[S-CG55] Dynamics in mobile belts

[SCG55-02] The Effect of Seamount Subduction on the Formation of Holocene Marine Terraces

*Junki Komori¹, Ryosuke Ando² (1.Earth Observatory of Singapore, Nanyang Technological University, 2.Graduate School of Science, University of Tokyo)

Keywords:Crustal deformation, Paleoseismology, Sagami trough, Subduction zone earthquakes, Holocene relative sea-level changes

Holocene marine terraces, which are exposed wave-cut benches or sandy beaches due to past relative sea-level (RSL) drops over the last 6,000–8,000 years, have been a subject of paleoseismology because they are believed to preserve evidence of recent crustal deformations resulting from great earthquakes. The objective of paleoseismology is to uncover the timing and source areas of past earthquakes. The timing of these events is estimated from the formation ages of marine terraces, by dating fossils of marine organisms found on each terrace. However, assessing the magnitudes and rupture areas of these earthquakes from the terrace distributions raises a challenge. As the present elevation of terraces encompasses both interseismic and coseismic vertical deformations, it becomes essential to isolate these factors to evaluate the coseismic contribution. However, neither the extrapolation of current geodetic observations nor the interpolation of long-term deformation deduced from tectonic landforms adequately accounts for the interseismic deformation between those events. Hence, a computational method to kinematically correlate these deformations in earthquake sequences is required.
In this study, we examined the characteristics of permanent deformation following an earthquake sequence, comprising coseismic and interseismic deformations, employing various kinematic and mechanical dislocation models, and proposed a potential mechanism behind marine terrace formation. The results of our model examination suggest that irregularities in the geometry of the plate interface, such as a subducted seamount, contribute to substantial permanent surface deformation after an earthquake sequence. By examining Holocene marine terraces in the southern part of the Boso Peninsula, central Japan, where seismic surveys indicated the subduction of a seamount, our crustal deformation model predicted a concentrated permanent uplift consistent with the observed distribution of the marine terraces. This study underscores that the elevation distribution of Holocene marine terraces around subduction zones cannot be directly correlated with coseismic deformation. Instead, a careful model examination is necessary to evaluate the patterns of past earthquakes.