Video titulado “Puerto Rico Earthquake Swarm: December 2019 – January 2020” producido por el Pacific Tsunami Warning Center (PacificTWC) y publicado en su canal de YouTube el 2 de febrero de 2020. El video muestra gráficamente el enjambre de sobre 3,000 terremotos que han afectado a al isla desde diciembre 2019. Esta animación muestra dos meses de actividad sísmica e incluye información sobre el movimiento de las fallas durante los terremotos de mayor magnitud. Como podrán apreciar en el video, Puerto Rico está localizado en un área geológicamente compleja y propensa a terremotos.
Según la descripción del video (en inglés):
The start of 2020 saw some significant earthquake activity in the Puerto Rico region. A M6.4 earthquake struck the southern coast of Puerto Rico on January 7, killing at least one person and damaging many structures including the homes of thousands of people. It also produced a small, non-hazardous tsunami. It was preceded by a series of foreshocks in late December and followed by a swarm of aftershocks that continues today (February 2, 2020). This animation therefore begins on December 1, 2019, to show the typical level of earthquake activity prior to the start of the earthquake swarm on December 28. It proceeds forward in time at a rate of one day per second through the end of January 31. Puerto Rico lies above an active plate boundary between the North American and Caribbean Plates. In fact, it makes up part of a “microplate” sandwiched between these two larger plates. Relative motions between these plates cause earthquakes at their boundaries. To the north the North American plate grinds beneath Puerto Rico in a subduction zone, a type of plate boundary that can produce megathrust earthquakes with large vertical motions that can cause tsunamis. A mirror image of this structure lies to the south of Puerto Rico such that the Caribbean Plate likewise grinds beneath the island. As it is squeezed between these megathrust fault systems Puerto Rico itself deforms with complex faulting that produced the earthquake swarm in this animation. For an earthquake to pose a tsunami hazard it has to be able to significantly move the sea floor in a vertical direction, either by suddenly dropping or popping up. Therefore, when an earthquake occurs PTWC scientists need to rapidly determine an earthquake’s location, including its depth. Is it on land or under the ocean? Is it shallow enough to move the seafloor, or is it so deep that it doesn’t pose a risk? They then determine its magnitude, since a larger earthquake will move more of the sea floor and over a larger area. These parameters can be determined within a matter of minutes. But over the course of the first hour following an earthquake they will continue to analyze their data and they may also be able to determine which direction the seafloor moved. It may have moved primarily in a vertical direction (either up or down), and thus pose a greater tsunami risk. Or it may have moved mostly sideways, posing a lesser tsunami hazard. Once these scientists have this information they can use it to better predict how dangerous a tsunami may be, but until they can figure it out they will assume the worst-case scenario of maximum vertical motion. If they figure out later that the earthquake is something else, such as an earthquake that mostly moved sideways, they may downgrade or cancel their tsunami alert. A graphical way to show this sense of motion for earthquakes is the “focal mechanism” often informally referred to as a “beach ball.” This animation includes these symbols to show which direction the largest earthquakes moved. Thankfully these largest earthquakes did not pose a significant tsunami hazard because they were not big enough and/or moved sideways. These five earthquakes are:
- January 6, 2020 10:32Z — M5.7 — strike-slip (sideways) motion
- January 7, 2020 8:24Z — M6.4 — normal (downward) motion, 6 cm/2.4 in. tsunami
- January 7, 2020 8:34Z — M5.6 — normal (downward) motion
- January 7, 2020 11:18Z — M5.8 — strike-slip (sideways) motion
- January 11, 2020 12:55Z — M5.9 — strike-slip (sideways) motion