Three-Dimensional P Wave Velocity Structure beneath Northern Part of the Tohoku District

Sato, Tamao; Nakayama, Kouji; Tanaka, Kazuo; Hasemi, Akiko

doi:10.4294/zisin1948.42.4_419

Cited by 6 publications

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“…Horiuchi et al (1982 a, b) revealed two-dimensional (2-D) depth distributions of the Conrad and Moho discontinuities beneath the central part of the Tohoku region using local seismic data. Sato (1981) showed a configuration of the subducting plate boundary together with a 3-D P and S wave velocity distribution by analyzing deep local earthquake data. Hasemi et al (1984) and Obara et al (1986) investigated a 3-D fine velocity structure beneath the whole Tohoku region.…”

Section: Introductionmentioning

confidence: 99%

Seismic Velocity Structure in the Crust and Upper Mantle beneath Northern Japan.

et al. 1994

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W e have applied an inverse method to P and S wave arrival time data observed at 52 seismic stations for 349 local earthquakes in order to estimate a three-dimensional (3-D) velocity structure beneath northern Japan. The method is characterized by a simultaneous determination of the two-dimensional depth distributions of velocity boundaries, 3-D velocity distribution and station corrections as well as hypocenter parameters of earthquakes. The Moho discontinuity under eastern Hokkaido is inclined from a depth of 32 km beneath the Hidaka Mountains to 20 km beneath the Konsen Plateau, while the thick crust with a thickness of 30-36 km distributes widely in western Hokkaido and the Tohoku region. The dip direction of the Moho in eastern Hokkaido is approximately perpendicular to the range of the Hidaka Mountains. The upper boundary of the subducting Pacific plate distributes in the depth range from 50 to 170 km in the surveyed area and the dip angle of the plate boundary in eastern Hokkaido (the Kurile arc) is slightly larger than that in the Tohoku region (the northeastern Japan arc).Estimated P (S) wave velocities are 5.8-7.0 km/s (3.4-4.1 km/s) in the crust, 7.4-8.0 km/s (4.2-4.5 km/s) in the upper mantle, and 8.1-8.6 km/s (4.6-4.9 km/s) in the plate, respectively. The velocities in the crust beneath northern Hokkaido are lower than those beneath eastern Hokkaido and the Tohoku region. The upper mantle has velocities which gradually increase with depth, and the subducted plate contains a high velocity zone with a P wave velocity faster than 8.3 km/s. VP/VS ratios derived from P and S wave velocities generally range from 1.75 to 1.80 in the crust and upper mantle, while the ratios in the high velocity zone within the plate are less than 1.75. These results suggest that the plate is composed of two layers: the first layer is a low-velocity and high-VP/VS zone with a thickness less than 20 km, and the second layer is a thick high-velocity and low VP/VS zone. The estimated plate thickness is about 90 km in the Kurile arc and about 110 km in the northeastern Japan arc.The relocated hypocenter distribution beneath Hokkaido clearly shows the double seismic plane in the subducting plate. The upper seismic plane is located in the first Received April 27, 1994; Accepted October 5, 1994 * To whom correspondence should be addressed . ** Present address: Faculty of Science , Kyushu University, Fukuoka 812, Japan. layer with a low velocity and the lower seismic plane is in the second layer with a high velocity. The seismic activity in the upper seismic plane is high in the depth range from 60 to 90 km, while that in the lower seismic plane is high at depths deeper than 100 km. The double seismic plane joins at depths from 90 to 130 km.

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Section: Introductionmentioning

confidence: 99%

Seismic Velocity Structure in the Crust and Upper Mantle beneath Northern Japan.

et al. 1994

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“…Therefore, in estimating the 3-D velocity structure, both elements must be included as parameters in the study model. The 3-D velocity structures beneath various regions in northern Japan have been investigated in putting emphasis on the block velocity distributions (Takanami, 1982;Miyamachi and Moriya, 1984;Hasemi et al, 1984;Obara et al, 1986;Sato et al, 1989). However, as pointed out by Koketsu (1991), Zhao (1991) and Zhao et al (1992), the effect of the velocity boundaries on the solution of the inversion must be taken into account.…”

Section: Discussionmentioning

confidence: 99%

“…Miyamachi and Moriya, 1984;Nakanishi, 1985;Hasemi et al, 1984;Obara et al, 1986;Sato et al, 1989;Zhao, 1991;Zhao et al, 1992;Zhao and Hasegawa, 1993). The methods for modeling the 3-D velocity structure used in these studies are basically classified into two: one is a discrete method represented by the "block configuration" of Aki and Lee (1976) or the "grid configuration" of Thurber (1983), and the other is the functional method using some continuous functions (Horiuchi et al, 1982;Ashiya et al, 1987).…”

Section: Introductionmentioning

confidence: 99%

“…On the other hand, the functional method uses some continuous functions such as power series (Horiuchi et al, 1982;Sato, 1981) and Chebyshev's function (Ashiya et al, 1987) to describe the depth distributions of the boundaries or the 3-D velocity distribution. In general, the number of unknown parameters in the functional method is less than that in the discrete method.…”

Section: Introductionmentioning

confidence: 99%

“…In the previous functional and discrete methods, the depth distributions of the boundaries have been estimated under the assigned velocity distribution (Horiuchi et al, 1982;Zhao et al, 1990) and the velocity distribution has been solved under the fixed boundaries (Miyamachi and Moriya, 1984;Hasemi et al, 1984;Obara et al, 1986;Sato et al, 1989). Recently, Koketsu (1991) pointed out that careful calculation of travel times is important in a velocity model with dipping boundaries.…”

Section: Introductionmentioning

confidence: 99%

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A Method for Determination of the Three-Dimensional Seismic Velocity Structure from Local Earthquake Data.

Miyamachi

1994

J,Phys,Earth

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We have developed an inverse method to estimate a three-dimensional seismic wave velocity structure using seismic arrival time data obtained from local earthquakes. The method is characterized by a simultaneous determination of a two-dimensional depth distribution of a velocity boundary, a three-dimensional velocity distribution, station corrections and hypocenters. The depth distributions of velocity boundaries and the distributions of the slowness perturbations for P and S waves are modeled by power series of latitude, longitude, and depth, This representation of the three-dimensional seismic structure is advantageous not only in reducing the number of unknown parameters, but also in analytically evaluating the boundary depths, velocities, and partial derivatives of travel times with respect to the unknown parameters. Ray trajectories connected between hypocenters and stations are determined by the so-called shooting method for ray tracing scheme under a simplified velocity distribution with the exact boundary depth distribution. Numerical experiments have proved that our inverse method can successfully estimate the three-dimensional velocity structure.

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A method for determination of the three-dimensional seismic velocity structure from local earthquake data

1995

International Journal of Rock Mechanics and Mining Sciences &am

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Three-Dimensional P Wave Velocity Structure beneath Northern Part of the Tohoku District

Cited by 6 publications

References 0 publications

Seismic Velocity Structure in the Crust and Upper Mantle beneath Northern Japan.

Seismic Velocity Structure in the Crust and Upper Mantle beneath Northern Japan.

A Method for Determination of the Three-Dimensional Seismic Velocity Structure from Local Earthquake Data.

A method for determination of the three-dimensional seismic velocity structure from local earthquake data

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