Newsrx Science. \"Earthquakes Actually Aftershocks

¶ … NewsRx Science. "Earthquakes actually aftershocks of 19th century quakes."

According to a collaborative study recently conducted at Northwestern University and the University of Missouri-Columbia, small earthquakes that occur in the Central region of North America are actually not contemporary earthquakes; they are actually aftershocks of much stronger earthquakes that occurred nearly two centuries ago. In principle, that is a result of the different rates of relative tectonic motion between the respective tectonic plates involved in active fault lines after large earthquakes of magnitude 7 and above.

Whereas fault lines closer to the coastal periphery (such as the San Andreas Fault in California) continue moving past one another at a rate of approximately an inch and a half per year after a major earthquake, those located closer to the central mass of tectonic plates (such as the New Madrid Faults) move 100 times slower. As a result, the aftershocks of earthquakes involving peripheral fault lines tend to subside after approximately one decade while those caused by large central plate faults can continue for centuries.

In principle, the researchers compared recorded patterns of large recent earthquakes to the known patterns associated with the gradual decrease in the magnitude of successive aftershocks. The results confirmed that the small earthquakes recorded in this decade are indeed part of the series of aftershocks from much earlier large earthquakes such as the 7.2 magnitude Hebgen Lake earthquake in Montana, Idaho, and Wyoming a half century ago.

Scientific Concepts

After large tectonic shifts that cause significant earthquakes, the tectonic plates involved continue their relative motion with respect to one another for many years afterwards. This is a function of the tectonic settling that scientists refer to as "swamping" their changes in position. In effect, the swamping process also reloads the fault once the plates stabilize at which point they regain their potential kinetic energy that will eventually be released in the next major tectonic shift resulting in a new series of a major earthquake followed by successively smaller aftershocks.

The tremendous difference in the rate of tectonic movement is likely attributable to the corresponding difference in the mass of large central-mass tectonic disturbances and the smaller masses involved in peripheral tectonic motion. More precisely, the length of the aftershock pattern that follows major earthquakes is proportional to the relative size and mass of the tectonic plates involved.