A magnitude 6.4 earthquake struck 5 miles (8 kilometers) south of Indios, Puerto Rico, on January 7, 2020, at 4:24 a.m local time. Pacific time. According to the United States Geological Survey (USGS), this event has been identified as the main shock event (with 97% probability). A magnitude 5.8 event that struck on January 6 in a similar location is now identified as a foreshock.
There have been numerous smaller earthquakes as part of the sequence. In fact, since the beginning of 2020, there have been approximately 210 earthquakes in this southern region of Puerto Rico, with a minimum magnitude of 2.5. These can be seen in Figure 1 below. There have been three 5.5 – 6.0 magnitude earthquakes, with the magnitude 5.8 being the furthest south and away from land. As a reminder, the magnitude 6.4 main shock releases eight times as much energy as the magnitude 5.8 foreshock. Both earthquakes are a result of shallow normal faulting, according the USGS.
As a result of the recent earthquake, power outages have been reported across the Puerto Rico, though reports expect power to be returned within the short term. According to the USGS PAGER, around 175,000 people along areas of the south coast of Puerto Rico experienced ‘very strong’ ground motions – motions at a Modified Mercalli Intensity (MMI) 7, that were observed from Ponce, a large city in southern Puerto Rico. The area to the north, within a 12.4 mile (20 kilometer) radius, observed “strong” ground shaking, at MMI 6. At MMI 7, we typically expect no damage to earthquake-resistant buildings and considerable damage in older or weaker buildings. Other areas may experience pockets of very light damage, but the vast majority of loss is expected to come from the city of Ponce, which has the largest level of infrastructure and highest areas of hazard.
Puerto Rico is still recovering from the effects of Hurricane Maria in 2017. Losses were felt across the island, with reported economic losses exceeding $1 billion in Ponce alone. With this earthquake sequence, CoreLogic expects the economic loss to be maintained within the $1 billion mark.
A unique case that may be experienced with this earthquake event is damage acceleration. This refers to the cumulative effect of damage from different events. However, in this case, it is not from the foreshock but rather from the buildings that were partially damaged from Hurricane Maria in 2017 that have yet to be repaired. This causes added uncertainty in loss estimation.
There is also uncertainty from the effect of demand surge, where materials are in short supply from the repairs from Hurricane Maria. As such, there could be an additional spike in the cost of building materials and reconstruction work. Because the island has been struck by two events in close proximity, repair costs could easily increase by up to 33% without federal or local government intervention.
While the probability that the mainshock of this sequence is the magnitude 6.4 that occurred on January 7, the aftershock forecast from the USGS does assign a 22% probability that at least a magnitude 6.0 may still occur. In Figure 1, a concentration of smaller earthquake activity can be seen. Much of this activity is occurring offshore, further away from land. This points to the maximum area of stress in the deformation zone from this normal intraplate faulting sequence. While we cannot be certain, the data points toward no further earthquakes in excess of magnitude 5.5, and if they do, they would occur further away from where the main concentration of exposure is located.
While an earthquake occurring in the sea in this region could be tsunamigenic, that is, having the potential of causing a tsunami, the magnitudes that are being discussed are not likely to cause significant wave heights on land and as such pose limited-to-no risk.
For more information about Puerto Rico’s spate of earthquakes or any other catastrophe insights, please visit Hazard HQ™.
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