Search

The 1975 Hokkiado-Oki earthquake (Ms7.0) was characterized as a tsunami earthquake which had anomalous long source process time and spectrum of seismic waves. Predominant period of seismic waves were situated in several tens to several hundreds seconds. This abnormal long-period seismic wave generated very weak intensity distribution compared to the seismic moment (Mo) and epicenter distance. Aftershock distribution and focal mechanismw indicated this event occurrde on very shallow plate boundary close to the trench axis, in where seismicity has been very low. If next tsunami earthquake will occur near Hokkaido, JMA will be likely to underestimate the M, because JMA seismographs are able to record the waves only less than several tens of seconds. This may cause error in judgment for urgent tsunami warning because of smaller M, than real Mo. Therefore, a reliable quick Mo estimation method is proposed by using near-and far-field strain seismograms and far-field broadband seismograms especially for tsunami earthquake warning. In this method Mo is estimated from source time function τ which is detected from the duration time of near-field huge pulses or far-field S-wave pulses for strainmeter and of P-wave pulse for broadband seismographs. Strain seismographs near the fault are able to fully record whole period strain change without falling off and saturation because of its flat response from several Hz to DC component. Trial Mo estimation from the observed near-field pulse associated with the 2003 Tokachi-oki earthquake (Mw8.0) shows in good agreement with Mo calculated by teleseismic waveform inversion. This implies that realtime Mo estimation will be done from only watching the growth process of strain seismographs near seismic fault.