Effects of Sedimentary Structure and Bathymetry Near the Source On TeleseismicPWaveforms From Shallow Subduction Zone Earthquakes

The effects of sedimentary layers and non-flat bathymetry on teleseismic P waveforms from shallow subduction zone earthquakes are examined by calculating synthetic seismograms. the actual bathymetry and shallow sedimentary layers revealed by the seismic refraction studies are incorporated in 2-D velocity models of the oceanic trench region. 2-D finite-difference calculation is employed to get a complete response of the assumed non-flat model structures. The result of the modelling of WWSSN long-period seismograms shows that not only the non-flat bathymetry but also thick sedimentary layers are essential to model the synthetic seismograms. the lack of non-flat bathymetry or sedimentary structure in the structural model often results in unrealistic synthetics. In general, the paths going across the trench axis are most affected. This is caused by the steeply dipping ocean bottom and the thick sedimentary layers at the trench slope. Observed seismograms of the 1968 Kurile thrust event (mb 5.5) and the 1967 Java normal event (mb 6.0) show remarkable large-amplitude later arrivals despite their moderate magnitudes. A realistic model of the trench structure, however, reproduces remarkably well this character of later arrivals, which shows that it is essential to incorporate the structure. Our results also indicate that we need precise knowledge on the trench structure to model the structural effects in detail. On the GDSN long-period seismograms the effect of the sedimentary layers is negligible because the rather narrow-band response of the GDSN seismograph lies in a frequency range lower than the affected band. However, the effect of the non-flat bathymetry is significant and can not be neglected.