Tectonic studies of the Caribbean: Pure GPS Euler vectors to test for rigidity and for the existence of a Northern Lesser Antilles Forearc block; constraints for tsunami risk from reassessment of the April 1, 1946 Alaska-Aleutians and August 4, 1946 Hispaniola events.

摘要

New Global Positioning System (GPS) data from nine sites within stable portions of the Caribbean (CA) plate have been used to constrain its motion in the International GPS Service reference frame (IGSb00) and with respect to North (NA) and South (SA) American plates. A comparison of GPS velocities between eastern and western Caribbean sites shows that there is no significant internal deformation occurring within the plate (0.9 mm/yr). The CA-NA Euler vector was used to predict azimuths at the northern Lesser Antilles arc, which slip vectors still misfit (∼ 5°--25°). This confirms slip partitioning in the forearc, where a forearc sliver, for the first time introduced here as the Northern Lesser Antilles Forearc block (NLAF) moves counter-clockwise with respect to Caribbean crust. Seismicity of the plate in conjunction with historical records of tsunamis in the Caribbean serves as evidence the region is prone to tsunamis. The Hispaniola August 4, 1946 earthquake generated a tsunami that 5 resulted in great loss of lives. A quantitative comparison of this event with that of the Aleutians April 1, 1946 show the tsunami originated from a submarine landslide and not as a "tsunami earthquake". The April 1, 1946 earthquake stands out as an exceptional case of slow rupture velocity. With a re-computed fault area of 181 x 115 km from aftershock relocations, and seismic moment of 5 x 1027 dyn-cm, the event features the smallest radiated energy to moment ratio (theta = -7.03) ever recorded to date. Tsunami earthquakes, such as the Hispaniola 1946, may have already occured in the past and could occur again in the Caribbean. Hence, potential rupture segments have been identified and used as loci of possible events in an effort to help assess the tsunami risk in the Caribbean and Atlantic region.