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 15th).

************** 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：2025年 5月 15日 (木) 13:30~
場所：京都大学 防災研究所 本館E-232D

Date and Time：2025-05-15(Thursday), 13:30~
Place：Uji Campus Main Building E232D

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Speaker 1（発表者）:　Reiju Norisugi

Title（題目）:
Multi-Scale Rate- and Roughness-Dependent Fault Constitutive Law and Dynamic Earthquake Sequence Simulation

Abstract（要旨）:
What physics governs the multi-scale source property of earthquakes, such as fracture energy scaling, is an interesting, long-lasting question. The fracture energy scaling indicates that energy dissipation during the dynamic event has a power law relationship with the coseismic displacement. Although many studies adopted rate- and state-dependent friction (RSF) law to simulate earthquake sequences, and various aspects of them can be expressed by the RSF framework, such a multi-scale property is absent in the conventional RSFs. Therefore, in this study, we pursue a way to properly incorporate the multi-scale source properties of earthquakes into the RSF framework.
We utilize the previously proposed slip- and time-dependent fault constitutive (STF) law. It accounts for the evolution of fault surface roughness by abrasion (flattening) and adhesion (healing), whose characteristic slip and time, respectively, depend on the wavelength. After defining an elemental length scale to coarse-grain the surface roughness, the Fourier components of surface roughness serve as the state variable. The shear strength is expressed by the sum of contributions from the roughness of different wavelengths. Particularly, the roughness of a larger wavelength evolves more slowly but has a larger contribution than that of a smaller wavelength. This property potentially replicates the fracture energy scaling. However, STF has no rate dependency (direct effect) which is crucial to modernizing the friction law, allowing us to treat diverse fault behaviors in a single framework (e.g., dynamic rupture, slow nucleation, slow slip, etc.). Thus, we modify the STF law by adding the direct rate dependency and treat it as a form of RSF law, named rate- and roughness-dependent fault constitutive (RRF) law. The rate dependency and Fourier roughness (state) dependency express the fault shear stress in the RRF framework.
We perform the dynamic earthquake sequence simulation with RRF law and find that the fracture energy scaling is reproduced. However, due to the steady-state property of RRF law, the significant afterslip is produced in the area of coseismic slip, unlike the natural earthquake sequences. Therefore, we further modify the friction law by, for example, adding saturation of the direct rate dependency. This result will add another line to replicate the multi-scale source property of earthquakes and offer insight into how we incorporate such multi-scales into the constitutive law to explain the general earthquake behavior.

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Speaker 2（発表者）:　Hernandez Abraham

Title（題目）:
Modelling the Shallow Crustal Structure of the Guerrero Seismic Gap from 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.
Gravity techniques are accurate methods to investigate the crustal configuration and define the structure in the subducting slab. In this project, 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. The shallow crust is of particular interest because lateral heterogeneity in the slab zone modifies subduction dynamics. These heterogeneities comprise seafloor structures (e.g., seamounts) and may be key to studying the seismogenic zone of the Mexican subduction and better assessing its risk.
Gravimetric inversion methods can solve subsurface mapping problems by determining the density and/or depth of the layers that comprise it. Here, we will use statistical methods of gravity inversion to relate the parameters (density) to the observed data. To do this, we will use a Bayesian approach, defining our likelihood functions, evaluating the forward map, and our prior function. The methodology used to generate our functions in the posterior probability function is presented here.

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