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New Seismic Hazard Maps for Puerto Rico and the U.S. Virgin Islands

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New Seismic Hazard Maps for Puerto Rico and the U.S. Virgin Islands New Seismic Hazard Maps for Puerto Rico and the U.S. Virgin Islands a) a) a) Charles Mueller, Arthur Frankel, M.EERI, Mark Petersen, a) M.EERI, and Edgar Leyendecker The probabilistic methodology developed by the U.S. Geological Survey is applied to a new seismic hazard assessment for Puerto Rico and the U.S. Virgin Islands. Modeled seismic sources include gridded historical seismicity, subduction-interface and strike-slip faults with known slip rates, and two broad zones of crustal extension with seismicity rates constrained by GPS geodesy. We use attenuation relations from western North American and worldwide data, as well as a Caribbean-specific relation. Results are presented as maps of peak ground acceleration and 0.2- and 1.0-second spectral response acceleration for 2% and 10% probabilities of exceedance in 50 years (return periods of about 2,500 and 500 years, respectively). This paper describes the hazard model and maps that were balloted by the Building Seismic Safety Council and recommended for the 2003 NEHRP Provisions and the 2006 International Building Code. ͓DOI: 10.1193/1.3277667͔ INTRODUCTION We present results of a new probabilistic seismic hazard assessment for Puerto Rico and the U.S. Virgin Islands (PRVI). The study area (Figure 1) is located along the bound- ary between the northeastern Caribbean Sea and the Atlantic Ocean, at the intersection of the Greater and Lesser Antilles island chains. These islands demarcate the active boundary between the North America and Caribbean tectonic plates, with the North America plate moving west-southwestward relative to the Caribbean plate at a rate de- termined from GPS geodesy of 18–20 mm/yr (Jansma et al. 2000, Mann 2005). West of PRVI, the two plates move along typical transform structures (the east-west-striking, left- lateral Oriente and Enriquillo-Plantain Garden faults and related structures). Southeast of PRVI, the North America plate subducts orthogonally westward beneath the Caribbean plate, forming a typical volcanic island arc (the north-south-trending Lesser Antilles). Plate inter- action near PRVI, in contrast, occurs in a 250-km-wide, east-west-trending zone of complex transpressional deformation, delimited by the Puerto Rico Trench in the north and the Muer- tos Trough in the south. Puerto Rico and the Virgin Islands lie on a shallow submarine bank within this complex deformational zone. The region is home to approximately four million U.S. citizens and has a long his- tory of destructive earthquakes. Historical records show that major earthquakes have struck PRVI several times during the past 500 years, although the locations and sizes of events that have occurred more than a few decades ago are poorly known. Major earthquakes a) United States Geological Survey, P.O. Box 25046 MS 966, Denver, CO 80225 169 Earthquake Spectra, Volume 26, No. 1, pages 169–185, February 2010; © 2010, Earthquake Engineering Research Institute 170 MUELLER ET AL. 22 Bahama Bank 19.4 mm/yr North American Plate 20 PT Main Rdg SFZ BunceFZ H SFZ BowinFZ MC EPGFZ MP PR 18 VI AP MT Caribbean Plate 16 deformation front 14 strike-slip fault normal fault 12 -74 -72 -70 -68 -66 -64 -62 -60 Figure 1. Map of North American—Caribbean plate boundary region. Inset box shows area of seismic hazard analysis. Arrow shows motion of North American plate relative to Caribbean plate from Jansma et al. (2000). Structural elements generalized from Grindlay et al. (1997), Dolan et al. (1998), LaForge and McCann (2005), and Grindlay et al. (2005). Abbreviations: H=Hispaniola, PR=Puerto Rico, VI=Virgin Islands, PT=Puerto Rico Trench, MT=Muertos Trough, MP=Mona Passage, AP=Anegada Passage, SFZ=Septentrional fault zone, EPGFZ =Enriquillo-Plantain Garden fault zone, and MC=Mona Canyon. have damaged Puerto Rico in 1615, 1670, 1787 (estimated moment magnitude, Mw, about 8.0–8.2, thought to have ruptured the plate interface south of the Puerto Rico Trench, off- shore northern Puerto Rico), 1844, 1846, 1867 (estimated Mw about 7.3, Anegada Passage, offshore southeastern Puerto Rico), 1875, 1906, 1918 (Mw 7.2–7.4, Mona Passage, offshore northwestern Puerto Rico), 1943 (Mw 7.6–7.8, plate interface, offshore northwestern Puerto Rico), and 1946 (Mw 7.8–8.0, plate interface, offshore northeastern Hispaniola). The 1867 and 1918 earthquakes were accompanied by destructive tsunamis. McCann (1985), Pacheco and Sykes (1992), Dolan and Wald (1998), McCann (2003a), Doser et al. (2005), and LaForge and McCann (2005) provide more information about PRVI historical earthquakes. Seismicity near PRVI is primarily related to 1) highly oblique subduction of the North America plate beneath the terranes of the plate boundary zone along the main plate interface south of the Puerto Rico Trench and 2) the interactions of several prob- able microplates within the complex boundary zone. Geodesy and seismicity data sug- gest the existence of a Puerto Rico-northern Virgin Islands microplate that is relatively rigid and seismically quiescent internally (Masson and Scanlon 1991, Jansma et al. NEW SEISMIC HAZARD MAPS FOR PUERTO RICO AND THE U.S.VIRGIN ISLANDS 171 2000, Jansma and Mattioli 2005). Most of the major seismogenic structures are located offshore; thus, estimates of activity rates can be highly uncertain, often based on indirect evidence such as seismicity patterns and focal mechanisms, bathymetry and shallow sea- floor seismic imagery, regional geodesy, kinematic reasoning, and tectonic analogs. Seismic sources related to deformation along the main plate boundary include mega- thrust faulting along the plate interface, southward-deepening intraslab faulting within the subducting North America plate, and strike-slip faulting on several structures that strike subparallel to the Puerto Rico Trench north and northwest of Puerto Rico (Figure 1). These include the Septentrional fault, a plate-boundary structure in central Hispani- ola that extends eastward across the northern Mona Passage toward Puerto Rico, as well as the Bunce fault zone (formerly North Puerto Rico Slope fault zone) and the Bowin fault zone (formerly South Puerto Rico Slope fault zone) and related structures. Other sources include two broad zones of roughly east-west tectonic extension, one west of Puerto Rico roughly coincident with the Mona Passage, and one southeast of Puerto Rico roughly coincident with the Anegada Passage. Extension within these zones is thought to be related to differences in rates of eastward motion (relative to North America) of crustal blocks south of the main plate boundary (e.g., McCann et al. 1987, Jansma et al. 2000); the Caribbean plate moves eastward relatively unrestricted, while blocks within the boundary zone are restricted by relatively high-standing tectonic fea- tures like the Bahama Bank and Main Ridge (Figure 1). The Great Northern and Great Southern Puerto Rico faults, major left-lateral strike-slip systems active on Puerto Rico from the early Cretaceous to the early Miocene are now considered largely quiescent, although they seem to be associated with very small earthquakes, and may represent in- herited zones of weakness (McCann 1985). Prentice et al. (2000) determine a recurrence rate for one onshore fault, the South Lajas fault in southwestern Puerto Rico, that they consider to be currently active. Several other candidate faults have been identified in western Puerto Rico, but not yet fully evaluated. Unspecified crustal faulting (and the intraslab faulting) is modeled with gridded historical seismicity (below). METHODOLOGY AND HAZARD MODEL We apply the probabilistic seismic hazard methodology developed by the U.S. Geo- logical Survey (USGS) as described by Frankel et al. (1996, 2000, 2002) and Petersen et al. (2008), and present maps of probabilistic ground motions: peak ground acceleration (PGA), and 0.2-second and 1.0-second spectral response acceleration (5%-damped) for 2% and 10% probabilities of exceedance in 50 years, corresponding to return times of about 2,500 and 500 years, respectively. Like others in the USGS seismic hazard series, these maps are intended to inform building codes and other structural-design guidelines, as well as earthquake-mitigation plans and emergency-response plans. Design maps derived from the hazard maps (Leyendecker et al. 2000) have been approved by vote of the BSSC for the NE- HRP Recommended Provisions for Seismic Regulations for New Buildings and Other Struc- tures, 2003 Edition (BSSC 2004), and accepted for the ASCE 7 guidelines (ASCE 2005) and the 2006 updates of the International Building Code and International Residential Code (ICC 2006a and 2006b). 172 MUELLER ET AL. 20 Hispaniola Seg Puerto Rico Seg BFZ SFZ H C ST V SJ SLF PR 18 Mona Extnl Zn Anegada SC Extnl Zn HISTORICAL SEISMICITY (M>=4.5,1963-2001) 0 =< depth (km) < 50 50=<d<80 80 =< d < 120 120 =< d 16 -70 -68 -66 -64 -62 Figure 2. Map showing modeled seismic hazard sources. Diagonal pattern=subduction seg- ments (sparse=Puerto Rico seg., dense=Hispaniola seg.). Vertical pattern=Mona and Anegada extensional zones. Abbreviations: SFZ=Septentrional fault zone, BFZ=Bowin fault zone, SLF =South Lajas fault, H=Hispaniola, PR=Puerto Rico, V=Vieques, C=Culebra, SC=St. Croix, ST=St. Thomas, SJ=St. John. The USGS methodology is based on 1) historical seismicity generalized using expo- nential magnitude distributions with regionally determined b values and 2) specific fault and subduction sources with geologically or geophysically constrained slip-rate or re- currence information. Where there is reason to suspect that the seismicity
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