A scaling model for quantification of earthquakes in and near Japan

Abstract

A simple method is developed to determine seismic moments of earthquakes. The method is qualified through criteria such as simplicity of calculations, coverage of wide magnitude range, and insensitivity to detailed instrumental response. The method is applied to 163 major earthquakes which occurred underneath Japan and the Japan Sea in the time from 1926 to 1977. Magnitudes of these earthquakes, which have been determined by the Japan Meteorological Agency, ( M JMA ) cover the range from 4.3 to 7.5. At first, source spectra are analyzed through a very simple way introducing two new parameters: characteristic period T c and seismic-moment factor M c . The former is defined as an average value of apparent periods of seismic waves with the maximum trace amplitude at many stations. The latter is an average of products of maximum trace amplitude and its apparent period multiplied by epicentral distance. It is shown that T c corresponds to the period of the corner frequency of an earthquake and M c to the seismic-moment density at the period of T c . A scaling model of earthquake source spectra is presented which satisfies the empirical relations between the surface-wave magnitude M s and M JMA , and M JMA and the body-wave magnitude m b . Those relations are independent of the Gutenberg and Richter relation between M s and m b , because M JMA is determined from maximum amplitudes of seismic waves with a period of about 4 sec. The static seismic moment of each earthquake can be estimated from calculated M c using the source spectra of the scaling model. Seismic moments of 18 earthquakes determined by conventional analyses from near- and/or far-field observations are consistent with static seismic moments thus estimated over the range from 2 × 10 23 to 3 × 10 27 dyne cm. This shows the potential in practice of the present method, especially in the routine processing of seismic data.