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The U.S. Commonwealth of Puerto Rico has a population of 3.8 million (2000 Census), a higher population density than any U.S. state. The island, approximately 160 km east to west by 50 km north to south, is bounded by offshore active faults on all sides. Numerous local and regional events in the recorded his-tory with M > 7.0, some of which have generated tsunamis, have caused extensive damage to local infrastructure, though the last significant ground motions were felt onshore in 1918. The U.S. Geological Survey hazard maps (Mueller et al. 2003) indicate that the seismic hazard is similar to the Basin and Range prov-ince in the Western United States, and the island is assigned Seismic Zone 3 in the current standard building code in Puerto Rico, the 1997 UBC. The significant hazard, combined with the large population and untested infrastructure, results in a potentially devastating combination for Puerto Rico. Efficient emergency response in the event of a large earthquake will be crucial to minimizing the resultant loss of human life and dis-ruption of lifeline systems. The first step in providing an appro-priate response to such a disaster is a timely knowledge of the magnitude, location, and expected ground shaking and damage patterns from a large earthquake. This requires a modern and dense seismic network, capable of not only recording the earth-quake ground motion without saturation but also doing so in real time and then providing data for near-immediate analysis that can be made available to emergency services and the com-munity at large.

The seismicity of the island, as well as the northeastern Caribbean region in general (including the U.S. and British Virgin Islands), is monitored jointly by the Puerto Rico Seismic Network (PRSN) and the Puerto Rico Strong Motion Program (PRSMP), both operating within the University of Puerto Rico at Mayagüez. They currently acquire, analyze, and archive broadband, short-period, and strong-motion seismic data in continuous real-time format, using both Antelope and Earthworm software, from stations on Puerto Rico and its sur-rounding islands, the U.S. and British Virgin Islands, and the

Dominican Republic. In addition, PRSN imports and pro-cesses data from various regional stations operated by other networks. PRSMP also operates dial-up and stand-alone instal-lations. Stations range from state-of-the-art seismic vaults for event location and magnitude determination to urban free-field installations and structural arrays for engineering studies and structural health monitoring. In the near future, the networks are expected to additionally monitor continuous streams of data from tidal gauges and buoys (for tsumani warning), GPS, and digital weather stations.

TECTONIC BACKGROUND

The island of Puerto Rico has a long history of damaging earthquakes and tsunamis. The island is bounded on all sides by major tectonic fault lines (figure 1). Major earthquakes produced damaging ground motions in Puerto Rico in 1615, 1670, 1751, 1776, 1787 (~M8.0 Puerto Rico Trench), 1867 (~M7.3 Anegada Passage) and 1918 (~M7.3 Mona Passage). Large events in 1943 (~M7.5) and 1946 (~M7.8) also caused much damage in the neighboring Dominican Republic (PRSN Historical Catalogue, http://temblor.uprm.edu/~victor/PRSN/history/; Shepherd and Lynch 1992). The 1867 and 1918 events were accompanied by destructive tsunamis (Reid and Taber 1919; Mueller et al. 2003).

Puerto Rico is located on a microplate sandwiched between the obliquely subducting North American and Caribbean plates (figure 1). Puerto Rico accommodates approximately 16.9 mm/yr of deformation relative to North America and 2.4 mm/yr relative to the Caribbean plate ( Jansma et al. 2000; Jansma and Mattioli 2005), primarily by left-lateral strike-slip motion along east-west striking faults. The main sources of seismic activity in the region are at the supposed boundaries of the microplate: the subduction zones to the north (the Puerto Rico Trench, which is the location of the largest gravity anomaly on earth) and south (the Muertos Trough: Carbó et al. 2005), and zones of extension at the Anegada Trough to the east and the Mona Canyon region to the west. All regions are capable of producing events greater than M7.0, and all have evidence of having done so in the recorded history of the island (Asencio 1980; Moya and McCann 1992; Macari 1994). On average, Puerto Rico is strongly shaken with Modified Mercalli Intensity (MMI) >VII

1. Puerto Rico Strong Motion Program, Department of Civil Engineering and Surveying, University of Puerto Rico at Mayagüez

2. Puerto Rico Seismic Network, Geology Department, University of Puerto Rico at Mayagüez

3. Now at Swiss Seismological Survey (SED), Institute of Geophysics, ETH Hönggerberg, Switzerland

The Current State of Seismic Monitoring in Puerto RicoJohn F. Clinton, Georgia Cua, Víctor Huérfano, Christa G. von Hillebrandt-Andrade, and José Martínez Cruzado

John F. Clinton1,3, Georgia Cua1,3, Víctor Huérfano2, Christa G. von Hillebrandt-Andrade2, and José Martínez Cruzado1

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once every 100 years, and MMI > VI is experienced on the island once every 50 years.

In addition to these offshore sources, recent trenching shows evidence of two surface-rupturing events on the inland’s South Lajas fault in southwestern Puerto Rico (Prentice et al. 2000; Prentice and Mann 2005), predominantly along a normal fault with a component of strike-slip motion, both within the past 5,000 years. This 50-km-long inland fault segment can pro-duce M7.0 events (LaForge and McCann 2003) and potentially could be part of a longer fault zone that extends toward Ponce, the second-largest city on the island (population 186,000: 2000 Census). Other shallow faults, mainly with east-west trends, are interspersed across the island. These include the Great Northern and Southern Puerto Rico fault zones. These structures have unknown potential for large magnitude events; as yet there is no evidence of Holocene rupture. Tectonic models (Prentice and Mann 2005) and GPS studies ( Jansma and Mattioli 2005)

indicate that active faults onshore accommodate some exten-sion across the island.

The most recent large event to cause widespread damage across the island occurred in the Mona Passage in 1918, with MS7.3 (Pacheco and Sykes 1992). This event caused substantial structural damage to the large towns of Mayagüez and Aguadilla on the west side of the island. The associated tsunami had a run-up of 6 m at Aguadilla and 2 m at Mayagüez, reaching more than 1 km inland (Mercado and McCann 1998), which killed more than 100 people. Widespread liquefaction was observed along the unconsolidated floodplains in the municipalities of Añasco and Mayagüez (Capacete et al. 1972; Moya and McCann 1992), areas now with pockets of dense population. With the island now having a far greater density of population and infrastruc-ture—infrastructure that has not been tested by strong motions since the 1918 event—a repeat of such ground motions would lead to a far more severe loss of life and property.

Figure 1. Map of northeastern Caribbean, showing major  tectonic structures and approximate  locations for damaging earthquakes in recent history. GPS displacement vectors are from Jansma et al. (2000). USVI: U.S. Virgin Islands; BVI: British Virgin Islands; Cul Is.: Culebra, P.R.; Vq Is.: Vieques, P.R.; PC: Punta Cana; SM: Samana; SLF: South Lajas fault; GSPRFZ: Great Southern Puerto Rico fault zone; GNPRFZ: Great Northern Puerto Rico fault zone.

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The tsunami risk in the northeast Caribbean is also very real. Aside from the potential for large-magnitude events (Huérfano 2003), recent bathymetry studies have shown numerous large landslide scarps and cliffs near the Puerto Rico Trench as well as the Muertos Trough (Grindlay, Hearne, and Mann 2005; Grindlay et al. 2005; ten Brink and Lin 2004). The offshore bathymetry is particularly severe to the north of the island where the Puerto Rico Trench drops to a depth of more than 8.3 km just 180 km north of the island. The interior of the island is mountainous, resulting in much of the population being concentrated in the at-risk low-lying coastal flood plains and alluvial basins.

SEISMIC NETWORKS IN PUERTO RICO

The Puerto Rico Seismic Network (PRSN) and the Puerto Rico Strong Motion Program (PRSMP) jointly monitor seis-mic activity in the northeastern Caribbean region. The pri-mary duty of PRSN is to identify and provide information on local, regional, and teleseismic earthquakes. PRSN also hosts the Emergent Tsunami Warning System for Puerto Rico and the Virgin Islands. PRSN is the reporting authority for event locations and magnitudes; PRSN maintains the authoritative event catalog. PRSN operates vault stations primarily in low-

noise locations. The objective of PRSMP is to record onscale ground motions from earthquakes affecting the island with as high a quality and station density as possible. PRSMP main-tains a dense urban network with both free-field stations and structural arrays. Both networks share the common goal of pro-viding high-quality data and information in response to needs of the emergency management, engineering, and scientific com-munities, as well as the general public.

PRSMP and PRSN run complementary operations and share all continuous data in real time (figure 2). This exchange provides an essential level of redundancy and robustness that will be important in ensuring timely and accurate information following a large earthquake.

PUERTO RICO SEISMIC NETWORK (PRSN)

The Puerto Rico Seismic Network (PRSN) historically has pro-vided locations and magnitudes for earthquakes in the Puerto Rico and Virgin Islands region. PRSN is the reporting author-ity for the region bounded by latitudes 17.0°N–20.0°N and longitudes 63.5°W–69.0°W (figure 1). The network has been operating since 1974, when USGS installed several short-period stations for the Puerto Rico Electric Power Authority that were used to evaluate local seismicity in response to concerns regard-

Figure 2. Data Acquisition and Processing at PRSN / PRSMP.▲

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ing the planned construction of two nuclear power plants. In 1982 the network was transferred to the University of Puerto Rico at Mayagüez, where it has been operating under the juris-diction of the Department of Geology since 1987.

The main objective of PRSN is to record, process, analyze, provide information, and research local, regional, and teleseis-mic earthquakes so as to quickly provide high-quality data and information as needed by the emergency management, aca-demic, and research communities, as well as the general public. It operates 13 broadband stations and 10 short-period stations throughout Puerto Rico and the U.S. and British Virgin Islands (figure 3). Six of these broadband stations also jointly house strong-motion sensors. The broadband (BB) sensors include Guralp CMG 40T, CMG 3ESP, and CMG 3T seismometers recording onto 24-bit Refraction Technology (130 and 72A) and NetDas DAQ or Quanterra Q330 digitizers, all sampling at 40 sps. The strong-motion (SM) sensors are all Kinemetrics EpiSensors, sampling at 100 sps. The broadband sensors are linked to the central data collection center in Mayagüez via Monitron UHF digital radios, DDS 56K telephone lines, spread-spectrum radios, and Internet service. The analog short-period (SP) seismic stations consist of Teledyne S-13 and Mark L-4 seismometers, and the data are digitized at 16 bits at 100 sps at the offices of PRSN.

PRSN runs Earthworm software ( Johnson et al. 1995; Earle et al. 2003) to acquire and write waveforms to disk for perma-nent archival. Automatic locations and alerts are generated for events in Puerto Rico, the Intra-America Seas, and the Atlantic Oceanby the EarlyBird system (Whitmore and Sokolowski 2002), which monitors PRSN stations as well as some 35 addi-tional stations run by networks operating in North, Central, and South America and other sites in the Caribbean. PRDANIS (Puerto Rico Data Analysis and Information System) software, developed by PRSN, supports manual locations and analyst

review of automatic locations of events within the PRSN area of responsibility (AOR), using all the broadband, strong-motion, and short-period waveforms. The location algorithm is based on the inversion schema HYPOINVERSE-2000 (Klein 2002), using the crustal model of Huérfano and Bataille (1994). Currently all magnitudes reported by PRDANIS are Md, dura-tion magnitude. All phase arrival times, locations, amplitudes, and magnitudes are reviewed by a seismic analyst, usually on the same day and always within one week. The continuous wave-form data is permanently archived at PRSN in GSE2.1 format, though all broadband waveforms are continuously exported to IRIS (Incorporated Research Institutions for Seismology, http://www.iris.edu), where they are archived in SEED format. Full dataless SEED volumes are available for all stations. Phase data is saved in HYPOINVERSE-2000 format.

PRSN hosts a Web site with a searchable event database, felt event reports, and general information about seismicity in the area of responsibility, which can be accessed from http://redsismica.uprm.edu/english/.

PRSN also is producing ShakeMaps (Wald et al. 2005) for all felt events in Puerto Rico and the Virgin Islands and pro-vides QDDS (Quake Data Distribution System) data to the USGS “Did You Feel It?” tool (http://pasadena.wr.usgs.gov/shake/prt/).

PUERTO RICO STRONG MOTION PROGRAM (PRSMP)

Strong-motion instrumentation efforts in Puerto Rico began in the 1970s and have continued at a steady pace due to the activi-ties of the Puerto Rico Strong Motion Program (PRSMP). In the immediate aftermath of a large earthquake, the mission of PRSMP is to provide timely information regarding the distribu-tion of peak ground shaking to emergency management officials

Figure 3. Stations with real-time continuous monitoring at PRSN. All are operated by PRSN except the IRIS station SJG and the PRSMP 18-bit strong-motion stations. Three joint broadband (BB) and strong-motion (SM) stations will be installed in locations outside the map on Mona Island and at Punta Cana and Samana in the Dominican Republic (see figure 1). Currently short-period stations IDE and CSB are out of operation.

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to guide response and recovery efforts. In the long term, PRSMP aims to provide relevant strong-motion data to the scientific and engineering communities to facilitate the construction of earth-quake-resistant structures. PRSMP focuses on (1) deploying and maintaining instrumentation for recording large earthquakes that will produce damaging ground motions on the island with as high a data quality and station density as possible; (2) pro-viding timely and appropriate information to local authorities, seismologists, and engineers; and (3) supporting and conducting research activities associated with these records, in particular the understanding of the effects of earthquakes in Puerto Rico and the mitigation of potential damage from future earthquakes.

PRSMP maintains a network of (1) 78 three-component 18-bit ETNA urban free-field stations; (2) six 24-bit EpiSensors alongside broadband sensors (jointly operated with PRSN) at remote sites with vault conditions; and (3) eight monitored structures with multichannel 19-bit K2 instrumentation (fig-ure 4). The density of strong-motion instrumentation across the

island is comparable to that of greater Los Angeles and greatly exceeds the density deployed in California as a whole.

Station sites for the urban free-field network are selected based on the proximity to highly populated urban areas and regions of high seismicity (i.e., regions exposed to the greatest seismic risk). Local cultural noise, site geology, ease of access for maintenance, availability of the Internet or telephone lines, and security are additionally important considerations. Of the 78 stations, nine have real-time continuous Internet communica-tion with PRSMP. These Internet stations are primarily located on different University of Puerto Rico campuses throughout the island. Of the remaining ETNA stations, 42 have dial-up telephone communications and 27 have no communications (they are stand-alone stations that must be visited to gather data). For reasons of accessibility, security, and ease of obtain-ing site permissions, a majority of the dial-up and stand-alone stations are installed in fire stations or on property owned by the Roman Catholic Church.

Figure 4. Strong-motion stations in Puerto Rico monitored by PRSMP. Additional details of the dense instrumentation in the three largest towns in Puerto Rico—San Juan, Ponce, and Mayagüez—are shown.▲

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In addition to the ETNA stations, PRSMP monitors eight structures across the island with multichannel sensors. These sites all are equipped with 19-bit Kinemetrics K2 dataloggers and Kinemetrics EpiSensors or FBA-23/FBA-11 sensors. The critical infrastructures monitored are: high-rise buildings (Plaza Immaculada Apartment Complex and Minillas Government Center, San Juan), concrete dams (Lucchetti, La Plata, and Carraizo), and bridges (Ponce Bypass, Mayagüez Viaduct, and Paso del Indio bridge). All structural installations are currently stand-alone.

PRSMP also shares the operation and maintenance of the six strong-motion sensors co-located with PRSN broadband sensors (figure 3), ensuring the entire seismic frequency and amplitude bandwidth is covered at these select stations.

The Antelope Real-Time System (ARTS, http://www.brtt.com; Malone 2000; Pavlis et al. 2004) facilitates communica-tion with the PRSMP ETNA stations as well as with strong-motion and other data collected at PRSN. Antelope software also provides automatic and reviewed location and magnitude determinations (Richter magnitude (ML) for local events; body wave magnitude (mb) and surface wave magnitude (MS) for regional and teleseismic events), which run as backup to PRSN operations. Data are archived in a temporary ring buffer of 15 days in SEED format, with permanent archiving of all data from all triggered events in SAC format. Event information, and selected event data, is available online at http://www.prsmp.uprm.edu/.

Data from nine strong-motion stations that recorded the recent 3 March 2006 Md 5.3 event off Anegada was submit-ted to the Consortium of Organizations for Strong-Motion Observation Systems (COSMOS) and is available from the COSMOS Web site, http://db.cosmos-eq.org.

BRIEF DESCRIPTION OF THE PRSN CATALOG

PRSN generates and maintains the earthquake catalog for Puerto Rico and the Virgin Islands. Before July 2004, the PRSN catalog was compiled using the PR-HYPO earthquake location code with both local (Richter magnitude, ML: Richter 1935) and duration magnitude (Md) computed using the equations of von Hillebrandt and Bataille (1994). In July 2004, the location schema HYPOINVERSE-2000 was implemented, and magni-tudes have been calculated from short-period and broadband data using the Eaton (1992) “full digital velocity” equations. PRSN now calculates a near real–time moment magnitude based on a Regional Moment Tensor (RMT) inversion code (Randall et al. 1995) and the ASPO search grid algorithm of Zahradnick et al. (2001).

Figure 5 shows the epicenters of the analyst-reviewed events in the PRSN local reporting region (AOR) between January 1986 and March 2006. There are more than 14,100 events in the catalog, nine with M>5.0 (six of these large events were shallow, with depths <35 km). The largest event was an Md5.6, occurring at 67 km depth, just south of Anegada, BVI,

Figure 5. Seismicity within PRSN local reporting region between January 1986 and March 2006. The largest event in this time period was a M5.6 located near Anegada, BVI. The most recent large event was a M5.3 on 2 March 2006, about 100km northeast of Anegada. There were nine events with M>5.0, denoted with blue stars: 1: M5.6, 1986; 2: M5.2, 1988; 3: M5.2, 1991; 4: M5.1, 1996; 5: M5.6, 1998; 6: M5.2, 2001; 7: M5.2, 2004, 8: M5.3, 2005; 9: M5.3, 2006. Black stars indicate large historical events labeled in figure 1.

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on 7 December 1998. The most recent large event was an Md5.3 event, 25 km deep, located 100 km northeast of Anegada, on 3 March 2006. Shallow seismicity is concentrated in regions along the Mona Canyon, the region between the Puerto Rico Trench and the 19°N fault zone, the Sombrero fault zone, and across the south of Puerto Rico. The oblique subduction of the North American plate under the Puerto Rican microplate appears to be the source of much of the deep seismicity, with events concentrated in the Dominican Republic and along northern Puerto Rico.

Short-period sensors dominated the network until 2000, when broadband sensors were introduced, although the sta-tion density has remained relatively constant. Figure 6A shows the number of events binned into three-month periods. It is clear that the level of recorded seismicity increased by about 20% around the turn of the century, once broadband sensors

were introduced to the network. The spike in events during January–March 2006 is due to the M 5.3 indicated in figure 5. This anomalously large number of events reflects significant recent improvements in automated event-picking algorithms using both Earthworm and Antelope modules. In figures 6B and 6C, the magnitude and depth distribution of the catalog is analyzed. From figure 6B, the average magnitude of complete-ness, MC, for the entire catalog can be estimated to be about Md 3.5. The majority of events are shallow (figure 6C), though the spike in event depth at 25 km is due to this being the initial default depth for events up until 2004. Figure 6D shows the Gutenberg-Richter distribution (Gutenberg and Richter 1944) for the catalog, with a b-value of 1.44. It is not clear whether this high b-value is real, and it could be due to magnitude com-pression of the Md calculations—this scale saturates for large events, so direct comparisons between the PRSN magnitude

Figure 6. Analysis of the PRSN catalog, January 1986–March 2006. (A) number of events over time, in three-month bins; (B) number of events in 0.1-magnitude bins; (C) number of events in 5-km–depth bins; (D) Gutenberg-Richter relation.▲

(C)

(D)

(A) (B)

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and independent magnitudes from global networks are not possible even for the infrequent large events. No other regional networks provide magnitudes from this region.

The spatial distribution of magnitude of completeness for the catalog across the PRSN reporting region is shown in figure 7. At best, the magnitude of completeness, MC, is Md 2.0, and is at least Md 3.3 across the island of Puerto Rico. MC is highest in the northeast of the island, where high-gain sensor coverage is poorest (figure 3). In the corners of the PRSN reporting region, which are the areas most distant from land (and thus stations), the MC degrades to about Md 3.8. Some degradation is inevi-table, an unavoidable consequence of the island geography, but the problem is significantly compounded by the frequent loss of communication with the most remote sites: the Virgin Islands to the east, and Desecheo and Mona islands to the west of Puerto Rico. The ongoing efforts to secure high-quality broad-band data with good communications at the more isolated sites such as Anegada and Mona, coupled with expected installa-tions in the Dominican Republic, will reduce the magnitude of completeness in the extreme regions. The density of seismicity is shown in figure 8. Although these data must be interpreted within the context of the spatial distribution of MC presented in figure 7, it is clear that that there are pockets of increased seismicity in the region. The seismicity is dominated by three regions: (1) north of the 19° N fault zone directly north of Puerto Rico; (2) the Sombrero seismic zone north of Anegada island; and (3) on Puerto Rico, in the southwest of the island, south of the Great Southern Puerto Rico fault zone.

In addition to the difficulties in detecting offshore events, exact earthquake location is also difficult in the region because most stations lie in a band between 18° N and 18.5°N, with poor azimuthal constraint on events to the north and south (Mendoza and Huérfano 2005).

CURRENT NETWORK INITIATIVES

ShakeMap for Puerto RicoShakeMaps are ground-shaking and intensity maps that com-bine instrumental measurements of shaking with information regarding local geology, earthquake location, and magnitude to estimate shaking variations throughout a geographic area (Wald et al. 2005). ShakeMap software is distributed and main-tained by USGS, with local network operators responsible for customizing the distributed software to their particular net-work environment. PRSN has the capability to manually gen-erate ShakeMaps within 1.5 hours following a significant felt event, after which they are posted on the Web site; an example for the recent 2 March 2006 M5.3 event can be found at http://redsismica.uprm.edu/spanish/sismos/sismarzo.php. PRSN ShakeMaps are constrained by peak ground-motion parameters obtained from real-time broadband and strong-motion instru-ments operated by PRSN and PRSMP; the initial ShakeMaps can be updated as strong-motion data from PRSMP dial-up stations become available. Site amplification effects are approxi-mated by assigning Vs30 values on a uniformly spaced 1.5-km grid throughout the island. A set of 14 Vs30 measurements

(Odum et al. 2006) sampling representative geologic units throughout mainland Puerto Rico was used to assign average Vs30 values to geologic units.

The efficiency and performance of the ShakeMap system in Puerto Rico can be improved by: 1) automating the trans-fer of data from the PRSN Earthworm data acquisition system to ShakeMap; 2) updating the current site-conditions map to take into account detailed microzonation studies available for the San Juan metropolitan area (Aponte et al. 2000), Ponce, and Mayagüez (Llavona 2004), the three largest urban centers on the island; and 3) educating potential users on how to use ShakeMap products to guide post-earthquake response and recovery efforts.

Caribbean Tsunami Warning SystemSince 2000, PRSN has been working toward establishing a local Tsunami Warning Center for the region. The original goal was to provide a platform for tsunami warnings to Puerto Rico (von Hillebrandt and Huérfano 2006), but this has now evolved into an initiative to create a Caribbean Tsunami Warning Center, which would be a fundamental component of the Tsunami and Other Coastal Hazards Warning System for the Caribbean and Adjacent Regions (UNESCO, 2005). Efforts to lay the foun-dation for this center included the installation of EarlyBird (Sokolowski 2002) at PRSN for the detection and reporting of potentially tsunamigenic earthquakes. This system moni-tors the seismic stations of PRSN and 35 other stations in and around the Caribbean that are available in real time through the GSN or bilateral agreements with regional seismic net-works. Earthworm (Earle et al. 2003) and SeisComP (Hanka et al. 2000, http://www.gfz-potsdam.de/geofon/seiscomp/) are used for the real-time exchange of seismic data. During 2006 the network will integrate additional broadband stations into the system: the nine GSN-quality stations that USGS will be installing in the region (McNamara et al. 2006) and stations to be installed in Dutch territories and the Cayman Islands. The goal is to be able to quickly and precisely detect all earthquakes of at least magnitude 5 in the Caribbean region. EarlyBird not only automatically locates earthquakes and provides different magnitudes for the events, it notifies PRSN personnel once specific regional dependent thresholds have been exceeded. In 2006, as part of the warning system, PRSN also will install six FEMA-funded tsunami-ready tide gauge stations in Puerto Rico, in addition to a Geostationary Operational Environmental Satellites (GOES) receiver at Mayagüez, to gather data from these and other regional tide gauges. The data from five Deep-Ocean Assessment and Reporting of Tsunamis (DART) buoys that were deployed in the spring of 2006 in the Caribbean Sea and adjacent seas also will be incorporated into the monitoring system.

To achieve a true Tsunami Warning System, PRSN com-plements improved monitoring capabilities with a series of research, education, and outreach initiatives. These activities include tsunami inundation modeling, seismic source char-acterization, protocol development, improved dissemination techniques, production of audiovisual materials, workshops,

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Figure 7. Geographical  distribution of  the magnitude of completeness,  MC,  for  the PRSN catalog  for  the  reporting  region, January 1986–March 2006. Colorbar units are magnitude of completeness, MC. MC is below Md 2.5 for a significant portion of the center and south-west of the island, and it is at least Md 3.3 throughout the island of Puerto Rico. As distance from the islands increases, the magnitude of completeness degrades; in some regions it is greater than Md 3.7.

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Figure 8. Density of seismicity in the reporting region, PRSN catalog, January 1986–March 2006. Color bar is in units of log (earthquakes per  km2).  Local  seismicity  is  concentrated  in  southwest  Puerto  Rico  (south  of  the  Great  Southern  Puerto  Rico  fault  zone);  the  region between the Puerto Rico Trench and the 19° fault zone; and the Sombrero seismic zone (see figure 1 for fault locations).

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talks, and drills. In May 2006, the National Oceanic and Atmospheric Administration (NOAA) declared the city of Mayagüez the first TsunamiReady community in Puerto Rico and the Caribbean. The TsunamiReady program has proven to be an excellent venue to promote and validate tsunami readi-ness and has been widely accepted by local public officials and the media.

All of these monitoring and complementary activities are being coordinated with other local, regional, and international institutions, including the Pacific Tsunami Warning Center, the institution presently responsible for providing tsunami warning guidance for the region.

Focal Mechanisms and Moment TensorsWith the support of the Sea Grant Network, PRSN is evalu-ating available seismological methods for the rapid identifica-tion of earthquake source parameters using the broadband seismic waveforms recorded by PRSN and the development of automated procedures for the emergent Tsunami Warning System in the Puerto Rico-Virgin Islands region. This work involves adaptation of existing codes for the numerical inver-sion of both local and regional broadband waveforms to allow for rapid implementation within the PRSN seismic-detection and information system. The results will be incorporated into the broadcast schemas as part of the tsunami protocol in the PR-VI region. The implementation of near real-time moment magnitude estimation based on the Regional Moment Tensor (RMT) inversion code of Randall et al. (1995), and regular calculation of first motion focal mechanisms are products of this work. For the recent 2 March 2005 (25-km deep) event, a moment magnitude of MW 5.5 was determined within one hour of event initiation. The focal mechanism has a strike/dip/rake of 83.2°/52.4°/53.6°, a thrust fault with some strike-slip com-ponent reflecting the compressional dynamics typical of sub-duction zones, and is likely related to the plate interface. This is consistent with the Harvard Quick CMT solution (http://www.seismology.harvard.edu/CMTsearch.html), which indicates an MW 5.5 high-angle thrust event with 103° strike.

Structural MonitoringPRSMP currently monitors eight structures, all stand-alone, with multichannel 19-bit K2 systems with ETNA free-field sensors. These are all located on critical infrastructure around the island: two high-rise buildings in San Juan, three bridges on the main road around the island; and three concrete dams. The instrumentation is summarized in table 1. Currently all these sta-tions are stand-alone, although cable Internet is being installed at the Plaza Immaculada building. Nine of the 15 channels at this site will be continuously monitored at PRSMP.

PRSMP is also in the process of installing dense instru-mentation at the 19-story El Castillo apartment complex in Mayagüez, a component of the Advanced National Seismic System (ANSS) Structural Response Monitoring System. This structural monitoring test bed will consist of at least 27 chan-nels of 24-bit strong-motion data recording on Quanterra Q330 data loggers.

At these test-bed locations, the natural frequencies and modes of the monitored building systems will be determined in real time. In addition to the response of the structures to earthquakes, severe wind loading conditions from hurricanes can be expected over the duration of the instrumentation. The detection of significant and permanent changes in the dynamic properties of the system during a heavy loading event is a key parameter for the remote identification of structural damage (Clinton 2004; Clinton et al. 2006).

NETWORK EXPANSION AND IMPROVEMENT

Upgrade at Remote Sites and New InstallationsExisting network infrastructure at Mona Island in Puerto Rico and Anegada in the British Virgin Islands (figures 1 and 3) will be upgraded by installing 24-bit broadband and strong-motion sensors and dataloggers. Satellite telemetry will be used to transfer data to PRSN. Both of these stations also will provide data to the emergent USGS earthquake and tsunami monitor-ing system for the Caribbean (McNamara 2006). In addition, the currently nonoperational short-period station at CSB (fig-

TABLE 1Summary of Instrumented Structures Operated by PRSMP

Locations for the sensors can be found in figure 4

Structure Name Location Station ID Etna Free field# K2

sensorsHigh Rise Apartment Plaza Inmaculada San Juan B02L SJ05 15

High Rise Building Minillas Government Center San Juan B01L SJ05 6

Highway Overpass Mayagüez Viaduct Mayagüez B01R MY09 24

Highway Overpass Ponce Bypass Ponce B02R PN10 12

Highway Bridge Paso del Indio Vega Baja B03R VGB1 24

Dam Lucchetti Yauco D01M YAC1 12

Dam Carraizo Trujillo Alto D02M TA02 9

Dam La Plata Toa Alta D03M TOA2 12

542  Seismological Research Letters  Volume 77, Number 5  September/October 2006

ure 3) is likely to be moved to an area near San Juan, which will improve the sensitivity of the network in the northeast region of the island.

Installations in the Eastern Dominican RepublicTwo sites have been selected and permitted in the eastern Dominican Republic, at Samana and Punta Cana (figure 1), where PRSN and PRSMP will jointly install and operate 24-bit co-located broadband and strong-motion stations with real-time continuous Internet data transmission.

Strong-Motion Monitoring in the British Virgin IslandsPRSMP has begun a three-year contract with the Department of Disaster Management in the British Virgin Islands to install a suite of free-field and structural strong-motion sensors at critical local infrastructure. The airport control tower on Beef Island and the three-story Government Building in Road Town, Tortola, will have 12-channel K2 instrumentation with both free-field and structural sensors. Both lie on reclaimed fill. Free-field sites on hard rock in Road Town on Tortola and Spanish Town on Virgin Gorda are being instrumented. All instrumenta-tion will be transmitted in real time by Internet to PRSMP.

ACKNOWLEDGMENTS

We would like to thank the PRSN and PRSMP personnel, technicians, administrative staff, and students who have made immense contributions to the success of both networks. The figures in this paper were made with the Generic Mapping Tools (GMT) software (Wessel and Smith 1991) and ZMAP (Wiemer 2001). We thank Stefan Wiemer for his comments and suggestions. Sue Hough provided helpful comments that greatly improved the quality of the manuscript. The authors gratefully acknowledge GNU software developers. The devel-opment of moment tensor solutions at PRSN was funded by the Sea Grant program (R-122-2-04).

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Swiss Seismological Service (SED) Institute of Geophysics

Swiss Federal Institute of Technology (ETH)Zurich, Switzerland

jclinton@sed.ethz.ch, georgia.cua@sed.ethz.ch ( J. C., G. C.)

Puerto Rico Seismic Network (PRSN)Geology Department

University of Puerto Rico at Mayagüez,Mayagüez, PR

victor@midas.uprm.edu(V. H.)

christa@midas.uprm.edu(C. H.-A.)

Puerto Rico Strong Motion Program (PRSMP)Department of Civil Engineering

University of Puerto Rico at Mayagüez,Mayagüez, PR

martinez@uprm.edu( J. M.-C.)