The islands of the Lesser Antilles are home to 3.2 million people. Let's
investigate the tectonics and earthquakes of the subduction zone along
that margin of the Caribbean Plate. The Caribbean Plate moves east at about
two centimeters per year with respect to the North and South
American Plates. There is essentially no relative motion,
and no earthquake zone marking the boundary between the North
and South American Plates in this region, so we will informally refer to the plate
subducting along the Lesser Antilles subduction zone as
the "Atlantic plate". As that oceanic lithosphere dives beneath the
Caribbean Plate it produces earthquakes of increasing
depth from east to west across the subduction zone.
Ocean-bottom seismometer observations have refined plate tectonic across
sections and these contours show the depth to the top of the subducting plate.
Also of note, is that volcanoes are located above the point where the top of
the subducting plate reaches 100 to 150 kilometers depth.
In the northern part of the subduction zone, the Atlantic plate subducts to
about 150 kilometers depth beneath the volcanic arc
that lies 300 kilometers from the trench. The accretionary prism of sediment,
scraped off the subducting plate onto the overriding plate,
is about five kilometers thick. Near the southern part of the
Lesser Antilles trench where the Orinoco River sheds sediment
from the South American continent, the Atlantic plate carries the sediment
load many kilometers thick towards the trench. Oceanic lithosphere
subducts at a shallow angle in this region.
Where distance from the trench to the volcanic arc is 460 kilometers,
the accretionary prism is nearly 20 kilometers thick
due to the sediment load delivered to the trench and piled onto the Caribbean Plate.
Now let's dig into the earthquake
history. In January 1843, over 120,000 people lived on the island of Guadaloupe.
On february 8th, the largest earthquake on record
shook the region. All masonry structures in Port-au-Petre
were destroyed resulting in 4,000-6,000 fatalities
This event illustrates the difficulty of
interpreting patterns of earthquake intensity from written accounts of
ground shaking and damage, then inferring earthquake properties
without seismometer recordings. For example, some researchers developed
intensity maps with isoseismal zones elongated parallel to the trench and
inferred the Guadaloupe earthquake was a magnitude 8 or larger
plate-boundary megathrust earthquake. Lack of tsunami indicated
that the rupture did not reach the ocean floor.
But other researchers developed intensity maps
with a smaller area of violent intensity shaking with more circular isoseismal zones.
Without knowing how deep the earthquake was,
this intensity pattern could indicate several possibilities.
Was it a shallow magnitude 7.5 within the overriding plate?
Was it a magnitude 8 plate-boundary megathrust? Or was it a larger event
within the subducting plate? So, it's not clear whether that
earthquake was a plate boundary megathrust event.
Since 1950, four major earthquakes have occurred within the subducting plate.
These events were mostly due to extension within the upper part of the
subducting plate as it bends into the subduction zone. Depth to the
hypocenter, below inhabited islands, resulted in low human impact.
In 1974, a magnitude 7.2 earthquake occurred northwest of Antigua
producing the strongest shaking in the Lesser Antilles since 1843.
Because the epicenter was nearly 20 kilometers from Antigua, and about 25
kilometers deep there were no fatalities. Though some
damage occurred to buildings and port facilities. As the Lesser
Antilles Trench curves from north-northwest
to east-west where it meets the Puerto Rico Trench, the Atlantic plate subducts
at an increasingly oblique angle. Resulting trench parallel forces
produced the Bunce Fault, an 850 kilometer long strike-slip fault
that transports a sliver of a accretionary prism along the trench.
This structure was named for Betty Bunce, a pioneering marine geophysicist who was
the first American woman to serve as chief scientist on a major
oceanographic expedition. Marine geophysics and
seismicity indicate the northern forearc of the
Lesser Antilles is broken by normal faults.
The Antigua earthquake occurred on one of these normal faults
that pose earthquake and tsunami hazards to the region.
With only five major earthquakes in the past 70 years,
and perhaps no great megathrust earthquakes in the past 200 years,
the unusually low seismicity here has puzzled seismologists for decades.
GPS observations can now monitor subduction zone deformation.
Most examples, like Japan or Alaska, have a high friction mega thrust fault that
is stuck for decades or centuries compressing and
pushing the overriding plate backwards. When forces eventually exceed friction,
an earthquake releases the stored elastic energy in a
major magnitude 7, or great magnitude 8 or magnitude 9
earthquake.
GPS observations reveal different dynamics for the Lesser Antilles
subduction zone. GPS stations on the islands show little
motion, indicating that the Caribbean Plate is
not being compressed or pushed back as the Atlantic plate subducts. Instead,
the low friction mega thrust boundary slips more smoothly
with fewer earthquakes, and perhaps without great magnitude eight or nine
earthquakes. Although the dynamics of the plate
boundary are complex and a topic of ongoing
research, two factors are likely to contribute to low friction on the Lesser
Antilles megathrust boundary. The Atlantic plate is old cold and dense
where it enters the subduction zone. It's pulled down by gravity, essentially
free falling into the deeper mantle, so frictional
coupling along the boundary is very low. Additionally, oceanic crust formed at a
slow spreading ridge, like the Atlantic plate, contains
hydrated minerals that release fluids as the subducting plate warms.
These fluids can further decrease friction along this plate boundary
so great mega-thrust earthquakes and the resulting tsunamis
may be extremely rare or entirely absent in the Lesser Antilles.
This does not mean there is no tsunami hazard
The great Lisbon earthquake of 1755 killed over 50,000 people from building
collapse, fires, and a tsunami. That tsunami
crossed the Atlantic in seven hours, and without warning swept
southward across the island arc with run-ups from seven meters at Saba
to 1.5 meters at Barbados. The largest historic tsunami of regional
tectonic origin was produced by the 1867 magnitude 7.2
earthquake in the Anagada Passage. Even at Granada
tsunami run-up reached 3 meters.
In terms of natural hazards, volcanic eruptions and hurricanes in the Lesser Antilles
have been far more deadly historically than have earthquakes or tsunamis.
In 1900 the town of Saint Pierre on Martinique was a bustling seaport.
The catastrophic eruption of Montpellier in 1902
killed 30,000 people in a matter of minutes when a pyroclastic flow barreled
down the mountain burying the town. Volcanic activity also
presents tsunami hazards from explosions landslides and collapse of volcano
flanks displacing ocean water. The great hurricane of 1780 pounded
across the lesser antilles with wind speeds as high as
320 kilometers per hour killing over 22,000 people, making it the
deadliest hurricane in recorded history in the Atlantic
Whereas modern weather and observations and modeling provide warnings that save
lives and mitigate storm damage, hurricanes remain a deadly and costly
natural hazard in the Lesser Antilles. Though less frequent than hurricanes,
major earthquakes can occur anywhere in the Lesser Antilles with no
warning. Protective actions like drop, cover, and hold on
should still be understood by all who live in or visit this region.