[S-CG50] Structure, evolution and deformation of island arcs associated with subduction processes

[SCG50-08] Stress loading on inland faults in Tohoku before and after the 2011 Tohoku earthquake

*Akinori Hashima¹, Hiroshi Sato², Tatsuya Ishiyama², Makoto MATSUBARA³ (1.Japan Agency for Marine-Earth Science and Technology, 2.Earthquake Research Institute, University of Tokyo, 3.National Research Institute for Earth Science and Disaster Resilience)

Keywords:Stress loading, Northeast Japan arc, Source fault, Finite element method, Crustal deformation

In the Tohoku region, there have been many reverse fault earthquakes associated with east-west compression since the late 1800s. In particular, since 2000, there have been a series of inland damaging earthquakes, such as the 2003 Miyagi earthquake, the 2004 Chuetsu earthquake, the 2007 Chuetsu-oki earthquake, and the 2008 Iwate-Miyagi earthquake. The activity of these reverse fault earthquakes has been decreasing since the 2011 Tohoku-oki earthquake. Recent geophysical surveys have estimated the deep geometry of the source faults in the Tohoku region. It is important to estimate loaded stress on these inland faults in order to predict seismic activity. In this study, we estimate the impact of the 2011 Tohoku-oki earthquake on these faults using the 3-D finite element method.
We developed a three-dimensional finite element model (FEM) of the Japanese islands in a previous study. The model consists of the Eurasian, Pacific, and Philippine Sea plates, and the geometry of the plate boundary is determined by previous studies based on the distribution of interplate earthquake. The shallow area below 80 km of the plate boundary was divided into about 800 subfaults, and the slip response of each subfault was calculated. In addition, the viscoelasticity of the asthenosphere is incorporated to handle long-term plate boundary processes. Using this model, we obtain the slip deficit distribution on the plate boundary before the Tohoku-oki earthquake (interplate locking) by inversion of geodetic data, and calculate the stress accumulated on the source fault due to interplate locking. Then, we superpose the stress changes caused by the 2011 Tohoku-oki earthquake to obtain the time evolution of stress on the source fault.
In the Tohoku region, the Coulomb stress was positive before the 2011 Tohoku-oki earthquake, but after 2011, the Coulomb stress became negative suppressing the occurrence of earthquakes. This is consistent with the change in seismic activity caused by the 2011 Tohoku-oki earthquake. However, the source faults in the northern Tohoku have a positive Coulomb stress due to the influence of the locking of the Kuril Trench. Such modeling study is useful for understanding the mechanism of inland earthquake occurrence by estimating the stress on the source faults, and verifying it by seismic activity.