Earthquake Prediction and ForecastingThe Holy Grail of Seismology

In recent years, a popular notion has taken root in the minds of many southern Californians: large earthquakes always happen in the morning! The magnitude 7.3 Landers earthquake and its largest aftershock, the Big Bear earthquake, shook awake a lot of people in 1992. Those events were still fresh on their minds when another large rupture, the magnitude 6.7 Northridge earthquake disturbed the sleep of millions in and around the Los Angeles area. Most recently, the Hector Mine earthquake of October 1999 struck at 2:47 am. Never mind that the Joshua Tree earthquake, which ultimately led to the Landers rupture two months later, struck just before 10:00 pm Pacific Daylight Time; a pattern was perceived independently by countless residents, especially those who could remember that the 1987 Whittier Narrows occurred just before 8:00 am. When this "discovery" was brought up over lunch or coffee with others who'd noticed the same thing, that only served to reinforce it. Now it is part of the "earthquake culture" in this area.

The question then becomes Can earthquakes be predicted? The answer is probably not, based on our current state of knowledge. A variety of prediction methods have been used for centuries, ranging from accounts of earthquake weather and the time of day, to the alignment of planets and jumpiness of animals, none of which are accepted.In general, for a prognostication to be referred to as a viable prediction it must include:1. Fault that will rupture2. Magnitude of earthquake3. Date (or date range) of earthquake

A successful prediction of a major earthquake in Hai'cheng, China in 1975 was hailed as the beginning of earthquake prediction worldwide. Abnormal behavior in domestic and wild animals led the administration to issue a warning that a large earthquake was about to hit. But what is often not told, is that preceding the main shock, came dozens of small foreshocks that shook Hai'cheng and the neighboring areas frightening most of the people enough to camp out in the open. This is what saved lives, not the prediction. A year later in 1976 similar animal precursors were observed at Tangshan, another Chinese city, but there were no foreshocks and so no prediction was issued. Close to 655,000 people were killed at Tangshan in a major quake and earthquake prediction was back to square one. Strange animal behavior cannot be ruled out altogether. But how does one differentiate between a genuine precursor and one that isn't. It is still not know to clearly what the animals sense, that scientific instruments cannot.

Hai'cheng and Tangshan

Damage to school resulting from the 1976 Tangshan earthquakeDamage to railroad buildingDamage to water main due to fault rupture

The New Madrid Earthquake Prediction of 1990In the fall of 1989, Dr. Iben Browning predicted that an earthquake similar in both size and extent to those which struck the area in 1811-1812 would strike the region on December 3rd, 1990, give or take 48 hours. His forecast was based on a 179-year cycle of tidal forces of the Sun and moon that produce stress on the Earth. Such forces were last felt in 1811.The New Madrid Seismic Zone, earthquake epicenters 1980-1990.

Dr. Browning, a climatologist, had been known to have predicted the 1989 Loma Prieta Earthquake a week in advance in an appearance before about 500 business executives and their wives at a convention. He also reportedly predicted the eruption of Mt. St. Helens in 1980.Collapse of the Cypress structure, Loma Prieta earthquake, 1989.Eruption of Mt. St. Helens, 1980.

The prediction was so specific and apocalyptic, it provoked near hysteria throughout the region. The media leaped on the prediction and suddenly the populace became all too aware of the threat. Schools and factories in the region closed and groups such as the Red Cross wasted precious funds in their efforts to calm the public. Unfortunately, Dr. Brownings prediction was scientifically groundless, and did not occur.

The ultimate responsibility for the misleading quake prediction has to rest with Browning and the scientific community. Scientists had the ultimate responsibility to call Browning a quack early on, yet wanted no part of Browning or his prognostications.

The 2004 Southern California PredictionMore recently, in 2004, an international research tema led by Dr. Vladimir Keilis-Borok, a UCLA seismologist, predicted that a magnitude 6.5 earthquake would strike the southern California area by September 5th of that year.Dr. Vladimir Keilis-Borok

Keilis-Borok, a seismologist and mathematical geophysicist thought he and his team had found that a precursory chain of small quakes that had occurred in the past and would lead to future larger earthquakes. Based on this method, the team had correctly predicted the Dec. 22, 2003 magnitude 6.5 Paso Robles earthquake, as well as an 8.1 quake in 2003 off Japans Hokkaido island.The region of southern California in which Keilis-Borok predicted the earthquake would hit

In the vicinity of a long chain of small earthquakes, the seismologists looked back and see the areas history over the preceding years. If the area had a certain pattern of seismicity, a nine-month alarm is released for the area of concern.By September 6th, seismologists had realized that this was just one more in a long line of prediction methods that havent worked reliably.

In 1976, the National Academy of Sciences published a list of suggested physical clues for earthquake prediction. These include changes in seismic P-wave velocity, ground uplift and tilt, radon emission, electrical resistivity, and the number of local earthquakes.

Unfortunately, as we have seen, predictions using these techniques usually do not pan out. Instead, long-term earthquake forecasts are made by studying paleoseismology. Paleoseismology is the study of the ancient earthquake record through fossil earthquakes. Several methods of this have been tried including the uplift of seashores produced by sudden fault slip, and measurement of growth rings in large trees whose root systems often cross the fault. More precise methods are now in place that can track sequences of great earthquakes by examining trenches across the fault.Trenching across the Hayward Fault, northern CaliforniaTracing rock layers within a trench across the Hayward Fault

Trenching usually takes place in faults near releasing bends. These releasing bends are generally swamps or marshes. During strong shaking of the ground during an earthquake, water-saturated sand layers beneath the surface may become liquefied. The weight of the overlying rocks and soil above then causes the water and sand to rise to the surface, forming a layer of sand called blows, boils or volcanoes. These sand layers may cover any organic material in the area, turning it over time into peat, which can be dated using C14 age dating techniques. Following this, the area may return to a marshy condition, until the next earthquake, when the cycle repeats.Peat layers offset by San Andreas at Pallet Creek site

16-foot excavation along the San Andreas Fault near Pallet Creek. Kerry Sieh has dated these peat layers to determine the dates of past earthquakes.

Black strata are peat layers, datable by 14C methods, that show increasing amounts of vertical displacement with depth, owing to cumulative slip from repeated earthquakes. Vertical component of displacement visible here is a few percent of net displacement, which is chiefly strike slip, approximately normal to trench wall, with block on right moving toward observer. Uppermost, unfaulted deposits postdate 1857 earthquake; lowermost peat bed on southwest side of fault was deposited about A.D. 800. Modified from Sieh (1978).

San Andreas fault exposed in southeast wall of 2a trench at Pallett Creek, Calif., 55 km northeast of Los Angeles.

Previous San Andreas fault ruptures at Pallett Creek (Mojave section)Preferred Event Date Possible Date RangeYears Until Next EventJanuary 9, 1857January 9, 1857 greater than 147December 8, 1812December 8, 181244.08 1480 A.D.1465 - 1495 A.D. 3321346 A.D.1329 - 1363 A.D. 1341100 A.D.1035 - 1165 A.D. 2461048 A.D.1015 - 1081 A.D. 52997 A.D. 981 - 1013 A.D. 52797 A.D. 775 - 819 A.D. 200734 A.D. 721 - 747 A.D. 63671 A.D. 658 - 684 A.D. 63before 529 A.D. ??? - 529 A.D. greater than 142 Based on the above data, it suggests the San Andreas ruptures with a large magnitude earthquake (Mag. 8.0+) every 145 years, on the average. But there is a large variation. The greatest time interval was over 300 years and the smallest as short as 52 years.

Wallace CreekA number of little gullies cross the San Andreas Fault along the Carrizo Plain. These gullies used to flow straight across the fault, but now are offset by the strike-slip motion of the fault. For example, Wallace Creek, is offset 420 feet (130 meters) across the fault. Sediments deposited in the channel of Wallace Creek prior to its offset are 3,700 years old based on C14 age dating. The rate of slip is the amount of the offset 130 meters divided by the age of the channel which is offset 3,700 years 3.5 cm (slightly less than 1 inches) per year.San Andreas Fault

During the great Fort Tejon Earthquake of 1857 the channel was offset as much as 9 to 12 meters. How long would it take for the fault to build up as much strain as it released in 1857? To find out, divide the 1857 slip 9 to 12 meters by the slip rate 3.5 cm/yr to get 257 to 342 years, an estimate of the recurrence interval for this part of the fault. Paleoseismic studies indicate that the last earthquake to strike this part of the fault prior to 1857 was around the year 1480, an interval of 370 to 380 years, which agrees with our calculations. Based on our lowest estimate of 257 years, we really shouldnt expect this section of the San Andreas to rupture until after the year 2100.Geologic map showing relationships observed in previous photos

The Parkfield ExperimentThe seismographic record in California has established that moderate-sized earthquakes (ML 5.5 to 6) have occurred near the town of Parkfield, located in the central portion of the state in 1901, 1922, 1934 and 1966. There is also evidence from felt reports of similar earthquakes in 1857 and 1881.

Simple subtraction suggests a pattern, with an almost constant recurrence time of about 22 years. If this pattern repeated, another Parkfield earthquake could have been expected about 1988. As a result, a prediction experiment was undertaken here, with placement of ultra-sensitive seismographs in order to measure any possible ground motion prior to the quake. Surface fault motions were monitored continuously by creep meters, and geodetic surveys began with special laser geodimeters that measure the distance across the fault between points. Anything that could be used as a reproducable precursor to an impending large earthquake.Laser geodimeter measures changes in distance across the San Andreas Fault near Parkfield. Changes in distances may indicate a precursor to an upcoming earthquake.

A geodolite shown here can measure changes in distance across a fault zone very accurately. Small changes in atmospheric conditions could cause variations in measurement, so a plane is employed to account for factors such as humidity and airborne particles. If changes in distance between points are detected, this could indicate a rapid buildup in strain along the fault and signal an impending quake.

The San Andreas fault in central California. A "creeping" section (green) separates locked stretches north of San Juan Bautista and South of Cholame. The Parkfield section (red) is a transition zone between the creeping and southern locked section. Stippled area marks the surface rupture in the 1857 Fort Tejon earthquake.

Significant earthquakes have occurred on the Parkfield section of the San Andreas fault at fairly regular intervals - in 1857, 1881, 1901, 1922, 1934 and 1966. The next significant earthquake was anticipated to take place within the time frame 1988 to 1993.

The similarity of waveforms recorded in the 1922, 1934 and 1966 events, shown below, is possible only if the ruptured area of the fault is virtually the same for all three events.Recordings of the east-west component of motion made by Galitzin instruments at DeBilt, the Netherlands. Recordings from the 1922 earthquake (shown in black) and the 1934 and 1966 events at Parkfield (shown in red) are strikingly similar, suggesting virtually identical ruptures.

Parkfield earthquake - Fulfillment of the forecast

The earthquake forecast for the Parkfield section of the San Andreas fault was fulfilled on 9/28/2004 with the Mw 6.0 earthquake at 10:17AM PDT.

Preliminary analysis of the 2004 Parkfield shows that it is dissimilar in some respects to the earlier quakes. The 2004 quake nucleated in the south and ruptured to the north. Unlike the 1934 and 1966 quakes, the 2004 quake was not immediately preceded (by 17 minutes, respectively) by a M4.5 foreshocks. Finally, the 38-year interval between the 1966 and 2004 earthquakes was the longest observed interval in the entire 147 year interval. However, the forecast of the place, magnitude, sense of slip, and rupture endpoints and likelihood of rupture was correct. This bolsters confidence in similar hazard forecasts for the Los Angeles and San Francisco Bay regions. Moreover, the 2001 forecast was based on analysis of geophysical data, published research, and fundamental physical principles. The 2004 quake demonstrates the validity to this approach and the value of collecting data that bears on the earthquake problem. However, accurately forecasting the time of damaging earthquakes remains as a significant challenge. Bridge across San Andreas Fault damaged by the magnitude 6.0 earthquake on September 28, 2004.

Forecasting earthquakesForecasting is not predictionless precise based upon analysis of earthquake return periods rather than identification of pre-cursor y signsActive faults or fault segments do not rupture in a random mannerthey have characteristic return periods (or at least return period envelopes)these reflect strain accumulation along the fault and the capacity of the fault to resist strain up to a given characteristic point - for that fault or fault segmentThere are complications:Rupture will not occur according to a rigid timetable - there is a return period envelope rather than specific dateStrain may be released by one large quake or a number of smaller ones (e.g. Marmara Sea south of Istanbul)this has implications for risk assessment

San Andreas examplePrior to 1906 M 8.25 San Francisco quake ~ 3.2m displacement across fault in 50 yearsPost-quake rebound on the fault was ~ 6.5mAmount of time for strain released in quake to accumulate(6.5/3.2) x 50 ~100 yReturn period until next comparable quake = 100yAssumesuniform strain accumulationquake did not alterfault properties

Problems with forecastingForecasts only as good as the available cataloguesHistorical catalogues good for well studied regions such as California, Japan, Europe, ChinaPoor for regions of low frequency-high magnitude seismicityCascadia subduction zoneNew MadridJamaicaWestern EuropeCatalogues need to go back further; requires geological studies

Cascadia subduction zone

The Seismic Gap conceptDefined as an area in an earthquake-prone region where there has been a below average level of seismic energy releaseThe 1989 Loma Prieta quake filled a gap that had been aseismic since 1906 Other gaps exist inAleutian arc (Alaska)south of IstanbulTokyosouthern California

Istanbul seismic gap

Seismic intensity forecastingOther parameters can be usefully forecast than just timing of a quakeForecasting seismic intensity at a particular site is vital for:siting structures such as dams, schools, hospitals & emergency centresconstructing seismic hazard mapsRequires detailed information on geology, ground conditions

Seismic intensity forecast map - Tokai (Japan)

Probabilistic forecastingMost useful way of expressing a forecast of a future quake is in terms of probabilitiesMost people are familiar with probabilities as a result of gamblingExample from San Francisco area (Bolt, 1999)5 quakes > M = 6.75 in 155 y between 1836 & 1991if events are random, another quake of 6.75 can be expected in 155/5 y = 31 y with high probabilityProblem: quakes not entirely random. On a particular fault system may be clustered (due to stress transfer) or follow certain trendsAlternative method of probabilistic forecasting is based on the ELASTIC-REBOUND modelBased upon estimates of strain accumulation across fault

Strain measurement and forecastingGeological mapping undertaken to define active fault segmentsAssumption made that a discrete segment will rupture in one goAs Seismic moment links magnitude with rupture length this gives measure of maximum expected earthquakeRelationship between M s and fault rupture length L: M s = 6.10 + 0.70 log L

Calculating probabilitiesNext: determine slip history of each segmentCalculate strain accumulation rate for each segmentSlip history for fault segment can then be plotted against timeAs slip is related to quake magnitude allows recurrence intervals between quakes greater than a given magnitude to be determined

The quake probability histogramConstruct histogram showing No. of quakes that occur with each specified recurrence timeMost probable recurrence interval is that which divides histogram into two equal areasIf time since last quake in the magnitude range is T1, the probability of the next quake occurring in T1 - T2 years = ratio of red area to yellow areaAs recurrence time T2 increases ratio approaches 1 and a quake becomes virtually certainThe more consistent the recurrence time the better the forecast

The quake probability histogram & the San AndreasSuited to California & San Andreas fault system because active faults exposed at surfaceEnables displacements to be measured easily and strain to be monitoredMethod crucially depends on constraining well the number of potentially destructive quakes in historic time and their agesFor more discussion of problems see Bolt (1999) p228 - 229)

Predicting earthquakesA highly controversial issue in seismologyInvolves giving a precise warning about the timing and size of a future quakeReliant upon the occurrence of pre-cursory signs in advance of a quakeMethod must be shown to be repeatable in order to be of any useIn a zone of high seismicity, any prediction is going to have greater than chance than zero of being rightOn the other hand - a prediction that is not fulfilled ensures that the method is invalid

Proposed earthquake precursorsChanges in seismic velocitiesCrustal deformationGroundwater changesGas releaseAtmospheric effectsAnomalous animal behaviourChanges in magnetic and electrical properties of the rocksthe so-called VAN method