Lateral variation of the cutoff depth of shallow earthquakes beneath the Japan Islands and its implications for seismogenesis

Abstract

We estimated the spatial distribution of the cut-off depth of shallow seismicity beneath the Japan Islands based on data from a recently constructed nationwide dense seismic network. In order to avoid the effect of heterogeneous seismic velocity structure, we determined the 3D seismic velocity structure for the whole of Japan, which was then used to relocate the hypocenters. The cut-off depth was determined by calculating D90, the depth above which 90% of the earthquakes occur. A spatial distribution of estimated D90 values shows a considerable lateral variation, ranging from about 5km to about 40km. The D90's are deep along the Pacific coast of eastern Japan. Locally shallow D90's extend along belt-like stretches of volcanic areas. Heat flow is low along the Pacific coast in eastern Japan and high in the volcanic areas. This indicates that the lateral variation of D90's is formed mainly by lateral geotherm variation. Cooling by the subducting old, very cold Pacific plate may cause the deep D90's along the Pacific coast in eastern Japan. Heating by magma supplied from the mantle wedge below is considered to be the main cause of the shallow D90's in the volcanic areas. Magma and/or aqueous fluids derived from solidified magma contribute not only to locally shallow D90's but also to the weakening of ambient rocks in the lower crust, resulting in stress concentrations in the upper crust just above them. Large shallow inland earthquakes tend to occur in such areas, suggesting that a model of shallow earthquake generation applied to Tohoku can be used in other areas in Japan as well.