EARTHQUAKE SUBMARINE GEOLOGY AND ESTIMATES OF FAULT SLIP RATES IN PUERTO RICO AND THE U.S. VIRGIN ISLANDS

A major hindrance to the accurate estimation of seismic hazard in the U.S. Caribbean is the lack of slip rates of active faults. As many inland faults extend offshore, earthquake submarine geology appears to be a useful tool for determining fault slip rates. We have developed a novel method of interpreting bathymetric data to estimate average/maximum slip rate information for an active fault. The digital dataset investigated is a series of detailed hydrographic surveys made in the 20th century. Identification of structures is fairly straightforward using a bathymetric grid.

Submarine terrace morphology can be found by evaluating 1st and 2nd derivatives of shelf bathymetry. As terraces are proxies for local sea-level at the time of their formation, they should be observed today as contour parallel structures if no deformation has disturbed them. We calculated the slope and slope aspect (slope direction) at each of the grid points to permit a more robust analysis of shelf morphology.

Preliminary observations for south central Puerto Rico include: 1- systematic deepening of the insular platform clearly related to distance to the investigator fault; 2-The smoothness of the shelf is interrupted by contour parallel scarps; 3-There is a SW trending high (Isla Caja de Muertos platform) near western end of shelf. The relative shallowness and relief on this feature appears caused by hanging wall uplift along the Ponce Basin fault; 4-Several ESE trending (north sloping) features are found in the slope aspect map for the shelf. Three of these are sub parallel to each other and appear as offshore continuations of strands of the Great Southern Puerto Rico Fault Zone (GSPRFZ); 5-There is an abrupt 15 m increase in shelf depth, deepening to the SW, where the eastern shelf is crossed by a linear scarp. The scarp is along strike of the Rio Jueyes segment of the GSPRFZ. If we assume that the south insular shelf was last beveled during a stand of the sea at about 105Ka or 80Ka, then we can estimate maximum vertical rate of slip on the offshore extension of the Rio Jueyes Fault. If the features observed on the shelf and along its edge were formed during the 80Ka sea level event, then the rate is 0.19mm/yr, or if during the 105Ka event, then the rate is 0.14mm/yr.

Similar observations are made in the Virgin Islands and western Puerto Rico.