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Simulation of Long-Period Ground Motions for the 1923 Kanto Earthquake (M≈8)

Takeo, Minoru and Kanamori, Hiroo (1992) Simulation of Long-Period Ground Motions for the 1923 Kanto Earthquake (M≈8). Bulletin of the Earthquake Research Institute, 67 (4). pp. 389-436. ISSN 0040-8972.

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We performed numerical simulations of long-period ground motions in Tokyo for the 1923 Kanto, Japan, earthquake (M_s=7.9 to 8.1) which devastated Tokyo, Yokohama, and their environs, and caused more than 130,000 fatalities. We used reflection-transmission matrices and the discrete wavenumber integration method to compute ground motions for fault models placed in layered structures. The objective of this study is twofold: 1) to estimate the long-period response spectrum for the 1923 Kanto earthquake; and 2) to investigate the effects of various source parameters on the ground motion so that we can assess the variability of estimated ground motions, and apply the results to a broader class of earthquakes than just a single design earthquake specifically fior the 1923 event. The Kanto earthquake was recorded in Tokyo with a Ewing seismograph and an Imamura seismograph. The ground motions estimated by earlier investigators from these two seismograms differed significantly. This conflict can be reconciled if we assume that the solid friction of the Ewing seismograph was very high during shaking. Solid friction can reduce the resonance of the instrument, and long-period ground motions with large amplitudes can be recorded correctly. We conclude that the ground motion of the Kanto earthquake had a very large long-period component with a velocity response spectrum of 120 cm/sec (5% damping) at a period of 13 sec. The velocity response spectrum at a period of 7.5 sec is estimated to be about 50 cm/sec. Numerical simulations produced a wide range of ground motions and response spectra, even with a given fault geometry and seismic moment. For a rupture model initiating from the southwestern end of the fault plane the most probable epicenter of the 1923 earthquake the computed response spectra have a range of 10 to 100 cm/sec at a period of 7.5 sec, which brackets the observed level. All the response spectra computed for this model have a peak in a period range of 10 to 13 sec. The slip distribution and the rupture direction significantly influence simulated ground motions. Large subevents in a shallow structure enhance the ground motion significantly, especially if the rupture propagation is toward the site. One of our extreme models, which has large slip of about 8 min the shallow crust at the western end of the fault plane, can produce a large ground motion comparable to that estimated from the Ewing seismogram. Reducing the rise time or increasing the rupture propagation increases the spectral amplitude at periods shorter than 5 sec. Also, if the site is located on a very soft sediment, significant (a factor of 1.4) amplification occurs. The basin structure beneath Tokyo would increase the duration of ground motion significantly. Although increased duration does not significantly affect the response spectrum, it will play an important role in the nonlinear response of structures.

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Kanamori, Hiroo0000-0001-8219-9428
Additional Information:© 1992 Earthquake Research Institute. We thank H. Yokota and S. Kataoka, Institute of Technology, Shimizu Co., who kindly provided us with the digitzed ground-motion data reproduced from the Imamura seismogram; and M. Yamada, Waseda University, who kindly provided us with the digitzed ground-motion data reproduced from the Ewing seismogram. M. Takeo was supported under the visitor program of the Southern California Earthquake Center and the exchange visitors program of the Ministry of Education, Science and Culture, Japan. This research was partially supported by the CUREe-Kajima Research Project. We have greatly benefitted from interaction with the Kajima research group on this subject. In particular, discussions with Drs. M. Niwa, M. Motosaka, M. Takemura, and H. Yamanaka have been very helpful during the course of this study. Contribution No. 5192, Division of Geology and Planetary Sciences, California Institute of Technology, Pasadena, California 91125.
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Southern California Earthquake Center (SCEC)UNSPECIFIED
Ministry of Education, Science and Culture (Japan)UNSPECIFIED
CUREe-Kajima Research ProjectUNSPECIFIED
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Caltech Division of Geological and Planetary Sciences5192
Issue or Number:4
Record Number:CaltechAUTHORS:20141205-094351478
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:52429
Deposited By: Tony Diaz
Deposited On:09 Dec 2014 22:51
Last Modified:09 Mar 2020 13:18

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