2:00 PM - 2:15 PM
[SCG52-14] Modeling of stress loading under Kanto constrained by geological data: towards elucidation of mechanism of damaging earthquakes under Tokyo
★Invited Papers
Keywords:Stress loading, Plate subduction, Intraplate earthquakes, Kanto Basin, Marine terraces
The ~M7 damaging earthquakes assumed to occur in the Kanto region in decadal scale are mainly intraplate earthquakes which occur within the plates or the slabs, rather than interplate earthquakes such as the 1923 Kanto earthquake. In general, earthquakes are a phenomenon in which stress accumulated around the source region is released by fault rupture. Therefore, in order to reveal the mechanism of earthquake occurrence, we must first clarify process of loading stress to be released and model it. In the case of interplate earthquakes, it is relatively easy to calculate the stress due to the kinematic conditions of relative plate motion. In contrast, the stress accumulation process for intraplate earthquakes (including active fault earthquakes) is still only qualitatively explained. The purpose of this study is to model the long-term stress accumulation process within the upper plate and subducting plate under the Kanto region, and thereby to quantitatively evaluate the risk of intra-plate earthquakes under the Kanto region.
Based on the plate subduction model of Matsu'ura & Sato (1989), we model the intra-plate stress accumulation due to the subduction of the Philippine Sea and Pacific plates. We model the crustal structure by an elastic-viscoelastic two-layered structure. Usually, the slip rates on the plate interfaces are calculated by a rigid plate motion model, but it cannot be directly applied to the Kanto region because of the internal deformation caused by the collision of the Izu-Bonin arc. GNSS data are strongly affected by the locking of the source region of the Kanto earthquake, and so, those data do not reflect long-term trend. Therefore, we estimated the distribution of slip rate (or slip-rate deficit) using the vertical deformation data from thermochronology, geology, and geomorphology over the Kanto Plain and surrounding mountains. Using this model, we constrained the range of the long-term locking area that satisfies the pattern of long-term vertical change in the Kanto region. Using the estimated distribution of slip rate, we calculate the stress accumulation rate within the plates by the plate subduction model.
The calculated stress accumulation pattern shows horizontal extension both above and below the Eurasian/Philippine Sea plate boundary under the Kanto Plain and the Boso Peninsula. In the southern Philippine Sea Plate, the stress accumulation pattern is strike-slip, and in the Izu collision zone, it is northwest-southeast compression. Then, we calculated the Coulomb failure function (ΔCFF) at the nodal plane of the observed earthquake mechanism using the stress accumulation rate. The ΔCFF is positive in the Kanto Plain, the Boso Peninsula, the southern Philippine Sea plate, the Izu collision zone, and the cluster in the northeastern part of the Boso Peninsula. This result indicates that the earthquakes occur to release the stress formed by the subduction of the Pacific and Philippine Sea plates, showing the validity of the stress accumulation model in this study.
The above model is based on the elastic-viscoelastic two-layer structure model and is valid for seismic activity at depths of 40 km and shallower. Meanwhile, the ~M7 damaging earthquakes that have occurred so far under Tokyo are thought to have occurred in deeper slabs. Thus, it is necessary to construct a model that can take these earthquakes into account. We are planning to build a three-dimensional finite element model that includes both the Pacific and Philippine Sea slabs, which will make it possible to calculate the stress accumulation in the assumed ~M7 damaging earthquakes.
Based on the plate subduction model of Matsu'ura & Sato (1989), we model the intra-plate stress accumulation due to the subduction of the Philippine Sea and Pacific plates. We model the crustal structure by an elastic-viscoelastic two-layered structure. Usually, the slip rates on the plate interfaces are calculated by a rigid plate motion model, but it cannot be directly applied to the Kanto region because of the internal deformation caused by the collision of the Izu-Bonin arc. GNSS data are strongly affected by the locking of the source region of the Kanto earthquake, and so, those data do not reflect long-term trend. Therefore, we estimated the distribution of slip rate (or slip-rate deficit) using the vertical deformation data from thermochronology, geology, and geomorphology over the Kanto Plain and surrounding mountains. Using this model, we constrained the range of the long-term locking area that satisfies the pattern of long-term vertical change in the Kanto region. Using the estimated distribution of slip rate, we calculate the stress accumulation rate within the plates by the plate subduction model.
The calculated stress accumulation pattern shows horizontal extension both above and below the Eurasian/Philippine Sea plate boundary under the Kanto Plain and the Boso Peninsula. In the southern Philippine Sea Plate, the stress accumulation pattern is strike-slip, and in the Izu collision zone, it is northwest-southeast compression. Then, we calculated the Coulomb failure function (ΔCFF) at the nodal plane of the observed earthquake mechanism using the stress accumulation rate. The ΔCFF is positive in the Kanto Plain, the Boso Peninsula, the southern Philippine Sea plate, the Izu collision zone, and the cluster in the northeastern part of the Boso Peninsula. This result indicates that the earthquakes occur to release the stress formed by the subduction of the Pacific and Philippine Sea plates, showing the validity of the stress accumulation model in this study.
The above model is based on the elastic-viscoelastic two-layer structure model and is valid for seismic activity at depths of 40 km and shallower. Meanwhile, the ~M7 damaging earthquakes that have occurred so far under Tokyo are thought to have occurred in deeper slabs. Thus, it is necessary to construct a model that can take these earthquakes into account. We are planning to build a three-dimensional finite element model that includes both the Pacific and Philippine Sea slabs, which will make it possible to calculate the stress accumulation in the assumed ~M7 damaging earthquakes.