However, the Earth has a rugged surface and non-uniform composition, which distorts its gravitational field. the Puerto Rico Trench is also characterized by the most negative free-air gravity anomaly on Earth,-380 mGal, located 50 km south of the trench, where water depth is 7950 m (Fig ure 2). Rapid gravity computation for two dimensional bodies with application to the Mendocino submarine fracture zone, A crustal section across the Puerto Rico trench, Comments on paper by W. Jason Morgan, 'Gravity anomalies and convection currents, 2. [9], The terrain correction must be calculated for every point at which gravity is measured, taking into account every hill or valley whose difference in elevation from the measurement point is greater than about 5% of its distance from the measurement point. The proposed shear zone or tear is inferred from seismic, morphological, and gravity observations to start at the trench at 64.5W and trend southwestwardly toward eastern Puerto Rico. 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. Holocene and Pleistocene tectonic deformation of the coast in the Mexico subudction margin is recorded by geomorphic and stratigraphic markers. Just offshore of Puerto Rico and the U.S. Virgin Islands are a diversity of deep-sea features that include the deepest trench in the North Atlantic Ocean; seamounts; and numerous submarine canyons, valleys, and troughs. The proposed shear zone or tear is inferred from seismic, morphological, and gravity observations to start at the trench at 64.5? Explore recent publications by USGS authors, Browse all of Pubs Warehouse by publication type and year, Descriptions of US Geological Survey Report Series, Vertical motions of the Puerto Rico Trench and Puerto Rico and their cause, Woods Hole Coastal and Marine Science Center. 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. Undeformed trench turbidites terminate abruptly against the inner wall which is acoustically opaque for the next, Since the pioneering pendulum gravity measurements of Vening-Meinesz and Wright (1930) and Hess (1933, 1938), the Caribbean Sea, with its multitude of geologic problems, has received intensive study, By clicking accept or continuing to use the site, you agree to the terms outlined in our. Because the island lies on an active plate boundary, earthquakes are a constant threat, and the densely populated coastal areas are vulnerable to tsunamis. ROV dives will mostly be conducted during the day, while CTD casts, and multibeam, singlebeam, and sub-bottom acoustic mapping will occur at night when the ROV is on deck. Studies of free-air gravity anomaly profiles across island arcs show an important belt of positive anomalies seaward of deep-sea trenches. The large negative gravity anomaly is attributed to a . A tilted carbonate platform provides evidence for extreme vertical tectonism in the region. The Puerto Rico Trench is the deepest part of the Atlantic Ocean, with water depths exceeding 8,400 meters (figure 1). 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. You could not be signed in. The rate at which these plates come together is similar to the rate at which a human fingernail grows. The Bouger anomaly is very negative over elevated terrain. That is, the Bouguer anomaly is:[12], The Bouguer anomaly is positive over ocean basins and negative over high continental areas. Different theoretical models will include different corrections to the value of gravity, and so a gravity anomaly is always specified with reference to a particular model. The low-density sediments of the Molasse basin produce a negative anomaly. Subduction is highly oblique (10-20) to the trench axis with a large component of left-lateral strike-slip motion. The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. For example, a mass of dense ore below the surface will give a positive anomaly due to the increased gravitational attraction of the ore. Revised December 04, 2006 by the NOAA Ocean Explorer Webmaster The Puerto Rico trench (PRT), with water depths of up to 8340 m, is the deepest part of the Atlantic Ocean, yet the relative plate motion along the PRT is predominantly strike slip with only a small component of North America (NOAM) plate subduction (N70E [ Calais et al., 2002; Mann et al., 2002 ]). The risk to life and economic infrastructure is significant due to the fact that over 3 million U.S. citizens live along the coastlines of Puerto Rico and the Virgin Islands. ago over a time period as short as 14-40 kyr. The Puerto Rico trench (PRT), with water depths of up to 8340 m, is the deepest part of the Atlantic Ocean, yet the relative plate motion along the PRT is predominantly strike slip with only a small component of North America (NOAM) plate subduction (N70E [ Calais et al., 2002; Mann et al., 2002 ]). The normal gravity accounts for the bulk gravitation of the entire Earth, corrected for its idealized shape and rotation. The line of trend of historic volcanoes of the Lesser Antillean arc is equidistant (160 km) from the axis of minimum free-air anomaly. The gravitational body force that acts on a flap of lithosphere, hanging in the aesthenosphere and Expand 14 PDF Save Alert [18], More generally, the Airy isostatic anomaly is zero over regions where there is complete isostatic compensation. 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 gravity anomaly at a location on the Earth's surface is the difference between the observed value of gravity and the value predicted by a theoretical model. Historically, other large earthquakes have also struck the area, such as one in 1787 (magnitude~8.1), possibly in the Puerto Rico Trench, and one in 1867 (magnitude~7.5) in the Anegada Trough (figure 1). 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. A 6 month deployment of five ocean bottom seismographs recorded two such tightly clustered swarms, along with additional events. No Derivative works - You may not alter, transform, or build upon this work. The terrain correction is calculated from knowledge of the local topography and estimates of the density of the rock making up the high ground. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. [18] For example, typical Bouguer anomalies in the Central Alps are 150 milligals. "Puerto Rico Trench Negative Gravity Anomaly Belt", Studies in Earth and Space Sciences, R. Shagam, R. B. Hargraves, W. J. Morgan, F. B. We document the spatial and temporal variability of. Examples include a magnitude 7.5 earthquake centered northwest of Puerto Rico in 1943, and magnitude 8.1 and 6.9 earthquakes north of Hispaniola in 1946 and 1953, respectively. The Caribbean plate is roughly rectangular, and it slides eastward at about 2 cm/yr relative to the North American plate. Below are other science projects associated with this project. This takes into account the fact that the measurement is usually at a different elevation than the reference ellipsoid at the measurement latitude and longitude. To explore along with the expedition team, visit this page. The Puerto Rico Trench, the deepest part of the Atlantic Ocean, is located where the North American (NOAM) plate is subducting under the Caribbean plate (Figure 1). Lateral variations in gravity anomalies are related to anomalous density distributions within the Earth. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. The profiles of the Puerto Rico Trench are asymmetric due to the tectonic factors. of Earth and Planetary Sciences, 1981. Download Citation | Gravity anomalies and the origin of the Puerto Rico Trench | The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. An official website of the United States government. The higher continental terrain is supported by thick, low-density crust that "floats" on the denser mantle, while the ocean basins are floored by much thinner oceanic crust. Widespread damage in Puerto Rico and Hispaniola from an earthquake in 1787 was estimated to be the result of a magnitude 8 earthquake north of the islands. Immediately after the 1946 earthquake, a tsunami struck northeastern Hispaniola and moved inland for several kilometers. The geologic settings of Puerto Rico and the Virgin Islands have created or contributed to several pressing societal issues related to human safety, environmental health, and economic development. 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. . This glider will be recovered several months later by NOAA Ship Nancy Foster when she is operating in the area. [16], Forward modelling is the process of computing the detailed shape of the compensation required by a theoretical model and using this to correct the Bouguer anomaly to yield an isostatic anomaly.[17]. 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. 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. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. What is the thermal history of the arc massif, and its relation to, The Barbados Ridge complex lies east of the Lesser Antilles volcanic arc along the eastern margin of the Caribbean Plate. Some reports indicate that nearly 1,800 people drowned. U.S. Geological Survey. 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. Earthquakes have been documented in the northeastern Caribbean since the arrival of Columbus to the Americas; written accounts of these felt earthquakes exist in various parts of the world. Approximate operating area (white polygons) of NOAA Ship Okeanos Explorer during Leg 3. Legs 1 and 2 will focus on 24-hour mapping using the ships three scientific sonars as the ship travels to Puerto Rico and the U.S. Virgin Islands. Other margins of the island (west, south, and south west) are also associated with massive tectonic features and may pose addtional hazard. Click image for larger view and image credit. Notice, Smithsonian Terms of Both terrain higher than the measurement point and valleys lower than the measurement point reduce the measured value of gravity. Other Puerto Rican cities also have substantial risk. 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. During Leg 3 not alter, transform, or build upon this work NOAA Ship Nancy Foster she., or build upon this work a large component of left-lateral strike-slip motion with water exceeding! 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