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Induced seismicity caused by man-made events, such as the filling of reservoirs has been well documented. In contrast, naturally induced seismicity has received little attention. It has been shown that a fluctuation of as little as several bars can trigger reservoir induced earthquakes. Naturally occurring phemonena generate similar fluctuations and could trigger earthquakes where the faults in ambient stress field are suitably oriented and close to failure.
The New Madrid Seismic Zone (NMSZ) presents an ideal test case for the study of naturally induced seismicity. The ideal data set for a study of triggering effects must contain a statistically significant number of events, a constant accumulated strain, and a limited focal region. New Madrid earthquakes are well documented from 1974 to the present, down to a magnitude ~1.8. They lie in a distinct fault pattern and occur as a reaction to the regional stress regime.
A statistical correlation was made between the earthquakes and a variety of different types of loads, to see if New Madrid seismicity could be triggered by natural fluctuations. The types of "triggers" investigated ranged from solid earth tides to variations in barometric pressure, rainfall, and stages of the Mississippi River. This analysis becomes complex because each factor investigated creates individual stresses, as well as having imbedded in it a reaction to other factors. For example, changes in barometric pressure influence the observed solid earth tides, as well as leading to rainfall, which in turn cause changes in the river stages. Most likely it is a combination of effects, reinforcing each other, that act as possible trigger sources.
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