Estimating source parameters of the earthquake swarm in the Hida Mountains Region, central Japan, using near‐field waveforms

Abstract

We estimated source durations for larger events in a strong swarm that occurred from August 7, 1998 in the Hida mountains region, central Japan. We used broadband seismograms recorded at the Kamitakara observatory at distances of about 30 km. The events had seismic moments from 1014 to 1017 Nm. To determine the source durations, we systematically searched for the half duration of source time functions which minimized the difference between synthetic waveforms and observed data. In the data we could clearly see the near‐field contributions to the waveforms, suggesting that the durations were longer than expected for typical events, considering their size and the distance to the station. We estimated the source durations (half duration of source time functions), source dimensions, and stress drops to study the source parameters of these event. Source dimensions are determined from these source durations under the assumption of circular faults and seismic moments are estimated from maximum amplitudes. These results show that source durations are longer and source dimensions are larger compared to most earthquakes with the same seismic moments.