Nonvolcanic tremor along the Oaxaca segment of the Middle America subduction zone

Abstract

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 that 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 nonvolcanic 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 were further supported by appropriate relative time moveouts in record sections and spectrograms with narrow frequency bands. Eighteen 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 semiautomated 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 toward 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°C–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.

Along-strike variations in temperature and tectonic tremor activity along the Hikurangi subduction zone, New Zealand

“…They tend to occur where a young oceanic plate (<50 Ma) is subducting, such as in the Nankai, Alaska, Cascadia, Mexico, Costa Rica, and south Chile subduction zones (e.g., Obara 2002; Rogers and Dragert 2003;Payero et al 2008;Brown et al 2009;Peterson and Christensen 2009;Brudzinski et al 2010;Ide, 2012). On the other hand, tectonic tremors have never been recorded where the old plate is subducting, except for the Hikurangi subduction zone located along the east coast of the North Island of New Zealand (Fry et al 2011;Kim et al 2011;Ide 2012).…”

Low Angle Contact Between the Oaxaca and Juárez Terranes Deduced From Magnetotelluric Data

“…We conclude that the slow slip events (SSE) and the ultra-slow velocity layer (USL) interpreted by BRUDZINSKI et al (2010) occur at the electrical interface between the subducting slab and the resistive lower crust. A highly hydrated oceanic slab under the impermeable and confined lower continental crust would reduce drastically frictional forces promoting aseismic subduction.…”

“…A third zone of aligned seismicity correlating with the horizontal conductor A may be also of regional dynamic significance. Nonvolcanic tremors (NVT) registered by BRUDZINSKI et al (2010) lie within and below the resistive lower Juárez continental crust (green squares) and are interpreted as being associated to dewatering related seismicity. Slow slip events (SSE) and an ultra-slow velocity layer (USL) interpreted by BRUDZINSKI et al -ENRIQUEZ et al 2013).…”

“…Projecting the location of two slow slip episodes (SSE) registered by BRUDZINSKI et al (2010) onto the magnetotelluric model profile (Fig. 6) it is observed that SSEs coincide with a sharp conductivity interface within an approximated depth range of 25-40 km.…”

“…6). Non-volcanic tremors registered by BRUDZINSKI et al (2010) have been proposed to be due to the dewatering of the oceanic crust at depths around 60 km (e.g., MANEA and MANEA 2010). The correlation of seismicity and anomalous conductivity in the temperature range of 600-700°C show that dewatering processes in the Oaxaca region start at a depth of 75 km, and precludes NVTs that are trigger above depths of *40 km as a consequence of fluid migration.…”