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New Seafloor Map of the Puerto Rico Trench Helps Assess Earthquake

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New Seafloor Map of the Puerto Rico Trench Helps Assess Earthquake Eos, Vol. 85, No. 37,14 September 2004 VOLUME 85 NUMBER 37 14 SEPTEMBER 2004 EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAGES 349-360 Deformation of the Subducting Plate New Seafloor Map of the The trench wall north of the eastern part of the trench is cut by a series of normal faults Puerto Rico Trench Helps Assess with vertical throw up to 1500 m. Deformation cuts across remnant abyssal-hill lineaments that were presumably created near the spreading Earthquake and Tsunami Hazards ridge, and is probably related to the bending of the NOAM plate around and under the PAGES 349,354 the plate boundary and to provide constraints curved subduction zone. Submarine slide for hazard assessment, the morphology of the scarps and slide toes were observed for the The Puerto Rico Trench, the deepest part of entire 770-km-long trench from the Dominican first time on the northern trench wall, the the Atlantic Ocean, is located where the North Republic in the west to Anguilla in the east largest of which (at 68°W) is 20 km wide. The American (NOAM) plate is subducting under the was mapped with multibeam echosounder areas to the north and the west of that slide, Caribbean plate (Figure l).The trench region during three cruises in 2002 and 2003. Parts of the including the steep slopes of Navidad Bank, may pose significant seismic and tsunami haz­ Puerto Rico Trench were previously surveyed show numerous submarine slide scarps. ards to Puerto Rico and the U.S.Virgin Islands, with side-scan sonar [Grindlay et ai, 1997] and where 4 million U.S. citizens reside. Widespread multibeam echosounder [Dillon et ai, 1998; damage in Puerto Rico and Hispaniola from Strain Partitioning in the Forearc Grindlay et al., 1997], often at lower resolution an earthquake in 1787 was estimated to be the and with line orientation and spacing that did A 535-km-long fault is located 10-15 km result of a magnitude 8 earthquake north of the not provide complete bathymetric coverage. south of the trench and passes through rounded islands [McCann et al, 2004]. A tsunami killed The data presented here were collected hills that form the accretionary prism (Figure 40 people in NW Puerto Rico following a mag­ using the SeaBeam 2112 multibeam system 1).The fault is interpreted to accommodate nitude 7.3 earthquake in 1918 [Mercado and aboard the National Oceanic and Atmospheric left-lateral motion because it is offset around a McCann, 1998].Large landslide escarpments Administration (NOAA) ship Ron Brown,W\\h pull-apart depression (Figures 1 and 2). Seismic have been mapped on the seafloor north of sufficient swath overlap and proper line orien­ reflection data show the steeply dipping fault Puerto Rico [Mercado et aL, 2002; Schwab et tation for hydrographic survey The data were to penetrate 5 km through the accretionary al., 1991],although their ages are unknown. gridded at 150-m grid size following resolution sediments before terminating in the subduc­ The Puerto Rico Trench is atypical of oceanic tests.Vertical resolution is estimated to be tion interface [ten Brink and Lin, Stress inter­ trenches. Subduction is highly oblique (10°-20°) 0.5-1% of the water depth (10-80 m; L. Mayer, action between subduction earthquakes and to the trench axis with a large component of oral communication,2003).Backscatter mosaic forearc strike-slip faults: modeling and appli­ left-lateral strike-slip motion. Similar convergence images derived from the multibeam bathymetry cations, submitted to J. Geophys. Res. Solid geometry is observed at the Challenger Deep data aided in interpretation.The total mapped Earth, 2004]. Part of this fault trace was first in the Mariana Trench, the deepest point on area is 100,000 km2, slightly smaller than the identified as a weak lineament on a Gloria Earth. In addition to its extremely deep seafloor, area of the state of Virginia. backscatter image [Masson and Scanlon, the Puerto Rico Trench is also characterized 1991] and was named the Northern Puerto by the most negative free-air gravity anomaly Rico Slope Fault Zone [Grindlay et ai, 1997]. on Earth,-380 mGal, located 50 km south of Subsidence of the Trench The authors propose renaming this distinct the trench, where water depth is 7950 m (Fig­ regional fault the "Bunce fault" after the late ure 2). A tilted carbonate platform provides The trench can be divided into two parts at Elizabeth Bunce, a pioneer marine geophysi- evidence for extreme vertical tectonism in the about 65°-66°WThe western part includes the cist who explored the Puerto Rico Trench in region.This platform was horizontally deposited deepest sector of the trench, and is associated the 1950s [Bunce and Fahlquist, 1962]. over Cretaceous to Paleocene arc rocks start­ with the most oblique convergence. This sector Bunce fault ends at the western end of the ing in the Late Oligocene.Then, at 3.5 Ma, the is 10-15 km wide and 8300-8340 m deep rela­ Puerto Rico Trench in several splays, typical of carbonate platform was tilted by 4° toward tive to mean sea level (Figure l).It is remark­ distributed deformation at the end of large the trench over a time period of less than 40 ably flat and is covered by nonreflective pelagic strike-slip faults, such as the Alpine fault of kyr, (U. S. ten Brink et al., manuscript in prepa­ sediments. Seismic profiles show it to be under­ New Zealand.The fault appears to be the only ration, 2004) such that its northern edge is at a lain by rotated blocks of the NOAM plate that active strike-slip fault along most of this highly depth of 4000 m and its reconstructed eleva­ indicate trench subsidence. The unusual depth oblique subduction zone. Its proximity to the tion on land in Puerto Rico is at +1300 m (Fig­ extends southward over parts of the forearc. trench suggests that slip along the subduction ures 1 and 2). The trench floor narrows to the west and abruptly interface is effectively oblique, which is in agree­ To help understand the origin of the unusual shallows to 4700 m as it turns into the Hispaniola ment with GPS measurements and earthquake bathymetry, gravity and vertical tectonics of Trench, where convergence is more perpendi­ focal mechanisms [ten Brink and Lin, Stress cular. The eastern part of the trench is shallower interaction between subduction earthquakes by 700 m and more rugged than the deep and forearc strike-slip faults: modeling and BY U.TEN BRINK,W DANFORTH, C. POLLONI, western part.The subducting NOAM plate is applications, submitted to J. Geophys. Res. Solid B.ANDREWS, PLLANES, S. SMITH, E PARKER,AND observed in seismic lines to be broken into Earth,2004].Another fault closer to Puerto Rico T. UOZUMI blocks, but the descending blocks are not rotated. (the South Puerto Rico Slope Fault Zone Eos, Vol. 85, No. 37,14 September 2004 69W 67W 66W 65W S4W &3W 62W 20N 19N 69W 68W 67W 66W 65W 64W 62W Relative pstiite motion Bank -19N Anguilla - 18N F/g. /. (?o#) Shaded relief of the new multibeam bathymetry along the Puerto Rico Trench illuminated from the northwest. Thin contours indicate bathymetry from ETOP02 at 500-m intervals, (bottom) Combined bathymetry map of the multibeam bathymetry data, single-beam bathymetry com­ pilation around Puerto Rico, Lidar data near shore, and topography of Puerto Rico. Contour interval is 500 m. Thick barbed white lines denote thrust faults; thick white lines denote normal faults; thin white lines denote strike-slip fault; thin black lines denote northern edge of tilted carbonate platform and southern edge on land; dashed lines indicate head scarp of slope failures; dotted line indicates debris toe; blue lines indicate fissures in the seafloor; A, pull-apart basin; B, location of probable extinct mud volcano observed on backscatter images. Large black box and adjacent grey arrow show location and viewing direction of Figure 2. Original color image appears at back of this volume. (SPRSFZ) [Grindlay et al., 1997]), which was fault, a major active fault in Hispaniola. Left- between a block comprising Puerto Rico and previously proposed to be active, has no clear lateral motion is inferred from the existence of a the Virgin Islands and the Lesser Antilles Arc expression in the bathymetry (Figure 2). small pull-apart basin along the fault (A in and, as such, extended the boundary to the Therefore, earthquake hazard to Puerto Rico Figure 1).The fault ends abruptly in a 1000-m- trench [e.g.,Masson andScanlon, 1991].This from strike-slip faults in the forearc appears low, deep circular depression 25 km west of Mona map shows that the Anegada Passage probably because only the Bunce fault appears active; Rift, contrary to previous suggestions that it ends in the Sombrero Basin and does not it is located ~110 km away, it is ~5 km deep, continues eastward across Mona Rift and along connect to the trench. and it offsets partially consolidated sediments. the base of the Puerto Rico slope. Bunce fault is deflected southward at 6S°W, Active Submarine Slides at the Edge perhaps because this section was perturbed Rifts in the Forearc and the Subduction Zone of the Carbonate Platform by the oblique subduction of Main Ridge. Main Ridge is interpreted to be underlain by a Mona Rift NW of Puerto Rico consists of Two semicircular escarpments, 30-50 km across, subducted ridge of seamounts because its three en-echelon depressions (5000,7800, and are mapped along the northern edge of the axis is perpendicular to the observed abyssal-hill 8150 m), which cut the carbonate platform carbonate platform at a distance of 35-50 km grain of the subducting NOAM plate (Figure 1).
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