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(January, 25).

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

科目：地震学ゼミナールIV B, D / Seminar on Seismology IV B, D（修士・博士）
日時：2024年 1月 25日 (木) 13:30~
場所：京都大学 防災研究所 本館E-232D または オンライン(Zoom)

Date and Time：2024-01-25, 13:30~
Place：Uji Campus Main Building E232D or Zoom (Hybrid)

　　　Please join the seminar on-site, especially students who need credit.

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Speaker（発表者）1:　Abraham Hernandez

Title（題目）:　Crustal Structure Model of the Guerrero Seismic Gap from seismo-geodetic data

Abstract（要旨）:
The Guerrero Seismic Gap (GGap) is a ~140 km segment at the Cocos-North America plate boundary. Since 1911 there has been no record of a large subduction thrust earthquake in the NW portion of the GGap, and taking into account the seismic evolution and subduction dynamics, specialists see a possible scenario of a Mw ~8.2 earthquake in the area. Therefore, understanding the nature of the rupture process in the crust is a fundamental question of this study.
Gravimetric and seismic techniques are two of the main methods to investigate the crustal configuration and define the structure in the subducting slab. The correlation between gravity field variations and seismological response can be a highly effective approach to develop an accurate model in the GGap.
In this project, from precision marine gravity data, together with data from the global satellite model of Sandwell et al., 2014, we intend to generate a model of the density distribution in the shallow crust in the GGap area. To validate this model, seismic data from the Ocean Bottom Seismometers of the seismo-geodetic network, recently established with the Japan/Mexico collaborative project 2016/2021, are expected to determine the Shear-wave (Vs) velocity structure to obtain the more accurate internal structure of the Earth, and to asses the seismic potential of the GGap.

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Speaker（発表者）2:　Viana Poveda

Title（題目）:　Ocean microseisms recorded by Cuban seismic stations: time variations and spectral features

Abstract（要旨）:
Marine microseisms are seismic waves in the spectrum of the ambient seismic wave-field caused by the interactions of ocean gravity waves. This work analyzed spectral characteristics of marine microseisms using seismic records from 13 broadband seismic stations in Cuba. We first performed an analysis of the mean power spectral density behavior in space, time and frequency, for the primary and secondary microseism peaks. Our results indicate that the dominant period of the secondary microseisms at almost all stations is between 2.9 s to 4.1 s, with slight variations from station to station. The main period of primary microseism peak is more stable at around 15.6 s for the majority of stations. The yearly main power spectral density values range from -118 to - 128 dB and -150 to -156 dB for the secondary and primary microseisms, respectively. The primary microseism peak is more stable in amplitude and frequency compared with the secondary microseism. We also analyzed the energy variation of microseisms in the two seasons of Cuba (i.e., dry and wet). The strongest signals at the main period of the secondary and primary microseism are found during the dry season although seasonal variation is weak. Meteorological events like tropical cyclones increase the intensity of microseisms on the days when these events’ path is close to Cuba although they do not influence the overall amplitude (power spectral density) in the year 2020. We also estimate the location of the sources that contribute to the generation of the secondary microseisms at three period’s ranges (1s-2s, 2s-4s, 4s-8s) for the same year. For that, we applied polarization analysis at a single station and estimated the distribution of the source's direction and how intense is the seismic energy in that direction. We focus our analysis on the Rayleigh and P waves’ polarization assuming both are uncorrelated. We found highly directional incident Rayleigh and P waves at periods between 1s-2s. Source direction at these periods imply that microseisms are developed in a close sea according to the station location. At 2s-4s, source contributions of Rayleigh waves are from the sea in the north and southeast Cuba. At 4s to 8s, the scatter source’s directions of Rayleigh waves can indicate that the source's contributions are coming from different directions; there is no strong dominant source. The seasonality distribution of the sources of the P waves is strong in most of the stations; sources are coming from south in the wet season and from north in the dry season, consistent with global distribution of P waves estimated by an ocean wave model.

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