Historical perspective on seismic hazard to Hispaniola and the northeast Caribbean region

This paper is not subject to U.S. copyright. The definitive version was published in Journal of Geophysical Research 116 (2011): B12318, doi:10.1029/2011JB008497.We evaluate the long-term seismic activity of the North-American/Caribbean plate boundary from 500 years of historical earthquake damage reports. The 2010 Haiti earthquakes and other earthquakes were used to derive regional attenuation relationships between earthquake intensity, magnitude, and distance from the reported damage to the epicenter, for Hispaniola and for Puerto Rico and the Virgin Islands. The attenuation relationship for Hispaniola earthquakes and northern Lesser Antilles earthquakes is similar to that for California earthquakes, indicating a relatively rapid attenuation of damage intensity with distance. Intensities in Puerto Rico and the Virgin Islands decrease less rapidly with distance. We use the intensity-magnitude relationships to systematically search for the location and intensity magnitude MI which best fit all the reported damage for historical earthquakes. Many events occurred in the 20th-century along the plate-boundary segment from central Hispaniola to the NW tip of Puerto Rico, but earlier events from this segment were not identified. The remaining plate boundary to the east to Guadeloupe is probably not associated with M > 8 historical subduction-zone earthquakes. The May 2, 1787 earthquake, previously assigned an M 8–8.25, is probably only MI 6.9 and could be located north, west or SW of Puerto Rico. An MI 6.9 earthquake on July 11, 1785 was probably located north or east of the Virgin Islands. We located MI < 8 historical earthquakes on April 5, 1690, February 8, 1843, and October 8, 1974 in the northern Lesser Antilles within the arc. We speculate that the December 2, 1562 (MI 7.7) and May 7, 1842 (MI 7.6) earthquakes ruptured the Septentrional Fault in northern Hispaniola. If so, the recurrence interval on the central Septentrional Fault is ∼300 years, and only 170 years has elapsed since the last event. The recurrence interval of large earthquakes along the Hispaniola subduction segment is likely longer than the historical record. Intra-arc M ≥ 7.0 earthquakes may occur every 75–100 years in the 410-km-long segment between the Virgin Islands and Guadeloupe

Abrupt along-strike change in tectonic style: San Andreas fault zone, San Francisco Peninsula

Seismicity and high-resolution aeromagnetic data are used to define an abrupt change from compressional to extensional tectonism within a 10- to 15-km-wide zone along the San Andreas fault on the San Francisco Peninsula and offshore from the Golden Gate. This 100-km-long section of the San Andreas fault includes the hypocenter of the Mw= 7.8 1906 San Francisco earthquake as well as the highest level of persistent microseismicity along that ~470-km-long rupture. We define two distinct zones of deformation along this stretch of the fault using well-constrained relocations of all post- 1969 earthquakes based a joint one-dimensional velocity/hypocenter inversion and a redetermination of focal mechanisms. The southern zone is characterized by thrust- and reverse-faulting focal mechanisms with NE trending P axes that indicate fault-normal compression in 7- to 10-km-wide zones of deformation on both sides of the San Andreas fault. A 1- to 2-km-wide vertical zone beneath the surface trace of the San Andreas is characterized by its almost complete lack of seismicity. The compressional deformation is consistent with the young, high topography of the Santa Cruz Mountains/Coast Ranges as the San Andreas fault makes a broad restraining left bend (~10 °) through the southernmost peninsula. A zone of seismic quiescence ~15 km long separates this compressional zone to the south from a zone of combined normal-faulting and strike-slipfaulting focal mechanisms (including a ML= 5.3 earthquake in 1957) on the northernmost peninsula and offshore on the Golden Gate platform. Both linear pseudogravity gradients, calculated from the aeromagnetic data, and seismic reflection data indicate that the San Andreas fault makes an abrupt ~3-km right step less than 5 km offshore in this northern zone. A similar right-stepping (dilatational) geometry is also observed for the subparallel San Gregorio fault offshore. Persistent seismicity and extensional tectonism occur within the San Andreas right stepover region and at least 15 km along-strike both to the SE and NW. The 1906 San Francisco earthquake may have nucleated within the San Andreas right stepover, which may help explain the bilateral nature of rupture of this event. Our analysis suggests two seismic hazards for the San Francisco Peninsula in addition to the hazard associated with a M = 7 to 8 strike-slip earthquake along the San Andreas fault: the potential for a M ≈ 6 normal-faulting earthquake just 5-8 km west of San Francisco and a M ≈ 6+ thrust faulting event in the southern peninsula

Three-dimensional upper crustal velocity structure beneath San Francisco Peninsula, California

This paper presents new seismic data from, and crustal models of the San Francisco Peninsula. In much of central California the San Andreas fault juxtaposes the Cretaceous granitic Salinian terrane on its west and the Late Mesozoic/Early Tertiary Franciscan Complex on its east. On San Francisco Peninsula, however, the present-day San Andreas fault is completely within a Franciscan terrane, and the Pilarcitos fault, located southwest of the San Andreas, marks the Salinian-Franciscan boundary. This circumstance has evoked two different explanations: either the Pilarcitos is a thrust fault that has pushed Franciscan rocks over Salinian rocks or the Pilarcitos is a transform fault that has accommodated significant right-lateral slip. In an effort to better resolve the subsurface structure of the peninsula faults, we established a temporary network of 31 seismographs arrayed across the San Andreas fault and the subparallel Pilarcitos fault at ~1-2 km spacings. These instruments were deployed during the first 6 months of 1995 and recorded local earthquakes, air gun sources set off in San Francisco Bay, and explosive sources. Travel times from these sources were used to augment earthquake arrival times recorded by the Northern California Seismic Network and were inverted for three-dimensional velocity structure. Results show lateral velocity changes at depth (~0.5-7 km) that correlate with downward vertical projections of the surface traces of the San Andreas and Pilarcitos faults. We thus interpret the faults as high-angle to vertical features (constrained to a 70 °-110 ° dip range). From this we concludet hat the Pilarcitos fault is probably an important strike-slip fault that accommodated much of the rightlateral plate boundary strain on the peninsula prior to the initiation of the modern-day San Andreas fault in this region sometime after about 3.0 m.y. ago