Strain accumulation along offshore faults of Puerto Rico and implications for tsunami hazard PUERTO RICO SEA GRANT FINAL REPORT Pamela Jansma, PhD Professor Department of Geosciences University of Arkansas Fayetteville, AR 72701
Overview and introduction The island of Puerto Rico is part of the deformation zone associated with east-west relative motion between the Caribbean and North American plates and thus has experienced destructive earthquakes throughout its history. Because most of the seismicity occurs offshore, the potential for tsunamogenic events is high. Indeed, the most recent large event in 1918 that originated in the Mona Rift offshore northwestern Puerto Rico was accompanied by a damaging tsunami of 4 to 6 m along the west side of the island (Mercado and McCann, 1998). Whether other offshore regions of Puerto Rico have the potential for tsunamogenic earthquakes is unclear. The number of active faults, their mechanical behavior, and their likelihood to accommodate dipslip motion (a necessary condition for tsunami generation) was unknown. Two end-members of mechanical behavior of faults are possible: 1) stable sliding, where opposite sides of the fault creep past each other and energy release is slow, producing little seismicity, or 2) stick-slip, where opposite sides of the fault are locked until a critical threshold is reached when the rocks can no longer absorb the accumulating slip, resulting in sudden motion of the two sides, rupture along the fault, and rapid energy release in the form of a devastating earthquake. Seismic risk is far greater when stick-slip behavior occurs. Determining the mechanical characteristics of fault zones traditionally has focused on calculating if the energy released from historical seismicity is enough to offset the amount of motion documented along active faults by geologic or geodetic methods. Problems include, but are not limited to, the large errors associated with dating offsets measured along faults and the potential incorrect location of historic earthquakes. The advent of the Global Positioning Sytem (GPS) made possible the direct measurement of relative plate motions and slip along faults, thereby, reducing much of the uncertainty associated with the evaluation of seismic risk. The primary objective of this two-year proposal, therefore, was to collect, process, and model GPS geodetic data that directly impact displacement rates across seismogenic structures in the northern offshore region of Puerto Rico. Although we cannot isolate the amount of displacement on offshore faults directly from the GPS geodetic data, we can test whether significant elastic strain accumulation occurs on any offshore structures.