ALBERT

Description: In this study, we quantify the seismic and tsunami hazard in the Lesser Antilles subduction zone, focusing on the plate interface offshore of Guadeloupe. We compare potential strain accumulated via GPS-derived plate motions to strain release due to earthquakes that have occurred over the past 110 yr, and compute the resulting moment deficit. Our results suggest that enough strain is currently stored in the seismogenic zone of the Lesser Antilles subduction arc in the region of Guadeloupe to cause a large and damaging earthquake of magnitude M w ~ 8.2 ± 0.4. We model several scenario earthquakes over this magnitude range, using a variety of earthquake magnitudes and rupture areas, and utilizing the USGS ShakeMap and PAGER software packages. Strong ground shaking during the earthquake will likely cause loss of life and damage estimated to be in the range of several tens to several hundreds of fatalities and hundreds of millions to potentially billions of U.S. dollars of damage. In addition, such an event could produce a significant tsunami. Modelled tsunamis resulting from these scenario earthquakes predict meter-scale wave amplitudes even for events at the lower end of our magnitude range ( M 7.8), and heights of over 3 m in several locations with our favoured scenario ( M 8.0, partially locked interface from 15–45 km depth). In all scenarios, only short lead-times (on the order of tens of minutes) would be possible in the Caribbean before the arrival of damaging waves.

Description: The M (sub w) 5.8 earthquake of 23 August 2011 (17:51:04 UTC) (moment, M (sub 0) 5.7X10 (super 17) N.m) occurred near Mineral, Virginia, within the central Virginia seismic zone and was felt by more people than any other earthquake in United States history. The U.S. Geological Survey (USGS) received 148,638 felt reports from 31 states and 4 Canadian provinces. The USGS PAGER system estimates as many as 120,000 people were exposed to shaking intensity levels of IV and greater, with approximately 10,000 exposed to shaking as high as intensity VIII. Both regional and teleseismic moment tensor solutions characterize the earthquake as a northeast-striking reverse fault that nucleated at a depth of approximately 7+ or -2 km. The distribution of reported macroseismic intensities is roughly ten times the area of a similarly sized earthquake in the western United States (Horton and Williams, 2012). Near-source and far-field damage reports, which extend as far away as Washington, D.C., (135 km away) and Baltimore, Maryland, (200 km away) are consistent with an earthquake of this size and depth in the eastern United States (EUS). Within the first few days following the earthquake, several government and academic institutions installed 36 portable seismograph stations in the epicentral region, making this among the best-recorded aftershock sequences in the EUS. Based on modeling of these data, we provide a detailed description of the source parameters of the mainshock and analysis of the subsequent aftershock sequence for defining the fault geometry, area of rupture, and observations of the aftershock sequence magnitude-frequency and temporal distribution. The observed slope of the magnitude-frequency curve or b-value for the aftershock sequence is consistent with previous EUS studies (b=0.75), suggesting that most of the accumulated strain was released by the mainshock. The aftershocks define a rupture that extends between approximately 2-8 km in depth and 8-10 km along the strike of the fault plane. Best-fit modeling of the geometry of the aftershock sequence defines a rupture plane that strikes N36 degrees E and dips to the east-southeast at 49.5 degrees . Moment tensor solutions of the mainshock and larger aftershocks are consistent with the distribution of aftershock locations, both indicating reverse slip along a northeast-southwest striking southeast-dipping fault plane.