Earthquake Focal Mechanisms in Northeastern China and Korea Determined by the Grid Search Algorithm

The grid search algorithm for determining earthquake focal mechanism and depth is tested by inverting regional-distance seismograms for a suite of five earthquakes in northeastern China and Korea. An improved crustal model for the region is used to compute the synthetic Green's functions for the eight fundamental fault types. To account for uncertainty in timing, location, and crustal structure, the synthetics are time shifted during the inversion to produce the best match. This process is repeated at a range of source depths. The focal mechanisms and depths for the earthquakes studied in this report are broadly consistent with teleseismic measurement of earlier large events in the region. The average time delays at each station and for each event provide a measure of the uncertainty in origin time and velocity structure. Either crustal velocity variations of 1-2 percent, or incorrect event locations of about 10 km, could explain the observed delays.