June 22, 2002 – The current seismic hazard map used to set building code standards and insurance rates places the Virgin Islands entirely in the most serious Zone 4, the same as the San Andreas Fault along the California coast.
Scientists working to create a new map for release in 2003 are expected to move St. Croix to the less-threatened Zone 3, which should be good news for vulnerable Hovensa storage, insurance rates and residents' comfort levels.
These were among the insights shared as about 40 people got an earful and an eyeful Thursday evening of the myths, "might's," "what if's," along with new scientific information about earthquakes, seismic activity and tsunamis in the region.
In response to community queries after the territory was rattled repeatedly in the second week of June, the University of the Virgin Islands set up an "Earthquakes in Paradise" lecture as part of UVI's 40th anniversary lecture series Co-sponsored by the Puerto Rico Seismic Network and the V.I. Territorial Emergency Management Agency, the public program was held at Chase Auditorium on the St. Thomas campus and teleconferenced to the St. Croix campus.
Roy A. Watlington, chancellor of the St. Thomas campus and a physics professor with a focus on marine science, served as moderator for the presentation. The featured speaker was Christa von Hillebrandt-Andrade, administrator of the Puerto Rico Seismic Network.
The network, von Hillebrandt-Andrade said, is responsible for tracking seismic activity in the region, including the Virgin Islands, responsibility assigned by the U.S. Geological Survey.
"Earthquakes are not natural disasters," she emphasized. "They are a natural phenomenon which can become a disaster if we do not take appropriate measures."
Myths in collective memory
Myths abound, since earthquakes are live long in collective memory, von Hillebrandt-Andrade said, and the worst one, in her opinion, is that "we can do nothing."
In her presentation she outlined a number of things that can be done — by individuals and by responsible officials. And she debunked a number of misconceptions:
Earthquakes can be predicted? No. Seismologists report; they do not predict. Through ever-improving measurement and scientific study, ground motion and reaction of structures can help seismologists get closer to prediction; they are making strides in their ability to provide warnings and watches, as are done in advance of hurricanes.
Earthquakes happen because it's hot? No. They happen in the state of Washington and in Alaska with snow on the ground. (Six of the 10 strongest quakes in the 20th century occurred in northern Russia and Alaska.)
Earthquakes are associated with hurricanes? No. It may seem that way in the Virgin Islands because people are extra-sensitive to natural occurrences during the hurricane season, and because of the events of 1867 — a tsunami on top of earthquakes on top of a recent devastating hurricane.
Earthquakes are associated with volcanoes? No. The Virgin Islands is responsible for this myth, von Hillebrandt-Andrade said, with newspaper and word-of-mouth reports that Saba Rock, off the present St. Thomas airport, was smoking during the traumatic events of 1867 in the area. Although the territory's islands were born of volcanoes, she said, those volacnoes have been dead for some 25 million years, and there is no chance of one acting up in connection with present-day earthquakes.
Earthquakes will destroy everything? No. Even in the1867 event so familiar to Virgin Islanders, many buildings remained standing and undamaged, or standing with damage. No gaping holes opened up to block roads. Today, she said, it is likely that many schools and business structures wouldl remain intact — depending on how well the construction industry is heeding the new building code adopted in the aftermath of Hurricane Marilyn.
Earthquakes generate tsunamis that might cover the entire island? No. Sometimes, earthquakes do generate tsunamis, but so do submarine landslides and volcanoes and, potentially, a large asteroid crashing into the ocean. But no tsunami has been recorded or recollected in this area of the world's oceans that reached higher than 20 feet, so scientists feel fairly confident in predicting that this is the maximum reach for the Caribbean region.
Seismic activity and its measurement
The Virgin Islands is affected by two distinct areas of seismic activity: the Sombrero Seismic Zone, off to the north of the islands, and the Virgin Islands Basin, between St. Thomas and St. Croix.
Earthquakes are a fact of life in this locale. "They have occurred in the past," von Hillebrandt-Andrade said, "and they're going to keep happening."
In the Sombrero Zone, it's uncertain if there ever has been a "big" one at the level of the 1867 event, which originated in the V.I. Basin.
The Sombrero Zone is very active — about 20 times more active than the V.I. Basin. Sombrero has no clear submarine faults, or rock fractures, whereas such faults are prominent in the V.I. Basin. The Sombrero Zone is where the North American and Caribbean plates meet beneath the floor of the sea, and the V.I. Basin is located away from that boundary. Sombrero is characterized by seismic swarms and lots of movement; the V.I. Basin has fewer swarms — more toward Vieques — and little motion.
Two kinds of earthquakes occur in the region: "subduction," at the boundary, where the North American plate slides beneath the Caribbean plate; and "strike-slip," the type of event characteristic of the San Andreas Fault. The Dominican Republic has a high likelihood of the strike-slip type, as a fault actually reaches that island, whereas V.I. and Puerto Rico events are usually offshore.
Puerto Rico network researchers have placed seismometers — instruments which measure and record data about earthquake activity — all over their area of responsibility. The graphic at the top of this article shows two kinds of seismometers:
– Short-period seismometers, shown in red, measure the larger events. One each was placed on St. Croix, St. John and St. Thomas (although the one for St. Croix was vandalized within a week and has yet to be re-sited).
– Broadband seismometers, shown in gray, measure events of 5.5 and less on the Richter Scale. These expensive instruments allow scientists to gather much more data regarding ground motion, plate motion and location of faults, and this allows them to extrapolate information to better inform and prepare officials and the public.
Hazard maps and building codes
The Virgin Islands in 1997 adopted the Universal Building Code, which addresses structural concerns posed by earthquakes. The code is based on the seismic hazard maps created for the states and territories by the U.S. Geological Survey's Seismic Hazard Mapping Group. The map in effect in 1997 places the Virgin Islands in Zone 4 along with the San Andreas fault.
The International Building Code, created in 2000, was adopted by the United States that same year but did not include the territories.
The Seismic Hazard Mapping Group is preparing a new map that will be released in April 2003. Of great interest locally is the fact that, while nothing is finalized yet, it is expected that St. Croix will be lowered one step in terms of vulnerability, to Zone 3, while the northern Virgin Islands will remain in Zone 4. The International Building Code will not be revised to reflect the new map until 2006; until then, it would be up to local governments to make the 2003 map apply legally. The hazard maps affect local and overall building codes, and insurance rates and availability.
The Geological Survey hazard maps are based on land with good soil. Actual local conditions affect how bad an earthquake will be. Fill land, sandy soil and soil with landslide potential are factors that local officials must consider.
What can be done, and what differen
ce it can make
Scientists know that:
– A tsunami 5 feet high has a force equivalent to a Category 5 hurricane (the strongest).
– A tsunami 10 feet high traveling at 12 miles per hour has the force of an air current moving at 379 mph.
– The effect of tsunami varies even in nearby shorelines. A 1755 earthquake in Lisbon sent a tsunami all the way across the Atlantic, and after eight hours it reached this area. It was noticeable in the northern British islands, but did not affect the Danish West Indies shorelines. Sometimes tsunamis approach land as a high tide, sometimes as a "wall of water."
– In the Northridge seismic event of 1994 in the Los Angeles area, 50 to 60 people died. A year later, in an event in Kobe, an area of Japan that had a low expectation of seismic activity, 5,000 people died. As a result of two earthquakes in 1999 in Turkey, 20,000 people died; 75 percent of them were killed by falling buildings. Ecuador is unusual in that area earthquake activity often includes a "precursor" event that precede a "big one" and allows timely evacuation of buildings, since Ecuador has paid attention to public education.
Why the differences? Factors include knowledge and education among officials and individuals, availability and dissemination of scientific data, and in-place procedures for dealing with after-stress. They vary greatly in different areas of the world.
Thus, scientists also agree that officials and individuals can do something about the threat of seismic activity. Aurelio Mercado at the Puerto Rico Seismic Network is leading the process of establishing a Caribbean Tsunami Warning and Mitigation Program similar to a well-publicized one in the Pacific area.
Public education is key, according to von Hillebrandt-Andrade. Put the information in local telephone books, she suggests, as many tourists have no understanding of earthquake and tsunami threats. Florida, for example, is in hazard category 0. Put brochures in hotels. If the earthquake is a big one, don't wait for the radio to tell you there's a tsunami coming: if you're at sea level, seek higher ground at once, on foot; don't head to the parking lot for your prized vehicle; save yourself.
Signage is important to mark evacuation routes, she said. And scientists must monitor and study even small activity to increase their knowledge; hence the importance of broadband instruments.
VITEMA's director, Harold Baker, reinforced the importance and local attention given to education, knowledge, construction practices, and dealing with after-stress. He and VITEMA official Clayton Sutton outlined specifics of what is being done and can be done locally.
Children are taught in school the procedure that earthquake experts 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. Get to inner walls, and stay away from windows so as to avoid being hit by flying glass.
Other tips: If you feel a tremor while in a vehicle, park the vehicle and stay in it. Do not check propane connections until after the tremor stops. Make a family plan that includes your extended family, and talk to your children about it. Every home and every classroom should have a battery-operated radio with fresh batteries on hand and a first-aid kit.
VITEMA has several booklets to help families prepare 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.
In the question-and-answer period following the formal presentations Thursday evening, some additional information came out.
– It's much easier to construct a wooden house than a concrete-block one to withstand earthquakes, and there's abundant information available on how to do it. What will happen with a concrete structure depends on the quality of the concrete mix, the exact location and interconnection of rebar, and other construction factors.
– The boating population is at risk. Von Hillebrandt-Andrade said sometimes there is nothing anyone can do. People who live on boats would probably not have time to move themselves or their vessels to safety in case of a tsunami generated by a nearby seismic event.
For updated information on earthquakes in the area, visit the Puerto Rico Seismic Center and UWI Seismic Research Unit web sites.
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