EPISODIC TREMOR AND SLIP ALONG THE OAXACA SEGMENT OF THE MIDDLE AMERICA SUBDUCTION ZONE AND COMPARISONS TO CASCADIA AND NANKAI

Convergent plate boundaries generate great earthquakes when tectonic stresses accumulate on the plate interface. Down-dip from the seismogenic zone where increasing temperatures, pressure and dehydration affect frictional behavior, episodic tremor and slip (ETS) has been shown to occur in the transitional zone. The Oaxaca subduction zone is an ideal area for detailed ETS studies as rapid convergent rates, shallow subduction, and short trench-to-coast distances bring the seismogenic and transition zones of the plate interface up to 250 km inland. Previously analyzed slow slip events occur over large areas in southern Mexico, and may even extend up-dip into the seismogenic zone, potentially playing a role in future megathrust earthquakes. A seismic deployment of 7 broadband seismometers dispersed along the Oaxaca segment provides the means to examine non-volcanic tremor (NVT) signals in detail. In this study, we analyze NVT envelope waveforms with a semi-automated process for identifying prominent energy bursts, and analyst-refined relative arrival times are inverted for source locations. Prominent NVT episodes that lasted upwards of a week were prevalent during the 15 months analyzed for this study, recurring every 2-3 months on average in a given region, but at different times in the eastern and western ends of our network. NVT burst locations primarily occur between the 40-50 km plate interface contours, except in eastern Oaxaca where they shift towards the 30 km contour as the slab steepens. NVT hypocenters correlate well with a high conductivity zone that is interpreted to be due to slab fluids. NVT is both more frequent, shorter in duration, and located further inland than GPS-detected slow slip, which in turn is associated with a zone of ultra-slow velocity interpreted to represent high pore fluid pressure. This zone of slow slip corresponds to approximately 350–450°C, with megathrust earthquakes, microseismicity, and strong long-term coupling occurring immediately updip from it. This leaves NVT primarily in a region further inland from the thermally defined transition zone, suggesting that transition from locking to free slip may occur in more than one phase.