Aftershocks arise directly from earthquake waves
In the days and months after severe earthquakes, a large number of other smaller tremors follow. According to measurements by two American geologists, however, they are not caused by new tension in the earth plates that were shifted during the event. Rather, the aftershocks are triggered by the seismic waves of the main tremor itself.
Karen Felzer of the American Geological Survey and Emily Brodsky of the University of California at Santa Cruz came to this surprising conclusion after looking at the relationship between the spatial and numerical distribution of aftershocks and the distance to the epicenter. The data was based on all earthquakes that had occurred in southern California between 1984 and 2002. In these cases, the number of subsequent tremors decreased rapidly and, above all, relatively linearly at a distance of 200 meters to 50 kilometers from the main tremor, so that the same triggering mechanism must have been active over the entire section.
According to previous theories, the aftershock active zone should be at most twice as long as the fracture zone active during the main quake. But even weak tremors still triggered after-effects fifty kilometers away, although their respective rupture zone was small and should therefore only have had an impact within a range of about a thousand meters. In addition, the amplitude of the aftershocks weakened along the entire route at the same rate as the energy of the original seismic waves. Brodsky and Felzer even conclude that the probability and magnitude of aftershocks is correlated with the magnitude of the mainshock - a completely new approach that completely contradicts conventional theory.
This connection does not make it possible to predict the first tremor, but it may at least make it possible to better predict the location, time and, above all, the strength of the aftershocks, the researchers continue.
