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On some geoelectric potential changes in Naha, Okinawa, Japan and their possible relationship with nearby earthquakes
Takahashi, I.; Nagao, T.; Uyeda, S. (1999). On some geoelectric potential changes in Naha, Okinawa, Japan and their possible relationship with nearby earthquakes. Bull. Inst. Oceanic Res. & Develop., Tokai Univ. 20: 31-40
In: Bulletin of Institute of Oceanic Research and Development. Tokai University. Tokai University. Institute of Oceanic Research and Development: Shimizu. ISSN 0289-680X, more
Peer reviewed article  

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  • Takahashi, I.
  • Nagao, T.
  • Uyeda, S.

Abstract
    We reexamined low noise long dipole geoelectric potential difference data obtained using NTT electrodes in Okinawa island, Japan, for the period 1987 to 1991. During the observation period, there were three distinct anomalous changes in the geoelectric potential difference.The first two were a clear co-seismic change and a possible precursor of the nearby M 6.0 earthquake of January 24th, 1988, and the third one was a less convicing precursor to a nearby M 3.9 event. In the case of the M 6.0 event, co-seismic signal occurred simultaneously with the arrival of seismic waves within the limit of timing accurancy and sampling rate, whereas possible precursory signals were recorded 6 days before the event. Electric potential contour maps were constructed for these anomalous changes all of which show a saddle point in th southern part of the network. These contour maps are noticeably different from those for potential changes induced by geomagnetic changes, which show a simple trend parallel to the strike of the island arc.Considering that the January 24th, 1988 was the only M 6 class event within 100 km of the network and that no other geoeletric potential changes of comparable significance were recorded during the observation period, it is inferred that nearby M>6 class earthquakes exhibit observable co-seismic and pre-seismic geoeletric changes, justfying further observation.

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