CRUSTAL SHEAR-WAVE ANISOTROPY IN SOUTHERN HAWAII - SPATIAL AND TEMPORAL ANALYSIS

Split shear wave arrivals are analyzed in seismograms from local earth quakes in southern Hawaii recorded at five temporary arrays and one pe rmanent network station. We identify split shear wave arrivals by thei r orthogonally polarized pulses, linear particle motions, and similar waveforms and estimate the delay time for the slow shear wave arrival (S-2) using a waveform cross-correlation method. Consistent leading sh ear wave polarizations were measured at the majority of our stations. Comparison of observed and predicted shear wave polarizations confirms that the former are due to anisotropy rather than earthquake source m echanism. Agreement between fast shear wave (S-1) polarizations and in dependently estimated directions of the maximum horizontal compressive stress (sigma(H)) for the Ainapo and Punaluu Gulch arrays leads us to conclude that the predominant source of the observed anisotropy for t hese two areas is stress-aligned cracks consistent with the extensive dilatancy anisotropy (EDA) hypothesis. Two distinct S-1 polarization d irections were observed over distances less than 1 km for the Bird Par k and South Flank arrays. S-1 polarizations parallel to the NE strikin g Kaoiki Pall fault system for the Bird Park array combined with a non horizontal maximum principal stress (sigma(1)) for the South Flank reg ion suggest stress-induced cracks aligned by nearby faulting as a sour ce for the observed anisotropy. Large station-to-station variations in S-1 polarization and the relationship between delay time and event de pth for arrays in the Kaoiki and South Flank regions provide evidence for anisotropy that is predominantly shallow rather than pervasive. Av erage delay times for the five arrays vary from about 100 to 230 ms, w ith standard deviations of the order of 30 ms. Estimated anisotropic v elocity variations and crack densities exceeding 10% indicate that the upper crust of southern Hawaii is highly fractured. A search for poss ible temporal changes in delay time associated with the 1983 Kaoiki ma in shock (M(L) = 6.6), at a station near the epicenter, finds no evide nce for change.