Determination of Hypocenters and Focal Mechanism Solutions for Semi-Historical Earthquakes by Using Template Matching Technique

Abstract:

Modern seismological analyses are difficult to carry out for earthquakes which occurred in the early period of instrumental observation (between 1870’s to 1920’s in Japan) because of sparse station distributions and low quality of data particularly clock errors. Source parameters of such old earthquakes can be estimated through comparisons of old data with recent seismological data with known hypocenters and focal mechanism solutions. In this study, we constructed a new method to determine hypocenters and focal mechanism solutions for semi-historical earthquakes using template matching technique. To quantify the similarity in hypocentral locations between recent and semi-historical earthquakes, we use RMS of S-P time differences. As for focal mechanism solutions, we calculated weighted (by the normalized P-wave amplitudes) misfit rate between observed first-motion polarities and expected polarities from recent focal mechanism solutions.

We confirmed the effectiveness of this method by applying it to recent earthquakes and comparing the distribution of RMS S-P time differences and weighted misfit rates with hypocenters and focal mechanism solutions determined by the Japan Meteorological Agency. RMS S-P time differences show small values around the true hypocenter and the weighted misfit rates become small for the true focal mechanism solutions.

We then preliminarily applied this method to several large earthquakes in semi-historical period. For the M6.8 earthquake of 1922 in the Kanto region, Japan, the six S-P times are similar to those reported from recent intermediate-depth earthquakes in the southern part of Chiba Prefecture. The thirteen first-motion polarities are consistent with those expected from recent strike-slip or normal-faulting types of earthquakes at depth 60-70 km within subducting Philippine Sea slab in this region. Such earthquakes are active along the western edge of slab-slab contact zone between the Philippine Sea and Pacific Plates.