P波の到来方向から傾斜計の設置方位を求める試み

Abstract

The Hokuriko Microearthquake Observatory is planning to set a vertical pendulum-typetiltmeter on the bottom of a 25 m bore-hole at a satellite station Mihama (MHJ). Thistiltmeter is composed of a vertical pendulum oscillating in mutually perpendicular directions, and so usable as a two component horizontal seismograph.As it is difficult to set the tiltmeter exactly in some determined direction, we intend toobtain the direction of tiltmeter by P wave amplitudes of the earthquake with knownhypocenter observed by a vertical pendulum.As the first step of this intent, this tiltmeter was set in the observation tunnel of theObservatory, so as for two pendulums to oscillate in NS and EW directions, respectively, inorder to check whether the direction of P wave incidence determined by the tiltmeterdeflects from the direction of epicenter determined by the seismic network or not. As aresult 15 degrees were obtained as the average of 35 earthquakes.Four reasons were investigated as the probable cause of this deflection. Namely, 1) themanner of setting of the tiltmeter, 2) difference in sensitivities of two components, 3) ani-sotropy of seismic waves in underground media in the Hokuriku region, and 4) complexityof the crustal structure in the same area. The average deflection of 15 degrees is too largeto explain by reasons 1), 2), and 3). By reason 4), the observed deflection may be explain-able, if we consider somewhat complicated structure.Finally a method to determine the direction of setting of the tiltmeter was proposed.