深発地震の地震波動伝播シミュレーションに基づく北海道下マントルウェッジの内部減衰構造

During deep earthquakes, it is well known that an anomalously large intensity has been observed in areas farther away from the epicenter than in areas closer to it, which is known as anomalous seismic zone. The deep earthquakes off the western coast of Sakhalin (d＝600 km) and off Cape Soya (d＝334 km) have shown a characteristic distribution of maximum amplitudes in northern Hokkaido, closer to the epicenter, in addition to the areas farther away from the epicenter according to the anomalous seismic zone. The maximum amplitude of the seismic wave at each distance displayed a V-shaped, decreasing with increasing epicentral distance and increasing again above a certain distance. The decrease in the northern portion of Hokkaido was very steep and could not be attributed to a simple geometrical spreading. Thus, the maximum amplitude distribution of deep earthquakes in northern Hokkaido could not be explained by the conventional knowledge of anomalous seismic zones. To explain this observation, we performed numerical simulations of 3-D seismic wave propagation using the deep earthquake off Cape Soya as a model and investigate the intrinsic attenuation structure of the mantle wedge under Hokkaido, Japan. We used a standard 3-D heterogeneous structure model under the Japanese archipelago with a plate geometry based on the latest seismic tomography, and the 3-D domain covered the epicenter and Hokkaido. An anisotropic von Kármán-type short-wavelength inhomogeneous medium was superimposed within the slab, and a seismic velocity gradient of 2% larger at the center than at the edges within the plate was set. We also set up a high attenuation zone of Qs＝20 in the mantle wedge to account for the dehydration of hydrous minerals in the oceanic crust. Under these conditions, we calculated the seismic waveforms and 3-D seismic wavefields at each Hi-net station in Hokkaido. The observed V-shaped amplitude distribution was successfully reproduced in the 1-2 Hz band. Results show that the high attenuation zone in the mantle wedge is necessary to reproduce the observed features, as the V-shaped amplitude distribution was not reproduced when the high attenuation zone in the mantle wedge was removed. To further constrain the location and size of the high-attenuation zone, we performed number of numerical simulations, with adjusting the location of the high-attenuation structure and the Q-value of the mantle wedge, and searched for the 3-D structural model that would best explain the observations by forward modeling. This allowed us to approximate constraint on the range of mantle wedge Q-values around and within which the high attenuation structure is present.