Nov. 1, 2001 – Scientists at two seismic research organizations which have long tracked earthquake activity in the Caribbean agree that the recent "earthquake swarms" in the region are in the waning stages of a "standard foreshock-main shock-aftershock sequence."
Both agencies — the Seismic Research Unit in Trinidad and Tobago and the Puerto Rico Seismic Network — are in the midst of projects that will increase and enhance earthquake data-collecting capability in the region that includes the Virgin Islands.
John Shepherd, head of the Trinidad unit, was in the British Virgin Islands last week to confer with officials and to establish a seismograph station in Road Town, Tortola. The station became operational Friday, and over the next three days it recorded seven earthquakes from the zone north of the Virgin Islands. The maximum magnitude was 4.3 on the Richter Scale, according to the SRU site, and it's unlikely any of the tremors were felt on land.
An SRU report concludes, "The pattern continues to be that of a declining aftershock sequence following the magnitude 6.0 earthquake of Oct. 17."
Shepherd expects the SRU to set up stations in Virgin Gorda and Anegada shortly.
Christa von Hillebrandt-Andrade, network administrator for the Puerto Rico Seismic Network, agrees with Shepherd's assessment of "petering off" from the standard sequence. On Monday evening she said the "seismicity" had decreased in both magniture and frequency over the preceding several days.
The ongoing Project PROBES (for Puerto Rico Ocean Bottom Earthquake Survey), operating out of von Hillebrandt-Andrade's agency, is placing additional stations throughout the region. The additional data collection and analysis capabilities will enable the network "to better understand the thermal structure beneath Puerto Rico and help predict the degree of ground shaking."
PROBES represents the first time that land-based and ocean-bottom probes are providing earthquake data jointly. Land-based stations are housed in a shelter about the size of a closet. The ocean probes, dropped to the ocean floor with a subsurface buoy attached, contain a cluster of measuring instruments to detect the different types of waves, notably P-waves and S-waves, emanating from earthquakes. Data collection and transmission from both types of probes is automatic.
A St. Thomas station is in place at the residence of Russell Crowther at the top of Upper Bonne Resolution and communications are being fine-tuned, von Hillebrandt-Andrade said. A St. John station is also in place, and one will be set up on St. Croix shortly.
Technicians have some logistic problems to resolve. Crowther said two to three receivers will allow data from St. John and Anegada stations to be fed into the computer at the St. Thomas location. Puerto Rico Seismic Network staff can dial in to the computer and retrieve data.
Since the Trinidad unit shares data with the Puerto Rico Network, all of these new measuring stations will benefit the Virgin Islands location with improved data, analysis, and pinpointed details.
The threat of tsunamis
With evidence of ongoing seismic activitiy in the region, is there a threat of tsunamis, so widely publicized on television for the Pacific Tsunami Warning System, to the Virgin Islands? Most definitely: The years 1530, 1689, 1692, 1751, 1755, 1775, 1780, 1842, 1867, 1907, 1918, 1929 and 1946 all saw destructive tsunamis in the wider Caribbean region.
Today, the threat is compounded by population growth and the choice of many people to live in the coastal zones of islands. And only the rudiments of a public-education and warning system are in place.
"In the last 150 years, tsunami-related fatalities in the Caribbean were nearly five times greater than in Hawaii, Alaska, and the U.S. West Coast combined," stated a report from a 1999 Tsunami Working Group led by Eric Geist of the U.S. Geological Survey and Aurelio Mercado Irizarry, a tsunami modeler based at the University of Puerto Rico.
Outlining a number of scientific initiatives, the working group emphasized the need for agencies and governments to develop a Puerto Rico and Virgin Islands tsunami warning system.
The science of tsunamis
A tsunami is more likely to occur from an earthquake than from a volcanic eruption, according to Roy Watlington, a marine science professor who also is chancellor of the St. Thomas campus of the University of the Virgin Islands.
A tsunami wave, with a wavelength of more than 60 miles, can travel across open ocean waters at 450 to 650 miles per hour and hardly be noticed by ships. But as it reaches shallower waters near a shoreline, the wave slows and can rise to a height of 100 feet. Historically, the waves of most destructive tsunamis have been 3 to 15 feet in height. The destructive 1867 tsunami that entered Charlotte Amalie Harbor and devastated the shore areas on St. Thomas as well as St. Croix, was reported at between 17 and 21 feet in height.
The Virgin Islands is in a seismically active area where the Caribbean and the North American tectonic plates meet far beneath the sea. The Caribbean plate is an east-west-oriented block that moves eastward with respect to the North and South American plates. Within the Caribbean plate's strike-slip motion at the northeastern edge is the deepest trench in the Atlantic, the Puerto Rican trench. That trench appears as the dark linear area at the top right of the graphic at the top of this article.
In addition to the increased activity always present at the edges of plates, the Virgin Islands area is complicated by "microplates" which have irregular intra-plate activity on a smaller scale. St. Croix especially is subject to these dynamics. Local measurement "stations" are particularly important to track these smaller movements.
Particularly active areas include the Anegada Passage east and north of St. Thomas and St. John, and the Grappler Channel and Jungfern Passage west of St. Croix. Stations placed throughout the area are used to establish epicenters through triangulation and to track dynamic activity. Stations as far away as Florida contribute to this process. The addition of more stations anywhere across the area will provide improved information on dynamics affecting the Virgin Islands.
Earlier regional and local interest
Two decades ago, Crowther recalls, a station at a house next door to his collected data for the Lamont Laboratory at Columbia University. Other residents remember an installation atop Crown Mountain on land later used by the U.S. Navy. Individuals had to periodically retrieve data tapes from these manual stations and physically transport them to data centers for organization and analysis. Computerization has dramatically overshadowed those early efforts.
Regional attention and planning for risk management has long been addressed in the Pacific area through the Intergovernmental Oceanographic Commission (IOC) of UNESCO and is well organized. And it has been tested by natural events, many seen on national television. IOCaribe, one of seven subcommissions of the IOC, formed a steering group of tsunami experts in 1996 to address the need for a tsunami warning system for the Caribbean similar to the Pacific-area system.
The first IOCaribe Caribbean Tsunami Workshop was held on St. John in May 1996, hosted by UVI's Eastern Caribbean Center, with Watlington as UVI chief facilitator. Attended by scientists and seismic network administrators from throughout the world, this "consultation" of experts on tsunamis spent two days at the V.I. Environmental Research Station at Lameshur Bay discussing the threat of tsunami and formulating ste
ps to bring about a regional warning network with the assistance of IOCaribe.
Subsequent tsunami workshops have been held annually, most of them in Puerto Rico, where the seismic network is based and where tsunami modeler Mercado is at work.
