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Seismological evidence for a lithospheric normal faulting — the Sanriku earthquake of 1933

Kanamori, Hiroo (1971) Seismological evidence for a lithospheric normal faulting — the Sanriku earthquake of 1933. Physics of the Earth and Planetary Interiors, 4 (4). pp. 289-300. ISSN 0031-9201. doi:10.1016/0031-9201(71)90013-6.

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The focal process of the Sanriku earthquake of March 2, 1933, is discussed in relation to the bending mechanism of the lithosphere. On the basis of the P times obtained at more than 200 stations, it is confirmed that the hypocenter of this earthquake is within the lithosphere beneath the Japan trench. The P wave fault plane solution, the amplitude of long-period (100 s) Love and Rayleigh waves and two near-field observations suggest, almost definitely, that the Sanriku earthquake represents a predominantly normal faulting on a plane dipping 45° towards N 90° W. A fault size of 185 × 100 km^2, in agreement with the size of the aftershock area, is required to yield a slip dislocation of 3.3 m, a value consistent with the tsunami data. This result suggests that the fracture took place over the entire thickness of the lithosphere, thereby precluding the possibility that the Sanriku earthquake merely represents a surface tensile crack due to the bending of the lithosphere. This large scale lithospheric faulting is presumably due to a gravitational pull exerted by the cold sinking lithosphere. The fracture probably took place on an old fault plane which had once fractured and healed up. The existence of this fracture zone which decouples, to some extent, the oceanic lithosphere from the sinking lithosphere accounts for the sharp bend of the lithosphere beneath oceanic trenches and also the abrupt disappearance of seismic activity across oceanic trenches. The sharp bend of the lithosphere is therefore a result, not the cause, of great earthquakes beneath oceanic trenches.

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Kanamori, Hiroo0000-0001-8219-9428
Additional Information:© 1971 Published by Elsevier B.V. Received 10 September 1970. I am grateful to Professor Takeo Matuzawa who kindly provided me with valuable information concerning the seismograms he collected. I greatly benefited from discussions with K. Abe and Y. Fukao in various aspects of this study. I thank Professor S. Miyamura for the discussion on the body wave and surface wave magnitudes of this earthquake. Dr. K. Oike of Kyoto University kindly checked the numerical calculations of near-field displacements. Dr. Tomowo Hirasawa generously allowed me to use his computer program for the focal mechanism determination from S wave data.
Group:UNSPECIFIED, Division of Geological and Planetary Sciences
Issue or Number:4
Record Number:CaltechAUTHORS:20150217-145257539
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Official Citation:Hiroo Kanamori, Seismological evidence for a lithospheric normal faulting — the Sanriku earthquake of 1933, Physics of the Earth and Planetary Interiors, Volume 4, Issue 4, April 1971, Pages 289-300, ISSN 0031-9201, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:54888
Deposited By: Ruth Sustaita
Deposited On:18 Feb 2015 03:56
Last Modified:10 Nov 2021 20:39

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