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Morning Update for Puerto Rico - January 7, 2020

January 7, 2020

On Jan. 7, a magnitude 6.4 earthquake struck the region at 4:24 am local time (08:24:26 UTC). Significant damage is possible. Over the past several weeks, hundreds of small earthquakes have occurred in the Puerto Rico region, beginning in earnest with a M 4.7 earthquake late on December 28 and a M 5.0 event a few hours.

The magnitude 6.4 earthquake was widely felt. According to ShakeMap, 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. The NOAA Tsunami Warning System states no tsunami warning or advisory. The USGS event page on this earthquake includes an aftershock forecast. Aftershocks will continue near the mainshock. 

Over the past several weeks, hundreds of small earthquakes have occurred in this same region, beginning in earnest with a M 4.7 earthquake late on December 28 and a M 5.0 event a few hours later. Since the M 4.7 event, over 400 M 2+ earthquakes have occurred in this region, ten of which were M 4+, including today’s M 6.4 event and yesterday's 5.8 quake. The preliminary location of today's 6.4 earthquake is within about 7.5 miles (12 km) of the January 6, 2020, M 5.8 earthquake. The proximity of these events to Puerto Rico, and their shallow depth, mean that dozens of these events have been felt on land, though with the exception of the latest two earthquakes, the M 6.4 and the M 5.8, none are likely to have caused significant damage.

The January 6 and 7, 2020, M 5.8 and M 6.4 earthquakes offshore of southwest Puerto Rico occurred as the result of oblique strike slip faulting at shallow depth. At the location of this event, the North America plate converges with the Caribbean plate at a rate of about 20 mm/yr towards the west-southwest. The location and style of faulting for the event is consistent with an intraplate tectonic setting within the upper crust of the Caribbean plate, rather than on the plate boundary between the two plates.

Topography and bathymetry map of the Northeastern Caribbean.
Map of the Northeastern Caribbean: topography is in shades of green and bathymetry in shades of blue. Fault traces are shown as lines with the following descriptions: barbed=thrust fault; solid=strike-slip fault with arrows showing relative direction of motion; black and white=normal fault. Faults outlined in red have a potential to generate a large earthquake. The arrow at the top right corner shows the direction of the North American plate motion relative to the Caribbean plate. All red stars with a date before the 20th century show the intensity center (MI) for that earthquake. The red star with a red ellipse shows the general location of several 17th century earthquakes with sparse felt data. Stars with 20th and 21st century years are earthquake epicenters located using instrument data. Years in boxes next to red outlines are reoccurrence rates. Red question marks indicate unknown reoccurrence rates for those respective faults. Those parts of the subduction zone not outlined by red are not expected from this study to generate large earthquakes (Public domain.)

Tectonics in Puerto Rico are dominated by the convergence between the North America and Caribbean plates, with the island being squeezed between the two. To the north of Puerto Rico, North America subducts beneath the Caribbean plate along the Puerto Rico trench. To the south of the island, and south of today’s earthquake, Caribbean plate upper crust subducts beneath Puerto Rico at the Muertos Trough. The January 6 earthquake, and other recent nearby events, are occurring in the offshore deformation zone bound by the Punta Montalva Fault on land and the Guayanilla Canyon offshore.

Visit the USGS event page for more information. For estimates of casualties and damage, visit the USGS Prompt Assessment of Global Earthquakes for Response (PAGER) website. If you felt any of these earthquakes, report your experience on the “USGS Did You Feel It?” website for this event.

USGS scientists expect that this event will trigger aftershocks, but these will decrease in frequency over time.

The USGS operates a 24/7 National Earthquake Information Center in Colorado that can be reached for more information at 303-273-8500. Learn more about the USGS Earthquake Hazards Program.


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