More than 500 tremblors have struck Puerto Rico during a 10-day period.
On December 28, 2019, a M 4.7 earthquake followed by another M 5.0 shock struck the Southwestern part of Puerto Rico resulting in minimum damage. Since then, more than 500 tremblors measuring over M 2,0 have occurred. "Over the past several weeks, hundreds of small earthquakes have occurred in this same region, beginning in earnest with a magnitude 4.7 earthquake late on December 28 and a magnitude 5.0 event a few hours later," the United States Geological Survey (USGS), stated.
A M 6,4 event struck the region on January 7, 2020, resulting in one fatality and infrastructure damage. "Strong to very strong shaking occurred across parts of Southern Puerto Rico closest to the event and moderate shaking occurred across the rest of the island," USGS reported.
The M 6,4 earthquake was the strongest among the numerous shocks that hit Puerto Rico. It caused residences to collapse, houses to remain without water and extensive power outages. After the shock, Wanda Vázquez Garced, Governor of Puerto Rico, declared the country at a state of emergency and asked people to remain poised. "We have responded to many difficult situations, and here we are once again," Governor Vázquez stated.
Since earthquake monitoring initiated in Puerto Rico back in 1974, no similar sequence of seismic shocks has been recorded.
The current sequence is rather unsual. The probability that an earthquake is followed by a larger shock is just about 6%. Therefore, the most common pattern consists of main earthquakes followed by a series of smaller ones. Nevertheless, this is not the case for the current event. “What we’re looking at here are earthquakes of more or less common size: There are still lots of small ones but quite a few fives and then up to the 6.4. That is a little more like what we would call an earthquake swarm, where there isn’t a clear dominant one early in the sequence and then a tapering,” Jackie Caplan-Auerbach, a seismologist at Western Washington University in Bellingham, said.
Throughout Puerto Rico's history, the seismic shocks recorded indicate that it is a region of moderate seismicity. However, the country's location is tectonically active since it is located in the convergence zone between the North America and Caribbean plates. Therefore, large seismic shocks are expected.
USGS prediction models show that about 160 smaller aftershocks (>M 3,0) will struck the region in the following days and the possibility of an earthquake larger than M 6,4 is about 7%.
Prediction modeling is still a "work in progress" according to Von Hillebrandt-Andrade, a seismologist who manages Puerto Rico Weather Service’s Caribbean Tsunami Warning Program (CTWP). Jim Reilly, USGS Director, stated that the procedures utilized for prediction modeling have been significantly improved and provide reliable results.
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