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(December, 26).

************** 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 12月 26日 (木) 13:30~
場所：京都大学 防災研究所 本館E-232D

Date and Time：2024-12-26, 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 366 M 6.5 or larger earthquakes from 2000 to 2024 in the ANSS earthquake catalog.
Our results show that approximately 2% 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. We also performed a similar analysis using the Japan Meteorological Agency catalog, a more complete local catalog in Japan, and obtained very similar result.

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Speaker 2（発表者）:　小出 鯉太朗 (Koitaro KOIDE)

Title（題目）:
Review of the crustal structure in Echigo Plain

Abstract（要旨）:
In general, assuming an elastic half space, large uplift is caused by a revere fault. Therefore, uplift fields such as mountains are expected in a compressive field. However, subsidence occurs in Echigo Plain, which is located in compressive field, called Niigata Kobe Tectonic Zone (e.g. Sagiya et al., 2000; Okazaki et al., 2021). In addition, there are west-dipping reverse faults in the east boundary of Echigo Plain. These geodetic features and fault geometries contradict to the geographical pattern in Niigata (Iio, 2009).
In my master thesis, I calculated vertical displacement fields occurred by reverse faults in elastic-viscoelastic layered medium. The result suggests subsidence can occur if a reverse fault is located only in the shallow part of elastic layer. This is because the crustal deformation after completing viscoelastic relaxation is considered as the deformation of an elastic plate occurred by a fault; when a fault is placed in the shallow part of the elastic layer, the shallow part is forced shorten by the fault motion, but there is not such a forced shortening in the deep part. Therefore, an imbalanced shortening occurs ,the elastic plate deforms as backing and subsidence occurs on the surface.This result can help understand the subsidence in Echigo Plain.
In the next seminar, I will talk about some researches of the structural feature in Echigo Plain (e.g. Nakajima & Hasegawa, 2017; Okada & Ikeda, 2012; Wada et al., 2017). Furthermore, I will report field excursion I conducted with Fukahata san in Niigata after the SSJ meeting 2024.

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