Japan Geoscience Union Meeting 2016

Presentation information


Symbol S (Solid Earth Sciences) » S-SS Seismology

[S-SS28] Seismic wave propagation: Theory and Application

Mon. May 23, 2016 1:45 PM - 3:15 PM A07 (APA HOTEL&RESORT TOKYO BAY MAKUHARI)

Convener:*Kiwamu Nishida(Earthquake Research Institute, University of Tokyo), Hisashi Nakahara(Solid Earth Physics Laboratory, Department of Geophysics, Graduate School of Science, Tohoku University), Jun Matsushima(School of Engineering, The University of Tokyo), Tatsuhiko Saito(National Research Institute for Earth Science and Disaster Prevention), Chair:Takuto Maeda(Earthquake Research Institute, the University of Tokyo), Yohei Yukutake(Hot Springs Research Institute of Kanagawa Prefecture)

2:00 PM - 2:15 PM

[SSS28-07] Evaluation of the generation and propagation mechanism of T-phase based on wave propagation simulation

*Mai Hoshihata1, Takashi Furumura1 (1.Earthquake Research Institute,The University of Tokyo)

Keywords:T-phase, Submarine topography, Underground structure, Wave propagation simulation

1. Introduction
The T-phase is the tertiary wave observed after P and S waves, which is acoustic wave propagating in the oceanic layer at speed of 1.5 km/s. T-phases are generated by P and S waves radiated from the earthquake below the ocean floor and transmitted into seawater directly and by S-to-P conversion at the solid-liquid interfaces of sea bottom, and then captured within the seawater by wide-angle reflections of P waves. In the presence of a sloping ocean floor above hypocenter, it is getting easier to make large-angle P reflections, and it is expected to enhance the excitation of T-phase. On the other hand, there is still question why T-phase is also generated on almost flat ocean floor (Okal 2008). For efficient propagation of T-phases, it is also explained that T-phases are trapped in the so-called SOFAR channel (SOund Fixing And Ranging) of minimum sound velocity. Also the T-phase reflected at large variation of topography such as seamounts (Obara and Maeda, 2009). In order to answer these questions and to understand the generation and propagation process of T-phases, we analyzed T-phase observed in ocean-bottom seismometer (OBS) and the conduct in 2D finite-difference method (FDM) simulation of seismic wave propagation.
2. T-phase data observed by OBS
We inspected the seismograms for T-phases in the broadband OBS station (WPAC) placed on North Pacific for 18 events occurred around Kuril and Aleutians in depth range 14-62 km and in distances range 788-1899 km. A band-pass filter of 2-8 Hz was applied to remove surface wave. To compensate the magnitude we examined relative amplitudes of T-phases normalized by P or S waves. We confirmed that T-phases propagated to far-field had a spindle shape of long duration properties. It is also confirmed that T-phase amplitudes were greater when slopes of seabottom above hypocenter is larger and longer. In addition, T-phase amplitudes were usually larger for shallow events. Also T-phase amplitudes attenuated drastically when propagation paths crossed seamounts.
3. Wave propagation simulation
For reappearance of such strong T-phases observed by OBS, we investigated the influence of submarine topography and underground structure by 2D FDM simulation of seismic wave propagation. For analyzing the relation between the generation of T-phases and seabottom topography, we computed the wave propagation using a flat topography model and a linearly sloping topography model. The crust and the mantle structural model was followed by Sereno and Orcutt (1985), and P waves velocity in oceanic layer were set to 1.5 km/s. In a reverse fault earthquake in 33 km depth, ground motion in maximum frequency of 8 Hz was calculated. As a result, in the sloping topography model, T-phases appeared after S waves, but T-phases did not generated in the flat topography model. When heterogeneous topography was added in the sloping model, T-phase amplitudes became somewhat weaker, and waveforms of T-phases became the spindle sphapes. Moreover, in the crust and the mantle model containing horizontally-elongated small-scale heterogeneous structure (Kennett and Furumura, 2014), waveforms had much longer duration of P and S waves (Po and So waves), but waveforms of T-phases were almost the same. Therefore, the generation of T-phase with the spindle shape relates strongly to sloping and roughly subsurface topography. Additionally, we could show that T-phase amplitudes became larger in the case of the shallow focal depth. Also, we could confirm that the attenuation of T-phase energy was weaker by being trapped in the SOFAR channel, and T-phases could propagate to longer distance easily.
Acknowledgements We acknowledge the Earthquake Research Institute of The University of Tokyo for the data of Ocean-Bottom Seismometer and the EIC computation.