Abstract
Abstract. For the prediction of the expected Tokai Earthquake (EQ), Japan Meteorological Agency (JMA) has been observing volumetric strain changes to detect its presumed pre-slip phenomena in Tokai and South Kanto regions. In 1998, an anomalous volumetric strain change, lasting for 5 days, which initially attracted much attention as a possible precursor to the Tokai EQ, was observed at Shimizu station in the Tokai area. Eventually, the Tokai EQ did not occur and the causal mechanism of the observed anomaly was unknown. However, we found variations of geoelectric potential differences (GPD) possibly associated with the anomaly by applying Principal Component Analysis (PCA) to the GPD data taken in the same area. From the polarities of the observed GPD changes, we infer that these GPD variations were caused by electrokinetic effects of groundwater motion, which might have been related with the strain changes. These results imply that the GPD observations may assist us in understanding the nature of EQ precursory phenomena.
Highlights
Large EQs recur in the subduction zones along Japanese Islands
An anomaly of volumetric strain, which looked like a precursor to the expected Tokai EQ at that point in time, was observed in October 1998 at Shimizu, Japan
The geoelectric potential difference (GPD) variations could have been generated by electrokinetic effect due to a local depression of groundwater
Summary
Large EQs recur in the subduction zones along Japanese Islands. For example, EQs in Nankai trough occur every 100– 150 yr (Ando, 1975; Thatchar, 1984; Strasser et al, 2009). An anomalous expansion began to appear on 11 October 1998 on the volumetric strainmeter in Shimizu (35.1016◦ N, 138.5102◦ E), which is one of the 36 stations in Tokai region (Yamasato, 1999). This anomaly was observed only at this station. Geoelectric field is affected by the variation of geomagnetic and tidal induction and artificial noises due to DC driven trains and factories These effects are not usually reduced by widening the electrode separation. We were conducting geoelectric potential difference (GPD) observation at Shimizu (SMZ) station with long dipoles in the area several kilometres northwest of the JMA strainmeter when the anomalous strain expansion was recorded (Fig. 2). We analyse the GPD data during the three months including the strain anomaly period to investigate their relationship with the strainmeter data using Principal Component Analysis (PCA)
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