Toward an integrated understanding of Holocene fault activity in western Puerto Rico: Constraints from high-resolution seismic and sidescan sonar data
Nancy R. Grindlay, Lewis J. Abrams, Luke Del Greco, Paul Mann, 2005. "Toward an integrated understanding of Holocene fault activity in western Puerto Rico: Constraints from high-resolution seismic and sidescan sonar data", Active Tectonics and Seismic Hazards of Puerto Rico, the Virgin Islands, and Offshore Areas, Paul Mann
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It has been postulated that the western boundary of the Puerto Rico–Virgin Islands microplate lies within the Mona Passage and extends onland into southwestern Puerto Rico. This region is seismically active, averaging one event of magnitude 2.0 or larger per day, and over 150 events of magnitude 3.0 or greater occurred during the past five years. Moreover, there have been at least 13 historical events of intensity VI (MM) or greater in the past 500 years. We conducted a high-resolution seismic and sidescan sonar survey of the insular shelf of western and southern Puerto Rico during May 2000 in an effort to identify Holocene faults and to further assess the seismic hazard in the region. We focus on an ∼175 km2 part of the surveyed area offshore of western Puerto Rico, extending from Punta Higuero to Boquerón Bay. This area was targeted as a likely place to image recent faults, because multi-channel seismic profiles offshore western Puerto Rico show numerous WNW-trending normal and strike-slip faults that offset Oligocene-Pliocene age carbonate rocks and underlying Cretaceous basement rocks. Analyses of these data identify three zones of active deformation within the survey area: (1) the Cerro Goden fault zone; (2) the Punta Algarrobo/Mayagüez fault zone that lies offshore the city of Mayagüez; and; (3) the Punta Guanajibo/Punta Arenas fault zone. Two of the offshore fault zones, the Cerro Goden and Punta Algarrobo, show strong correlation with fault zones onland, Cerro Goden and Cordillera, respectively. Many of the mapped faults offshore appear to reactivate older WNW-trending basement structures and show evidence of some component of right-lateral motion that is consistent with geodetic measurements. The offshore deformation zones are also associated with headlands and linear NW-SE magnetization lows (serpentinite dikes?) mapped offshore. Elongate outcrops of serpentinite in western Puerto Rico are colinear with the fault zones we have mapped offshore, suggesting that either the presence of serpentinite has localized fault activity or that fault activity has remobilized serpentinite. This offshore study improves assessments of the seismic hazard in Puerto Rico by identifying targets for onshore paleoseismic studies and by better defining the total length of offshore Holocene faults.