

process affected densely populated areas, as well as

agricultural slopes. The thickness of the material

involved in such events ranged from a minimum of

40 cm up to 1.5 m. In particular, the sliding surface

corresponded with the transition to the lower, clay-

rich paleosol layer, which separated the pumice/

scoria ashfall sequence from the basal ignimbrite.

The total area affected by Type 2 events has been

0.07 km2, lower than the Type 1 events, but with the

striking difference that, in the Type 2 cases, the soil

slides transformed into very mobile debris flows that

caused the deaths of hundreds of people. In fact, the

failure at Taxcala Hill (number 21, Figs. 5 and 9),

the summit of which is occupied by a cemetery,

mobilized a soil mass of approximately 5000 m3 that

was transformed into a debris flow that killed 150

people.

The volume of each soil slide of Type 2 can

easily be obtained by multiplying the areas reported

in Table 1 by the effective thickness of the removed

layered sequences of 1–2 m (see model below). This

Fig. 6. Selected stratigraphic columns showing relation between fall units and paleosols. Section numbers correspond to those used to indicate

the areas affected by mass failure on Fig. 5.


estimation does not consider a possible volume

increase due to the bulking process caused by water

and by rock or soil fragments picked up as the mass

moves downslope.

6. Physical properties of volcanic deposits and

associated soils

6.1. Sedimentology

Sedimentological analyses were carried out for the

volcanic horizons, as well as for the soils/paleosols

found in the stratigraphic record. The global sedimen-

tological spectrum of the deposit (from � 5/ to 10/)was obtained by dry-sieve analyses (� 5/ (4 mm) to

4/ (0.0625 mm)), and by a laser sedimentograph (5/(0.031 mm) to 10/ (0.001 mm)). The results are

reported in Table 2.

The ignimbritic sequence in the Northern Sierra is

constituted by a basal pumice flow deposit (sample

TZ01 and TZ06d) with up to 84% in sand, and barren

of the clay fraction. The paleosol that lies between the

ignimbrite and the pumice-fall deposit (TZ06c) is silt

loam with up to 16% in clay fraction. The upper

pumice-fall deposit (TZ06b) consists only of gravel

and sand fractions (57% and 43%, respectively). The

upper laminated pumice-flow deposit (TZ06a) is rich

in sand (up to 72%) with low amounts of gravel and

silt, and is barren of clay.

The scoria/pumice-fall sequence, where the towns

of Teziutlan and San Diego are located, shows differ-

ent sedimentological characteristics. All of the scoria/

pumice-fall layers that have been recognized are made

up mainly of gravel and sand, with practically no silt

or clay fractions (Table 2). All of these layers are

interbedded with clay-rich paleosols. The analyzed

paleosols are mainly silty (varying from silt, silt loam,

to loam sand; USDA soil textural classification; Table

2) and with an important content in the clay fraction,

varying from a minimum of 4% to a maximum of

24% (Table 2). For example, section TZ30 (Fig. 6)

shows, from bottom to top, a 40-cm-thick dark silt

loam paleosol (up to 21% in clay fraction), a 60-cm-

thick brown silt loam paleosol (up to 4.5% clay), and

a 75-cm-thick pumice-fall deposit, consisting of

gravel and sand (56% and 44%, respectively), and

without silty or clay fractions. The modern soil caps

the section. Section TZ02 shows a similar stratigraphy

(Fig. 6, Table 2). It is worth mentioning that in all the

analyzed sections the clay content in the paleosols

increases downward, and that the lowest soil layer,

which separates the upper fallout sequence from the

older ignimbritic deposits, shows the highest clay

content. A triangular graph (Fig. 10) shows the later

trend, where the clay content, from top to bottom,

migrates toward the clay vertex (see, for example,

samples TZ02b-c-d and TZ22a-c-e).

6.2. Permeability

The saturated permeability (Ks) for the analyzed

paleosols and sediments has been calculated from

theoretical pedotransfer models available in the liter-

Table 1

Type of mass movement, scarp, slope, and areal extent for each

recognized event

Location Type Area

(m2)

Slope

(j)Scarp type

1 1 16,100 40 not defined

2 2 17,800 38 well defined

3 1 24,500 40 well defined

4 1 9400 40 well defined

5 1 16,200 39 slightly defined



8 2 22,500 17 not defined

9 2 2200 28 not defined












21 2 2200 21 slightly defined










See Fig. 5 for point location.


Fig. 7. Panoramic view of the Sierra Norte showing the Type 1 mass movement. Notice the small detrital fan (black arrow), which originated at

the base of the canyon wall.

Fig. 8. Aerial view of points 10–14 (see Fig. 5 for locations) showing an example of Type 2 mass movement.


ature (Lalibert et al., 1968; Brakensiek et al., 1984),

and implemented in the web applet PEDON-SEI 1.1

(CNR-IGES, 2001). As observed by Wosten et al.

(2001), these functions often prove to be satisfactory

predictors for missing soil hydraulics characteristics,

as is the case of Teziutlan, where geohydraulic and

geotechnical parameters are not available. Table 3

shows all the values obtained for the soils and

pyroclastic layers discussed in the text. Based on

these results, it is evident that the scoria/pumice-fall

sequence shows important differences in terms of

permeability (Fig. 11). In fact, the ashfall horizons,

due to their grain-supported texture, lack fine sedi-

ments (total absence of silt and clay fraction), which,

in turn, results in a highly porous and permeable soil,

with average values of Ks up to 500 mm/h (Table 3).

In contrast, the soils that separate the fallout horizons

are clay-rich and relatively impermeable with values

that range from 5 to 9 mm/h, approximately two

orders of magnitude lower than the ashfall units. As

deduced from the sedimentological analysis, the clay

content increases downward; so, the permeability

Fig. 9. Aerial view of the soil slide/debris flow at Taxcala Hill. This debris flow caused the deaths of 150 people. Dashed line indicates the limits

of the debris flow.


decreases in the same direction. These variations in

permeability represent important discontinuities in

ground-water infiltration that correspondingly pro-

mote the formation of perched water tables. In the

next section, we present a triggering model for Type

2 movement based on the infiltration process in the

multi-layer succession (pyroclastic deposits/paleo-

sols). In contrast, the ignimbritic sequence is quite

homogeneous. Its permeability is lower as compared

to the ashfall layers (Ks of 400 mm/h, Table 3),

because the fine-grained material fills all the cavities

between the clasts and boulders, but its permeability

is still higher in respect to the clay-rich paleosol

horizons.

7. Mass movement triggering mechanisms

7.1. Type 1

The Type 1 mass movements have been recog-

nized as affecting only the ignimbritic unit that

characterizes the northern region where it constitutes

vertical walls of deep canyons. This phenomenon

involved only the erosion of external portions of the

exposures, which are very unweathered (fresh) due

to the high erosion rate. In fact, Yokota and Iwa-

matsu (1999) demonstrated that a slope with such

characteristics becomes gradually unstable as the

weathering of rocks proceeds inward from its sur-

Table 2

Textural and sedimentological characteristics of analyzed deposits and paleosols

Sample Type Gravel

(>2 mm)

Sand

(2 mm–62 Am)

Silt

(62–2 Am)

Clay

( < 2 Am)

Mud

(silt + clay)

TZ01 ig 0.00 71.54 24.93 3.53 28.46

TZ06a ig 14.95 71.17 13.88 0.00 13.88

TZ06b pf 57.38 42.62 0.00 0.00 0.00

TZ06c SIL pls 13.17 22.01 58.34 6.48 64.82

TZ06d ig 8.46 83.74 7.80 0.00 7.80

TZ10 SIL sl 15.01 24.79 58.21 1.72 59.93

TZ02a pf 38.68 59.75 1.57 0.00 1.57

TZ02b SI pls 0.00 1.96 97.74 0.31 98.05

TZ02c SI pls 0.00 1.57 94.11 4.32 98.43

TZ02d SIL pls 0.00 0.35 75.16 24.50 99.66

TZ30a pf 56.17 43.83 0.00 0.00 0.00

TZ30b SIL pls 6.67 36.82 52.03 4.49 56.52

TZ30c SIL pls 0.00 0.00 78.39 21.61 100.00

TZ22a LS pls 17.38 73.13 9.32 0.16 9.48

TZ22b pf 67.50 32.50 0.00 0.00 0.00

TZ22c SI pls 0.00 3.44 91.56 5.00 96.56

TZ22d pf 88.33 11.67 0.00 0.00 0.00

TZ22e SI pls 0.00 13.72 76.22 9.86 86.08

TZ23a ig 24.19 65.71 10.10 0.00 10.10

TZ23b pf 72.48 27.52 0.00 0.00 0.00

TZ23c SI pls 0.00 0.87 99.12 0.00 99.12

TZ14a SIL pls 0.00 25.29 68.44 6.21 74.65

TZ14b pf 72.18 27.82 0.00 0.00 0.00

TZ14c SI pls 0.00 14.86 70.68 14.46 85.14

TZ14d sf 85.02 14.98 0.00 0.00 0.00

TZ14e SI pls 0.00 13.78 76.34 9.89 86.23

Abbreviation for U.S. Dept. of Agriculture (USDA) soil textural classification: LS, loam sand; SIL: silt loam; SI: silt. Other abbreviations: ig:

ignimbrite; pf: pumice fall; sf: scoria fall; pls: paleosol; sl: soil.


face, typically failing during heavy rainfall. Frequent

failures along steep slopes composed of soft, degrad-

able pyroclastic rocks are believed to be mainly

attributable to rock weathering because individual

failures are very shallow and they tend to repeat with

time (Shimokawa et al., 1989). The weathering of

volcanic rocks is due to a geochemical reaction in

which the volcanic glass degrades to clay minerals

such as allophane and halloysite (Pettapiece and

Pawluck, 1972; Neall, 1975; Parfitt and Wilson,

1985). These chemical changes accelerate the phys-

ical and mechanical deterioration. In this sense, rock

porosity increases and both dry density and strength

decrease with time. It is likely that this process is

occurring in the study area where Type 1 movements

were recognized. In fact, a sample taken from the

ignimbrite deposit (TZ01, Table 2) indicated the

presence of a very low percentage of clay fraction,

for which X-ray analysis yielded a halloysitic com-

position. Yokota and Iwamatsu (1999) found that the

estimated rate of weathering ranges from 10� 2 to

100 cm year� 1 in this type of rock. The October 4–

5 meteorological event probably accelerated this

process, resulting in massive superficial erosion with

the formation of detrital fans at the base of the

canyon walls (Fig. 7). The direct consequence of

this phenomenon was the erosion of the surface of

roads that in some places drastically reduced their

usable widths.

7.2. Type 2

Rainfall triggering of debris flows in steep terrains

has been the object of intensive study (e.g., Varnes,

1978; Johnson, 1984; Reid et al., 1988; Iverson et al.,

1997; Phillips and Davies, 1991; Reddi and Wu,

1991; Cruden and Varnes, 1996; Terlien, 1997). The

key hydrologic requisites for debris-flow mobilization

are sufficient water to saturate (or nearly saturate) the

soil and sufficient pore-water pressure and/or weight

to satisfy the Coulomb criterion of failure (Iverson et

al., 1997). In fact, most studies indicate that debris

flows result from development of positive pore

pressures that accompany saturation. Saturation and

positive pore pressures commonly develop when

infiltrating water encounters soil with low perme-

ability, and transient perching of the water table

occurs (Campbell, 1975; Reid et al., 1988). In this

sense, based on Darcy’s law, if the rainfall intensity

rate (mm/h) is greater than the hydraulic conductivity

Ks of the soil, a saturated zone will develop.

It is important to note that slope failure can occur

even though a significant fraction of the soil is not

fully saturated. In fact, when positive pressure at

Fig. 10. Sand–silt –clay triangular diagram showing sedimentological variation between the analyzed horizons. Observe the increment on the

clay fraction toward the lower paleosols.


depth triggers failure, the water content in the unsa-

turated zone can rise to saturated or near-saturated

levels as a consequence of soil contraction that

originates at the sliding surface but that spreads to

adjacent portions by conduction of granular temper-

ature (Iverson, 1997; Iverson et al., 1997; Van Asch et

al., 1999).

This process seems to be most probably respon-

sible for the slope failure at the Teziutlan area that

caused the mobilization of a debris flow (Type 2

event). In fact, the particular intercalation of paleosols

and ashfall units with different hydraulic conductiv-

ities caused the formation of perched water tables. In

this scenario, during an intense rainfall a saturated

zone formed in correspondence of those low-perme-

ability soils and propagated upwards. When the pos-

itive pore pressure raised up to the limit of the soil

strength and cohesion forces, the failure occurred and,

for soil contraction, positive pore pressure increased

also in the non-saturated part of the soil liquefying the

mass completely to form the debris flow.

Figs. 12 and 13 present an infiltration model for

two typical sections observed in the Teziutlan area,

referring to the simplest case of single layer (TZ22)

and the more complicated of multi-layers (TZ02).

The model used is based on the Green–Ampt alge-

braic solution (Green and Ampt, 1911) modified by

Mein and Larson (1973) for a constant rainfall rate.

In this model, water is assumed to infiltrate into the

soil as piston flow resulting in a sharply defined

Table 3

USDA soil texture Green–Ampt infiltration parameters

Sample Type Ks

(mm/h)

Sf(mm)

/(m3/m3)

hi(m3/m3)

TZ06a ig 300 50 2 –

TZ06b pf 500 20 2 –

TZ06c SIL pls 26.58 220 0.566 0.055

TZ06d ig 300 50 2 –

TZ10 SIL sl 35 180 0.621 0.05

TZ02a pf 500 20 2 –

TZ02b SI pls 10.18 330 0.441 0.051

TZ02c SI pls 8.93 350 0.453 0.03

TZ02d SIL pls 4.89 500 0.411 0.023

TZ30a pf 500 20 2 –

TZ30b SIL pls 55 180 0.51 0.037

TZ30c SIL pls 5.9 500 0.415 0.024

TZ22a LS pls 370 50 2 –

TZ22b pf 500 20 2 –

TZ22c SI pls 9.6 350 0.434 0.032

TZ22d pf 500 20 2 –

TZ22e SI pls 9.75 330 0.509 0.042

TZ23a ig 300 50 2 –

TZ23b pf 500 20 2 –

TZ23c SI pls 9.75 360 0.441 0.039

TZ14a SIL pls 27 220 0.566 0.052

TZ14b pf 500 20 2 –

TZ14c SI pls 13 350 0.509 0.051

TZ14d sf 500 20 2 –

TZ14e SI pls 18 330 0.517 0.0441

Abbreviation for USDA soil textural classification: LS, loam sand;

SIL: silt loam; SI: silt.

Other abbreviations: ig: ignimbrite, pf: pumice fall, sf: scoria fall,

pls: paleosol, sl: soil, Ks: saturated hydraulic conductivity; Sf:

effective suction at the wetting front; /: soil porosity; hi: initialwater content.

Fig. 11. Typical stratigraphic relationship between the pumice-fall

layer (PF) and paleosols (Pls). Section TV21. Scale: 60 cm.


wetting front, which separates the wet and unwet

zones. The integrated form that describes the infiltra-

tion rate is:

Ksðt � tp � tpVÞ ¼ F � Sf ð/ � hiÞln 1þ F

Sf ð/ � hiÞ

� �

ð1Þ

tp ¼Fp

Rð2Þ

Fp ¼Sf ð/ � hiÞ

RKs

� 1ð3Þ

where, Ks [L/T] is the saturated hydraulic conductiv-

ity; Sf [L] is the effective suction at the wetting front;

/ [L3/L3] is the soil porosity; hi [L3/L3] is the initial

water content; F [L] is the accumulated infiltration;

Fp [L] is the cumulative infiltration at time of

ponding; tp [T] is the time to surface pounding; tpV[T] is the time to infiltrate volume Fp; R [L] is the

rainfall rate.

Immediately prior to surface ponding, the infiltra-

tion rate is equal to the rainfall rate, and after finding

tp, Eq. (1) can be applied to determine the infiltration

rate (t). For layered soils, where the hydraulic con-

ductivity of the successive layer decreases with depth,

as the studied case (i.e., section TZ02), when the

wetting front enters the successive layer, Ks is set as

the harmonic mean Kh=(Ks1�Ks2)1/2 and the effective

suction at the wetting front is set equal to Sf of the

second layer.

As stated before, Ks to solve this problem is

obtained from pedo-transfer functions, and the other

infiltration parameters needed to apply Eqs. (1)–(3)

are deduced from theoretical tables (Rawls et al.,

1992). Table 4 lists the values used for each layer.

We used a constant rainfall rate of 40 mm/h, twice

the average of the total rainfall measured during

October 4 and 5, 1999. We applied this value

because the rainfall rates available are averages

computed over 24 h, whereas the abnormal rainfalls

accumulated during only a few hours. It is worth

mentioning that the rain started on September 30, and

Fig. 12. (a) Infiltration rate through section TZ22. (b) Stratigraphic sections showing the migration of the wetting front and the location of the

perched water table at different time intervals.


the layers were probably already partially saturated

on October 4. This implies that the calculated infil-

tration times are probably lower with respect to

reality. Despite this approximation, the model dem-

onstrates that the lowest, clay-rich paleosol that

separates the pumice/scoria-fall sequence from the

ignimbrite deposit corresponds to the hydraulic dis-

continuity where a water table begins to grow

upward, saturating the upper layers where positive

water pressure initiates.

Fig. 13. (a) Infiltration rate through section TZ02. (b) Stratigraphic sections showing the migration of the wetting front and the location of the

perched water table at different time intervals.

Table 4

Geotechnical parameters used to determine the Fs for sections TZ02 and TZ22

TZ22 TZ02b and c TZ02d TZ02d (multi-layer)

Min Max Min Max Min Max Min Max

cV(kPa) 0 0 0 0 0 0 0 0

/V(j) 35 40 35 40 35 40 35 40

ru 0.3 0.47 0.07 0.164 0.385 0.474 0.35 0.395

b (j) 28 40 28 40 28 40 28 40

cp (kN/m3) 10.7 13.7 9 13 9 13 11.86 14.36

z (m) 0.8 0.9 0.2 0.3 0.2 0.3 0.9 1

%Fs < 1 100 62 100 100

%Fs>1.3 0 1.23 0 0

Abbreviations: cV: cohesion; /: friction angle; ru: coefficient of interstitial pressure; b: slope angle; cp: pondered soil unit weight; z: sliding

surface depth.


The infiltration model for section TZ22 (Fig. 12a

and b) shows that as soon as the wetting front reaches

the base of the paleosol TZ22c, the upper ashfall

horizon and the modern soil are already saturated

and positive pore pressure can trigger the slide.

According to the model, because the saturated

hydraulic conductivity of layers a and b are higher

than the rainfall rate, after only 24 h the wetting front

is on the limit between layers b and c. Here, after an

initial time of ponding of 1.11 h the wetting front

begins to move downward, reaching the base of layer

c after approximately 62 h.

Section TZ02 shows a different scenario, because

the ashfall layer rests on top of a multi-layer sequence

of paleosols, where the saturated hydraulic conduc-

tivity decreases downward. Based on the model (Fig.

13a and b), after 39 h the wetting front is at the base of

layer d with a rainfall excess of 609 mm, which means

that the upper sequence is almost completely saturated

and a slide can be triggered. According to the model,

after 7.5 h the wetting front is at the limit between

layer a and b (Ks of layer a is higher than the rainfall

rate; so, the rainfall rate is equal to the percolation

rate) and after a ponding time of 1.10 h it starts to

enter layer b. From the diagram of Fig. 13a, it is

possible to observe an important change of the infil-

tration rate migrating from layer c to layer d (corre-

sponding to a decrease from 23 to 16 mm/h) that

promotes the formation of a perched water table from

layer c and upward.

In the next section, we present a slope stability

analysis to determine whether or not under the

hydraulic conditions determined by this hydrological

model the slopes are in equilibrium.

8. Infinite slope-stability analysis

The hydrological model evidences that the clay-

rich paleosols constitute an important hydraulic

discontinuity over which a perched water table

forms. In this section we present infinite slope-

stability analyses for sections TZ22 and TZ02.

Because this method considers a homogenous layer

that slides over a failure surface parallel to the

slope, we consider that the ashfall cover slides on

the paleosol horizons, as observed in some of the

analyzed cases.

The geotechnical parameters such as friction angle

and unit weight of soil will be taken from previous

studies in which volcanic materials similar to those of

the Teziutlan area were tested. The authors fully

understand that these geotechnical parameters are

difficult to determine and that their accuracy under

particular pore-pressure and saturation conditions is

difficult to estimate. For example, it has been proved

that the inter- and intra-particle voids in pumice soils

determine their response to rainfall/ground water

conditions and that the usual in situ material phases

must be modified, as must the unit weight of the

materials, which will increase as the water penetrates

into them (Esposito and Guadagno, 1998). In addi-

tion, triaxial stress tests simulating rainfall infiltration

in volcanic slopes have evidenced an atypical volu-

metric behavior (Charles and Chiu, 2001). For the

present study, we used the values proposed by Amanti

et al. (2000) for the slope movement, which in 1999

affected the circum-Vesuvian areas of Italy, because

they fit to our model.

For the slope stability analysis we used a routine

based on the Monte-Carlo Method applied to the

general model of infinite slope (Eq. (4); Selby, 1993):

Fs ¼cVcz þ ðcos2b � ruÞtan/V

� �sinb cosb

ð4Þ

where, Fs is the factor of safety; b is the slope angle

(j); /Vis the friction angle (j); cVis the cohesion (kPa);c is the soil unit weight (kN/m3); z is the sliding

surface depth (m); and ru is the coefficient of inter-

stitial pressure where

ru ¼cwZiwXn

i¼1

ciZi þ cwZiw

ð5Þ

with 0.0 < ru < 0.5 for slopes not completely saturated;

Z =Si = 1n Zi is the total thickness of sliding layers; Ziw

is the saturated thickness of each single layer.

A computer program calculates approximately

6000 values of Fs, after the generation of random

values for each variable, using a uniform distribution

defined by lower and upper bounds. A frequency

analysis for the obtained sample of Fs, finally, gave

the percentage of cases for which the Fs is < 1.0 and


Fig. 14. Diagrams showing the frequency of the Fs value (PDF) and its cumulative frequency (CDF) for sections TZ22 (a) and TZ02 (b–d).


>1.3 (where the limiting conditions for stability were

defined by Fs = 1.0 and, acceptable condition for

stability of natural slope was Fs>1.3). The range of

parameters used in the analysis is shown in Table 4.

Cohesion will be considered equal to zero because the

ashfall layers do not have clay fractions. Fig. 14

shows the results obtained from the simulation for

TZ22 and TZ02b sections.

Section TZ22 represents the simplest case observed

in the Teziutlan area, where an ashfall layer slides

directly on top of a paleosol. For this case, we

considered an interval of time between 48 and 62 h

that, based on the geohydrological model, corre-

sponds to a piezometric level ranging from 586 to

928 mm. Table 4 shows the parameters used, from

which we concluded that the Fs is always less than

unity (f 0.5) (Fig. 14a).

For section TZ02 we provided three different

scenarios. The first (Fig. 14b) is a slope stability

analysis in which the wetting front moves from layer

b through c, during a span of time from 12 to 21 h.

Here we considered that the sliding surface lies

between layers a and b (situation similar to section

TZ22). Under these conditions, the Fs is for the 62%

under unity (f 0.9) and higher than Fs = 1.3 for the

1.2% only (Fig. 14b). However, when the wetting

front reaches the base of layer d (second scenario),

after approximately 40 h, the ashfall layer is almost

completely saturated and the Fs is 100% less than

unity (f 0.4) (Fig. 14c).

In the third scenario (Fig. 14d), we approximated a

multi-layer situation with a sliding surface deeper than

the previous cases, at 1 m, where an ashfall layer

could be present. The results are interesting because

even in this case the Fs is always below unity

(f 0.5). This last example demonstrates the high

susceptibly of sliding even for deeper volcanic layers,

a really common situation in the studied area.

9. Hazard assessment

Two different types of mass-movement processes

were identified in the Teziutlan area. The Type 1 mass

movement produced on the ignimbrite deposit does not

result in debris flows, but only forms small detritus fans

at the base of the canyon walls, without reaching

human development. The only damage derived from

Type 1 flows is to roads that, because of their proximity

to the ravines, suffered from erosion, resulting in the

reduction of their width. In contrast, the debris flows

that originated from Type 2 mass movements were

responsible for deaths and destruction during the Octo-

ber 5, 1999, event. In fact, soil slides transformed into

highly mobile debris flows that buried human settle-

ments. Under this scenario, the area with the highest

susceptibility for formation of debris flows corre-

sponds to the distribution of the ashfall lithological

units, as delimited in Fig. 5. In particular, as resulted

from the slope-stability analysis, in areas with slopes

between 28j and 40j, where the ashfall layer lies on topof the clay-rich paleosols, the probability that the Fs is

below one unity (f 0.5) is almost always 100% for

rainfalls condition similar to those that occurred in

1999. From Fig. 5, one can deduce that important

human settlements can be threatened by debris flows

that, even if they are small in volume, can inflict serious

damage on a dense population and its infrastructures.

10. Conclusions

This work shows the importance of the study and

hazard assessment of mass movements in tropical

volcanic terrains, such as those of Mexico. The

1999 event on eastern Mexico demonstrated the high

susceptibility of these terrains to failure during intense

rainfall. The Teziutlan case is only a small example of

such phenomena that during 1999 affected several

Mexican states. Damage from landslides accounted

for 384 deaths; 66 people are missing; 212 municipal-

ities incurred damages; 39 rivers overflowed their

banks; and approximately 198,000 people and

48,000 houses were adversely affected to varying

degrees. Economic losses were massive; in the State

of Puebla alone, it is estimated that more than US$200

million will be needed for restoration of damaged

infrastructures (Vazquez-Conde et al., 2001).

The present study utilizes a simple model to

explain mass failure on incoherent volcanic terrains

but, at the same time, indicates that more quantitative

work is needed. Without understanding this type of

phenomenon it is impossible to prevent and mitigate a

catastrophe like the 1999 event, wherein as observed

here, after a period of 12–48 h of heavy rainfall, a soil

slide can occur.


Acknowledgements

This work was supported by the Instituto de

Geografıa, Universidad Nacional Autonima de Mex-

ico (UNAM), CONACYT-CNR project, and Civil

Protection of Puebla State, with special thanks to Prof.

G. Melgarejo. Dr. M. Abrams from the Jet Propulsion

Laboratory (California Institute of Technology) pro-

vided Landsat imagery. Dr. Renato Castro from

UNAM provided technical support during field work.

We appreciated the constructive revisions of Th. Van

Ash and R.L. Schuster, which substantially improved

this manuscript, and the valuable suggestions of K.M.

Scott and G.F. Wieczorek of the U.S. Geological

Survey for the final version.

References

Amanti, M., Chiessi, V., Conte, G., Pantaleone, A., Motteran, G.,

Pantaloni, M., Ventura, R., 2000. Rapporto preliminare relativo

agli eventi idrogeologici che hanno colpito nei giorni 15 e 16

Dicembre del 1999, i comuni di Cervinara e S. Martino in Valle

Caudina in provincia di Avellino. Servizio Geologico d’Italia

(http://www.dstn.it/sgn/u_geoapp/cervinara.htm). 15 pp.

Brakensiek, D.L., Rawls, W.J., Stephenson, G.R., 1984. Modifying

SCS hydrologic soil groups and curve numbers for rangeland

soils. American Society of Agricultural Engineers (Paper No.

PNR-84-203, St. Joseph, Michigan).

Campbell, R.H., 1975. Soil slips, debris flows, and rainstorms in the

Santa Monica Mountains and vicinity, southern California. U.S.

Geological Survey Professional Paper 851, 51.

Charles, W.W., Chiu, A.C.F., 2001. Behaviour of a loosely com-

pacted unsaturated volcanic soil. Journal of Geotechnical and

Geoenvironmental Engineering 127, 1027–1036.

CNR-IGES, 2001. PEDON-SEI: Pedotransfer Functions for Esti-

mating Hydraulic Soil Parameters. http://www.area.fi.cnr.it/iges/

software.htm.

Cruden, D.M., Varnes, D.J., 1996. Landslide types and processes.

In: Turner, A.K., Schuster, R.L. (Eds.), Landslides—Investiga-

tion and Mitigation. Special Report - U.S. National Research

Council, Transportation Research Board, vol. 247, pp. 36–75.

Washington, D.C.

Esposito, L., Guadagno, F.M., 1998. Some special geotechnical

properties of pumice deposits. Bulletin of Engineering Geology

and the Environment 57, 41–50.

Ferriz, H., 1985. Zoneamiento composicional y minarologico en los

productos eruptivos del centro volcanico de Los Humeros, Pue-

bla, Mexico. Geofısica Internacional 25 (1), 97–158.

Ferriz, H., Mahood, G.A., 1984. Eruption rates and compositional

trends at Los Humeros volcanic center, Puebla, Mexico. Journal

of Geophysical Research B89, 8511–8524.

Green, W.H., Ampt, G.A., 1911. Studies on soil physics: 1. The

flow of air and water throughout soils. Journal of Agricultural

Science 4, 1–12.

Iverson, R.M., 1997. The physics of debris flows. Reviews of Geo-

physics 35 (3), 245–296.

Iverson, R.M., Reid, M.E., LaHusen, R.G., 1997. Debris-flow mo-

bilization from landslides. Annual Reviews in Earth and Plane-

tary Sciences 25, 85–138.

Johnson, A.M., 1984. Debris flow. In: Brunsden, D., Prior, D.B.

(Eds.), Slope Instability. Wiley, Chichester, pp. 257–361.

Lalibert, G.E., Brooks, R.H., Corey, A.T., 1968. Permeability cal-

culated from the saturation data. Journal of the Irrigation and

Drainage Division, Proc. ASCE 94, 57–69.

Lugo-Hubp, J., Vazquez-Conde, M.T., Melgarejo-Palafox, G.,

Garcıa-Jimenez, F., Matias-Ramirez, G., 2001. Procesos grav-

itacionales en las montagnas e Puebla. Ciencias y Desarrollo

27, 24–33.

Mein, R.G., Larson, C.L., 1973. Modeling infiltration during steady

rain. Water Resources Research 9 (2), 384–394.

Neall, V.E., 1975. Climate-controlled tephra redeposition on Poua-

kai ring plain, Taranaki, New Zealand. New Zealand Journal of

Geology and Geophysics 18 (2), 317–326.

Pareschi, M.T., Favalli, M., Giannini, F., Sulpizio, R., Zanchetta,

G., Santacroce, R., 2000. May 5, 1998, debris flows in circum-

Vesuvian areas (southern Italy): insights for hazard assessment.

Geology 28 (7), 639–642.

Parfitt, R.L., Wilson, A.D., 1985. Estimation of allophane and hal-

loysite in three sequences of volcanic soils, New Zealand. In:

Fernandez Caldas, E., Yaalon Dan, H. (Eds.), Volcanic Soils.

Catena Supplement, pp. 1–8.

Pettapiece, W.W., Pawluck, S., 1972. Clay mineralogy of soils

developed partially from volcanic ash. Soil Science Society of

America Journal 36, 515–519.

Phillips, C.J., Davies, T.R.H., 1991. Determining rheological pa-

rameters of debris flow material. Geomorphology 4, 101–110.

Pla Sentıs, I., 1997. A soil water balance model for monitoring soil

erosion processes and effects on steep lands in the tropics. Soil

Technology 11, 17–30.

Quintas, L., 2000. EricII, documentacion de la base de datos

climatologica y del programa extractos. Instituto Mexicano

de Tecnologıa del Agua, Mexico. 23 pp. and CD-ROM. ISBN

968-7417404.

Rawls, W.J., Ahuja, L.R., Brakensiek, D.L., Shirmohammadi, A.,

1992. Infiltration and soil water movement. In: Maidment, D.R.

(Ed.), Handbook of Hydrology. McGraw-Hill, New York, pp.

5.1–5.51.

Reddi, L.N., Wu, T.H., 1991. Probabilistic analysis of ground-water

levels in hillside slopes. Journal of Geotechnical Engineering

117, 872–890.

Reid, M.E., Nielsen, H.P., Dreiss, S.J., 1988. Hydrologic factors

triggering a shallow hillslope failure. Bulletin of the Association

of Engineering Geologists 25, 349–361.

Scott, K.M., Kerle, N., Macias, J.L., Strauch, W., Devoli, G., 2003.

Catastrophic precipitation-triggered lahars at Casita Volcano, Ni-

caragua—flow transformations, flow bulking, and future mitiga-

tion strategies. Geological Society of America Bulletin (in press).

Selby, M.J., 1993. Hillslope Materials and Processes, 2nd ed. Ox-

ford Univ. Press, Oxford. 451 pp.


http:\\www.dstn.it\sgn\index.html

http:\\www.area.fi.cnr.it\iges\software.htm

Sheridan, M.F., Bonnard, C., Carreno, C., Siebe, C., Strauch, W.,

Navarro, M., Calero, J.C., Trujilo, N.B., 1999. Report on the 30

October 1998 rock fall/avalanche and breakout blow of Casita

Volcano, Nicaragua, triggered by Hurricane Mitch. Landslide

News 12, 2–4.

Shimokawa, E., Jitozono, T., Takano, S., 1989. Periodicity of shal-

low landslide on Shirasu steep slopes and prediction of potential

landslide sites. Transactions of the Japanese Geomorphological

Union 10, 267–284.

Terlien, M.T.J., 1997. Hydrological landslide triggering in ash-cov-

ered slopes of Manizales (Colombia). Geomorphology 20,

165–175.

Van Asch, T.W.J., Buma, J., Van Beek, L.P.H., 1999. A view on

some hydrological triggering systems in landslides. Geomor-

phology 30, 25–32.

Varnes, D.J., 1978. Slope movement types and processes. In:

Schuster, R.L., Krizek, R.J. (Eds.), Landslides—Analysis and

Control. Special Report - U.S. National Research Council,

Transportation Research Board, vol. 176, pp. 11–33.

Vazquez-Conde, M.T., Lugo, J., Matıas, L.G., 2001. Heavy rainfall

effects in Mexico during early October 1999. In: Gruntfest, E.,

Handmer, J. (Eds.), Coping with Flash Floods. Kluwer Academic

Publishers, Netherlands, pp. 289–299.

Wosten, J.H.M., Paschepsky, Y.A., Rawls, W.J., 2001. Pedotransfer

functions: bridging the gap between available basic soil data and

missing soil hydraulic characteristics. Journal of Hydrology 251,

123–150.

Yokota, S., Iwamatsu, A., 1999. Weathering distribution in a steep

slope of soft pyroclastic rocks as an indicator of slope instability.

Engineering Geology 55, 57–68.


Documents

Mass movements in tropical volcanic terrains: the case of Teziutlán (México)