HR: 10:35h
AN: S52A-02    [Abstracts]
TI: Seismic sequences in the Sombrero Seismic Zone
AU: * Pulliam, J
EM: jay@ig.utexas.edu
AF: UTIG, Institute for Geophysics, J.J. Pickle Research Campus, Bldg. 196; 10100 Burnet Road (R2200), Auxtin, TX 78758, United States
AU: Huerfano, V A
AF: Puerto Rico Seismic Network, P.O. Box 9017, Mayaguez, PR 00681, United States
AU: ten Brink, U
EM: utenbrink@usgs.gov
AF: USGS - Woods Hole, Woods Hole Science Center Eastern Region, U.S.G.S., United States
AU: von Hillebrandt, C
EM: christa@midas.uprm.edu
AF: Puerto Rico Seismic Network, P.O. Box 9017, Mayaguez, PR 00681, United States
AB: The northeastern Caribbean, in the vicinity of Puerto Rico and the Virgin Islands, has a long and well-documented history of devastating earthquakes and tsunamis, including major events in 1670, 1787, 1867, 1916, 1918, and 1943. Recently, seismicity has been concentrated to the north and west of the British Virgin Islands, in the region referred to as the Sombrero Seismic Zone by the Puerto Rico Seismic Network (PRSN). In the combined seismicity catalog maintained by the PRSN, several hundred small to moderate magnitude events can be found in this region prior to 2006. However, beginning in 2006 and continuing to the present, the rate of seismicity in the Sombrero suddenly increased, and a new locus of activity developed to the east of the previous location. Accurate estimates of seismic hazard, and the tsunamigenic potential of seismic events, depend on an accurate and comprehensive understanding of how strain is being accommodated in this corner region. Are faults locked and accumulating strain for release in a major event? Or is strain being released via slip over a diffuse system of faults? A careful analysis of seismicity patterns in the Sombrero region has the potential to both identify faults and modes of failure, provided the aggregation scheme is tuned to properly identify related events. To this end, we experimented with a scheme to identify seismic sequences based on physical and temporal proximity, under the assumptions that (a) events occur on related fault systems as stress is refocused by immediately previous events and (b) such 'stress waves' die out with time, so that two events that occur on the same system within a relatively short time window can be said to have a similar 'trigger' in ways that two nearby events that occurred years apart cannot. Patterns that emerge from the identification, temporal sequence, and refined locations of such sequences of events carry information about stress accommodation that is obscured by large clouds of unrelated events in plots of the general catalog. One characteristic of these sequences is that their magnitudes tend to be consistently small (1.0 - 3.5 mb, with only five events greater than 3.5 mb) and they typically do not include an event that could confidently be identified as a "main" shock. Nevertheless, the numbers of events, temporal and geographic distribution of shocks in each sequence suggests that these are aftershock sequences, yet none includes an event that could confidently be identified as a "main" shock. This observation suggests several questions. Do these sequences truly represent aftershocks? If so, where are the main events? Are they perhaps related to "silent" or "slow" earthquakes in the subduction zone? If so, could such slow earthquakes be related to the dropping away of the subducting slab beneath the deep Puerto Rico Trench? Or do the sequences indicate tearing of the NA lithosphere of the North America plate as it subducts beneath the Caribbean plate?
DE: 3070 Submarine landslides
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7294 Seismic instruments and networks (0935, 3025)
DE: 8107 Continental neotectonics (8002)
DE: 8123 Dynamics: seismotectonics
SC: Seismology [S]
MN: 2007 Joint Assembly