Puerto Rico in the northeast Caribbean Sea is home to over 3 million people with an additional
150,000 people living on the nearby Virgin Islands. In January 2020, Puerto Rico was
rattled for more than three weeks by over 400 earthquakes greater than M3. Before addressing
that sequence, let’s look at the broader tectonic setting and earthquake history.
The Caribbean Plate moves east at about 2 cm per year with respect to the North American
Plate. The convergent plate boundary is curved so that oceanic lithosphere of the North American
Plate enters the Puerto Rico Trench at an oblique angle. North of Hispaniola, oblique
collision of the Bahama Platform produces additional trench-parallel forces. The result
is a zone of distributed deformation with the Caribbean Plate broken into microplates,
including the Puerto Rico–Virgin Islands microplate between the Lesser Antilles and
Hispaniola. Examining over 1800 recent earthquakes with
M>4 shows shows the depth of the deepest earthquakes increasing with distance from the Puerto Rico
Trench. This map of depth to the subducting plate shows the oceanic lithosphere has reached
about 100 km depth beneath northern Puerto Rico which lies over 160 km from the trench.
The distance from the trench to Puerto Rico AND the depth to the subducting plate beneath
it, both contribute to seismic waves losing amplitude, and thus shaking potential, as
they travel from the megathrust to Puerto Rico.
The most recent megathrust earthquake occurred beneath Mona Canyon in 1943. with magnitude
7.7 earthquake. Rupture started at 35 km depth and moved upward toward the trench, but stopped
at 25 km depth so no tsunami was produced. Ground shaking in northwest Puerto Rico was
strong but damage was minimal. This is the largest earthquake on this sector of the subduction
plate boundary in the 500-year written record. It would be foolish, however, to assume it
is the largest earthquake possible. Because great magnitude 8 and 9 earthquakes often
have recurrence intervals over 1000 years, we do not know how large future Puerto Rico
subduction zone earthquakes might be. Returning to the cross section, we see the
capping layers of the limestone carbonate platform of Puerto Rico dip toward the trench.
Let’s go back 3 million years, when this shallow marine coral growth was originally
flat to examine the evolution of the subduction zone. At about this time, subduction became
steeper and depth of the trench increased to become the deepest part of the Atlantic
Ocean. These changes in geometry may have decreased friction on the plate boundary so
that great magnitude 8 or 9 megathrust earthquakes could be less likely.
However, this northward tilt makes the slope vulnerable to submarine landslides and two
large amphitheater-shaped features are probably sites of immense prehistoric landslides. If
the largest of these failed in a single landslide event, the resulting tsunami could have had
runups approaching 16 meters along the north coast of Puerto Rico. Indeed, submarine landslides
are an important tsunami hazard throughout the Greater and Lesser Antilles.
Let’s now examine the tectonic boundaries of the Puerto Rico – Virgin Islands microplate.
When we hold the Caribbean Plate fixed, we observe a small component of North America
Plate motion perpendicular to the trench and a larger component parallel to the trench.
[Pause 1-2sec] As shown by the earthquake history and modern GPS observations, this
oblique subduction drives separation of the Puerto Rico–Virgin Islands microplate from
the Lesser Antilles to the southeast and from Hispaniola to the west.
An example earthquake between the Virgin Islands and Lesser Antilles occurred in 1867. Six
meters of combined normal and left-lateral strike-slip displacement produced a fault
scarp on the north wall of the Virgin Islands Basin during this earthquake with approximate
magnitude 7.2. Uplift of the ocean floor generated a tsunami with runups over 7 meters at St
Croix and 6 meters at St Thomas. At least 23 and perhaps as many as 50 fatalities occurred
in the Virgin Islands. The 1785 and 1867 earthquakes document rifting of the Puerto Rico – Virgin
Islands microplate from the Lesser Antilles through the Anegada Passage.
On October 11, 1918 a magnitude 7.2 earthquake with 3 meters of normal-fault displacement
occurred in Mona Passage. Severe ground shaking on the west coast of Puerto Rico caused 76
fatalities. Five minutes after the earthquake, the sea receded from the shore at Punta Borinquen
and Punta Higuero, then returned two minutes later in a wave that reached over 6 meters
runup. Over the next 40 minutes, the tsunami swept down the west coast causing 140 drownings.
Bathymetry reveals a scarp caused by a submarine landslide 9 km across, and 150 meters thick
at the south end of Mona Canyon. Although tsunami modeling demonstrates this landslide
could have caused the 1918 tsunami, close visual observations by submersible suggest
it could be a much older landslide. The rifting in Mona Passage that resulted in the 1918
earthquake and continued extension of Mona Canyon is produced by oblique collision of
the Bahama Platform with northern Hispaniola which causes Hispaniola to be pulled away
from the Puerto Rico-Virgin Islands microplate. A series of moderate to strong normal- and
strike-slip-faulting earthquakes started in late December 2019 offshore of southwestern
Puerto Rico. Recent GPS observations indicate that the boundary between the Hispaniola and
Puerto Rico - Virgin Islands microplates passes through southwest Puerto Rico. The largest
of the 2020 earthquakes, at magnitude 6.4, occurred on January 7 south of Guayanilla.
Ponce, a city with population over 153,000, experienced very strong ground shaking with
one person killed and 8 injured. Damage to buildings and infrastructure was estimated
at $3.1 billion. Many onland faults, like the left-lateral Punta Montalva strike-slip
fault zone, and offshore structures, like the Investigator and Caja de Muertos normal-fault
zones, are located in this region. Ongoing research through the Puerto Rico Seismic Network
and offshore submarine geophysical surveys will clarify the faults responsible for the
2020 earthquake swarm. Plate boundary subduction zone earthquakes
are important hazards for Puerto Rico and the Virgin Islands. However, as demonstrated
recently, shallow earthquakes on the microplate boundaries also present important earthquake
and tsunami hazards.