… danger above and below

Last Friday, the Earthquake Unit at UWI Mona confirmed what many residents of Portland and St. Andrew had already guessed.  Thursday afternoon (October 7), there was a moderate quake (4.5 on the Richter scale), centred near Hope Bay in Portland.

About 200 earthquakes occur in Jamaica every year, most too small to feel.  The most active areas are the Blue Mountain block in eastern Jamaica and the Montpelier-Newmarket belt in western Jamaica. Other areas with notable activity include the near offshore southwest of Black River on the south coast, offshore Buff Bay and on the northeast coast.

The reason for all this activity lies in how the floating chunks of the earth’s crust (“plates”) move, and what happens when they rub against each other.  The Caribbean Plate is squeezed like a knee-cap between the North American Plate (ending just south of Cuba) and the South American Plate (just north of Venezuela).  A long sliver, the Gonave Micro-plate, is breaking away like a bone chip from the north edge of the Caribbean Plate.  Jamaica sits at the bottom of this micro-plate, about half-way along.

The North American Plate is moving west and the Caribbean Plate is moving (more slowly) eastward.  Their combined drift adds up to about an inch a year.  When friction stops a section of the edge from moving, a lot of momentum builds up.  Sooner or later, the “stuck” spot lets go, and the released energy shakes everything around it.  The micro-plate is along for the ride, which sometimes gets bumpy.

Last week, there was apparently a lot of friction at the “hot corner” of the Puerto Rico Trench, where the Caribbean Plate is pushing the North American Plate down.  On October 11, a tightly spaced clump of seven small shakes north of the Virgin Islands was followed by similar shocks north of Puerto Rico and the Dominican Republic. In the week before, the U.S. Geological Service reported a dozen quakes in this area, including a 4.0 in the Mona Passage.

On October 4, USGS reported a 5.1-magnitude quake off Cuba’s southeast coast.  This occurred on the edge of the Oriente Fracture that separates the Caribbean and North American Plates, almost straight north of the tremor that shook Jamaica on Thursday.

Jamaica has had only two major earthquakes in its history, in 1692 and 1907.  Are we due again?  A major earthquake anywhere in the Caribbean now would be more than usually dangerous.  An active storm season with a lot of heavy rain has left the soil everywhere soaked and sloppy.  A good shake could trigger landslides and weaken foundations in areas that would otherwise not be troubled.

We don’t get advance warning of earthquakes, as we do for storms, but there are a few things we can do to prepare.  First, find out whether you live in a fault zone, either from UWI Mona’s Earthquake Unit or from your local ODPEM office.  Second, check your surroundings to make sure your property is well drained, especially around foundations, and that any nearby trees are securely upright.

ODPEM does annual earthquake drills, to test the readiness of emergency services to deal with a lot of disasters happening at once.  This is useful.  With earthquakes as with hurricanes, you can never be prepared too early, or too well.