Stress, Anisotropy and Strain Rate in Kyushu, Japan, from Earthquakes and GPS: Tectonic implications

Abstract:

We compare crustal stress and strain rates on the Island of Kyushu as measured from inversions of focal mechanisms, GPS and shear wave splitting on shallow and deep earthquakes to determine how well anisotropy can be used to predict stress or strain. Shear wave splitting of local and regional earthquakes was measured on the NIED network between 2004 and 2012, and on Aso, Sakurajima, Kirishima and Unzen volcano networks. Strain measurements were made from the Geonet stations. JMA-determined S arrival times processed with the MFAST shear wave splitting code measure fast polarisations (phi), related to the orientation of the anisotropic medium and time delays (dt), related to the path length and the percent anisotropy. We apply the TESSA 2-D delay time tomography and spatial averaging code to the highest quality events with the most nearly vertical incidence angles, separating the 3455 shallow (depth < 40 km) from the 4957 deep (>=40 km) earthquakes. Using 30 km grids for all the inversions and interpolating where measurements are not available, the average difference between maximum horizontal stress axes (Shmax) and phi is nearly identical to that between maximum compressional strain rate (Mstrain) and Shmax (35 degrees). Shmax and mstrain are each also significantly correlated with Shmax, but less so with each other. Amplitudes of splitting delay time/km, strain eigenvalues and stress ratio R follow similar patterns. All three orientations are E-W in central Kyushu, where the compressional strain is highest. Both splitting and stress suggest plate-boundary-parallel maximum principal stress just off the coast of Kyushu, where strain measurements are sparse. Southwestern Kyushu has the largest difference between strain and stress. Phi from shallow and deep earthquakes is not well correlated, suggesting that the deep earthquake waveforms are not simply re-split in the crust. Causes for the anisotropy may be olivine crystals aligned by drag of the subducting Phillipine Sea plate in the mantle and stress-aligned microcracks in the crust. We have begun inverting for 3-D anisotropic structure to determine the relation of the mantle anisotropy to plate flow.