今週のうなぎセミナーについてお知らせいたします。

Here is information of the Unagi-seminar(May, 16).

************** Seminar on Seismology IV A, C /地震学ゼミナールIV A, C (Unagi Seminar) **************

科目：地震学ゼミナールIV A, C / Seminar on Seismology IV A, C（修士・博士）
日時：2024年 5月 16日 (木) 13:30~
場所：京都大学 防災研究所 本館E-232D

Date and Time：2024-05-16, 13:30~
Place：Uji Campus Main Building E232D

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Speaker（発表者）1:　Kentaro Kuniyoshi

Title（題目）:　Spatiotemporal feature of tectonic tremors in the southeastern Guerrero seismic gap

Abstract（要旨）:
In general, the sources of tectonic tremors can be determined more accurately compared to that of slow slip events which do not radiate seismic wave. Determining the sources of tectonic tremors is essential for understanding the spatial characteristics of slow earthquakes in high resolution. However, It is hard to estimate the sources of tectonic tremors by reading raw waveform because the signal level of tectonic tremors is quite low and the phase of them is not clear. A lot of methods have been developed to estimate the sources of tectonic tremors. A method called “envelope cross correlation method” made it possible to estimate the source tectonic tremors by using travel time differences obtained by cross correlating envelope waveform of multiple adjacent stations (Obara, 2002). In this method, a single source is assumed at a single time window, therefore multiple tectonic tremors that occurred at the same time cannot be detected. On the other hand, Mizuno and Ide (2019) developed the method called “modified envelope cross correlation method”. In this method, multiple sources can be detected that occurred at the same time window because they calculate averaged cross correlation function over stations and find local maxima by using grid search and gradient method. They introduced this method in western Japan where there is the dense seismic station network.
There is a seismic gap in the south coast of Guerrero in Mexico in which earthquakes larger than M7 have not been observed for the past 100 years (e.g., Suarez et al., 1990). The gap is called Guerrero seismic gap. Plata-Martinez et al. (2021) spatially and temporally demonstrated the feature of activity of slow to fast earthquakes including tectonic tremors occurred in the northwestern part of the gap from 2017 to 2018.
This study aims to identify spatiotemporal characteristics of tectonic tremors that occurred in the southeastern part of the Guerrero seismic gap. The sources of tectonic tremors have been detected by modified envelope cross correlation method using the data got from 8 ocean bottom seismometers that were installed for one year starting from March 2022. I modified some parameters of the envelope correlation as well as the velocity structure (Spica et al., 2016) to make it more suitable for the sparse station network in the ocean bottom environment. The detected tectonic tremors are distributed in some clusters and their distinctive duration are also different between each cluster. Some of them are seen in the region where the residual gravity changes from negative to positive, and this feature suggests that the tectonic tremors occurred in the southeastern part of the Guerrero seismic gap are related to the subsurface structure.

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Speaker（発表者）2:　Ruan Yihuan

Title（題目）:　Anisotropic Velocity Structure beneath Shikoku, Japan: Insights from Receiver Function and Shear Wave Splitting Analyses and Ambient Noise Tomography

Abstract（要旨）:
The analysis to receiver function (RF) analysis and shear wave splitting allows us to reveal the Earth's velocity structure exhibiting anisotropic properties. The fast polarization direction (FPD) and split time are treated as key parameters for inferring anisotropic properties. Plate subduction, particularly in the mantle wedge-a wedge-shaped region between the subducting oceanic plate and the Mohorovi?i? discontinuity (Moho) of the overlying continental plate-significantly influences seismic anisotropy. The previous findings that anisotropic strengths are relatively weaker in the tectonic tremor-band, especially in northwestern Shikoku, and stronger in the northern part of central and eastern Shikoku around the no-tectonic-tremor area. These variations in anisotropy may provide insights into the geological structure beneath Shikoku Island.
Because the anisotropy analysis done previously only consider the particle motion in horizontal panel. By taking account into the shallow several kilometer sedimentary layer’s low velocities. The further inversion analysis of RF will be strongly affected by the intrinsic trade-off property between thickness and velocity different. Thanks for the relatively density distribution of Hi-net stations, the ambient noise tomography is conducting to investigate the shallow velocity structure. Then, further putting the tomography result as a priori into MCMC inversion may enhance result stability. The exploration of these properties can enhance our understanding of the geological evolution in one of the world's most active slow-earthquake areas.

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