Three‐dimensional fine velocity structure and hypocentral distribution of earthquakes beneath the Kanto‐Tokai District, Japan

Abstract

We determine the three‐dimensional P wave velocity structure beneath the Kanto‐Tokai district, central Japan, and simultaneously relocated by inverting travel time data for 91 events that were observed at 61 stations operated by the National Research Center for Disaster Prevention. The variance of the travel time data is reduced by 75%, as compared to the variance for the routinely used laterally homogeneous model. In general, our velocity model is consistent with tectonic models of the Kanto‐Tokai district. Our velocity model is also in good agreement with seismic explosion data and Bouguer gravity anomalies. The uppermost layer (from 0 to 16 km depth) is characterized by lateral velocity differences of more than 20%. We find low‐velocity zones beneath Tokyo Bay, the middle of the Boso Peninsula, and Omae‐zaki, where there are thick sedimentary layers. A marked 30‐km‐thick high‐velocity zone extends to the northeast and the west of the Izu Peninsula. This zone dips gradually from the surface to a depth of 70 km in the eastern region and to a depth of 40 km in the western region and is interpreted as the Philippine Sea (PHS) plate, which is being subducted at the Sagami and the Suruga troughs. The data unquestionably require a high‐velocity zone beneath Mount Tsukuba that cannot be explained in terms of present tectonic models. We use our three‐dimensional velocity model to relocate about 23,000 earthquakes that took place from 1983 through 1985. The accuracy of the hypocentral parameters improved considerably. As a result, the double seismic zone within the subducting Pacific plate is more clearly delineated than in previous studies. The inclined seismic zone for the subducting PHS plate coincides fairly well with the high‐velocity zone of our three‐dimensional velocity model.

P wave velocity tomography of the Venezuelan region from local arrival times

“…The damped least squares method [Maquardt, 1963] Ishida and Hasemi [1988]. The method provides fast and suffidenfly acoarate arrival time calculation and has been widely used in local earthquake tomography for these reasons.…”

Crustal Structure in the Southern Kanto-Tokai Region Derived from Tomographic Method for Seismic Explosion Survey.

“…8. region at 16 km in depth from the earthquake data reported by Ishida and Hasemi (1988) and Ishida (1992). The low-velocity region is located in the upper portion of the subduction seismic zone dipping from Suruga Bay to the area beneath the region east of MTL (Fig.…”

“…These results have helped and improved our understanding of geophysical and geological processes in the earth. Ishida and Hasemi (1988) studied three-dimensional fine velocity structure and hypocentral distribution of earthquakes beneath the Kanto-Tokai district by tomographic inversion using the observed travel times of earthquakes. The obtained results were successfully employed in the interpretation of plate tectonics for movements of the Pacific and Philippine Sea Plates in the Kanto region.…”

Step-wise relocation of ISC earthquake hypocenters for linearized tomographic imaging of slab structure

“…However, the use of inaccurate hypocenter locations can prohibit the detailed investigation of relationships between seismicity and slab structure, e.g. double WadatiBenioff zones (Engdahl and Scholz, 1977;Hasegawa et al, 1978a, b;Ishida and Hasemi, 1988). Because of the danger of structural signal being mapped into biased hypocentral parameters, large mislocations may have a significant effect on investigations where both the locations and travel times of seismic waves from deep earthquakes are used.…”