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Focal mechanism of the Tokachi-Oki earthquake of May 16, 1968: Contortion of the lithosphere at a junction of two trenches

Kanamori, Hiroo (1971) Focal mechanism of the Tokachi-Oki earthquake of May 16, 1968: Contortion of the lithosphere at a junction of two trenches. Tectonophysics, 12 (1). pp. 1-13. ISSN 0040-1951. doi:10.1016/0040-1951(71)90063-1.

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The focal mechanism of the Tokachi-Oki earthquake of 1968 (M_s∼ 8.0) and its aftershocks is studied on the basis of P-wave first motion, S-wave polarization angle, and long-period surface-wave data. The major objective is to understand the nature of the deformation of the oceanic lithosphere at a junction of two trenches. The main shock is interpreted as a low-angle thrust fault with a considerable strike-slip component, the oceanic side underthrusting beneath the continent. This type of faulting is common with other great earthquakes of the northwestern Pacific belt, and is considered to represent a major tectonic movement in this region. The largest aftershock (M_s≈ 7.5), that occurred about 10 hours after the main shock, suggests a faulting in which the slip direction is almost opposite to that of the main shock. Other aftershocks are grouped into either the main shock type or the largest aftershock type. A simple model is proposed to explain this unusual aftershock sequence. In this model a contortion of the underthrusting lithosphere at a junction of two trenches, the Kurile and the Japan trenches respectively, plays a key role. Because of this contortion of the lithosphere, the source region of the 1968 Tokachi-Oki earthquake interacts mechanically with a neighboring region where the 1952 Tokachi-Oki earthquake occured. This interaction causes aftershocks whose faulting is in a direction opposite to that of the main shock. The source parameters of the main shock are as follows: plane a (fault plane) dip angle = 20°, dip direction = S66°W; plane b dip angle = 78°, dip direction = S60°E; seismic moment = 2.8·10^(28)dyn·cm; slip dislocation = 4.1 mm; stress drop = 32 bar; strain drop = 0.71·10^(−4); strain energy release (residual strain is assumed to be zero) = 1.0·10^(24) erg. In these calculations, the fault dimension and the rigidity are assumed to be 100 × 150 km^2 and 4.5·10^(11) dyn/cm^2 respectively.

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Kanamori, Hiroo0000-0001-8219-9428
Additional Information:© 1971 Published by Elsevier B.V. Received 23 March 1971, Available online 11 April 2003. I am indebted to Katsuyuki Abe, Yoshio Fukao and Mizuho Ishida for helpful discussions on the mechanism of the unusual mainshock-aftershock sequence. Dr. Kenshiro Tsumura kindly gave me permission to use the map of the distribution of aftershocks. I am grateful to Professor Tomowo Hirasawa for giving me the computer program for the determination of source parameters from S-wave polarization angles, and to Professors Kazuaki Nakamura and Seiya Uyeda for kindly reviewing the manuscript. I owe the design of Fig. 11 to Miss Takako Yamada and Professor Kazuaki Nakamura. I gratefully acknowledge the assistance of Miss Tatoko Hirasawa throughout this work.
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Official Citation:Hiroo Kanamori, Focal mechanism of the Tokachi-Oki earthquake of May 16, 1968: Contortion of the lithosphere at a junction of two trenches, Tectonophysics, Volume 12, Issue 1, July 1971, Pages 1-13, ISSN 0040-1951, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:54928
Deposited By: Ruth Sustaita
Deposited On:18 Feb 2015 17:02
Last Modified:10 Nov 2021 20:39

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