By Ruth Ludwin
Because of their devastating potential, there is great interest in predicting the location and time of large earthquakes. 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.
Worldwide, each year there are about 18 earthquakes magnitude (M) 7.0 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 we are not able to predict individual earthquakes, the world's largest 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 each year, and the Pacific is shrinking as ocean floor is pushed beneath Pacific Rim continents. Geologically, earthquakes around the Pacific Rim are normal and 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. Although there are clear seismic hazards in our area, Pacific Northwest faults are complex and it is not yet possible to forecast when any particular fault segment in Washington or Oregon will break.
Along the San Andreas Fault, an earthquake prediction was made in the 1980s for the segment near Parkfield CA considered likely to rupture. Earlier this century it produced a series of identical earthquakes (about M 6.0) at fairly regular time intervals. Using a set of assumptions about fault mechanics and the rate of stress accumulation, the USGS predicted that a Parkfield earthquake of about M 6.0 earthquake would occur between 1988 and 1992. USGS scientists monitored Parkfield for a wide variety of possible precursory effects, but the predicted earthquake did not materialize until 2004, long after the prediction window expired. "Capturing" the magnitude 6.0 Parkfield earthquake in a dense network of instrumentation was a significant accomplishment, providing data to determine whether precursory effects exist (none were found), and give new insights on the mechanics of fault rupture More from the USGS...More from CISN.
The segment of the San Andreas fault that broke in the 1989 M 7.1 Loma Prieta or "World Series" earthquake had been identified by the USGS as one of the more likely segments of the San Andreas to rupture. Magnitude 5+ earthquakes 2 and 15 months before the damaging earthquake were treated as possible foreshocks, and the USGS issued 5-day Public Advisories through the California Office of Emergency Services. Even in areas where foreshocks are fairly common, there is no way of distinguishing a foreshock from an independent earthquake. In the Pacific Northwest, there is no evidence of foreshock activity for most historic earthquakes.
One well-known successful earthquake prediction was for the Haicheng, China earthquake of 1975, when an evacuation warning was issued the day before a M 7.3 earthquake. In the preceding months changes in land elevation and in ground water levels, widespread reports of peculiar animal behavior, and many foreshocks had led to a lower-level warning. An increase in foreshock activity triggered the evacuation warning. Unfortunately, most earthquakes do not have such obvious precursors. In spite of their success in 1975, there was no warning of the 1976 Tangshan earthquake, magnitude 7.6, which caused an estimated 250,000 fatalities.
Earthquake prediction is a popular pastime for psychics and pseudo-scientists, and extravagant claims of past success are common. Predictions claimed as "successes" may rely on a restatement of well-understood long-term geologic earthquake hazards, or 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 can not state that the predicted earthquake will not occur, because an event could possibly 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. Scientific earthquake predictions should state where, when, how big, and how probable the predicted event is, and why the prediction is made. The National Earthquake Prediction Evaluation Council reviews such predictions, but no generally useful method of predicting earthquakes has yet been found.
It may never be possible 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. While it may eventually be possible to accurately diagnose the strain state of faults, the precise timing of large events may continue to elude us. In the Pacific Northwest, earthquake hazards are well known and future earthquake damage can be greatly reduced by identifying and improving or removing our most vulnerable and dangerous structures.