Tomographic Model and JHD Microearthquake Locations Confirm Existence of Muertos Trough Megathrust at Shallow Depth Beneath Southern Puerto Rico
Abstract
A one-dimensional P-wave velocity model and station delays were developed for the Puerto Rico region using the program Velest. Five different starting models were used along with 5472 P-wave arrivals at 42 stations. The best P-wave velocity model is a simple 2-layer crust over a half space. The topmost layer has Vp 6.45 km/sec to 17 km depth and overlies a layer with Vp 7.1 km/sec. A mantle half space Vp 8.1 km/sec underlies the 31 km thick crust. More than 18,000 locally recorded events were jointly located using this new 1-D model and station delays. The JHD results clearly show that the bulk of the shallow seismicity in Southern and Southwestern Puerto Rico is not related to related just to shallow or surficial crustal features as had been previously presumed, but rather to a downgoing seismic zone of the subducted lithosphere of the Caribbean Plate. Microearthquakes along southern Puerto Rico define a shallowly dipping seismic zone starting about 10 km depth, 40 km landward of the axis of the Muertos Trough, and extending northward to about 30 km depth. Shallow crustal activity, above what we interpret to be the Muertos megathrust, also deepens to the north. Some 20 km beneath this zone of shallow seismicity is another northerly dipping zone of events that extends to at least 75 km depth. We interpret these events as earthquakes internal to the subducted Caribbean Plate. Simul2000 was used with 1500 events (nearly 9,000 P-wave arrivals) recorded at 21 stations to estimate a 3-D P-wave velocity model of Puerto Rico composed of cells 10 km on a side. A northerly dipping velocity anomaly overlaps and underlies the similarly dipping seismic zone described above. This velocity anomaly is strongest (3% fast) at about 30 km depth and slowly decreases to about 0.5% at 70 km depth. Both the velocity anomaly and seismicity are clearest in the western and central portions of southern Puerto Rico and weakest or absent in the southeast. These data suggest that at least 70 km of Caribbean lithosphere lies beneath western and central portions of Puerto Rico. Seismicity defines the position of the megathrust and crustal faulting associated with it. Both are found at shallow depths, about 10-20 km along the south coast, and slowly deepen to the north to about 20-30 km depth. The Muertos megathrust is believed capable of producing events of about magnitude 8 every few thousand years, so the presence of the Caribbean slab at such shallow depths beneath the island represents a significant earthquake hazard to the Island of Puerto Rico.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.S43D..02M
- Keywords:
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- 7212 Earthquake ground motions and engineering seismology;
- 7230 Seismicity and tectonics (1207;
- 1217;
- 1240;
- 1242);
- 7240 Subduction zones (1207;
- 1219;
- 1240);
- 7270 Tomography (6982;
- 8180)