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June 18, 2002 – Scientists in and around the Virgin Islands are keeping tabs on a phenomenon known as a "seismic swarm" that shook up the landscape — and the people living on it — last week.
Seismic swarms, or continuous series of small earthquakes, have come and gone in the Eastern Caribbean for eons. But one expert says the series that jostled Virgin Islanders last Thursday is of particular interest. "The location of the three earthquakes we had would certainly get our attention," said Roy Watlington, who is chancellor of the University of the Virgin Islands St. Thomas campus but also a physics professor and marine science specialist.
Other scientists are casting a wary eye just north of the Virgin Islands, to a series of tremors that began last October. The most troubling aspect, they say, is that the swarm in an area near Tortola mimics a sequence that has been observed elsewhere just before the arrival of big, disastrous earthquakes. Since Hurricane Marilyn struck in 1995, observers have been warning that the territory is overdue for a major quake — of magnitude 7.0 or more on the Richter Scale. They say such tremors occur roughly once in a century in the area, and the last one took place in 1867.
This, to the mind of geophysicist Jose Molinelli, is a very good reason to keep an eye on small sequential quakes occurring in brief intervals.
Molinelli works with the Physics Department at the University of Puerto Rico at Hato Rey. Within a seismic swarm, "There is the possibility that a bigger earthquake may occur," he said. "After a while they disappear. Everything comes to a rest. But there are times when an earthquake swarm may precede a big event."
John B. Shepherd, a professor of geophysics at the University of the West Indies in Trinidad, says there are clues to be found in sequential tremors and some can signal the start of a major quake. "The terminology of earthquake sequences like this is imprecise," he says. "The word I have just used, 'sequence,' is neutral. It simply means that there have been a number of earthquakes, and it avoids drawing conclusions."
He continued, "A 'swarm' is a sequence of earthquakes which are all about the same size. Swarms can go on for months or even years without ever reaching a climax. If the present sequence is truly a swarm, then there is no cause for alarm. It is unlikely that any single earthquake will be bigger than the ones which have already happened."
Shepherd heads UWI's Seismic Research Division. He said the event to beware of is the foreshock-main shock-aftershock sequence, similar to the swarm experienced in the Virgin Islands last week. "This consists of a series of moderate-sized earthquakes called fore-shocks followed by a main shock, which is two to three magnitude units bigger than the foreshocks," he said. "The main shock is then followed by a diminishing sequence of aftershocks. If this is a foreshock sequence, then we might expect a main shock with magnitude in the range 6 to 7."
Last year, at the request of Gov. Frank Savage of the British Virgin Islands, UWI installed seismic monitors on Tortola that are linked by computer to the research station in Trinidad. In October, he said, sensors picked up a distinctive sequence that resembled the pattern preceding a major quake. "The sequence last year started in the middle of October and looked much more like a foreshock sequence than a swarm," Shepherd said, "except that there were two main shocks, each of magnitude about 5.5."
The location of the events was around 50 kilometers, or 31 miles, north of Tortola. The situation still bears watching, he said, because the Tortola tremors are still going on.
"The sequence which began last October has never really stopped," he said. "We have recorded over one hundred earthquakes of magnitude greater than 3.0 since then. What has happened is that the epicenters have migrated irregularly southward from north of Tortola to south of St. Thomas."
Because the BVI and the USVI are geologically one and the same, Shepherd said, he would like to set up additional seismic stations on St. Thomas, St. John and St. Croix. A Tortola research station staff member is in the territory this week for meetings, he said. However, Shepherd said involvement beyond "an itinerant interest" is unlikely.
UPR's Seismic Center at Mayaguez, Puerto Rico, measures seismic activity south of Sir Francis Drake Channel, and for June 12-14, it recorded 17 tremors. However, tremors by themselves are not unusual; scientists agree there's a lot of seismic activity in the Eastern Caribbean. What could be considered somewhat unusual is that seven tremors in a row ranging in magnitude from 3.3 to 4.6 originated Wednesday and Thursday exclusively on the Virgin Islands Plate.
The pattern then shifted, with the epicenters of subsequent tremors scattered between the Virgin Islands and northern and central Puerto Rico. Scientists were also able to pick up related activity as far away as Antigua and Montserrat.
Activity along the Virgin Islands Plate was of special interest to Watlington, he said, because the quakes originated not far below the ocean floor, while the historic quake of 1867 "may have occurred at a greater depth." And from its depths, that quake generated a catastrophic tsunami wave that claimed lives in and around the Charlotte Amalie harbor.
Watlington is involved in IOCARIBE, a United Nations-sponsored regional group studying tsunamis. The group's aims include the creation of tsunami early warning systems and educational outreach for emergency managers in the Caribbean.
Given the magnitude of Thursday's largest tremors on the Virgin Islands Plate — at least three in the Richter range of 4.0 to 4.6 — along with the geological sensitivity of the area and the center of activity below the ocean floor, Watlington and Molinelli say concerns about tsunamis are well founded. "You can have a lot of earthquake activity without a tsunami, but it all depends on the type of earthquake," Molinelli said.
If a foreshock sequence were to occur, he said, the main shock generated during a series of 4.0 magnitude quakes could be of a magnitude 6.0 or higher. A wave produced by a quake of magnitude 6.2 or more would be enough to endanger shipping, he said.
And while ships at sea might weather such an event without much harm, Watlington said, ships in port would not be so lucky. "We could have a very significant concern if we had one of our cruise ships in port," he said.
For more information…
On Thursday evening, the public is invited to attend a free presentation on recent seismic activity in the territory. It will be presented at 7:30 p.m. in Chase Auditorium on the University of the Virgin Islands St. Thomas campus and teleconferenced to the St. Croix campus in the Melvin Evans Center.
Sponsored by UVI in collaboration with the V.I. Territorial Emergency Management Agency and the Puerto Rico Seismic Network at UPR/Mayaguez, the presentation will provide information and address concerns about the unusual clustering of seismic events in the area. For further details, see "Earthquakes in Paradise".

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