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(July 9).

************** うなぎセミナーのご案内 (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：2020年7月9日（木）14:00～
場所：オンライン(Zoom)

Date and time：July 9 (Thursday), 14:00～
Site：Online by Zoom

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*JpGU rehearsal *

*1)* [発表者 (Presenter)]
Yuki Noda

[題目 (title)]
10kHzサンプリングによるP波初動波形の相似性を用いたP波到達時刻の差の推定
Estimation of P-wave arrival time differences using P-wave first motion similarity with sampling frequency of 10 kHz

[要旨 (Abstract)]
In the Western Nagano Prefecture, central Japan, seismic observations with a sampling frequency of 10 kHz have been conducted since June 1995. The seismic stations are located mainly in the eastern part of the aftershock area of the 1984 Western Nagano Prefecture Earthquake, and seismic activity has been high around this area since August 1976. In this area, many earthquakes have shallow focal depths and then large signals can be observed immediately above their hypocenters. Inelastic attenuation is small probably due to homogeneous hard rocks, and the surroundings of the stations are quiet and have low noise levels, so the environment in the area is very suitable for seismic observations. Therefore, we can obtain a large number of microearthquake data and simple waveforms. In addition, the observation with a sampling frequency of 250 Hz of the Manten system has been performed since August 2008.
In this study, we defined mainshocks and their foreshocks of small to moderate earthquakes spatiotemporally based on magnitudes, origin times, and distances between hypocenters. Then, we estimated P-wave arrival time differences of the earthquakes by cross-correlating their P-wave onset waveforms.
We use the vertical component of velocity waveforms. In some spatiotemporally defined foreshock activities, there were many earthquakes that were not similar in the whole waveform but similar in the P-wave initial rise recorded at an identical station. Therefore, we calculated the time differences that maximize cross-correlation coefficients of each pair of earthquakes in a series of foreshock activities, using only windows of a length of 0.01s around P-wave onsets. We treated such time differences as P-wave arrival time differences between two earthquakes at each station. Using these time differences, we will perform relative hypocenter determination with a few meters precision and elucidate the nucleation process of a series of foreshocks which is illustrated by preslip model or cascade model.

*2)* [発表者 (Presenter)]
Yutaro Mori

[題目 (title)]
3-D Numerical Simulation of Island Arc Deformation Due to Steady Plate Subduction

[要旨 (Abstract)]
Island arc-trench system is characterized by a pair of anomalies in topography and free-air gravity, which is high in the island arc and low around the trench. Although this feature is observed without exception in the world, physical mechanism of the formation of this feature has not been well understood. Fukahata and Matsu'ura (2016) calculated deformation of the island-arc lithosphere due to steady plate subduction for a 2-D problem and explained the deformation mechanism based on the combination of lithospheric rotation and gravity. However, 3-D geometry of the plate interface may have an important effect on lithospheric deformation. For example, a large negative gravity anomaly is observed around a bend of a trench axis (Sandwell and Smith, 1997). This feature has been well reproduced by a numerical model (Hashimoto et.al., 2008), but the physical mechanism of the formation has not been clarified yet.
In this study, we develop a 3-D numerical model based on elastic-viscoelastic dislocation theory and compute lithospheric displacement rates in the island arc-trench system caused by steady slip on a plate interface, which has a convex bend toward the island arc along the trench axis. Computed results show that the island arc lithosphere significantly subsides around the bend, where horizontal extension parallel to the trench axis is observed. This result suggests that the slip vectors that have the direction away from the bend along the trench in the overriding plate results in mass deficit around the bend and subsidence of the island-arc lithosphere.

*3)* [発表者 (Presenter)]
Yuuki Kurihara

[題目 (title)]
開口を許した断層の動的破壊シミュレーション:間隙水圧と地震モーメン トの等方成分の生成について
Dynamic Rupture Simulation on a Fault with Crack Opening : On High Pore Pressure and Generation of Isotropic Component of Seismic Moment

[要旨 (Abstract)]
Recently, some non-volcanic earthquakes with significant isotropic (ISO) components were observed. For example, Stierle et al. (2014) reported some aftershocks of the 1999 Izmit, Turkey, earthquake had more than 15 % ISO components.
In this study, we investigated how we can reproduce a rupture with such a large ISO component in a simulation of mixed Mode (I & II) dynamic rupture.
In our simulation, we used a space-domain 2-D Boundary Integral Equation Method (BIEM), which can handle mixed-mode rupture on a fault of complex geometry. We discretized the fault with linear elements, and used integration kernels showed in Tada & Madariaga (2001). For the time-marching scheme, we used a second-order accurate predictor-corrector method by Noda & Lapusta (2010) which enhances numerical accuracy and stability compared with that by Cochard and Madariaga (1994). For the friction law, we used a linear slip-weakening friction law.
First, in order to validate our code, we solved the benchmark problems TPV14&15 2-D which were defined by the SCEC/USGS Spontaneous Rupture Code Verification Project (Harris et al., 2009). Our results compared very well with those produced with other methods.
Second, we simulated dynamic ruptures on a fault with a step and evaluated the ratio of the ISO component. The fault was piecewise linear and consisted of three straight segments, L-fault C-fault and R-fault. L and R-faults are parallel, and C-fault connects them. The rupture was nucleated on C-fault by stress perturbation. We parameterized the ambient stress condition with three parameters, Φ(angle of the maximum stress axis from C-fault), Ψ(angle of the fault bends) and a S-value. With keeping the differential stress, the lower S-value corresponds to the higher pore pressure.
In the parameter study, lower S-value generally produced higher ISO components. However, if Ψis small (~15°), no matter how low S-value we used, ISO components could not exceed 10 %. On the other hand with high Ψ(~75°), the failure criterion is often satisfied on the fault without the stress perturbation for rupture nucleation, and thus such initial conditions cannot be realized on the natural fault. ISO component became large for intermediate Ψ, a small S-value, and Φ< -30°. Our results indicate that not only high pore pressure, but also the fault geometry and the orientation of the stress field is important for generation of seismic events with a significant ISO component.

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