Shear-wave polarizations from local seismic events

Shear-wave trainsfrom three-component seismograms of local seismic events are analysed for diagnostic effects of wave propagation through effectively anisotropic media. There is recent evidence to suggest that cracks within the upper crust are differentially aligned by non-lithostatic stresses to produce effectively anisotropic structures to short period seismic waves. Shear-wave splitting the propagation of two, or more, shear-waves with approximately orthogonal polarizations and different velocities along a single ray path and the orientation of shear-wave polarizations can be used to identify effective seismic anisotropy. The effects of the shear-wave interaction at the free surface and the shear-wave radiation pattern from seismic sources are also considered. Three data sets are available for shear-wave particle motion analysis. The data sets consist of three-component short period seismic records of (i) reservoir-induced seismic events from Monticello, South Carolina, USA; (ii) hydraulic-induced seismic events from a Hot Dry Rock site at Cornwall, UK, and (iii) natural seismic events associated with the San Andreas fault system in the Livermore Valley, California, USA. The shear-wave particle motion is displayed in polarization diagrams. A few seismic records from Monticello display shear-wave splitting, but the presence of many transversely-polarized shear-waves recorded at angles of incidence beyond the critical angle suggests that shear-wave interaction at the free surface masked any anisotropic effects. The data set from the HDR site, Cornwall, provides a demonstration of seismic wave propagation through in situ cracked rock as both sources and receivers are contained within a granite batholith. The acoustic events induced by the hydraulic injection were recorded on a surface network of singlecomponent and three-component stations centred on the HDR site. The observation of shear-wave splitting, alignment of shear-wave polarizations, and time delay patterns suggests that the in situ cracks and joints are effectively anisotropic to seismic waves. The shear-wave polarizations and time delays are displayed on equal-area projections of the focal sphere and show patterns characteristic of wave propagation through vertical parallel liquid filled cracks. The effective anisotropy appears to pervade the whole granite rock mass. Focal mechanisms are determined for the acoustic events using P-wave polarities. The source mechanism is shear as opposed to jacking. A comparison of theoretical shear-wave polarizations from the seismic sources to the observed polarizations supports the anisotropic interpretation of the data. The Livermore data set was recorded on a permanent three-component seismic network operating in the Livermore Valley area. Shear-wave splitting and alignments of shear-wave polarizations at several stations suggests the presence of effective anisotropy in the upper crust. Random patterns of polarizations at other stations, and the reverberative nature of many seismograms suggests that much seismic energy is scattered. There is good correlation between theoretical shear-wave polarizations from seismic sources and observed shear-wave polarizations at several stations.