Three-dimensional P-wave Research Articles

Three-dimensional P-wave velocity structure beneath Central Japan: low-velocity bodies in the wedge portion of the upper mantle above high-velocity subducting plates

The three-dimensional (3-D) P-wave velocity structure beneath Central Japan has been investigated in detail by an inversion method. 7490 P-wave arrival times from 120 shallow and intermediate depth earthquakes that have occurred in this region are used to estimate velocity anomalies in 3-D subdivided blocks and hypocentral perturbations, simultaneously. The results reveal complex 3-D structures, with low-velocity zones in the wedge portion of the upper mantle above the high-velocity Philippine Sea and Pacific plates subducting beneath this region. Prominent low-velocity bodies exist just beneath active volcanoes, particularly in the Hida mountain range. Low-velocity bodies are spatially correlated with the low- Q zones estimated from seismic intensity data. One low-velocity body coincides with an anisotropic body detected from the study of shear-wave splitting. Dome-shaped low-velocity masses seem to represent partially melted mantle diapirs. No clear evidence on velocity contrast has been identified across the Fossa Magna, which is a tectonic boundary between Northeast and Southwest Japan.

How deep can we see the high velocity anomalies beneath the Japan Islands?

High velocity anomalies are found in the lower mantle beneath the Japan Islands and their vicinity through an analysis of detailed three‐dimensional P‐wave velocity structure based on a tomographic inversion of ISC data. Beneath the Sea of Japan, the high velocity anomalies in the lower mantle are slab‐like and appear to extend below the deep focus earthquakes of the high velocity Pacific slab descending from the Japan trench. This slab‐like high velocity body extending into the lower mantle steepens below a depth of 500‐600 km. Resolution analyses confirms the existence of a slab‐like high velocity region in the lower mantle, though the depth extent is not well resolved. On the other hand, the Pacific slab descending from the Izu‐Bonin trench seems to be fingering, that is, high velocity anomalies are penetrating into the lower mantle beneath latitude 26°N, and not penetrating but horizontally bending in the deeper portion of the upper mantle around 29°N.

The spatial distribution of earthquake hypocenters and the three-dimensional velocity structure in the Kanto-Tokai district, Japan.

We examined the spatial distribution of earthquake hypocenters and the three-dimensional velocity structure of P-wave for the Kanto-Tokai District, central Japan, in order to investigate their relation. About 5, 000 earthquakes which occurred during the period from January 1980 to December 1981 were relocated. The double-planed structure of the intermediate-depth seismic zone is observed clearly beneath the Kanto District. The seismic zone underthrusting from Suruga trough toward the west and from Sagami trough toward the east were clearly traced throughout the troughs. Their dip angles are found to be steeper at the northern end compared with the southern part. These inclined seismic zones, however, could not be traced through the inland region to the north of the Izu Peninsula.The three-dimensional P-wave velocity structure was investigated by applying an inversion method for local earthquake data. The velocity struc-ture between 0 and 145 km in depth was modeled by six layers. Each layer was divided into 10 (latitude)×6 (longitude) blocks, the size of which is 28×26 km2. A high velocity zone appears in and around the Izu Peninsula (northernmost tip of the Philippine Sea plate) in the first layer (0-20 km) and to the east of the Izu Peninsula in the second layer (20-45 km). Low velocity zones appear beneath Tokyo Bay and in the western region in the first layer and to the northwest of the Izu Peninsula in the second layer. Coincidence of the high velocity zone with the inclined seismic zone is found under Sagami trough but such a correlation is not found under Suruga trough owing presumably to poor resolution.