[SSS10-P01] Spatial-temporal and seismogram characteristics of foreshock and aftershock activities for tsunami earthquakes
Keywords:tsunami earthquake, seismic activity, waveform analysis
First, we use seismic catalogs and waveform recording provided by ISC/USGS and IRIS respectively to extract the foreshock and aftershock activity, then evaluate the spatio-temporal features. We have found that four foreshock sequences (1992/08/10 06:09:20 Mb 5.2, 1992/08/10 06:34:14 Mb4.7, 1992/08/11 05:23:51 Mb 5.1, 1992/08/13 08:41:17 Mb 4.7) near the epicenter of mainshock area were observed before the occurrence of the Nicaragua mainshock. We have also analyzed the regional and spatio-temporal characteristics of three tsunami earthquake regions by dividing the area of 5*5 around the epicenter into twenty-five segments with the size of 1*1. As a result, in each of the three regions, we have found out that there is a seismic quiescence period in the vicinity of the mainshock area in each tsunami earthquake. Meanwhile, regarding aftershocks of the Nicaragua earthquake, seismic quiescence area is still detected in the same segment as the foreshock.
Migration phenomenon is observed in the foreshock activity near the focal region of the mainshock, especially before the mainshock of the Nicaragua earthquake. Meanwhile, the waveforms of aftershocks occurring in the same area as the events included in the migration have been compared with those of the mainshock. Next, we have calculated the ratio of maximum absolute amplitude of surface wave to that of body wave, in order to address features on waveforms of tsunami earthquake. As a result, the ratio of the mainshock, such as the tsunami earthquake shows extremely high value, whereas for the foreshocks and aftershocks, it does not show the same characteristic as the tsunami earthquake does. This suggests that small earthquakes occurring within the tsunami seismogenic region show the same phenomenon as ordinary earthquakes, but only the mainshock has the characteristics of the tsunami earthquake.