Earthquake Prediction and Control

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Earthquake Prediction and Control

based on the lecture notes of Prof. Stephen A. Nelson - Tulane University

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.

 - 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 the recorded historical record.  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 14C 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.


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 the 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 example, there were clusters of earthquakes around a zone that included a major earthquake on Jan 30, 1973, another cluster around an earthquake of March 14, 1979, and two more cluster around earthquakes of July 1957 and January, 1962.  Between these clusters were large areas that had produced no recent earthquake activity.  The zones with low seismically are called seismic gaps.  Because the faulting had occurred at other places along the subduction zone it could be assumed that strain was building in the seismic gaps, and earthquake would be likely in such a gap within the near future.   Following a magnitude 8.1 earthquake on September 19, 1985, a magnitude 7.5 aftershock on Sept. 21, and a magnitude 7.3 aftershock on Oct. 25, along with thousands of other smaller aftershocks, the Michoacan Seismic gap was mostly filled in.  Note that there still exists a gap shown as the Guerrero Gap and another farther to the southeast.   Over the next 5 to ten years we may expect to see earthquakes in these gaps.


Example - The San Francisco, Loma Prieta, and Parkfield Seismic Gaps

Shown below are two cross-sections along the San Andreas Fault in northern California.  The upper cross section shows earthquakes that occurred along the fault prior to October 17, 1989.  Three seismic gaps are seen, where the density of earthquakes appears to be lower than along sections of the fault outside the gaps.   To the southeast of San Francisco is the San Francisco Gap, followed by the Loma Prieta Gap, and the Parkfield Gap. Because of the low density of density of earthquakes in these gaps, the fault is often said to be locked along these areas, and thus strain must be building.  This led scientist to issue a prediction for the Parkfield gap that sometime between 1986 and 1993 there would be an earthquake of magnitude 6 or greater south of Parkfield.  No such earthquake has yet occurred.  However a magnitude 7.1 earthquake occurred in the Loma Prieta gap on Oct. 17, 1989, followed by numerous aftershocks.  Note how in the lower cross-section, this earthquake and its aftershocks have filled in the Loma Prieta Gap.  This still leaves the San Francisco and Parkfield gaps as areas where we might predict a future large event.


Short-Term Prediction
Among the precursor events that may be important are the following:
Controlling Earthquakes Although no attempts have yet been made to control earthquakes, earthquakes have been known to be induced by human interaction with the Earth.  This suggests that in the future earthquake control may be possible. 
Examples of human induced earthquakes
In the first two examples the increased seismicity was apparently due to increasing fluid pressure in the rocks which resulted in re-activating older faults by increasing strain. 
The problem, however, is that of the energy involved.  Remember that for every increase in earthquake magnitude there is about a 30 fold increase in the amount of energy released.  Thus, in order to release the same amount of energy as a magnitude 8 earthquake, 30 magnitude 7 earthquakes would be required.  Since magnitude 7 earthquakes are still very destructive, we might consider generating smaller earthquakes.   If we say that a magnitude 4 earthquake might be acceptable, how many magnitude 4 earthquakes are required to release the same amount of energy as a magnitude 8 earthquake?   Answer 30 x 30 x 30 x 30 x 30 = 810,000!  Still, in the future it may be possible to control earthquakes either with explosions to gradually reduce the stress or by pumping fluids into the ground.