Seismic Ground Motion and Residual Deformation near a Vertical Fault

Ground motion and residual ground deformation at strategic points in the epicentral region of an idealized, vertical shear fault that may correspond to an intermediate magnitude (6–6½) earthquake are displayed and analyzed. For analytical purposes, Haskell's expressions for the elastic displacements near a propagating fault are separated into two parts, the near-field and the far-field terms. The distance from the fault at which the far-field terms are sufficient to describe the total field varies, depending upon the orientation of particle motion relative to that of the dislocation (slip) vector. For the component of particle motion parallel to the dislocation vector, this distance is comparatively greater than that for the perpendicular component. The Earth Physics Branch strain gauge, which has a threshold sensitivity of about 5 × 10−10, has the capability of detecting residual strain at least as far as 100 km from the hypothetical fault. The Earth Physics Branch tiltmeter, which can detect a step change in tilt in the range 5 × 10−9 to 5 × 10−10 radians, can detect residual tilt at least as far as 50 km from the causative fault.The high-frequency slope of the theoretical Fourier amplitude spectrum of ground acceleration (FS), is not appreciably steepened by the addition of the near-field to the far-field terms. The implication of this observation is that the steep slope observed at high frequencies on FS curves of predominantly California earthquakes cannot be attributed, even in part, to the source mechanism assumed but must be attributed entirely to travel path effects such as attenuation and scattering.