S52A-01
Do Fault Intersections Play a Role in Concentrating Seismicity along the Northern Caribbean Plate Boundary? Insights from 2-D Numerical Modeling
Analytical studies spanning several decades have shown that stresses concentrate at fault intersections and bends when subjected to background tectonic loading. This phenomenon has also been demonstrated in geological systems, both within intraplate (continental and oceanic) and plate boundary settings. In particular, studies of stress interactions between strike-slip faults and the relationship between these stresses and seismicity in the northern Caribbean plate boundary region have produced a better understanding of regional tectonics. In this study we use two-dimensional mechanical modeling to investigate the role of fault intersections in concentrating seismicity along a part of the northern Caribbean plate boundary. We use a numerical method called "Distinct Element Method" implemented by a commercially available code called "UDEC". Our block model comprises of the structural geometry of a part of the northern Caribbean plate boundary spanning Hispaniola and Puerto Rico, and includes the major faults. In our model we use realistic mechanical properties (bulk and shear moduli, and density for the blocks, and stiffnesses, friction, and cohesion for the faults) based on the known geology. Assuming a linear, elastic behavior, we load our model tectonically for a specified period along the direction of the regional plate motion at a rate obtained from GPS studies in the region. At the end of the tectonic loading period we observe the spatial distribution of shear stresses in the model and along the individual faults. We find that relatively high shear stresses are concentrated at the intersections of the Hispaniola and Puerto Rico trenches, Septentrional and Bruce faults, and other fault intersections northeast of Puerto Rico. These locations of relatively high shear stresses also coincide spatially with clusters of seismicity in the region. Some of these fault intersections also have been postulated to be locations of major historical earthquakes. The areas of largest shear stresses also coincide with increased seismicity of greater magnitudes. Our modeling results also predict the appropriate senses of motion along individual faults. Additional, simple three-dimensional analysis is underway to investigate a spatial correlation of the seismicity with depth. These preliminary modeling results demonstrate that fault intersections concentrate stresses when tectonically loaded and influence the spatial distribution of seismicity. Such an analysis has the potential to identify the locations of major earthquakes in the future.
S52A-02
Seismic sequences in the Sombrero Seismic Zone
The northeastern Caribbean, in the vicinity of Puerto Rico and the Virgin Islands, has a long and well-documented history of devastating earthquakes and tsunamis, including major events in 1670, 1787, 1867, 1916, 1918, and 1943. Recently, seismicity has been concentrated to the north and west of the British Virgin Islands, in the region referred to as the Sombrero Seismic Zone by the Puerto Rico Seismic Network (PRSN). In the combined seismicity catalog maintained by the PRSN, several hundred small to moderate magnitude events can be found in this region prior to 2006. However, beginning in 2006 and continuing to the present, the rate of seismicity in the Sombrero suddenly increased, and a new locus of activity developed to the east of the previous location. Accurate estimates of seismic hazard, and the tsunamigenic potential of seismic events, depend on an accurate and comprehensive understanding of how strain is being accommodated in this corner region. Are faults locked and accumulating strain for release in a major event? Or is strain being released via slip over a diffuse system of faults? A careful analysis of seismicity patterns in the Sombrero region has the potential to both identify faults and modes of failure, provided the aggregation scheme is tuned to properly identify related events. To this end, we experimented with a scheme to identify seismic sequences based on physical and temporal proximity, under the assumptions that (a) events occur on related fault systems as stress is refocused by immediately previous events and (b) such 'stress waves' die out with time, so that two events that occur on the same system within a relatively short time window can be said to have a similar 'trigger' in ways that two nearby events that occurred years apart cannot. Patterns that emerge from the identification, temporal sequence, and refined locations of such sequences of events carry information about stress accommodation that is obscured by large clouds of unrelated events in plots of the general catalog. One characteristic of these sequences is that their magnitudes tend to be consistently small (1.0 - 3.5 mb, with only five events greater than 3.5 mb) and they typically do not include an event that could confidently be identified as a "main" shock. Nevertheless, the numbers of events, temporal and geographic distribution of shocks in each sequence suggests that these are aftershock sequences, yet none includes an event that could confidently be identified as a "main" shock. This observation suggests several questions. Do these sequences truly represent aftershocks? If so, where are the main events? Are they perhaps related to "silent" or "slow" earthquakes in the subduction zone? If so, could such slow earthquakes be related to the dropping away of the subducting slab beneath the deep Puerto Rico Trench? Or do the sequences indicate tearing of the NA lithosphere of the North America plate as it subducts beneath the Caribbean plate?
S52A-03
The Muertos Subduction Zone as a Major Earthquake and Tsunami Hazard for Puerto Rico
A revised catalog of 25 years of locally recorded earthquakes located using newly developed 1-D and 3-D velocity models allows us to better understand the spatial distribution of these events and their relation to known tectonic structures. In map view, the relocated events still show a pattern generally lacking in lineations that might be related to crustal faults. However, the previous cloud of crustal events in vertical cross sections now display a clear northerly dipping seismic zone as shallow as 10 km depth along the island's south coast extending to 40 km depth along the north coast of the island. This dipping seismic zone is interpreted as the Muertos Megathrust along which buoyant Caribbean Plate is obliquely underthrust beneath Puerto Rico. 3-D velocity anomalies associated with the region of high seismicity in western Puerto Rico confirm subducted Caribbean crust and lithosphere. The dipping seismic zone is seen best in SW Puerto Rico but also extends to the easternmost part of the island. Seismicity and historic earthquakes in the Dominican Republic also indicate the presence of the Muertos Megathrust beneath the SE part o that country. A large earthquake in 1751 generated a damaging tsunami. These new data suggest a new paradigm for earthquake hazards from "crustal" or "shallow" earthquakes in Puerto Rico. That is, after removing events associated with the dipping megathrust, little "random crustal seismicity" is left in the crustal wedge above the plate interface, and the amount and location of random crustal events depends heavily upon exactly which events are chosen to be part of the megathrust seismicity.
S52A-04
Active Subduction on Both Coasts of Costa Rica Does not Represent an Important Tsunami Hazard
Costa Rica, on the southern terminus of the Middle American Trench is being affected by active subduction on both, along the Pacific coast as well as on its Caribbean coast. Three main subduction segments can be recognized along the Pacific coast: 1) under northwestern Costa Rica, off Papagayo Gulf and Nicoya peninsula, the Cocos plate is subducting under the Caribbean plate; 2) under southern Costa Rica (Osa and Burica peninsulas) the Cocos plate subducts under the Panama Block and 3) in the central Pacific coast (between Nicoya and Osa peninsulas) the Cocos plate subducts under a shear zone that marks the transition between the Caribbean and the Cocos plate. Along the Caribbean coast, south of Puerto Limon, the Caribbean plate subducts under the Panama block. Large subduction earthquakes occur under the Nicoya peninsula, Osa peninsula and south of Limon. Most of the rupture area of these large events lies below land so the deformation of the ocean floor is minimal and therefore the tsunamis they generate are small. No large subduction earthquakes occur under the central Pacific coast of Costa Rica due to the subduction of small sea mounts that act as small asperities without potential to accumulate large amounts of slip. For this reason the region between Nicoya and Osa peninsulas is not an important tsunamigenic zone.
S52A-05
Tsunami Warning Services for the Caribbean Region
Tsunami warning and watch services are currently provided to the Caribbean region through a collaborative effort between the two NOAA Tsunami Warning Centers (TWCs): the Pacific Tsunami Warning Center (PTWC) in Ewa Beach, Hawaii, and the West Coast/Alaska Tsunami Warning Center (WCATWC) in Palmer, Alaska. The WCATWC, in coordination with the Puerto Rico Seismic Network (PRSN), provides fast-response warning services to the U.S. territories of the Commonwealth of Puerto Rico and the U.S. Virgin Islands (PR/VI). The PTWC provides regional watch services to other countries throughout and surrounding the Caribbean Sea as part of the Intergovernmental Coordination Group for the Caribbean Sea and Adjacent Regions. This collaboration is analogous to the TWC's responsibilities in the Pacific basin: the WCATWC provides fast-response warning services for the U.S. west coast states, Alaska, and British Columbia in Canada, while the PTWC provides regional services for countries throughout and surrounding the Pacific Ocean (as well as a fast-response service for the U.S. State of Hawaii). Caribbean seismic data are transmitted to the TWCs through several means. The PRSN directly exports data to the WCATWC, providing the Center sufficient seismic data for the PR/VI region. Additionally, the PRSN provides the TWCs with data gathered from other Caribbean nations. Using modern communication capabilities, the seismic data can be processed at the TWCs at the same time it is processed locally. Another source of high- quality seismic data is the new USGS nine-station array that circles the region. The Global Seismic Network maintains several stations in Caribbean, Central American, and South American nations which are available in real-time to the TWCs. Unfortunately, sea level data coverage is sporadic in the region. The PR/VI has a relatively dense array of coastal tide gages, but coastal tide gage coverage is very sparse for the rest of the Caribbean basin. Three deep-ocean pressure sensors are installed in the region: two north of PR/VI and one in the central Caribbean Sea. Together, these seismic and sea level data sets provide the TWCs with the necessary data to provide effective tsunami warning products and services for the Caribbean region.
S52A-06
USGS contributions to earthquake and tsunami monitoring in the Caribbean Region
USGS Caribbean Project Team: Lind Gee, Gary Gyure, John Derr, Jack Odum, John McMillan, David Carver, Jim Allen, Susan Rhea, Don Anderson, Harley Benz Caribbean Partners: Christa von Hillebrandt-Andrade-PRSN, Juan Payero ISU-UASD,DR, Eduardo Camacho - UPAN, Panama, Lloyd Lynch - SRU,Gonzalo Cruz - UNAH,Honduras, Margaret Wiggins-Grandison - Jamaica, Judy Thomas - CERO Barbados, Sylvan McIntyre - NADMA Grenada, E. Bermingham - STRI. The magnitude-9 Sumatra-Andaman Islands earthquake of December 26, 2004, increased global awareness of the destructive hazard posed by earthquakes and tsunamis. In response to this tragedy, the US government undertook a collaborative project to improve earthquake and tsunami monitoring along a major portion of vulnerable coastal regions, in the Caribbean Sea, the Gulf of Mexico, and the Atlantic Ocean. Seismically active areas of the Caribbean Sea region pose a tsunami risk for Caribbean islands, coastal areas along the Gulf of Mexico, and the Atlantic seaboard of North America. Nearly 100 tsunamis have been reported for the Caribbean region in the past 500 years, including 14 tsunamis reported in Puerto Rico and the U.S. Virgin Islands. Partners in this project include the United States Geological Survey (USGS), the Smithsonian Institute, the National Oceanic and Aeronautic Administration (NOAA), and several partner institutions in the Caribbean region. This presentation focuses on the deployment of nine broadband seismic stations to monitor earthquake activity in the Caribbean region that are affiliated with the Global Seismograph Network (GSN). By the end of 2006, five stations were transmitting data to the USGS National Earthquake Information Service (NEIS), and regional partners through Puerto Rico seismograph network (PRSN) Earthworm systems. The following stations are currently operating: SDDR - Sabaneta Dam Dominican Republic, BBGH - Gun Hill Barbados, GRGR - Grenville, Grenada, BCIP - Barro Colorado, Panama, TGUH - Tegucigalpa, Honduras. These stations complement the existing GSN stations SJG - San Juan, Puerto Rico, SDV - Santo Domingo, Venezuela, TEIG - Tepich, Yucatan, Mexico, and JTS - Costa, Rica. 2007 will see the construction of two additional stations in Guantanamo Bay, Cuba and Barbuda. Planned stations in Jamaica and Grand Turks are awaiting local approval. In this presentation we examine noise conditions at the five operating sites and assess the capabilities of the current seismic network using three different measures of capability. The three measures of network capability are: 1) minimum Mw detection threshold; 2) response time of the automatic processing system and; 3) theoretical earthquake location errors. The new seismic stations are part of a larger effort to monitor and mitigate tsunami hazard in the region. Destructive earthquakes and tsunamis are known to be a threat in various parts of the Caribbean. We demonstrate that considerable improvement in network magnitude threshold, response time and earthquake location error have been achieved.
S52A-07
Development of Rapid Seismic Monitoring Procedures for Incorporation into the Caribbean Tsunami Warning System
"Middle America", the geographic area between North and South America, including the Caribbean, has been a site of great plate tectonic activity in the most recent 35 million years. The region continues to be highly geologically active today as the Caribbean tectonic plate advances to the east, interacting with the North American and South American plates. Type examples of many of the salient features of plate tectonics—subduction zones, deep trenches, transform faults, pull-apart basins, subduction-to-strike-slip transitions, subduction-related volcanism, and volcano-free subduction zones—exist in a relatively small geographical area. The circum-Caribbean region has a documented history of destructive earthquakes and large damaging tsunamis that have affected coastal areas, including the events of Virgin Islands in 1867 and Mona Passage in 1918. These tsunamis have been triggered by large earthquakes that deformed the ocean floor. Tsunami waves originating in the prominent fault system around the Caribbean are considered to pose a near-field hazard because they can reach populated coastal areas within a few minutes after the earthquake. Because of this situation, the need to establish a system of rapid notification for tsunami alerting in the Caribbean has been recognized by emergency management agencies and the scientific community. In the wake of the December 26, 2004 devastating earthquake and tsunami in Indonesia, attention has been focused worldwide on the establishment of local and regional tsunami warning systems. The objective of the monitoring component of the Caribbean tsunami warning system under development in the Puerto Rico Seismic Network since 2000 is to detect, inform and confirm as rapidly and accurately as possible potential tsunamigenic events. In this presentation, we will describe the application of waveform analysis procedures for the rapid identification of shallow earthquake source parameters (geometry and size) in the Caribbean using three-component digital broadband records recorded at local and regional distances. For locally-recorded events, a grid-search method is investigated that uses the entire wave train recorded at multiple stations. A comparative search is conducted between observed and theoretical amplitude spectra at frequencies up to the corner frequency of the recording instrument to identify the best-fitting strike, dip, rake, and focal depth. In the regional procedure, a formal moment-tensor inversion scheme is used to derive the focal mechanism that best reproduces the waveforms recorded at a single or multiple stations.
S52A-08
Short-term Inundation Forecasting for Tsunamis in the Caribbean Sea Region
After the 2004 Indian Ocean tsunami, the USA Congress gave a mandate to the National Oceanographic and Atmospheric Administration (NOAA) to assess the tsunami threat for all USA interests, and adapt to them the Short-term Inundation Forecasting for Tsunamis (SIFT) methodology first developed for the USA Pacific seaboard states. This methodology would be used with the DART buoys deployed in the Atlantic Ocean and Caribbean Sea. The first step involved the evaluation and characterization of the major tsunamigenic regions in both regions, work done by the US Geological Survey (USGS). This was followed by the modeling of the generation and propagation of tsunamis due to unit slip tsunamigenic earthquakes located at different locations along the tsunamigenic zones identified by the USGS. These pre-computed results are stored and are used as sources (in an inverse modeling approach using the DART buoys) for so-called Standby Inundation Models (SIM's) being developed for selected coastal cities in Puerto Rico, the US Virgin Islands, and others along the Atlantic seaboard of the USA. It is the purpose of this presentation to describe the work being carried out in the Caribbean Sea region, where two SIM's for Puerto Rico have already being prepared, allowing for near real-time assessment (less than 10 minutes after detection by the DART buoys) of the expected tsunami impact for two major coastal cities.