earthquake hazards and prediction Many of the world's greatest natural disasters have been due to earthquakes (Table 1). In part, this is because earthquakes, like hurricanes and storms, can generate severe effects over large areas. The hazards from earthquakes can be classified into two categories, based on whether they are primary or secondary effects. Primary hazards include ground shaking, surface fault rupture, and uplift or subsidence. Liquefaction, landslides, and water waves such as tsunamis and seiches are secondary effects because they are either caused by strong ground shaking or, in the case of tsunamis, coseismic (i.e. at the same time as the earthquake) elevation changes (uplift or subsidence).

Ground shaking is the result of seismic waves (see earthquake mechanisms and plate tectonics) reaching the Earth's surface. This is the most damaging of all earthquake hazards because of its far-reaching effects. Ground shaking or ground motions are a function of earthquake size and mechanism, distance from the causative fault, the attenuating properties of the Earth along the path of travel of the seismic waves, and the near-surface geological conditions beneath the location of the observer (see strong-motion seismology).

Surface fault rupture or surface faulting occurs when an earthquake is generated along a fault that is expressed at the Earth's surface. As a result, movement generated at depth along the fault is propagated upward, resulting in displacement of the ground. For example, in the 1992, magnitude (M) 7.3, Landers, California earthquake, the maximum displacement along the fault observed at the ground surface was about 6 m horizontally (see earthquake seismology for discussion of magnitude). Any structure situated along a fault which has a surface expression is subject to damage in a future earthquake. In the United States, some state laws require that certain facilities should not be built along active fault zones. Not all faults reach the Earth's surface, including the greatest faults—the megathrusts located in subduction zones (see earthquake mechanisms and plate tectonics).

Significant coseismic elevation changes can also occur in large earthquakes; these are generally associated with large subduction zone megathrust earthquakes. For example, in the great 1964 Alaskan earthquake, M 9.2, portions of Montague Island were uplifted 11 m on the upthrown side of the megathrust fault and about 2 m on Prince William Island which is located on the downthrown side. Because of the coincidence of extensive coastlines along the Pacific Rim and the subduction zones within the ‘Ring of Fire’ (see earthquake mechanisms and plate tectonics), this hazard can be significant over a large part of the Earth. Also, because these elevation changes are generally long term in nature relative to sea level, they can lead to permanent loss of function of coastal facilities, such as harbours.

A possible secondary and dramatic effect of coseismic elevation changes are tsunamis. These giant sea-waves are generated by sudden displacement of the sea floor or by submarine landslides. In the open ocean, tsunamis have small wave heights but they can travel up to speeds of 500–800 km s⁻¹. As the waves approach the coast, the water depth decreases, resulting in increases in wave heights up to 20 m. In the 1964 Alaskan earthquake, the resulting tsunami caused extensive damage along the Pacific coast of Canada and western USA and Hawaii, and killed more than 100 people. Another type of water wave, a seiche, is generated in an enclosed body of water such as a lake. Earthquakes can induce seiches, such as was the case in the 1959, M 7.3, Hebgen Lake, Montana earthquake in the western United States.

When water-saturated sandy and/or silty soil is subjected to strong earthquake ground shaking, a phenomenon called liquefaction can occur. Shaking realigns the soil particles, resulting in an increase in pore pressure and a decrease in strength. Soils in such a state acquire a degree of mobility sufficient to permit deformation. In extreme cases, the soil particles become suspended in groundwater and the deposit reacts as a fluid giving rise to sand or mud volcanoes. There have been spectacular examples of damage due to liquefaction as it often occurs in large earthquakes.

Earthquake-induced landslides are like other types of slope failure, except they are triggered by strong ground shaking. One of the most catastrophic landslides to ever occur took place in the 1970 Peru earthquake (Table 1). The landslide was actually a large debris avalanche which originated from a peak of the Nevado Huascaran volcano, travelled at a speed of about 320 km s⁻¹, and destroyed two villages. The death toll was more than 18 000 lives.

One of the most important applications of the Earth sciences today is the evaluation of earthquake hazards. Such evaluations generally comprise two major steps: the identification and characterization of the sources of the earthquake and the assessment of the associated hazards. The former requires a multidisciplinary approach, utilizing not only seismology but also other fields of geophysics and, most importantly, geology and geomorphology. Possibly the most important breakthrough in our understanding of earthquake processes has been due to the relatively new field of palaeoseismology. Palaeoseismic studies consist of geological investigations of prehistoric earthquakes through evaluation of their impacts on the environment, such as surface faulting, tsunami deposits, liquefaction features, and buried marshes.

The assessment of specific earthquake hazards has involved seismologists, geologists, and earthquake engineers. Strong earthquake ground shaking has been the subject of greatest attention and studies (see strong-motion seismology).

Earthquake prediction

Motivated by the desire to mitigate the often disastrous effects and hazards of earthquakes, the early prediction of such events has long been attempted. However, modern attempts at earthquake prediction were not initiated until about the 1960s, and such efforts were generally concentrated in Japan, China, the former USSR, and, to a lesser extent, the United States.

Earthquake prediction has revolved around the search for earthquake precursors, those physical indicators that might signal the future occurrence of a seismic event. Precursors might include unusual changes in:(1) seismicity in the region of an impending event;(2) crustal deformation of the ground, including horizontal and vertical land changes, water-well fluctuations, and changes in the Earth's seismic velocities;(3) changes in the Earth's gravity, magnetic, or electrical fields;(4) emission of gases such as radon;(5) unusual animal behaviour.

All or most of these effects might be related to strain and stress changes in the Earth before earthquake.

Earthquake prediction involves answering the three basic questions (see earthquake seismology) of where, when, and how big. Thus, earthquake precursors can be classified on the timescale at which they may be useful. The classifications include: (1) long-term or up to a few hundred years; (2) intermediate-term or up to several years; and (3) short-term or on the order of days to months. A successful example of a long-term precursor is based on the concept of a ‘seismic gap’. First applied to the world's subduction zones, a seismic gap is a region where large earthquakes have occurred in the past but none in past decades or longer and where any seismicity appears to be nearly absent. Eventually these gaps are filled by large earthquakes and their aftershocks. The concept has been applied successfully in the circum-Pacific belt of subduction zones.

Although of considerable value, the weakness in long-term precursors is that the uncertainties in predicting the time of a future earthquake can be large, on the order of years or decades. Herein lies the basic problem in present earthquake prediction efforts. Great strides have been made in predicting the place and the size of future earthquakes. The problem of predicting the specific time of an event has, for the most part, remained totally unsolved. No reliable intermediate or short-term precursors have been identified to date. This is not to say there have not been successes. Probably the most spectacular example was the short-term prediction of the 1975, M 7.3, Haicheng earthquake by the Chinese, based on earthquake foreshocks, unusual animal behaviour, and a few other precursors. By evacuating Haicheng, probably ten of thousands of lives were saved. Unfortunately, this event was followed a year later by the catastrophic 1976, M 7.7, Tangshan, China earthquake which took up to 650 000 lives with no apparent warning and no prediction.

In the United States, efforts in earthquake prediction accelerated in the 1970s as a result of reported successes by Soviet seismologists who had observed changes in travel times and velocities of seismic waves as they passed through the source region of an impending earthquake. In general, however, US scientists failed to observe similar anomalies. Recently, the only significant US effort has been focused on an experiment along a section of the San Andreas fault near the town of Parkfield, California. At this location, an M 6 earthquake had been predicted to occur at some time in the time frame 1988± 5 years, based on an apparent regularity of previous earthquakes. Although this event has not yet occurred, efforts are continuing to monitor the proposed epicentral area with a wide variety of instrumentation, including seismometers, water-well gauges, tiltmeters, levelling surveys, triangulation and trilateration arrays, radon gas monitors, gravimeters, and magnetometers. Currently, efforts in the United States are focused on long-term forecasting, specifying the location and size of future earthquakes and their probability of occurrence within a time period of several decades.

Ivan G. Wong

Bibliography

Bolt, B. A. (1993) Earthquakes. W. H. Freeman, New York.
Brumbaugh, D. S. (1999) Earthquakes, Science and Society. Prentice Hall, New Jersey.
Reiter, L. (1990) Earthquakes hazard analysis. Columbia University Press, New York.