In a new report released today, USGS experts estimated that aftershocks from the magnitude 6.4 Jan. 7, 2020, earthquake will persist for years to decades, although with decreasing frequency. Additionally, earthquakes will likely be felt on a daily basis for several more months.
The forecasts found in this report can be used to guide public policy decisions or other actions.
The report forecasts aftershock duration potential for up to ten years after the magnitude 6.4 mainshock in southwestern Puerto Rico. This sequence is very active, and the probabilities of magnitude 5 and 6 or greater aftershocks remain high now and into the future. More detailed findings are in bullets below.
Puerto Rico lies in a tectonically active region where earthquakes have occurred for centuries, but because the country has not experienced a quake of this level of impact since 1918, the recent quakes, their aftershocks, and resulting damage took many islanders by surprise. About 7,500 people have left their homes for other kinds of shelter, including, in some cases, cars and tents.
“Puerto Rico’s recent large earthquake and its aftershocks were destructive and unsettling to people on the island,” said USGS Director Jim Reilly. “USGS and our partners at the Puerto Rico Seismic Network began deploying additional seismic stations shortly after the mainshock. We will continue to work diligently to provide timely earthquake information — such as this aftershock duration report — to help support and protect the local communities of our nation.”
The study results do not imply a change in the risk of earthquakes in other parts of Puerto Rico. The results in this report are based on the current behavior (as of January 17, 2020) of this aftershock sequence and may need to be modified if that behavior changes, including if a larger earthquake occurs.
Key findings of the report include:
- People in the area affected by the aftershocks may feel shaking from magnitude 3 aftershocks on a daily basis for the next 2 to 6 months. The rate of aftershocks is expected to decline, but they may still feel such shaking on a weekly basis for between 1.5 years and a decade.
- The annual chance of having one or more magnitude 5 or greater aftershock will remain above 25 percent for up to a decade or more.
- The annual chance of a magnitude 6 or greater aftershock will remain above 25 percent for 3 months to 3 years.
- Within the next year, there is a 20-30 percent chance of an aftershock as large as the mainshock or larger.
- Within the next year, there is a 5-10 percent chance of a magnitude 7 or greater earthquake.
- Future aftershocks are likely to be located where aftershocks have already been occurring — within about 30 kilometers (about 20 miles) of the epicenter of the magnitude 6.4 mainshock.
- The aftershocks discussed in this report would be located in the same general area as the aftershocks that have already occurred. Our results do not imply a change in the risk of earthquakes in other parts of Puerto Rico.
- Changes in the behavior of the aftershock sequence, including the occurrence of a new large aftershock, could require new estimates.
