Risk of Tsunamis in the Northern Caribbean, The

Phi Kappa Phi Forum, Winter 2006 by Mann, Paul

Many of us enjoy quiet vacations lolling on Caribbean beaches in places such as Jamaica and the Virgin Islands or crisscrossing the Caribbean Sea on cruise ships to these and other exotic ports of call. Because many of these tourist destinations are at sea level, - should we be concerned during our next Caribbean vacation about being engulfed by a tsunami, or earthquake-triggered sea wave, similar to the Sumatra earthquake and tsunami that killed more than 280,000 people in the circum-Indian Ocean region on December 26, 2004? In this article, I will review some basic information about the active tectonic setting of the northern Caribbean region, its history of past earthquakes and tsunamis, and its potential for future catastrophes, including whether far-traveled tsunamis originating in the Caribbean could threaten the southern and eastern coast of the United States.

BOUNDARIES OF CONCERN

The North American-Caribbean plate boundary extends more than 3,200 km from northern Central America through the Greater Antilles (Jamaica, southern Cuba, Hispaniola, Puerto Rico, and the Virgin Islands) to the northern end of the Lesser Antilles subduction zone (Figure 1). This immense strike-slip plate boundary ranks with the great seismogenic, strike-slip plate boundaries of the world including the San Andreas fault zone of California (1,500 km in length), the Alpine fault zone of New Zealand (600 km in length), and the North Anatolian fault zone of Turkey (1,000 km). Motion along all of these plate-boundary fault systems is mostly horizontal and conservative, so no large areas of the plate are lost or subducted beneath the neighboring plate - nor is significant space generated by opening gaps between the plates. The plates grind past one another at rates that we humans would consider glacial (20 mm/yr) but are significant in geologic terms.

Localized and sometimes subtle curvatures along all of these large strike-slip faults produce localized areas of topographic uplift that are responsible for some of the spectacularly rugged and scenic coastlines found along these plate boundaries (for example, Northern California, South Island of New Zealand). This tectonically related topographic uplift is good news for the long-term tectonic manufacture of scenic islands in a tropical setting but bad news for generating large earthquakes and accompanying tsunamis. The uneven plate grinding along faults produces earthquakes because the fault slippage in the upper part of the crust becomes frozen or "locked," sometimes for hundreds of years. Continued interplate slip will break these asperities with a sudden snap and release of seismic energy: Earthquake! When this snapping motion occurs underwater, the seabed can move upwards or downwards to set in motion a tsunami that can travel thousands of kilometers. The earthquake also can trigger a large submarine landslide or slump, usually when an unstable part of the shelf breaks off and slides into deeper water.

During the past fifteen years, GPS-based geodetic studies on the Caribbean islands and mainland areas have shown that the Caribbean plate is moving eastnortheastward at a rate of 18 to 20 /-3 mm/yr relative to North America (Figure 1). This direction implies that the segment of the plate boundary from northern Central America to southern Cuba is strike-slip with varying degrees of transtension (strike-slip opening) and transpression (strike-slip closure) (Figure 1). Maximum interplate transpression occurs between the island of Hispaniola (Dominican Republic and Haiti) on the Caribbean plate and the thick crust of the Bahama carbonate platform on the adjacent North American plate (Figure 1). For this reason, the earthquake belt is wider in the Hispaniola region than in tectonically simpler parts of the plate boundary to the west.

Transpression between the two plates has produced higher topography in central Hispaniola than in any other area of the northern and eastern Caribbean: Pico Duarte in the Dominican Republic attains a maximum elevation of 3,087 meters, or about the same elevation as some of the lower peaks of the Rocky Mountains in the western United States. Transpressive plate motion is also taken up along onshore strike-slip faults and convergence across a trench-like feature off the north coast, and also along widespread folds and faults in young rocks on the island itself (Figure 1). The tectonic setting of Hispaniola is analogous to the Big Bend region of southern California, where the San Andreas fault curves to produce various mountain ranges and deep intervening valleys.

THE HISTORICAL RECORD

The largest earthquakes generated along the North American-Caribbean plate boundary cluster in the collisional zone of Hispaniola, where a magnitude (M) 8.1 thrust-type event and related tsunami occurred in 1946 and resulted in several hundred fatalities. The largest strike-slip event recorded on the western plate boundary was the M7.2 Guatemala event of 1976. This event took the lives of 22,780 Guatemalans and left more than a million homeless in a country with a total population then of about 5.5 million people. The potential for human loss of life in developing countries is staggering because of poor building standards and complacency about the dangers of large earthquakes.