Is the seismic gap theory valid and reliable?
The seismic gap theory says that, if there have been large earthquakes on an active fault's neighboring faults in the past, will that unbroken piece of fault be the most likely location for the next major earthquake? A considerable amount of research has tried to answer this question, and at the present time the answer seems to be "no." One reason might be that the unbroken piece of fault is just stronger than the surrounding faults, and it takes much more tectonic stress to break it. Or it may be that the tectonic forces on the unbroken fault have actually been reduced by the earthquakes on the neighbors (this may happen for various reasons arising from the physics of the way stress is transferred through the ground).
In either case, it is possible the unbroken "gap" will persist for some time before it eventually does rupture. This is what makes earthquake forecasting and prediction so difficult.
A frequently cited example of a seismic gap being filled was the magnitude 7 Loma Prieta earthquake of October 17, 1989. An example of an expected great earthquake failing to happen is the Tokai earthquake southwest of Tokyo. This magnitude 8+ event has been anticipated for several decades, but has yet to occur.
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