5:15 PM - 6:45 PM
[SSS07-P11] Feature of S-coda Envelopes of Interplate Events in Tohoku, Northeastern Japan
Keywords:S-coda, northeastern Japan, interplate earthquake, scattering, reflection
We investigated feature of S-coda envelopes of interplate events in Tohoku, northeastern Japan. Mean square envelope seismograms of 66 events with various distances from the Japan Trench were generated from seismograms recorded at the NIED S-net and Hi-net stations. Band-pass filtered three component seismograms at each station are squared, averaged over moving windows with a 1-s length, and multiplied by medium density. These envelopes of an event mean spatiotemporal energy density of the event, which can be related to its source energy, focal mechanism, site factors at stations, and heterogeneous structure in the crust and the upper most mantle. The obtained distributions of energy density for the near shore events show energy concentration around their wavefronts of S-wave first arrivals even in lapse times with 50 s to 60 s from the origine times. This means “mean free time” in this area is 50 s or longer, which is similar to or slightly longer than that of the inland area in Tohoku. On the contrary, the distributions of energy density for the near trench events show unclear S-wave fronts and relatively high energy density around epicenters in the same lapse time. This indicates that strong scattering and/or reflections by heterogenous structure especially around the hypocenters. Asano (2023SSJ) investigated the distribution of the P-wave polarities of the same 66 interplate events and showed that “up” and “down” arrivals related upgoing direct p-waves from near trench events could be observed only in small area just above hypocenter. This is interpreted to be caused by steeply bending ray in this area probably with strong velocity gradient. Such structure may easily generate sS, sSS and other later phases and consistent with the present shown unclear S-wave fronts of the near trench events.