Quick Estimation of Moment Magnitude Using Strong Ground Motion Records

The 2011 off the Pacific coast of Tohoku earthquake was the greatest earthquake in Japan with the moment magnitude of 9.0. However, the first report of JMA (Japan Meteorological Agency) magnitude estimated 3 minutes after the earthquake initiation was 7.9. Furthermore, the moment magnitude which should be calculated about 15 minutes after the earthquake initiation was not calculated due to the saturation of broad-band seismometers. As a result, the tsunami height was underestimated causing loss of many lives unfortunately. Now southwestern Japan faces the possibility of the great earthquake of Nankai Trough, so it is extremely important to develop a quick estimation system of unsaturated magnitude. In this study, we propose a method to calculate the permanent displacement accurately based on an acceleration record and a scheme to estimate moment magnitude quickly. The moment magnitude estimation method described in this study is based on the theoretical relation between coseismic displacement and hypocentral distance. By applying the method to the 2011 off the Pacific coast of Tohoku earthquake and 2003 Tokachi-Oki earthquake, the moment magnitude was estimated at 8.9 and 8.5, respectively. These values are close enough to the other estimations. In addition, by applying the method to the scenario earthquake at Nankai Trough, the correct value of moment magnitude was estimated. Finally, we propose using the equivalent hypocentral distance for large earthquake in order to consider the spatial extent of the seismic fault. Although real-time detection of the spatial extent of the fault is required, the moment magnitude can be estimated more rationally by this method.