掛川と春野に埋設されている石井式3成分歪計で観測された歪の磁場応答係数の変化

抄録

The Ishii-type three-component strainmeter that can detect strain change as small as 10^-10 is expected to catch a precursor of the anticipated Tokai earthquake. The instrument uses magnesensor, so that the output is also affected by magnetic fields. The response of the sensor to the geomagnetic field makes a noise that might mask signals produced by changes of the tectonic stress. However, that feature of the instrument can be utilized to get information about the underground electric conductivity, because the geomagnetic fields recorded by the magnesensor include those generated by induced electric currents. With the expectation to detect change in the electric conductivity beneath the strainmeter stations at Kakegawa and Haruno, we investigated temporal change in the pro-portionality between the outputs of the three-component strainmeters and the geomagnetic field at Kakioka situated on a stable crustal block about 250km to the northeast from the Tokai region. Using one-minute sampling data at times of geomagnetic storms during the period from April 1998 to November 2002, we calculated the response coefficient of strain to geomagnetic field at each of the storm times. It was found that the response coefficient increased with time at first, but the tendency changed in the midst of 2000 for both stations at Kakegawa and Haruno. The temporal change of the response coefficient and the long-term change of the strain are alike when the linear trend is subtracted from the latter. We think the change of the trend in the response coefficient in mid 2000 might have been related to changes in the electromagnetic conductivity beneath the Tokai region.