Thursday, March 17, 2011

Thrust faulting for the Japan earthquake

The images below are taking from a USGS poster on the Japanese earthquake.

As in the last post, the balloon diagrams are "fault plane solutions." These are based on the P-waves arriving at different seismic stations, and whether the first pulse of energy is a compression or a dilatation (stretching apart). These patterns reveal the orientation of the fault on which the earthquake occurs, and the nature of the faulting. Note that in the top figure, the boundary between the black and white portions of the balloon diagrams are parallel to the major plate boundary between the Pacific plate (to the east) and the Eurasian pate (to the west). This tells us what the strike of the fault is.  Also, the pattern of black (compressions) and white (dilatations) shown corresponds to a thrust fault occurring on a fault plane shallowly dipping to the northwest, as suggested by the pattern of earthquake locations shown on the second diagram. Thus, we can use seismic waves to tell us what kind of faulting of the Earth's crust occurred 25 km below the surface, even though no one is there to directly observe it.

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