Regional and Spatiotemporal Characteristics of Foreshock Activities for Three Typical Tsunami Earthquakes

Tsunami earthquakes and their source process are one of open questions in earthquake seismology because the tsunami earthquakes are rare events comparing to ordinary earthquakes with the same magnitude. The body waves observed from tsunami earthquake are also not remarkable when it comes to the large-scale tsunami that usually brings tremendous disaster to coastal areas, which means that it is essential to deepen the comprehensive understanding toward destruction process and related physical phenomena concerning this kind of earthquake. In order to understand a source process of tsunami earthquakes, we focus on three typical examples of tsunami earthquake and their foreshock sequences which are Nicaragua(1992/09/02, Mw7.7), Jawa island(2006/07/17, Mw7.7) and Mentawai(2010/10/25, Mw7.8) to verify whether there were any foreshock activities before the mainshock occurred by investigating catalogs. We have found that in the case of the 1992 Nicaragua earthquake, three foreshock sequences near the epicenter of the mainshock from August 10 to August 13 were observed, and each of these epicenters were located within the range of ten kilometers west of the mainshock. At the same time, the migration of the activities heading from the northeast to the southwest was also observed. We have also analyzed the regional and spatiotemporal characteristics of the three regions by dividing 5゜* 5゜epicenter area into twenty-five segments and each segment is actually 1゜* 1゜. We have found out six seismic quiescence segments in Nicaragua, one segment in Jawa island, and two segments in Mentawai, respectively; all of them were around the epicenter. Seismic quiescence may be a universal phenomenon before tsunami earthquakes. We will use the results to do a research on the destruction process as well as the mechanism of tsunami earthquake in our future work.