The societal response to natural hazards posed by earthquakes and volcanoes is couched in terms of the specific hazards that they present, as well as the viable options for dealing with the phenomena. We'll consider some of the specific hazards associated with each, and discuss the mitigation strategies that have evolved.
Earthquake hazards:
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Primary: |
Fault Displacement/Ground Rupture |
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Ground Shaking |
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Secondary: |
Landslides |
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Liquefaction |
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Tsunamis |
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Dam Failures |
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Fire |
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Nuclear/Chemical Plant Failure |
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Induced Earthquakes |
Earthquake Hazard Mitigation Approaches:
Key to making decisions regarding earthquake mitigation is use of the historical earthquake behavior and understanding of earthquake faulting driving mechanisms to develop seismic risk maps. These involve a consideration of the probability of certain levels of ground shaking during a given interval of time (often 30 or 50 years, a typical building life expectancy). Risk assessment also entails some consideration of the exposure to damage, which is influenced by population density, historical building practices, geography and other factors.
At some level, any attempt to define the probability of future ground shaking entails forecasting of future events. When seismologists discuss earthquake prediction, there is a very specific connotation of the term: an earthquake prediction must specify the location, magnitude and time of the event, given in terms of ranges or windows of parameters, with some assessment of the statistical likelihood of the event. This requires a very complete understanding of the physical processes producing an earthquake, and while there have been a few accurate 'predictions', based on various arguments, there is no accepted universal criteria for prediction specificity. But, we do have some general understanding of past earthquake behavior upon which we can base statistical projections of future events, albeit with great uncertainty. In this regard, earthquake prediction is akin to weather forecasting, entailing a future projection of the statistical likelihood of a particular outcome given a recent type of behavior, relative to past observations.
Earthquake forecasting is significantly more complex than weather forecasting, both because we have a less complete fundamental understanding of earthquakes and far fewer opportunities to measure the complexity of stress and deformation ongoing in the Earth relative to our ability to monitor global atmospheric conditions and variations of solar heating and other driving forces for weather.
We subdivide earthquake prediction efforts into
Long term forecasts exploit our general understanding of plate tectonics, in that a given plate boundary fault is anticipated to have a cumulative faulting offset with time that corresponds to the relative plate motion. Each segment of the fault must slip a comparable total distance, even if some regions fail more often in smaller events while other regions fail rarely in large events. This notion has observational support in that earthquake rupture zones tend to fill in the entire plate boundary as a function of time, with rerupturing of the same segment being very common. For example, if we look at the Pacific/Eurasia plate boundary along Japan, where oceanic plate is underthrusting the islands, we find that in this century earthquakes have pretty uniformly covered the plate contact. In the previous century the same is true, with each segment of fault tending to rupture every 50-100 years on average. The slip in each event tends to be 5 m or so, which corresponds to the average time between ruptures multiplied by the plate convergence rate of about 9 cm/yr. In some regions we have over a thousand year history of repeated failure of a particular stretch of fault. Along the Tonankai-Nankaido region south of Honshu the average repeat time is about 180 years, although there are large fluctuations in the time between events. The Tokai Gap south of Tokyo is a region that failed in 1854, but did not fail when adjacent areas of the fault ruptured in 1944 and 1946.
A seismic gap is a region that is known to have previously failed in earthquakes (versus a continuously creeping section of fault), and has not failed for a length of time close to the average recurrence interval for that stretch of fault. Long-term forecasts try to identify which faults are 'mature' in terms of approaching their average recurrence delay since the last event. This notion assumes some uniformity in the rate of plate motions and the limiting strength of rock (bounding the strains that can accumulate prior to frictional sliding). Seismic gaps can be assigned probabilities for failure based on the spread in repeat times between earthquakes, although the latter is not well known for many faults, and for some faults has huge variance. Long-term forecasts based on recurrence behavior are useful for societal planning (construction codes, etc., acknowledging the regional hazard), but are not useful for evacuation or other short-term decisions.
Short-term prediction or forecasting is based on the notion that there will be some preparatory phenomena in the rock mass around a fault as it approaches the limiting strain conditions which induce frictional instability. The idea is that the instability must have some detectable precursory effects, since the highly strained rock is on the edge of an instability prior to sliding. One of the most commonly sought effects is precursory seismicity.
Seismicity patterns that are observed prior to some earthquakes include:
Other effects that are sought include strain effects that result in anomalous behavior just before an earthquake. Some of the things that are measured are:
The hope is that the volume of rock that is undergoing final strain accumulation right up to the point of sliding failure is both large enough to result in a measurable surface effect, and that it will be temporally recognizable. We find that some events do seem to be preceded by one or more of these strain like effects, but other events have no precursor.
More exotic approaches include
A few prediction case histories
I. 1975 Haicheng earthquake. In this northern China city, a prediction was made based on anomalous foreshock earthquake activity in a normally quiet region. Well levels, tilting of the ground and anomalous animal behavior were observed, along with curious effects such as spontaneous ignition of gas bubbles in swamps.
The prediction evacuated the town of 100,000 for 2 nights, with a magnitude 7.3 earthquake occurring on the second day. 90% of the houses collapsed or were damaged, and few lives were lost. The evacuation was successful in part because of the totalitarian regime which enforced it, along with popular awareness of earthquake hazard and detection of the small foreshocks. It also was fortunate that the event happened within a short time, or else the evacuation may not have been sustained.
II. 1978 Oaxaca, Mexico. In 1977 U. Texas researchers observed temporal quiescence in a substantial seismic gap, which began in 1973. They estimated a magnitude 7.5 event could occur. In August 1978, small foreshocks were detected by a small seismic array that had been installed in the region. In November 1978, a magnitude 7.8 event occurred. This is an example of a medium-term forecast (no precise time was given for the event, although a magnitude and location were).
III. 1981 Peru. A major seismic gap along the coast has persisted for over 100 years since magnitude 8.0+ earthquakes occurred last century. U.S. Scientists Brady and Spence developed a complex theory of earthquake occurrence. They made very specific predictions based on their theory, for a magnitude 8.8 on August 10, 1981 and a 9.8 on Sept. 15, 1981. The U.S. National earthquake prediction council evaluated this prediction and found no credibility to the theory. Nonetheless there was great panic in Peru, with devastation of tourism, exodus of wealthy people and extension depression. The prediction was withdrawn as the specific occurrence of a small foreshock did not take place, but the damage was done. This was a major international fiasco.
IV. Parkfield, California. On a small stretch of the San Andreas fault in central California, a moderate size, magnitude 6.0 earthquake occurred in 1966. Investigations showed that the same stretch of fault had failed in 1934, 1922, 1901, 1881, and 1857. The times between events were then 32, 12, 21, 20, and 24 years, with a mean repeat time of 22 years and a standard deviation of 3 years. Based on this behavior, the U.S. Geological Survey made its only official prediction, for a magnitude 5.5 earthquake in 1988+/- 5 years. They set up a monitoring system to seek any short term precursors as well. At the end of 1993 the prediction window was exceeded and the prediction formally failed. It is now thought that there will be an earthquake in the future at Parkfield, but there appears to be more variability than the last 6 events had suggested. A concern is that the 1857 event occurred shortly before the 1857 rupture of the southern San Andreas fault in a great, magnitude 8 earthquake. Will the next Parkfield event trigger the Big One?
V. Tokai Japan. Based on the presence of a seismic gap south of Tokyo, which last ruptured in 1854, and before that in 1707, it is expected that there will be an event as large as 7.5-8.0. The adjacent area of fault ruptured in 1944. The Japanese have set up a massive earthquake prediction program, with extensive measurement of all viable phenomena which may show precursors (seismicity, radon gas, electrical, magnetic, tilting, water, etc.). Official procedures have been set up with a set of prediction criteria and an advisory panel that will make the prediction. Failure is not allowed, nor is uncertainty.
While specific predictions have a checkered history, there are now maps of probability of earthquake shaking for many regions of the San Andreas, based on historic activity. Southern California is deemed to be more likely to have the next major event than Northern California, but the Parkfield experience has increased skepticism.
In some cases, we can exploit local detection of an earthquake to phone ahead and warn that the seismic waves are on the way. This idea is used in Tsunami warning system for the Pacific. In this case, seismic recordings are used to locate events around the Pacific, to determine their size, depth, and faulting mechanism. If the event appears to be a good candidate for having displaced a lot of water, a tsunami warning is issued. This alerts areas far from the earthquake that a sea wave may be approaching. This is possible because the seismic waves travel much faster than tsunamis in the ocean do (they travel about the speed of a jet plane, so it takes 4-10 hours to traverse the Pacific). Events can be studied by seismic waves in a few tens of minutes to an hour. This allows alerts to be broadcast for places like Hawaii, Alaska, Japan, and other areas that are vulnerable to tsunamis from earthquakes at various positions around the Pacific. It is not useful for the very local tsunami, as that arrives before the seismic signals can be analyzed.
An analogous system was set up in 1989 after the Loma Prieta earthquake. The collapse of the Cyprus Freeway in Oakland required emergency crews to risk their lives going into the pancaked freeway. Aftershocks threatened to make further collapses. What was done was to set up a phone system, by which aftershocks that were felt strongly in San Jose led to alerts in Oakland via electronic transmission, far faster than the speed of seismic waves. Thus, there were a few tens of seconds for rescue workers to get out prior to the arrival of shaking from aftershocks 100 km to the south.
Aside from the scientific issues of earthquake prediction there are major socioeconomic considerations that influence the process. An earthquake prediction can cause great economic impact, decreasing property values, business activity, leading to civic disruption. Prediction assessment panels have been set up to try to maintain control on the process, but there is much concern about the liability incurred by a scientist who makes a prediction that fails. There are also dilemmas about whether a prediction is actually beneficial in some cases, as it may prompt response that exacerbates the losses even if the prediction is correct. For example, panic evacuations may clog freeways that collapse during an event or may prevent emergency vehicles from being able to respond to fires and injuries. It is no surprise that few people are bold enough to publicly make a prediction.
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