Saturday, December 1 2007

THE CARIBBEAN is vulnerable to devastating earthquakes, several of which have generated deadly tsunamis. Thursday afternoon’s 7.3 magnitude quake, which shook several islands in the region, is just the latest in more than a dozen major earthquakes of magnitude 7.0 or greater that have occurred in the region in the past 500 years.

Information obtained by Newsday shows that one major earthquake, a magnitude 8.1 recorded in 1946, resulted in a tsunami that killed a reported 1,600 people.

Close to 20 million people live in the Caribbean and, according to scientists, a major earthquake occurs on average every 50 years and it is not a question of if it will happen but when the next major event will take place.

In a study published in December 2004 in the Journal of Geophysical Research from the American Geophysical Union, geologists Uri ten Brink of the US Geological Survey in Woods Hole and Jian Lin of the Woods Hole Oceanographic Institution (WHOI) report a heightened earthquake risk of the Septentrional fault zone, which cuts through the highly populated region of the Cibao valley in the Dominican Republic.

They warned that the geologically active offshore Puerto Rico and Hispaniola trenches are capable of producing earthquakes of magnitude 7.5 and higher. The Indonesian earthquake on December 26, which generated a tsunami that killed (to date) an estimated 150,000 people, came from a fault of similar structure, but was a magnitude 9.0, much larger than the recorded quakes near the Puerto Rico Trench.

The Puerto Rico Trench, roughly parallel to and about 75 miles off the northern coast of Puerto Rico, is about 900 kilometres (560 miles) long and 100 kilometres (60 miles) wide. The deepest point in the Atlantic Ocean, the trench is 8,340 metres (27,362 feet) below the sea surface. The Hispaniola Trench parallels the north coast of the Dominican Republic and Haiti, and is 550 kilometres (344 miles) long and only 4,500 meters (14,764 feet) deep.

Earthquakes typically occur near faults or fractures in the Earth’s crust where rock formations, driven by the movements of the crustal or tectonic plates that make up the Earth’s surface, grind slowly past each other or collide, building up stress. At some point, stress overcomes friction and the rocks slip suddenly, releasing seismic energy in the form of an earthquake, which drops the stress in one area but raises the stress elsewhere along the fault line. Eighty percent of earthquakes occur on the sea floor and most of them along the plate boundaries.

Hispaniola, Puerto Rico and the US Virgin Islands sit on top of small crustal blocks that are sandwiched between the North American and Caribbean plates. The island of Hispaniola faces a double risk: an earthquake from the Septentrional fault on the island itself as the plates move past each other, and an earthquake deep in the earth in the subduction zone on which the island sits. Both could cause severe damage and loss of life, although the researchers say an earthquake in the subduction zone could be more devastating and has the potential to cause a tsunami.

“Every earthquake has its own character,” says Lin, who has studied mid-ocean ridges, hotspots and undersea volcanoes as well as earthquakes in Southern California, China and the Pacific. “And not all earthquakes generate tsunamis, which form when large areas of the seafloor rise or drop suddenly, causing the ocean above them to move. Many factors come into play in tsunami formation, including the size and type of an earthquake and how much the quake has ruptured the seafloor.”

Lin, a senior scientist and a marine geophysicist in the WHOI Geology and Geophysics Department, says that each time an earthquake occurs on the offshore Puerto Rico and Hispaniola trenches, it adds stress to the Septentrional fault zone on Hispaniola. Since the fault is in a highly populated region and is capable of generating magnitude 7.7-7.9 earthquakes, the public should be educated about the risk of this earthquake prone area.

Ten Brink, who studies earthquakes, tsunamis and geology in the Caribbean and Puerto Rico region, and has studied earthquake hazards in the Dead Sea in the Middle East, says there are a number of possible sources for tsunamis in the Caribbean.

“The threat of major earthquakes in the Caribbean, and the possibility of a resulting tsunami, are real even though the risks are small in the bigger picture,” ten Brink said. “Local earthquakes, such as from the fault on Hispaniola, or effects from distant earthquakes can be severe. Landslides and volcanic eruptions can also cause major earthquakes and potential tsunamis in this region. It has happened before, and it will happen again.”

The Puerto Rico Trench, which is capable of producing earthquakes of magnitude 7 to 8 or greater, faces north and east into the Atlantic Ocean. There are few land areas or islands to block a tsunami generated near the Puerto Rico Trench from entering the Atlantic Ocean. The direction of the waves would depend on many factors, including where in the trench the earthquake occurred.