Dynamic Stress Drop of Recent Earthquakes: Variations within Subduction Zones

Stress drop is a fundamental parameter of earthquakes, but it is difficult to obtain reliable stress drop estimates for most earthquakes. Static stress drop estimates require knowledge of the seismic moment and fault area. Dynamic stress drop estimates are based entirely upon the observed source time functions. Based on analytical formulas that I derive for the crack and slip-pulse rupture models, the amplitude and time of the initial peak in source time functions can be inverted for dynamic stress drop. For multiple event earthquakes, this method only gives the dynamic stress drop of the first event. The Michigan STF catalog provides a uniform data base for all large earthquakes that have occurred in the past four years. Dynamic stress drops are calculated for the nearly 200 events in this catalog, and the resultant estimates scatter between 0.1 and 100 MPa. There is some coherent tectonic signal within this scatter. In the Sanriku (Japan) and Mexico subduction zones, underthrusting earthquakes that occur at the up-dip and down-dip edges of the seismogenic zone have correspondingly low and high values of stress drop. A speculative picture of the stress state of subduction zones emerges from these results. A previous study found that the absolute value of shear stress linearly increases down the seismogenic interface to a value of about 50 MPa at the down-dip edge. In this study, the dynamic stress drop of earthquakes at the up-dip edge is about 0.2 MPa, while large earthquakes at the down-dip edge of the seismogenic plate interface have dynamic stress drops of up to 5 MPa. These results imply that: (1) large earthquakes only reduce the shear stress on the plate interface by a small fraction of the absolute level; and thus (2) most of the earthquake energy is partitioned into friction at the plate interface.