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[SCG45-09] Crustal hetergeneity around the source area of large earthquakes in Japan(II)
Keywords:Large crustal earthquakes, Lower crustal heterogeneity, Seismic tomography
Previous study [Zhao et al., 2000] shows that most of the large crustal earthquakes (magnitude 5.7 to 8.0, depth less than 20km,) occur nowhere but at the edge of low velocity zones. The validity of this conclusion is not carefully tested. In previous study, earthquakes clustered above the same low velocity zone are counted in, and only shows the percentage of earthquakes at the edge of low velocity zones to support the conclusion.
In this study, 3-D seismic tomography[Zhao et al., 1992] beneath Japan is conducted including 51162 earthquakes and 2318 stations recorded by JMA from 2000 to 2024.4, and a denser vertical interval of grids in the range of crust and uppermost mantle is set compared with previous tomography study [M.Matsubara et al., 2008]. Resolution matrix is calculated to evaluate the reliability. Velocity profiles beneath contemporary earthquakes(magnitude greater than 6.0, depth less than 25km, 1923-2024.4), historical earthquakes(magnitude greater than 7.0, 599-2000), and faults are included and selected to distribute uniformly. Several kinds of hypothesis tests are used to test the deviation and difference between the velocity profiles beneath earthquakes and the back
ground.
Our results show that velocity anomaly appears beneath the source area of large crustal earthquakes with about -2% S wave velocity perturbation at the depth from 25km to 35km. We conclude that most large crustal earthquakes occur at the edge of low velocity zones inside the lower crust. These low velocity zones represent weak zones in the lower crust. The relationship between location of large crustal earthquakes and low velocity zones could reveal the mechanism of these earthquakes. The shear stress accumulates in the upper crust on the faults above the edge of low velocity zones in the lower crust. The heterogeneity of lower crust may be caused by high temperature in volcanic areas, or dehydration of subducting slab in other areas.
In this study, 3-D seismic tomography[Zhao et al., 1992] beneath Japan is conducted including 51162 earthquakes and 2318 stations recorded by JMA from 2000 to 2024.4, and a denser vertical interval of grids in the range of crust and uppermost mantle is set compared with previous tomography study [M.Matsubara et al., 2008]. Resolution matrix is calculated to evaluate the reliability. Velocity profiles beneath contemporary earthquakes(magnitude greater than 6.0, depth less than 25km, 1923-2024.4), historical earthquakes(magnitude greater than 7.0, 599-2000), and faults are included and selected to distribute uniformly. Several kinds of hypothesis tests are used to test the deviation and difference between the velocity profiles beneath earthquakes and the back
ground.
Our results show that velocity anomaly appears beneath the source area of large crustal earthquakes with about -2% S wave velocity perturbation at the depth from 25km to 35km. We conclude that most large crustal earthquakes occur at the edge of low velocity zones inside the lower crust. These low velocity zones represent weak zones in the lower crust. The relationship between location of large crustal earthquakes and low velocity zones could reveal the mechanism of these earthquakes. The shear stress accumulates in the upper crust on the faults above the edge of low velocity zones in the lower crust. The heterogeneity of lower crust may be caused by high temperature in volcanic areas, or dehydration of subducting slab in other areas.