An array study of ground tremors and its application to exploration

Abstract

Micro ground tremors were observed by an array of multiple 3-D geophones of 1 Hz with a short spacing along a traverse in a volcanotectonic zone of northeastern Japan. Instead of line spectral methods by single observation point for earthquake engineering purposes, a continuous spectrum was processed by cross correlation and 2-D filtering in time and then in the frequency domain. In addition to the power spectral profiling, 2-D representation of ground motion vectors assists in understanding a realistic structure. Frequency to depth conversion was made by using the law of a quarter wavelength in assumption of velocity distribution of S- or P-waves. Advanced discussion is proposed on this point considering dispersion of surface waves dominant in the tremors, especially of the longer period. As an example of interpretation of the results, geomagnetic intensity was correlated in detail, and the volcanotectonism was seen as double caldera walls (post Miocene).