Tomographic imaging of P and S wave velocity structure beneath northeastern Japan

Abstract

The seismic body wave tomography method has been improved and extended to adapt to a general velocity structure with a number of complexly shaped seismic velocity discontinuities (SVDs) and with three‐dimensional variations in the velocities in the modeling space. An efficient three dimensional ray tracing algorithm which iteratively uses the pseudobending technique and Snell's law is developed. The large and sparse system of observation equations is solved by using the LSQR algorithm. This method is applied to 18,679 arrival times from 470 shallow and intermediate‐depth earthquakes in order to study P and S wave tomographic images beneath northeastern Japan. In addition to first P and S wave arrivals, clear later arrivals of SP waves converted at the Moho and PS and SP waves converted at the upper boundary of the subducted Pacific plate (UBPP) are also used in the inversion. The UBPP, Conrad and Moho are taken as three SVDs, and their depth distributions obtained by previous studies are used. High‐resolution P and S wave tomographic images down to a depth of 200 km have been determined. Large velocity variations amounting to 6% for P wave and 10% for S wave are revealed in the crust and upper mantle. In the crust low‐velocity (low‐V) zones exist beneath active volcanoes. In the upper mantle the low‐V zones dip toward the west from the volcanic front. A high‐velocity (high‐ V) zone corresponding to the subducted Pacific plate is clearly delineated. Most earthquakes in the lower plane of the double‐planed deep seismic zone are found to occur in relatively high‐V areas. The obtained tomographic images are also found to explain other seismological observations well.

Three-Dimensional Seismic Velocity Structure of the Crust and Uppermost Mantle beneath Taiwan.

“…In this study, we used the tomography method of Zhao et al (1992a). Although the conceptual approach of this method parallels that of Aki and Lee (1976), it has additional features.…”

“…Three-dimensional grid nodes are set in the modeling space and velocities and the nodes are taken as unknown parameters. Travel times and ray paths are calculated by using an efficient 3-D ray-tracing technique (Zhao et al, 1992a). The LSQR algorithm of Paige and Saunders (1982) is used to conduct the inversion, thus allowing the use of a great number of data to solve large tomographic problems.…”

Mathematica as a powerful and practical tool for displaying animated three‐dimensional structures of the Earth's interior

“…The file dv-3 is a set of points of À3% velocity perturbation, while dv-4 and dv-5 consist of the data tracing outlines of the north-south cross sections of À4% and À5% velocity perturbations, respectively. These data files are from the 3-D velocity structure by Zhao et al [1992]. Parameter values ''{À2.84, À1, 1.15}'' given in ''ViewPoint'' draw Figure 1a.…”

“…Many seismic events, either large-or micro-earthquakes, are observed, that lead seismologists to determine detailed velocity structure beneath Japan [e.g., Zhao et al, 1992;Hasegawa et al, 1993]. For example, Zhao et al [1992] analyzed 14,045 arrival times including both P and S waves.…”

“…[5] Figures 1a to 1e show 3-D mapping of the crust and the upper mantle to 120 km depth beneath NE Japan (36.5-41.6N 138.5-142.5E), based on the P-wave seismic tomography by Zhao et al [1992]. Program codes for 3-D views and animations by Mathematica 4 are given in appendicies A to C. Even simple Mathematica codes yield 3-D Earth structure in quite detail.…”