June 13, 2002 – Wednesday's spate of earthquakes in the Virgin Islands is not cause for alarm, according to Waverly Person, a geophysicist with the U.S. Geological Survey's National Earthquake Information Center in Golden, Colorado.
While the Golden equipment measured only the 8:48 a.m. earthquake, which it placed at 4.5 on the Richter Scale, the Puerto Rico Seismic Network in Mayaguez recorded a dozen earthquakes on Wednesday, ranging from 2.8 to 4.6 on the scale — the highest figure being the Seismic Network's rating of the same morning tremor that the USGS classified at 4.5.
The Seismic Network recorded two others above 4.0 — a 4.1 at 3:59 p.m. and a 4.2 at 10:47 p.m. Its measurements indicated that all three quakes occurred nearby and close to the surface of the ocean floor.
Research assistant Victor Huérfano said that a staff shortage has kept the Seismic Network from listing all of Wednesday's quakes on its web site.
Person said the USGS equipment records only earthquakes that are greater than 4.0 on the scale and also that differences in equipment used there and in Puerto Rico usually result in the numbers in Golden being a bit lower. So, he said, the Seismic Network's reported tremors of 4.1 and 4.2 could have been read as 4.0 or below in Colorado, which would explain why the center did not record those earthquakes.
The University of the West Indies, which monitors earthquake activity from the British Virgin Islands for its Seismic Research Unit in Trinidad, noted in its Thursday web site summary that "the sequence of earthquakes" that began Wednesday morning "continued through the night and into today." It said UWI researchers "have refined the location of these events, and it now seems that they are all roughly in the same location … significantly further south than our original estimate." They placed that location at about 18.2 degrees north latitude and 64.75 degrees west longitude, the same coordinates given by USGS on Wednesday.
Those tremors recorded by UWI ranged from 2.5 to 4.0. on the Richter Scale. Its equipment recorded the strongest at 1:58 p.m. Wednesday, "but because of the shallow depth, at least 14 others may have been felt," the Thursday summary stated.
On Wednesday, Person told the Source that he doubted the second of the day's earthquakes was an aftershock from the first. On Thursday, he amended those remarks to note that the dozen earthquakes recorded by the Seismic Network would seem to indicate there were some aftershocks involved. When the USGS center "eventually" receives the data from the Seismic Network, he said, it will have a fuller picture.
Person said on Wednesday that because the tremors occurred relatively close to the surface of the Earth, people heard the rumbling before they felt the movement. He called Wednesday's events a series of earthquakes, as did the UWI Seismic Research Unit, but Huérfano called what occurred a small cluster.
"It is not normal activity," Huérfano said. He said that the Puerto Rico-Virgin Islands area experienced more than 200 earthquakes in October 2001, and that phenomenon was termed a cluster. Like Wednesday's tremors, they caused no damage other than to people's nerves.
"Yesterday was so active," St. Thomas resident Sherri Merritt said, noting that she also felt or heard several of the smaller quakes that occurred throughout the day. She said that by the time the 10:47 p.m. tremor occurred, she and the four children in her household were ready to come up with a plan in case "a big one" should hit. "We have a meeting place on the property," she said.
The territory hasn't seen "a big one" since the 1867 earthquake that in turn caused a tsunami that caused widespread destruction in the Charlotte Amalie and Frederiksted harbors.
As a precaution, however, schoolchildren have been taught the procedure that earthquake experts all espouse: "Duck, cover and hold." This means get under something substantial, such as a sturdy table or bed, until the earthquake passes. If that item moves, grab the legs and move with it. And stay away from windows so as to avoid being hit by flying glass.
The V.I. Territorial Emergency Management Agency has several booklets to help families prepare response plans to be put into action in the event of a tremor that causes damage. People who deal with children will find "What Every Child Care Provider Should Know" and a coloring book called "A Guide to Being Ready and Safe for Earthquakes in the U.S. Virgin Islands" especially helpful. For information about obtaining copies, call VITEMA at 774-2244 on St. Thomas, 773-2244 on St. Croix or 776-6444 on St. John.
For frequently updated information on earthquakes in the area, visit the Puerto Rico Seismic Center and UWI Seismic Research Unit web sites.
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