Attenuation-based tomography of the 2015 Illapel, Chile earthquake rupture region

Abstract:

Along-strike limitation of megathrust subduction earthquake rupture areas indicates that subduction zones are segmented, inspiring interest in understanding the structures at which ruptures nucleate and also terminate. We examined ray paths of 819 aftershocks of the 2015 M_w 8.3 Illapel, Chile earthquake as recorded by a network of 72 broadband seismic stations from September 2015 to October 2016. We measured the attenuation (by shear wave quality factor, Q_s) of these ray paths by employing a body wave spectral ratio method to determine Q_s for each path. We performed spectral division of S-wave to P-wave amplitudes on the basis of corresponding wavelength instead of corresponding frequency in order to hold constant the scale of structure being sampled by both waves. Then, multipaths for each ray path were identified and isolated, allowing us to estimate Q_s for the minimum-time ray path, as well as for multipathed waves. We used the quality factors of each ray path and their trajectories to construct an attenuation-based tomography of the Illapel region. Since Q_s is sensitive to temperature differences in the subsurface, attenuation-based tomography provides an image of the material rheology of the study volume, and allows for location and interpretation of structure which could act to limit or initiate earthquake rupture.