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(October, 9th).

************** 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 10月 9日 (木) 13:30~
場所：京都大学 防災研究所 本館E-232D

Date and Time：2025-10-09(Thursday), 13:30~
Place：Uji Campus Main Building E232D

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Speaker（発表者）:　Nishizawa Takashi
Title（題目）:
Formation Mechanism of Central Lowland Zones based on Dislocation Model for Plate Subduction

Abstract（要旨）:
　In subduction zones, a characteristic pattern of topography and gravity anomalies generally appears: subduction in oceanic trenches and uplift in island arcs (outer arc to volcanic arc). Matsu'ura & Sato (1989) applied a dislocation theory to plate subduction and calculated crustal deformation in subduction zones. Applying this method to long-term plate subduction, they successfully reproduced subsidence in oceanic trenches and uplift of island arcs. However, the most important internal structure in island arcs is the relative subsidence between the outer arc and the volcanic arc (Central lowland zone: Kaizuka, 1972), which has not been reproduced. This means that while the general uplift of island arcs can be explained, it is not well understood why the lowland zones are observed in the Seto Naikai (Inland Sea) and the Kitakami-Abukuma in Japan, as well as in the Cascadia and Nicaragua. However, it is interesting that some island arcs, such as Aleutian, Kermadec, and Mariana, do not have it. Therefore, it is expected that different subduction parameters (e.g., dip angle, plate age) and mechanical properties (e.g., viscosity) in each region control its formation.
　This study aims to understand the formation of the central lowland zone through a theoretical model. Specifically, in the former part, I will introduce the results of numerical experiments based on the dislocation model for plate subduction. In Fukahata & Matsuura, the direct predecessor of this study, the crust and mantle system were modeled by an elastic-viscoelastic medium. In this study, the crust is modeled as a viscoelastic material with high viscosity, as in Sato Matsuura (1993), and its response is compared to that of an elastic model. In the latter part, we compare observed gravity anomalies with calculated uplift rates for several subduction zones. On the basis of these results, we will discuss the formation mechanism of the central subduction zone.
　Additionally, although not directly related to the formation mechanism, we are currently studying seismic activity in Northern Costa Rica. Since Northern Costa Rica, like the Nankai and Cascadia, is a subduction zone with a central subsidence zone, we will briefly introduce it at the end and hope to discuss the relationship between its geomorphology and tectonics.