Shear Wave Splitting Intensity of the Maule, Chile Rupture Zone: Results from Teleseismic and Local Aftershock Datasets

Abstract:

We calculated the shear wave splitting intensity (SI) of the Maule, Chile rupture zone (32°S-39°S) to constrain the seismic anisotropy of the region. Our data are from 80 of the temporary seismometers deployed as part of the IMAD (International Maule Aftershock Deployment) geophysical networks to capture the aftershocks of the Mw 8.8 megathrust event in 2010. We implemented the multichannel analysis method of Chevrot (2000) to measure the SI of 64 teleseismic SKS phases in addition to the fast orientations ϕ and splitting delays δt measured with the method of Silver & Chan (1991). To measure the SI of local aftershocks, we modified the method to allow for use of the upgoing S phase from local events in and above the Nazca slab after correcting for the initial event polarization. We compared our results with other measurement methods (Silver and Chan 1991, Wolfe and Silver 1998) that solve for splitting parameters to examine the robustness of the shear wave splitting intensity method, particularly for local datasets. The results we obtained using the splitting intensity method for the teleseismic data show an overall fast direction that is parallel to the absolute plate motion of the Nazca plate that is subducting beneath the South American plate. These results are consistent with the results we calculated using the Wolfe and Silver method. SI deriving from S waves that originate in the Nazca slab or deeper SA lithosphere are likely to reveal patterns of crustal fabric, and hence differ from the SI of the teleseismic shear waves.