地層構造と表面波群

Abstract

The seismic prospectings have been carried out at various places, by the members of ourInstitute in view of determining the relation between the elastic properties of the earth'ssurface and earthquake damages experienced on it.According to these studies, the bed rock is from 10 m to 100 m in depth and is overlaidby an alluvial layer. Surface waves which are of Rayleigh type are propagated along thesurface. The ratio of the surface wave velocity (VL) to the longitudinal wave velocity (Vp)was found to be closely connected with earthquake damages on the surface.The surface waves in the layered media, however, have complicated characters. The ex-periments were carried out at various fields, to determining the characters of ground motionscaused by small dynamite charges.Seismometers were set up at various depths within bore-holes, and varying the depth ofshot-hole, the explosive waves were recorded.In most of experiments predominant two types of surface waves which could be consider-ed as M1 (Rayleigh-type). And M2 (Sezawa-type)-waves were observed, and their amplitudedistributions with depth could be observed in detail.The characters of observed waves are in good agreement with those predicted by thetheories of Mr and M2-waves in a solid layer over solid half space.In one field, there was hardly any velocity difference between the two separated layers, boundary waves which could be considered as Stoneley-type waves were observed when theseismometers were placed at or near the boundary surface of the two different layers.The observed M1-waves have similar character of the Rayleigh wave in half space me-dia. The ratio of V1/v~ must be selected the ratio of M1-wave velocity to longitudinal wavevelocity.The ground has its own predominant period, which is the well known fact among theseismologists. The predominant period has been found on the record of frequency analyzedseismic waves at Hond6 observing room (near the crater of Volcano Aso).It is considered that the predominant wave is the 'leaking mode" of the layered media, and its period is as same as the period of the 3rd kind volcanic microtremors which can beconsidered as the Airy-phase of the 'normal mode wave" (Rayleigh-type wave) in the samelayered media,