Three-dimensional seismic attenuation structure beneath the Japanese islands and its tectonic and thermal implications

Abstract

A three-dimensional (3-D) seismic attenuation structure beneath the Japanese Islands is determined by inversion of seismic intensity data. The result clearly reveals a remarkable contrast in the attenuation structure and reflects the lateral variation of thermal structure beneath the Japanese Islands. For the inversion, the method developed by Hashida and Shimazaki ∗ ∗ (1984). is used: the intensity which is assumed to be a measure of the maximum acceleration of the S-wave at the observation point is inverted to estimate the attenuation structure and the acceleration at the seismic source. The data consist of about 15,000 intensity readings for about 800 earthquakes reported by the Japan Meteorological Agency between 1951 and 1983. By combining 3-D structures of eight districts in Japan, 3-D attenuation maps consisting of three layers are constructed to a depth of 90 km. The obtained structure reveals the following features: first, a distinct difference exists in the crustal structure between NE and SW Japan. In NE Japan, low-Q (high-attenuation) areas on the continental side of the arc are dominant whereas some high-Q (low-attenuation) areas exist on the Pacific side of the arc. In SW Japan, except for the Kyushu district, high-Q areas are dominant and small low-Q areas exist mainly on the Pacific coast. These crustal features generally continue down to the uppermost mantle. Second, the obtained crustal structure shows that attenuation is age dependent. low-Q areas correspond to the distribution of Quaternary volcanic materials and high-Q areas correspond to the geological provinces where pre-Cenozoic rocks are distributed. Since the thermal structure strongly depends on the crustal age, this feature suggests that the attenuation structure reflects the thermal structure. Third, in the upper mantle, low-Q loci are found, which correspond to the distribution of active and other Quaternary volcanoes forming the arc-volcanic chain. It should be noted that even on the continental side of the volcanic front these loci do not exist beneath areas where no volcano exists. This suggests that the upper mantle attenuation is controlled by the existence of upwelling diapirs, which are considered to lie beneath a volcano or a group of volcanoes. Fourth, high-Q zones in the upper mantle exist on the Pacific side of the volcanic front along the Kurile-northeast Honshu arc and along Ryukyu arc. The high-Q mantle lying above the sinking slab seems to reflect a cooling effect of the subducting cold oceanic plates. The thickness of the lithosphere may reach 60 km or more in such outer arcs, although the thickness has been considered to be about 30 km or so. In SW Japan, where the high-Q uppermost mantle also lies, a thick lithosphere of about 60 km may exist. Possible thermal cross sections are proposed for NE and SW Japan, based on the obtained 3-D attenuation structure and other geophysical considerations. In the uppermost mantle of NE Japan a higher temperature is expected than in SW Japan. The thermal structures reflect the difference of tectonic settings in the two regions, such as the subducting plate, overriding lithosphere and volcanic activity.