Non-Volcanic Tremor along the Oaxaca Segment of the Middle America Subduction Zone

Non-Volcanic Tremor along the Oaxaca Segment of the Middle America Subduction Zone Map of the study region focused along the Oaxaca segment of the Middle American Subduction Zone. Grey triangles (seismic) and white squares (GPS) show the state of the joint network in 2006-2007 that is used to determine the extent of non-volcanic tremor in July 2006 (green line, this study) and the 100 mm slip contour for slow slip events in early 2006 and early 2007 (white ovals) [Correa-Mora et al., 2008; 2009]. Black ovals are approximate rupture zones of large subduction thrust earthquakes over the past 50 years as estimated from locations of rupture aftershocks. Straight line is a profile of magnetotelluric measurements with areas of high conductivity near the subduction inter-face highlighted in yellow [Jödicke et al., 2006]. Dotted lines are isodepths of the subducting plate from analysis of seismicity [Pardo and Suarez, 1995]. Blue circle indicates where an ultra-slow velocity layer has been detected, and dashed circle indicates where it is absent.
<p>The Oaxaca subduction zone is an ideal area for detailed studies of plate boundary deformation as rapid convergent rates, shallow subduction, and short trench-to-coast distances bring the thermally defined seismogenic and transition zones of the plate interface over 100 km inland. Previous analysis of slow slip events in southern Mexico suggests they may represent motion in the transition zone, defining the downdip edge of future megathrust earthquakes. A new deployment consisting of broadband seismometers distributed inland along the Oaxaca segment provide the means to examine whether non-volcanic tremor (NVT) signals can also be used to characterize the boundary between the seismogenic and transition zones. In this study, we established that NVT exists in the Oaxaca region based on waxing and waning of seismic energy on filtered day-long seismograms that were correlated across neighboring stations, and further supported by appropriate relative time moveouts in record sections, and spectrograms with narrow frequency bands. 18 prominent NVT episodes that lasted upwards of a week were identified during the 15 months analyzed (June 2006 to September 2007), recurring as frequently as every 2-3 months in a given region. 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. NVT burst epicenters primarily occur between the 40-50 km contours for depth of the plate interface, 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 more frequent, shorter in duration, and located further inland than GPS-detected slow slip, while the latter 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.
</p><p>Acknowledgements: NSF EAR-510812</p>


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