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International Journal of Geophysics
Volume 2013 (2013), Article ID 651823, 18 pages
http://dx.doi.org/10.1155/2013/651823
Research Article

Field Detection of Microcracks to Define the Nucleation Stage of Earthquake Occurrence

1Risk Control Association, Kanda Hirakawa Chiyoda-ku, Tokyo 101-0027, Japan
2Tierra Tecnica Ltd., Musashimurayama, Tokyo, Japan
3Communication Research Laboratories, Koganei, Tokyo, Japan
4OKI Engineering Co., Ltd., Nerima-ku, Tokyo, Japan

Received 4 July 2013; Revised 21 September 2013; Accepted 28 October 2013

Academic Editor: John P. Makris

Copyright © 2013 Y. Fujinawa et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Main shocks of natural earthquakes are known to be accompanied by preshocks which evolve following the modified Ohmori’s law in average over many samples. Individual preshock activity, however, is far less systematic for predictive purposes. On the other hand, the microcracks in laboratory rock experiments are always preceded to final rupture. And, previous investigations of field acoustic emissions showed that the activity increases prominently before and after the main shock. But there is no detection of any phenomena to identify the nucleation stage. Here we show that a special underground electric field measurement could detect microcracks. Pulse-like variations were classified into three groups (A, B, C) by frequency. The B-type is suggested to define the nucleation period: activity increases sharply following the modified Omori’s law before the main shock and there is no activity afterward. The B-type is subgrouped into three types possibly corresponding to crack-rupture modes. The variations are supposed to be induced by crack occurrence through electrokinetic effects in the elastic-porous medium. The detection distance is suggested to be several orders larger than that of the acoustic emission due to the effective smallness of dissipation rate, and the waveform can be used to infer the rupture mode.