Lateral variation of Pn velocity and crustal delay time in the Kanto-Tokai district, Japan

Abstract

Abstract The Pn arrivals recorded from the Kanto-Tokai array of National Research Center for Disaster Prevention were used to demonstrate the lateral variation of Pn velocity and the crustal delay time. Nearly 1000 travel times were used in the analysis. The classical time-term method was extended to allow the Moho velocity to vary in a block-type structure. The obtained Pn velocity ranges from 7.2 to 8.0 km s −1 within the array. Low Pn velocity is found around the Mt Fuji region. High Pn velocity is found in northern Kanto, in Tokyo Bay to the southern Boso peninsula, and offshore of the Tokai district. The high velocities in the later two regions are related to the Philippine Sea plate subducting from the Sagami and the Suruga troughs, respectively. Distinct low velocity around the Mt Fuji region where there is no seismicity related to the Philippine Sea slab at a depth of 20–50 km, shows ambiguous Moho discontinuity owing to ‘hot’ mantle under the volcano or the complicated colliding structure between the Philippine Sea and the Eurasian plates. The crustal delay times depend on both crustal velocities and the Moho depth. The region near Tokyo Bay has largest crustal delay in this district, mainly owing to thick sediments. The regions of the Izu peninsula and Kanto mountains show small crustal delays which indicate high-velocity crust on average.