Abstract
Various instruments have been installed for monitoring crustal strains, seismicity and ground water in the Tokai district, Japan where the next large-scale earthquake is anticipated. Most of the data are telemetered on real time to the Japan Meteorological Agency (JMA) and the Earthquake Prediction Council is in charge of an immediate scientific evaluation of possible signals of imminent precursors. An attempt is made to assess the observability of precursory crustal deformations caused by a foreslip in this area. Foreslips are assumed to take place on a model fault proposed by Ishibashi in 1976 or its lateral and deep extensions. The same focal mechanism as that of the main shock is assumed. The results of static calculation for volumetric strain change at JMA stations are presented by a ratio of strain change to the seismic moment of a foreslip. Since the calibration of the dilatometers has not been completed, it is assumed that the elasticity contrast between the instrument and the surrounding rock amplifies 1.5 times the signal of crustal strain changes. It is found that almost all localized foreslips with seismic moment exceeding 5.0 × 1026 dyne-cm occurring in a three hour period, would produce the signals large enough to satisfy one of the criteria for convening the Còuncil in emergency session. However, the probability of reaching to the criterion drops less than one-half if the seismic moment of a foreslip is smaller than 5.0 × 1024 dyne-cm. It is assumed that a localized foreslip takes place equally likely at any place on the assumed fault plane and its extensions. In general, the recorded strain from a foreslip tends to be buried in the noises at most stations when its seismic moment is small and it takes place near the southern end of the fault, which is often assumed to be a starting location of the earthquake rupture. Recent studies on the relationship between fault length and seismic slip suggest that the seismic slip of the anticipated Tokai earthquake could be smaller than the expected one which is based on the estimation of the seismic slip of the 1854 Tokai earthquake. The estimated seismic moment of the future Tokai earthquake is 6.9 × 1027 dyne-cm, which is roughly equal to that of the 1923 Kanto earthquake of magnitude 7.9.
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Shimazaki, K. (1985). Observability of a Foreslip on a Hypothesized Fault of the Anticipated Tokai, Japan Earthquake. In: Kisslinger, C., Rikitake, T. (eds) Practical Approaches to Earthquake Prediction and Warning. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-2738-9_25
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DOI: https://doi.org/10.1007/978-94-017-2738-9_25
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