Geodetic Matched Filter Search of Low Amplitude Slow Slip Events on the Mexican Subduction Zone

Abstract:

The description of aseismic processes sheds a new light on the fault dynamics and provokes new questions, particularly about the interplay between seismic and aseismic processes. Exploring new slow slip detection methods at the GPS noise level is a key challenge to complete the catalog of aseismic events. The Mexican subduction zone exhibits a wide range of rupture phenomena, from classic earthquakes, low frequency earthquakes (LFEs), and tremors to the largest magnitudes slow slip events (SSEs) in the world. SSEs have been observed both in Guerrero and Oaxaca regions, with larger magnitudes and recurrence times in Guerrero than in Oaxaca. A singular feature of this subduction zone is that in both regions, SSEs are located up-dip compared to tremor locations. We take advantage of the Mexican continuous GPS network (40 stations) within these two regions to attempt to detect low amplitude slow slip events. We first build synthetic templates of small amplitude events that we cross-correlate with the GPS time-series. We then stack all correlation functions across the GPS network. Positive detections on the stacked correlation function are strengthened by the coincidence of tremors and LFEs bursts. Spatially, positive detections correspond to clustered source template locations. We group all the detections of a given location in what we call a family and then we stack all the events, aligned at detection times, in order to reduce the signal to noise ratio. We detect several families that present a motion compatible with previously observed slow slip events in both the Guerrero and Oaxaca regions. While some families include events within previously observed large SSEs that seem to highlight small asperities located within large SSEs contours, other families are situated outside of these contours, some of them coincident with tremor and LFEs locations.