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.
译文:
Ruth Ludwin的地震预测
由于大地震具有毁灭性的潜质，人们对于其发生的位置和时间的预测极为关注。尽管人们对于哪里可能会有地震已经有了很多了解，但目前还没有可靠的方法来预测在哪几天或哪几个月里地震将会发生在具体哪个地方。
在世界范围内，每年约有18次7.0或以上级别的地震。事实上，从1968年开始，1986年和1990年每年的次数为6-7次，在1970年，1971年和1992年，则每年高达20-23次。尽管我们不能预测单次的地震，但世界最大的地震的确有明确的空间形式，因而对未来的一些大地震的发生方位和震级是可以做出预测的。大多数地震发生在环太平洋边缘的长断层。这是由于大西洋每年都会变宽几英寸，而太平洋却随着洋底被环太平洋大陆挤压在下方而缩小。从地质学上说，环太平洋地区的地震是正常且预计会发生的。环绕太平洋的长断层区被地质的不规则性分成了更小的单独断裂的断层片断。地震的震级和时机受断层大小，岩石硬度以及累积起的压力的多少控制。人们已经了解了断层和板块运动的位置，最容易断裂的断层块也可以被确定。如果知道断层片断在过去的大地震中已经破裂，可以根据断层片断的大小、破裂的历史以及压力的累积来估计地震再次发生的时间和可能的强度。这种预测方法只能用于对其了解较多的断层，比如圣安德列斯断层。对于了解甚少的断层，比如造成了1994年加州北岭地震和1995年日本神户地震的那些断层，就无法做出这种预报。尽管太平洋西北地区有明显的地震危险，但是该区域的断层很复杂，至今尚不能预测在华盛顿或是俄勒冈州的具体哪块的断层何时将会破裂。
沿着圣安德鲁斯断层，在80年代曾作出过一次地震预测，认为邻近加州帕克菲尔德的断层可能会断裂。本世纪初，在相当规律的时间间隔里发生了一系列相似的地震（约6.0级）。运用一系列关于断层力学和压力累积率的假设，美国地质调查局预测在1988年到1992年间在帕克菲尔德将会发生大约6.0级的地震。美国地质调查局的科学家对帕克菲尔德可能存在的诸多前兆效应进行监测，但直到2004年，预测窗口已经失效很久以后，地震才发生。在密集的仪表网络系统中捕获到6.0级的帕克菲尔德地震是个意义重大的成就，这为判断前兆效应是否存在（最终没有发现）提供了数据并对断层断裂力学提出了新见解。
在1989年7.1级的洛马-普雷塔地震，或称为“世界系列”地震中遭破坏的圣安德鲁斯断层被美国地质调查局确认为是圣安德鲁斯断层中较可能断裂的片断之一。在这场破坏性地震前的两个月和15个月发生的5级以上地震被认为可能是前震，美国地质调查局通过加州紧急服务办公室发布了持续5天的公共警报。即使是在前震相当普遍的地区，也无法将前震与独立地震区分开来。在太平洋西北地区，历史上的大多数地震都没有前震活动的证据。
一次广为人知的成功地震预测是1975年中国的海城地震预测，在这场7.3级地震发生的前一天发布了疏散警报。在此前几个月里，陆地高度和地下水位的变化、关于动物反常行为的大量报告以及几次前震已经对此作了低级别警报。前震活动的增加引发了疏散警报。遗憾的是，大多数地震没有这么明显的前兆。尽管在1975年成功地进行了预报，但1976年的唐山地震却没有任何预警，这场7.6级的地震造成了约250,000人死亡。
地震预测在特异功能人士和伪科学者中是一项流行的活动，对过去成功预测的夸大吹嘘也很常见。被称为成功的预测可能靠的是对众所周知的长期地质地震危险的重新叙述，或是预测范围宽泛且模糊，以至他们得以用典型的背景地震活动来证实自己。不论潮汐力或是异常的动物行为对地震预测都没有帮助。如果有人作出了非科学的预测，科学家们不会说预测的地震不会发生，因为尽管没有理由认为这个预测日期会比其他日期更有可能发生地震，但地震可能会恰巧发生在所预测的时间里。科学的地震预测应该指出所预测地震的位置、时间、强度、可能性以及为何会作出这个预测。美国国家地震预测评估理事会回顾了这类预测，但至今仍未发现任何普遍有用的地震预测方法。
也许我们永远无法预测到破坏性地震发生的确切时间，因为当累积了足够的压力时，断层可能会变得极其不稳定，任何小的背景地震都可能会继续造成断裂从而演变为一场大地震。尽管准确判断断层的压力状态最终是有可能的，但我们可能仍无法知道大地震发生的确切时机。在太平洋西北地区，地震危险是众所周知的，但未来地震的破坏性可以通过发现、改造或拆除最脆弱最危险的建筑物来大大降低。
注:Ruth Ludwin 为西雅图华盛顿大学地球和空间科学系的研究科学家