Observations of shear wave splitting on the southeast flank of Mauna Loa Volcano, Hawaii

Abstract

We use a combination of particle motion covariance matrix decomposition, particle motion plots, and cross correlation to estimate the polarization and split times of shear wave arrivals from local earthquakes in the Kaoiki and Hilea regions of southern Hawaii. Consistent leading shear wave polarizations and identifiable fast and slow shear wave arrivals were frequently observed. Average delay times for the four arrays vary from 100 to 230 ms. The leading shear wave polarizations at stations in the Kaoiki region agree with the direction of the local maximum horizontal compressive stress, σH, determined from focal mechanism studies and field observations of ground rupture. For the Bird Park (BP) array, in the Kaoiki region, the relationship between delay time and event depth shows evidence for both dominant shallow and weaker pervasive anisotropy. The correlation between delays and event depth is much weaker for the Ainapo (AN) array than for BP, however a smaller slope is apparent. Conservative estimates of the crack densities for the BP and AN arrays are e=12% and e=6%, respectively. In the Hilea region, the leading shear wave polarization for the Punaluu Gulch (PG) array differs by approximately 30° from the direction of σH determined from focal mechanism studies, while the Waihaka Gulch (WG) array shows no predominant polarization direction. No correlation between delay time and event depth was observed for either of the two Hilea arrays.