I explain these vertical movements by a sudden increase in the slab's descent angle that caused the trench to subside and the island to rise. From February 24 through the end of April 2015, three NOAA Ship Okeanos Explorer expeditions legs will involve traveling to areas identified as priority regions for exploration by multiple groups within the U.S. Geological Survey, the NOAA National Marine Fisheries Service, the NOAA National Ocean Service, the Caribbean Fisheries Management Council, Puerto Rico Department of Natural Resources, and other territory and federal partners. The gravitational body force that acts on a flap of lithosphere, hanging in the aesthenosphere and, The authors propose that the Caribbean (Ca)-North American (NA) plate boundary zone (pbz) from the Puerto Rico Trench to the Venezuelan Basin from Mona Canyon east has been in left-transtension over, Summary. This latter deformation caused the disappearance of a trench opposite the southern Lesser Antillean arc and a displacement of the Puerto Rico trench axis oceanward away from the axis of the negative free-air anomaly belt near the northeast corner of the Caribbean plate. Dynamically triggered offshore aftershocks, caused by passing seismic waves from main shocks located on land, are currently not considered in tsunami warnings. A lock ( Puerto Rico and the Virgin Islands are located at an active plate boundary between the North American plate and the northeast corner of the Caribbean plate. ago over a time period as short as 14-40 kyr. The gravitational attraction of the high terrain is balanced by the reduced gravitational attraction of its underlying low-density roots. The theoretical value of gravity can be corrected for altitude and the effects of nearby terrain, but it usually still differs slightly from the measured value. The . It is postulated that differential shifts between the Caribbean plate and the underthrust Atlantic plate have occurred at least twice, once in late Eocene to early Oligocene, during which the outer island chain of the northern Lesser Antillean arc was formed, and again sometime since the late Miocene, when the Barbados ridge and associated uplifted topography were formed. No Derivative works - You may not alter, transform, or build upon this work. The low-density sediments of the Molasse basin produce a negative anomaly. U.S. Department of Commerce Both terrain higher than the measurement point and valleys lower than the measurement point reduce the measured value of gravity. The gravity anomaly data used in this study were extracted from the International Center for Global Earth Models (ICGEM), and our land data (254 stations) that were collected in western Puerto Rico in 2010. The USGS has an ongoing program to identify and map the faults in this region using various geophysical and geological methods in order to estimate the location and magnitude of potential earthquakes. Immediately after the 1946 earthquake, a tsunami struck northeastern Hispaniola and moved inland for several kilometers. 5, Puerto Rico Trench negative gravity anomaly belt, The Caribbean: gravity field and plate tectonics, Geophysical investigation of the Puerto Rico Trench and outer ridge, Seismic refraction and reflection measurementsPuerto Rico outer ridge, Continuous seismic profiles of the outer ridge and Nares basin north of Puerto Rico, Geophysical study of Antilles outer ridge, Puerto Rico trench, and northeast margin of Caribbean Sea, On the applicability of a universal elastic trench profile, Seismic refraction and reflection in Caribbean Sea, Crustal and upper mantle structure of the Central Aleutian Arc, Evidence for high density crust and mantle beneath the Chile trench due to the descending lithosphere, The Kuril Trench-Hokkaido Rise system: Large shallow earthquakes and simple model of deformation, The present day motions of the Caribbean plate, Paleogeography and geologic history of Greater Antilles, The relationship between bathymetry and gravity in the Atlantic Ocean, Tectonics of the Caribbean and Middle America regions from focal mechanisms and seismicity, Gravity anomalies and convection currents 1. The gravitational body force that acts on a flap of lithosphere, hanging in the aesthenosphere and more dense than it, pulls the lithosphere down at the trench. Eyewitness reports of an 1867 Virgin Islands tsunami gave a maximum wave height of >7 m in Frederiksted, St. Croix, where a large naval vessel was left on top of a pier. [15] The isostatic correction is dependent on the isostatic model used to calculate isostatic balance, and so is slightly different for the Airy-Heiskanen model (which assumes that the crust and mantle are uniform in density and isostatic balance is provided by changes in crust thickness), the Pratt-Hayford model (which assumes that the bottom of the crust is at the same depth everywhere and isostatic balance is provided by lateral changes in crust density), and the Vening Meinesz elastic plate model (which assumes the crust acts like an elastic sheet). of Earth and Planetary Sciences, 1981. (or is it just me), Smithsonian Privacy The line of trend of historic volcanoes of the Lesser Antillean arc is equidistant (160 km) from the axis of minimum free-air anomaly. Van Houten, C. A. Burk, H. D. Holland, L. C. Hollister. The tensile stresses necessary to deform or tear the slab could have been generated by increased curvature of the trench following a counterclockwise rotation of the upper plate and by the subduction of a large seamount. Many tectonic models have been proposed to explain this geologically fascinating, tectonically active region; however, none has gained acceptance, and the region remains poorly understood, largely because its underwater location makes it difficult to study. U.S. Geological Survey. [25] The Hawaiian gravity anomaly appears to be fully compensated within the lithosphere, not within the underlying aesthenosphere, contradicting the explanation of the Hawaiian rise as a product of aesthenosphere flow associated with the underlying mantle plume. The gravity anomaly is the difference between the observed acceleration of an object in free fall (gravity) near a planet's surface, and the corresponding value predicted by a model of the planet's gravitational field. For full access to this pdf, sign in to an existing account, or purchase an annual subscription. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. Other margins of the island (west, south, and south west) are also associated with massive tectonic features and may pose addtional hazard. Over a few hundreds or thousands of years several meters of motion accumulate resulting inoccasionalearthquakes and associated landslides and tsunamis. An official website of the United States government. Peter Molnar, Gravity anomalies and the origin of the Puerto Rico Trench, Geophysical Journal International, Volume 51, Issue 3, December 1977, Pages 701708, https://doi.org/10.1111/j.1365-246X.1977.tb04215.x. The ridge and its flanks appear to be fully isostatically compensated. The Caribbean plate is roughly rectangular, and it slides eastward at about 2 cm/yr relative to the North American plate. Click image for larger view and image credit. A sphere and cylinder sinking beneath the surface of a viscous fluid, Gravity anomalies and convection currents 2. The rapid (14-40 kyr) and uniform tilt along a 250 km long section of the trench is compatible with scales of mantle flow and plate bending. E-mail Updates | User Survey | Contact Us | Report Error On This Page | Privacy Policy | Disclaimer | Site Info | Site Index Approximate operating area (white polygons) of NOAA Ship Okeanos Explorer during Leg 3. Search ADS The USGS has an ongoing program to identify and map the faults in this region using various geophysical and geological methods in order to estimate the location and magnitude of potential earthquakes. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. The increased dip could have been caused by shearing or even by a complete tear of the descending North American slab, although the exact nature of this deformation is unknown. The unusually deep sea floor is not limited to the trench and extends farther south toward Puerto Rico. National Oceanic and Atmospheric Administration. This belt of positive anomalies is called the, Gravity models of oceanic trenches computed prior to the advent of plate tectonic concepts fell into two classes of solutions: (1) if a homogeneous mantle density was assumed, then the gravity models, The free air gravity anomaly and depth are sampled at 2-km intervals along two long, reasonably straight ship tracks across the Atlantic Ocean. Official websites use .gov The terrain correction is calculated from knowledge of the local topography and estimates of the density of the rock making up the high ground. Site Info, NOAA Ocean Exploration Then, at 3.5 Ma, the carbonate platform was tilted by 4 toward the trench over a time period of less than 40 kyr, such that its northern edge is at a depth of 4000 m and its reconstructed elevation on land in Puerto Rico is at +1300 m (Figures 1 and 2). Note that the Puerto Rico Trench, a deep trough where the North American plate slides past and underneath the Antilles, becomes deeper and wider north of Puerto Rico. Mo, Marine overwash from the north a few centuries ago transported hundreds of angular cobbles and boulders tens to hundreds of meters southward from limestone outcrops in the interior of Anegada, 140 km eastnortheast of Puerto Rico. Location of earthquakes as a function of depth and size in the northeastern Caribbean. The slumped material comprises carbonate blocks, which are cohesive and the edge of the carbonate platform is steeper than most continental slopes, resulting in a higher potential runup To help understand the origin of the unusual bathymetry, gravity, and vertical tectonics of the plate boundary and to provide constraints for hazard assessment, the morphology of the entire 770-km-long trench from the Dominican Republic in the west to Anguilla in the east was mapped with multibeam echosounder during three cruises in 2002 and 2003. Share sensitive information only on official, secure websites. [27], At scales between entire mountain ranges and ore bodies, Bouguer anomalies may indicate rock types. Crossref. Larger surveys across the region provide evidence of a relict subduction zone. Use, Smithsonian This new type of lander will likely provide the first full-depth CTD measurements in the Puerto Rico Trench. However, he found that the clock ran too slowly in Cayenne, compared with the apparent motion of the stars. Trenches in the Pacific are located in places where one tectonic plate subducts or slides under another one. [26], Local anomalies are used in applied geophysics. At a minimum, these include the tidal correction gtid, the terrain correction gT, and the free air correction gFA. A positive anomaly is associated with the Ivrea body, a wedge of dense mantle rock caught up by an ancient continental collision. Gravity anomalies are modelled assuming that the crustal thicknesses of the Atlantic and Caribbean Oceans are constant (but different . below sea level under the ocean basin, 12 km under the Caribbean Sea, at about 16 km under the trench, and at slightly shallower depth under Puerto Rico. The trench region may pose significant seismic and tsunami hazards to Puerto Rico and the U.S. Virgin Islands, where 4 million U.S. citizens reside. Thesis (Sc.D.) The crustal thickness and the free-air anomaly computed for sinkers of different sizes and depths are then compared with the measured freeair anomalies and seismic depths of the Puerto Rico trench and the mid-Atlantic rise. Plate movements have caused large magnitude earthquakes and devastating tsunamis. Geophysical monograph 14, Am. We use accounts of historical earthquakes to assign intensities and the intensity assignments for the 2010 Haiti earthquakes to derive an intensity attenuation relation for Hispaniola. The gravitational body force that acts on a flap of lithosphere, hanging in the aesthenosphere and more dense than it, pulls the lithosphere down at the trench. The unusually deep sea floor is not limited to the trench, but also extends farther south toward Puerto Rico. The isostatic anomaly includes correction terms for both effects, which reduces it nearly to zero as well. The Bouger anomaly is very negative over elevated terrain. For a measurement point above the reference ellipsoid, this means that the gravitational attraction of the bulk mass of the earth is slightly reduced. However, present-day slope seafloor-morphology is complicated by shaping processes, which are not readily separable through pure bathymetric analysis. The trench itself is very negative,[23] with values more negative than 250 mgal. [3], Gravity anomalies were first discovered in 1672, when the French astronomer Jean Richter established an observatory on the island of Cayenne. Many tectonic models have been proposed to explain this geologically fascinating, tectonically active region; however, none have gained acceptance, and the region remains poorly understood, largely because its underwater location makes it difficult to study. Trenches in the Pacific are located in places where one tectonic plate subducts or slides under another one. The hazard from tsunamis is also apparent. Many tectonic models have been proposed to explain this geologically fascinating, tectonically active region; however, none have gained acceptance, and the region remains poorly understood, largely because its underwater location makes it difficult to study. If the Earth were an ideal oblate spheroid of uniform density, then the gravity measured at every point on its surface would be given precisely by a simple algebraic expression. Click image for larger view and image credit. And moved inland for several kilometers Department of Commerce Both terrain higher than measurement! Of the stars use, Smithsonian this new type of lander will provide. Very negative over elevated terrain have caused large magnitude earthquakes and devastating.... Terrain is balanced by the reduced gravitational attraction of the Molasse basin produce a negative anomaly, D.... Ivrea body, a wedge of dense mantle rock caught up by an continental! 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