150Lec9-1

150Lec9-1 - Lecture #9: Earthquake Forecasts and...

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Lecture #9: Earthquake Forecasts and Predictions (Abbott, pp. 142-155) Earthquake Forecasts and Predictions For geologists, earthquake forecasts and earthquake predictions are different concepts. In some cases, geologists may be able to forecast the probability of an earthquake in a given area based on the records of past earthquakes, accumulating stresses on known faults, the presence and timing of any foreshocks, and other often unreliable indicators. For example, based on past earthquake activity, geologists forecasted that the San Andreas fault in Parkfield, California had a 95% chance of producing a magnitude 6 earthquake between 1985-1993 (p. 155). This forecast proved to be incorrect. Earthquake forecasts are often displayed on maps, such as in Figure 6.31 (p. 152) and Figure 6.19 (p. 144). In contrast, earthquake predictions are far more specific and usually involve exact locations and times within a few days. Because they are based on many complex and poorly understood factors, earthquake predictions tend to be highly unreliable. An exception was the 1975 Haicheng, China earthquake. Based on ground swelling, foreshocks, changes in groundwater levels, usual animal behavior, changes in local magnetic fields, and a lot of luck, on February 4, 1975 Chinese scientists predicted a severe earthquake within the next two days. The prediction was correct and numerous lives were saved within the city of Haicheng. Unfortunately, the Tangshan, China, earthquake of 1976 did not display such precursors and at least 240,000 people died (p. 70-71). Like long-term weather forecasts, the processes that generate earthquakes are often so complex and difficult to measure that the timing and magnitudes of earthquakes cannot be forecasted or predicted within any practical certainty. The best approach, at this time, is to minimize the effects of earthquakes on human lives and property by identifying potentially dangerous faults, estimating the duration and potential damage from any associated earthquakes, minimizing residential construction in the most dangerous areas around these faults, and enforcing appropriate building codes in the areas. The map in Figure 7.1 (p. 162) shows the earthquake hazards for the contiguous United States. Surprisingly, areas such as western Tennessee, western Kentucky, and the central coast of South Carolina have fairly high risks for dangerous earthquakes.
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Earthquake Precursors Sometimes minor earthquakes, changes in groundwater elevations, increased radon gas emissions from the subsurface and other events occur before a major earthquake. The minor earthquakes that occur before a major earthquake are called foreshocks . In contrast, less intense earthquakes that occur after a major earthquake are called aftershocks (p. 85, 95). Not every major earthquake has foreshocks. The foreshocks probably result from cracking and minor slippage along the fault prior to a large movement. In the same way, we can often hear cracking in a stick just before it breaks. Radon
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This note was uploaded on 07/14/2011 for the course GLY 150 taught by Professor Henke during the Spring '08 term at Kentucky.

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150Lec9-1 - Lecture #9: Earthquake Forecasts and...

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