Southern Puerto Rico was affected by a Mw 6.4 earthquake early on January 7, 2020, after experiencing a Mw5.8 earthquake the day before. Since late December, the region had been shaken by more than 200 M4+ earthquakes. The North American and Caribbean tectonic plates converge along the southwest and west of Puerto Rico, and this was the source of the recent stronger shaking activity. Estimated maximum shaking intensities in the populated areas of Ponce (estimated population ~153k) and Guyanilla (~5k) were considered very strong (MMI VII), and another 300k+ were estimated to have experienced strong shaking (MMI VI). Initial reports of impacts include at least one fatality, regional electric power and telecommunications blackouts, and structural damage in Ponce, Puerto Rico’s second largest city. Aftershocks are expected to continue for several days or weeks and could exacerbate damage in the region, which is still recovering from the impacts of Hurricane Maria (2017).
Mw6.4 Earthquake Estimated Ground Shaking Intensities Affecting Puerto Rico. Source: USGS.
Seismological Background
The Caribbean Tectonic Plate is bordered by 4 other plates: North American, Nazca, South America, and Cocos. The island of Puerto Rico is sandwiched between two trenches of subduction activity: The North American plate is converging in a westward direction with the Caribbean plate, which is also subducting under the island to its south. This southern trench (Muertos Trough) was the source of recent seismic activity, including the Mw6.4 earthquake on January 7th. The recent earthquakes are attributed to a deformation zone between two fault systems: Punta Montalva and Guayanilla Offshore.
Impacts
More than 10 days of seismic activity has induced a variety of damaging impacts to southwestern Puerto Rico. Prior to the largest shock of Mw6.4, a smaller earthquake of Mw5.8 on January caused isolated structural damage to non-engineered structures and collapse of the iconic Punta Ventana. The island entered a state of emergency on January 7th according to the Governor’s office. Regional electric power and telecommunication outages were slowly being restored later in the day on January 7th with a majority of restoration across the island anticipated in the next few days. More than 300 thousand people in Puerto Rico rely on electric powered pumps for their water supply. In the Ponce municipality, all five regional hospitals were open and functional following the earthquake.
Severe structural damage was reported in smaller towns like Guánica and Guayanilla located in closer proximity to the earthquake and thus likely to experience more significant impacts. Initial reports show collapse or severe damage to non-engineered or older structures. A number of landslides were reported to have impacted a major highway near Ponce. Inspections of schools and other government buildings are in process, while services are suspended due to the declared state of emergency.
Sources: Reuters, Associated Press, Agence France Presse, USGS.