Summary
A technique is presented for computing quasi‐static strains and displacements at the surface of a vertically heterogeneous elastic or viscoelastic half‐space caused by a dislocation of finite size. The distribution of displacement discontinuity and orientation of the dislocation surface are arbitrary. The technique is applied to the problems of deep stress relaxation on the San Andreas fault. It is found that modern geodetic data do not allow ready distinction between stress relaxation due to deep seismic slip and stress relaxation due to viscoelastic creep. We also investigate tectonic motions in the Imperial Valley and conclude that most of the motion on the southern part of the San Andreas system is occurring in the Imperial Valley. It may also be inferred that deep relaxation of stress has occurred on the Imperial fault since the earthquake there in 1940.
S U M M A R YA theoretical understanding of the mechanisms by which quarry blasts excite seismic waves is useful in understanding how quarry blast discriminants may be transported from one region to another. An experiment in Texas with well-placed seismic stations and a cooperative blasting engineer has shed light on some of the physical mechanisms of seismic excitation at short periods (0.1-3 Hz). Azimuthal radiation patterns of the 0.2-3 Hz Rayleigh and Love waves are diagnostic of two proposed mechanisms for non-isotropic radiation from quarry blasts. Observations show that the Love and Rayleigh wave radiation patterns depend upon the orientation of the quarry benches. Two possible mechanisms for non-isotropic radiation are (1) the lateral throw of spalled material and (2) the presence of the topographic bench in the quarry.The spall of material can be modelled by vertical and horizontal forces applied to the free surface with time functions proportional to the derivative of the momentum of the spalled material. We use wavenumber integration synthetics to model the explosion plus spall represented by seismic moment tensor sources plus point forces. The resulting synthetics demonstrate that the magnitude of the SH (Love) compared with the SV (fundamental Rayleigh or Rg) in the short period band (0.5-3 Hz) may be explained by the spall mechanism. Nearly all of the available mass must participate in the spall with an average velocity of 2-5 m s −1 to provide sufficient impulse to generate the observed Love waves. Love wave radiation patterns from such a mechanism are consistent with the spall mechanism.We modelled the effects of the topographic bench using 3-D linear finite-difference calculations to compute progressive elastic wavefields from explosion sources behind the quarry bench. These 3-D calculations show SH radiation patterns consistent with observations while the SV radiation patterns are not consistent with observations. We find that the radiation patterns from the explosion behind the 3-D bench cannot be modelled by a modified moment tensor. The 3-D effects of the bench are more complicated than the representation by a moment tensor with a single reduced horizontal couple. The 3-D finite-difference synthetics exhibit strong azimuthal asymmetry and polarity reversals in the outgoing P-SV waves (P, S and Rg) radiated behind the bench for V p /V s ratios between 2 and 3.Both mechanisms may contribute to the non-isotropic radiation patterns but the spall mechanism is the simplest physical mechanism that explains the bulk of the observations. Adjustments to the time functions for the horizontal force, the vertical force and the explosion source may further refine the remaining differences between prediction and the observations.
High gain portable seismographs have been operated at 22 sites along the San Miguel fault zone, Baja California, Mexico. We report results in terms of normalized rates of microearthquakes per day. Very high rates, exceeding 100 events/day, were observed near the southeast end of the San Miguel fault. Sixteen of the 22 sites recorded rates exceeding 20 events/day. The overall micro‐seismicity is higher than along most of the San Andreas fault in Southern California. At two sites, arrays were operated to obtain accurate locations. Locations near the southeast end of the San Miguel fault were very near the fault trace and at depths of 8 to 14 km. Relatively high microearthquake rates were observed at a site 70 km from Tijuana, suggesting that the northwestern San Miguel fault zone is active and might pose a significant seismic hazard to the cities of Tijuana and San Diego.
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