eqprediction&cntrl

eqprediction&cntrl - EQ Prediction & Control EENS...

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This page last updated on 21-Jan-2004 EENS 204 Natural Disasters Tulane University Prof. Stephen A. Nelson Earthquake Prediction and Control Earthquake Prediction and Control Long-Term Forecasting Long-term forecasting is based mainly on the knowledge of when and where earthquakes have occurred in the past. Thus, knowledge of present tectonic setting, historical records, and geological records are studied to determine locations and recurrence intervals of earthquakes. Two aspects of this are important. z Paleoseismology - the study of prehistoric earthquakes. Through study of the offsets in sedimentary layers near fault zones, it is often possible to determine recurrence intervals of major earthquakes prior to historical records. If it is determined that earthquakes have recurrence intervals of say 1 every 100 years, and there are no records of earthquakes in the last 100 years, then a long-term forecast can be made and efforts can be undertaken to reduce seismic risk. { Example: The diagram below shows a hypothetical cross-section of a valley along a fault zone. The valley has been filled over the years with clays, sands, and peat (decaying organic matter). The upper peat layer is not yet cut by the fault. Peat is a useful material to geologists, since it contains high amounts of Carbon that can be dated using the 14 C method. The ages for each of the peat layers are shown. The dates suggest that a major faulting event cut the lower peat layer sometime after it was deposited 440 years ago. The dates also show the middle peat layer was cut by a faulting event after it was deposited 300 years ago. If these faulting events were associated with earthquakes, this suggests a recurrence interval of about 140 years. Since the upper peat layer has not yet been cut by the fault and is 135 years old, we can speculate that within the next 10 years or so there may be another earthquake. This assumes, of course, that the two previous events are an accurate measure of the recurrence interval. 1/21/2004 Page 1 of 5
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z Seismic gaps - A seismic gap is a zone along a tectonically active area where no earthquakes have occurred recently, but it is known that elastic strain is building in the rocks. If a seismic gap can be identified, then it might be an area expected to have a large earthquake in the near future. { Example - The Mexico Earthquake of 1985 The map below shows the southern coast of Mexico. Here the Cocos plate is subducting beneath the North American Plate along the Acapulco Trench. Prior to September of 1985 it was recognized that within recent time there had been major and minor earthquakes on the subduction zone in a cluster pattern. For
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eqprediction&cntrl - EQ Prediction & Control EENS...

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