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(November, 27th).

************** 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：2025年 11月 27日 (木) 13:30~
場所：京都大学 防災研究所 本館E-232D

Date and Time：2025-11-27(Thursday), 13:30~
Place：Uji Campus Main Building E232D

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Speaker 1（発表者）:　Kai Koyama
Title（題目）:
Global investigation of foreshock acceleration prior to large earthquakes

Abstract（要旨）:
The acceleration of foreshocks has gained significant attention in the context of earthquake forecasting. Bouchon et al. (2013) claimed that many large earthquakes occurred at plate boundaries were preceded by accelerating foreshock activities. Nishikawa & Ide (2018) also reported an accelerating foreshock activity immediately before the M 6.9 interplate earthquake in the Japan Trench subduction zone in 2008. However, Bouchon et al. (2013) have been criticized for inadequately accounting for the effects of earthquake clustering (Felzer et al., 2015). Furthermore, Nishikawa & Ide (2018) analyzed only a few large earthquakes, and it is unclear whether similar acceleration phenomena can be observed globally.
We investigated foreshock activity before large earthquakes globally using the epidemic-type aftershock-sequence (ETAS) model (Ogata, 1988), a standard statistical model of seismicity. Based on previous numerical simulations and rock experiments (e.g., McLaskey, 2019), we incorporated a new term representing the accelerating foreshock activity into the ETAS model. This term, similar to the inverse Omori law, describes a power-law acceleration of the seismicity rate leading to a mainshock (= L/(Teq - t + d)q), where t is time, Teq is the mainshock origin time, and L, d, and q are new model parameters. We applied the model to seismicity (M 4.5 or larger and within 100 km of a large earthquake) preceding 240 M 6.5 or larger earthquakes from 2000 to 2024 in the ANSS earthquake catalog.
Our results show that approximately 4% of the large earthquakes exhibited significant foreshock acceleration. For example, the M 6.9 earthquake analyzed by Nishikawa & Ide (2018) and a M 7.3 interplate earthquake in the Vanuatu subduction zone in 2008 showed notable foreshock acceleration. Comparisons with randomly selected seismic activity and synthetic catalogs demonstrate that the observed acceleration cannot be explained by aftershock cascades, implying that it results from aseismic processes, yet it is not clearly regarded as unique to large earthquakes.
We also performed a similar analysis using the earthquake catalogs of the Japan Meteorological Agency (JMA) and the Taiwan Geophysical Data Management System, which are more complete local catalogs in Japan and Taiwan, respectively. Focusing on M 6.0 or larger events, we obtained consistent results from both datasets.

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Speaker 2（発表者）:　Reiju Norisugi
Title（題目）:
What is preserved and lost by a coarse-graining of fault constitutive law?

Abstract（要旨）:
We model earthquake sequences by using continuum mechanics which is well-established coarse-grained model of molecular dynamics. To determine the boundary condition across the fault interface within the continuum, we employ some constitutive laws (e.g., friction). The typical constitutive law for earthquake modeling is rate-and-state friction which express the friction as a function of fault slip rate and state variable. However, it is questionable whether the rate-and-state friction can be properly coarse-grained. In other words, it is unclear how small scales we need to resolve for modeling of earthquake sequences. Given the strong heterogeneity of natural fault zones, it is crucial to seek a coarse-grainable constitutive law.
We employ the hierarchical patch model as the type of heterogeneity on the fault interface and investigate coarse-grainability of rate-and-state friction law. The coarse-graining process is based on the introduction of multiple state variables, and characteristic weakening distances. In the present coarse-graining process that we will explain in the talk, we do not need additional tuning of frictional parameters, and coarse-graining process can be done straightforwardly for the given heterogeneity. The hierarchical patch model is characterized by the brittlness of seismological patches which is the ratio of seismological patch size to the relavant estimation of nucleation size. We will present how the gorse-grainability and characteristics of earthquake sequence change depending on the brittleness of hierarchical patch model.