Determination of Hypocenters and Focal Mechanism Solutions for Semi-Historical Earthquakes in Kanto region, Japan by Template Matching Technique

Abstract:

We determine the hypocenters and focal mechanisms for earthquakes occurred in Kanto region, Japan between 1885 and 1923. In this period, available seismological data are limited because of sparse station distributions and low quality of data particularly in absolute timing. The number of reported phase arrival times and first-motion polarities of earthquakes are fewer by more than an order of magnitude compared to those from recent earthquakes. This makes it difficult to apply recently-developed techniques in determining source parameters for semi-historical earthquakes. Instead, we try to constrain source parameters by using the template matching technique with S-P times, first-motion polarities, and focal mechanisms of recent earthquakes. To quantify the degree of similarity for hypocentral locations and focal mechanisms, we use the root mean squares differences in S-P times and weighted misfit rate, respectively.

For example, root mean squares differences in five S-P times between the 1922 Uraga-channel earthquake and recent earthquakes are small in SW Chiba prefecture. The weighted misfit rates are small for earthquakes with strike-slip focal mechanisms in this region, resulting from the complete matching between fourteen first-motion polarities reported for the 1922 earthquake and radiation pattern for these earthquakes.

The number of target earthquakes (M≥6.5) are 30, for which only several S-P times and/or first-motion polarities are available for each earthquake. We use template S-P times of earthquakes from two catalogues; one form the Japan Meteorological Agency (JMA) with >100,000 events between 1923 and 2011, the other from the Metropolitan Seismic Observation Network (MeSO-net) with >3,000 events between 2008 and 2012. We compile >55,000 focal mechanisms from the five networks (i.e., the Kanto-Tokai observation network, Japan University network, F-net, JMA, and MeSO-net) as template.