Earthquake Prediction: A Long Way to Go

Photo by Sanej Prasad Suwal

Damodar Maity

The catastrophic event of the Sumatra earthquake on 26 December 2004 and the consequent devastating loss of life and property posed a billion-dollar question to us: Can we not predict earthquakes? The incidence of rumors about striking earthquakes in India, especially in the Northeast is increasing day by day The panic of the striking quake in Assam on 8 September 2006 shows the need to create awareness among the common people. The people got panicky over the prediction made by a scientist of the University of Madras that a quake of the magnitude of 7 to 8 on the Richter scale will rock the State (in Dibrugarh) on the morning of September 8 at 8:21 am. The rumor reached such a height that the State Government had to make it clear on 7th September that it had not declared a holiday for the educational institutions Despite this, many parents did not send their wards to school in the morning out of fear! Thousands spent sleepless nights and poured out into the open after the earthquake alert in Assam One of the positive fallouts of the recent earthquake is a growing concern amongst the general public and government authorities of preparedness of possible future earthquakes Now even common people know the areas, which are highly seismically active. But the people want to know exactly when and where the next quake will come. It would thus be natural to consider if such a devastating earthquake could be predicted. Unfortunately under the current state of the art technology. it is next to impossible to predict an earthquake with precision Although a great deal is known about where earthquakes are likely, there is currently no reliable way to predict the days or months when an event will occur in any specific location The panic of quake will reduce if we know the current state of the art knowledge on earthquake prediction Without entering into the detailed interdisciplinary technological knowledge, let us see why such type of prediction is vague.

Definition of Earthquake Prediction:

Earthquake prediction is usually defined as the specification of the time, location and magnitude of a future earthquake within stated limits. Prediction would have to be reliable and accurate to justify the cost of response. The goal of earthquake prediction is to give warning of potentially damaging earthquakes early enough to allow appropriate response to the disaster enabling people to minimize loss of life and property. Because of their devastating potential, there is a great interest in predicting the location and time of large earthquakes. Scientific earthquake predictions should state where, when how big, and how probable the predicted event is, and why the prediction is made.

Goals of Prediction:

The significance of the prediction studies may be classified into two categories: scientific and socio-economic significance.

Scientific Significance: It is one of the most rewarding scientific challenges, and if attained, a monumental scientific and technological achievement. The research program, involving cutting-edge science and technology, contributes to further extending the frontiers of scientific and technological knowledge.

Socio-Economic Significance Earthquake hazard has to be better assessed to contribute to its mitigation. Innumerable lives and much valuable property are to be saved, and the effects on social and industrial activities are minimized.

Scientific understanding of earthquakes is of vital importance to any nation. As the population increases expanding urban development and construction works encroach upon areas susceptible to earthquakes. With a greater understanding of the causes and effects of earthquakes, scientists may be able to reduce damage and loss of life from this destructive phenomenon.

Estimation of Earthquake Probabilities:

Scientists estimate earthquake probabilities in two ways by studying the history of large earthquakes in a specific area and the rate at which strain accumulates in the rock. This research includes field, laboratory, and theoretical investigations of earthquake mechanisms and fault zones. Ultimately, scientists would like to be able to specify a high probability of a specific earthquake on a particular fault within a particular time An earthquake results from a sudden slip on a geological fault. Such fracture and failure problems are notoriously Intractable. The heterogeneous state of the Earth and the inaccessibility of the fault zone to direct measurement impose further difficulties. Except during a brief period in the 1970s, the leading seismological authorities of each era have generally concluded that earthquake prediction is not feasible.

Worldwide, each year there are about 18 earthquakes of magnitude 70 or larger Actual annual numbers since 1968 range from lows of 6-7 events/year in 1986 and 1990 to highs of 20-23 events/year in 1970, 1971, and 1992. Although scientists are not able to predict individual earthquakes, few large earthquakes do have a clear spatial pattern, and "forecasts of the locations and magnitudes of some future large earthquakes can be made. Most large earthquakes occur on long fault zones around the margin of the Pacific Ocean. This is because the Atlantic Ocean is growing a few inches wider in each year. and the Pacific is shrinking as the ocean floor is pushed beneath Pacific Rim continents Geologically, earthquakes around the Pacific Rim are expected. The long fault zones that ring the Pacific are subdivided by geologic irregularities into smaller fault segments, which rupture individually. Earthquake magnitude and timing are controlled by the size of a fault segment, the stiffness of the rocks, and the amount of accumulated stress. Where faults and plate motions are well known, the fault segments most likely to break can be identified. If a fault segment is known to have broken in a past large earthquake, recurrence time and probable magnitude can be estimated based on fault segment size, rupture history, and strain accumulation. This forecasting technique can only be used for well-understood faults, such as the San Andreas. No such forecasts can be made for poorly understood faults, such as those that caused the 1994 Northridge, CA, and 1995 Kobe, Japan quakes.

Scientific Prediction:

Earthquake prediction is a popular pastime for psychics and pseudo-scientists, and extravagant claims of past success are common. Predictions claimed "successes" may rely on a restatement of well-understood long-term geologic earthquake hazards, or may be so broad and vague that they are fulfilled by typical background seismic activity. Neither tidal forces nor unusual animal behavior have been useful for predicting earthquakes. If an unscientific prediction is made. scientists cannot state that the predicted earthquake will not occur, because an event could occur by chance on the predicted date, though there is no reason to think that the predicted date is more likely than any other day. The National Earthquake Evaluation Council Prediction reviews such predictions but no generally useful method of predicting earthquakes has yet been found.

One well-known successful earthquake prediction was for the Haicheng China earthquake of 1975 when an evacuation warning was issued the day before an M 7.3 earthquake in the preceding month's changes in land elevation and groundwater levels widespread reports of peculiar animal behavior, and many foreshocks had led to a lower level warming. An increase in foreshock activity triggered the evacuation warning Unfortunately most earthquakes do not have such obvious precursors. Despite their success in 1975, there was no warning of the 1976 Tangshan earthquake, magnitude 7.6. which caused an estimated 250,000 casualties.

Methods of Prediction:

Scientists study the past frequency of large earthquakes to determine the future likelihood of similar large shocks. For example, if a region has experienced four magnitude 7 or larger earthquakes during 200 years of recorded history, and if these shocks occurred randomly in time then scientists what percent probability that is just as likely to happen as not to happen) to the occurrence of another magnitude 7 or larger quake in the region during the summers But in many places the assumption of random occurrence with time may not be true because when strain is released along one part of the fault system, it may increase on another part. Four magnitude 6.8 or larger earthquakes and many magnitude 66.5 shocks occurred in the San Francisco Bay region during the 75 years between 1836 and 1911 For the next 68 years (until 1979), no earthquakes of magnitude 6 or larger occurred in the region. Beginning with a magnitude 6.0 shock in 1979, the earthquake activity in the region increased dramatically; between 1979 and 1989, there were four magnitude 6 or greater earthquakes, including the magnitude 7.1 Loma Prieta earthquake This clustering of earthquakes leads scientists to estimate that the probability of a magnitude 6.8 or larger earthquake occurring during the next 30 years in the San Francisco Bay region is about 67 percent.

Another way to estimate the likelihood of future earthquakes is to study how fast strain accumulates. When plate movements build the strain in rocks to a critical level, like pulling a rubber band too tight, the rocks will suddenly break and slip to a new position. Scientists measure how much strain accumulates along a fault segment each year, how much time has passed since the last earthquake along the segment, and how much strain was released in the last earthquake.

This information is then used to calculate the time required for the accumulating strain to build to the level that may result in an earthquake. This simple model is complicated by the fact that such detailed information about faults is rare. In the United States, only the San Andreas fault system has adequate records for using this prediction method. Large earthquakes repeatedly occur along a large-scale fault, and the entire recurrence process includes the following stages:

1. ">Fault healing and re-strengthening Just after the previous earthquake occurred
2. Accumulation of the elastic strain energy with tectonic stress loading
3. Local concentration of deformation and rupture nucleation at the final stage of tectonic stress buildup in which enough amount of the strain energy has been stored
4. Main shock earthquake rupture, and
5. Rupture arrest and its after effect.

An earthquake cannot occur without any accumulation of the elastic strain energy in the medium surrounding the fault. In this sense, stage til may be regarded as the preparatory process in a broad sense, which is characterized by the process of elastic deformation. In this stage, the tectonic stress level is below critical, and the amount of the stored strain energy is insufficient to cause the ensuing large earthquake on the same fault. As the tectonic stress continues to build up, the stress eventually reaches a critical level at this final preparatory stage (10 a necessary amount of the strain energy has been stored Laboratory studies demonstrate that local concentration of deformation inevitably occurs and rupture begins to nucleate if the fault is characterized by mechanical and/or structural inhomogeneity (CAN WE USE heterogeneity?). The nucleation may be accelerated by a triggering effect of stress transfer due to fault-fault interaction. Fluid-solid rock interaction may be activated at this stage as well because of high tectonic stress, and the nucleation may also be accelerated by activation of the fluid-rock interaction. Despite these possible accelerating effects, a large earthquake will be preceded by a slow growth of the nucleation, though the time- gap for the ensuing earthquake may be shortened by these possible effects. This stage may be regarded as the preparatory process in a narrow sense. The process of earthquake dynamic rupture during which short period seismic waves are generated is characterized by rapid stress drop with ongoing slip on the fault and almost complete release of the stored strain energy. The phase of rupture arrest and its effect are characterized by independent stress relaxation and stress transfer as a consequence. aftershocks and after-effects of crustal deformation follow.

Another trend is the increasing use of probabilities in communicating earthquake information to the public Instead of predicting the time, place, and magnitude of a future earthquake, recent public policy documents attempt to estimate an earthquake probability with a given window of time, space, and magnitude, take recourse to the method of the hypothesis testing Probability is a relatively new concept in human history: The origin of the theory of probability goes back to the 17th century when B. Pascal and P de Fermat exchanged letters on dice. The concept appears to be useful in dealing with difficult problems in human society.

In addition to this, physical modeling is recognized as a more important way to simulate the complex space-time magnitude behavior of earthquake occurrence. A central issue in these attempts is what lies behind the complex behavior of seismicity the non-linear dynamics inherent in earthquake rupture, or the interaction of rupture with heterogeneities of fault zone structure This issue is critical to the validity of the characteristic earthquake concept and is being debated vigorously. A sound development of a physical theory of earthquake prediction requires sustained Interaction between geology (heterogeneous earth) and physics (non-linear dynamics of fault rupture).

A wide array of monitoring techniques are being tested along part of the San Andres fault. For the past 150 years, earthquakes of about magnitude 6 have occurred on average once every 22 years on the San Andreas fault near Parkfield, California. The last shock was in 1966. Because of the consistency and similarity of these earthquakes, scientists have started an experiment to "capture" the next Parkfield earthquake. A dense web of monitoring instruments was deployed in the region during the late 1980s. The main goals of the ongoing Parkfield Earthquake Prediction Experiment are to record the geophysical signals before and after the expected earthquake; to issue a short-term prediction, and to develop effective methods of communication between earthquake scientists and community officials responsible for disaster response and mitigation. This project has already made important contributions to both earth science and public policy.

New Trends in Research:

In recent years several new trends have appeared in the study of earthquake prediction, especially in the U.S.

First, this involves the adoption of more objective testing of prediction methodology by researchers other than the advocators as well as efforts on the part of predictors to make prediction testable by others. Secondly, earth science information on future earthquake hazards is regularly transmitted in terms of probabilities to officials in charge of emergency preparedness, earthquake engineers, and the public. This trend is welcome because the probability conveys the information in a form, which can be dealt with in response planning and cost-benefit analysis of mitigation. Thirdly, physicists outside the community of earth science have started to get involved in the study of earthquake prediction.

The Need:

It is extremely difficult to predict the exact time when a damaging earthquake will occur, because when enough strain has built up, a fault may become inherently unstable, and any small background earthquake may or may not continue rupturing and turn into a large earthquake. In the Pacific Northwest. earthquake hazards are well known and future earthquake damage can be greatly reduced by identifying and improving or removing the most vulnerable and dangerous structures.

The Increased frequency of earthquakes around the world has brought the urgency for a planned program to marriage and mitigate disastrous natural catastrophes. To achieve the goal of earthquake prediction, there is a clear need for enhanced scientific contribution and technological (or observational) breakthroughs. The government has to come forward to fund the research and invest in the technology required to enhance predictive capability for natural calamities.

Selected References:

An Evaluation of the Animal-behavior theory for earthquake Prediction, 1988.http://www.nature.com/nature/debates/earthquake/equakeframeset html R.B. Schall, California Geology. Vol. 41. No. 2, pp. 41-45.

Predicting the Next Great Earthquake in California, 1985, R.L. Wesson and R.E. Wallace. Scientific American, Vol. 252. No. 2. pp. 35-43.

Prediction Probabilities from Foreshocks, 1991, D.C. Agnew and L.M. Jones. Journal of Geophysical Research, Vol. 96. No. 87. pp. 11.959-11.971.

Earthquake Prediction: A Challenging Area of Research, 2004. D. Maity. Kriti Journal of the Association of Civil Engineers, IIT Guwahati, Vol 1, pp. 15- 18

Shock Alarm, 2006. D. Maity. The Assam Tribune, Horizon. Saturday Special, pp. 1 & III. September 23.

Can We Predict Earthquakes?, 2005, D Maity, The Telegraph, Guwahati Edition, pp. 18, March 07.

http://quake.exit.com/

http://www.nature.com/nature/debates/earthquake/equakeframeset html

http://www.geo.arizona.edu/K12/azpepp/education/history/china/pred iction.html

http://earthquake.usgs.gov/hazards/pre diction.html

http://earthquake.usgs.gov/regional/worl d/byyear.phpeqstats.html

http://wwwnelc.cr.usgs.gov/neis/eqlists/

Dr. Maity is in the faculty of the Civil Engineering Department, Indian Institute of Technology, Kharagpur