Resource Data

January 2020 Puerto Rico earthquake provides valuable data for ground failure models

Field surveys in the days following the Jan. 7, 2020, earthquake in Puerto Rico documented more than 300 landslides and severe liquefaction in southern coastal regions, according to a new study by researchers. from the US Geological Survey and the University of Puerto Rico Mayagüez.

The ground failure that resulted from the magnitude 6.4 earthquake was not exceptional, especially compared to the catastrophic ground failure during recent events such as the August 2021 earthquake in Haiti. But the ground failure survey led by USGS research geophysicist Kate Allstadt and her colleagues provides a valuable resource for Puerto Rico, which lacks an island-wide hazard map for landslides or liquefaction triggered by an earthquake.

In their article published in Seismological research lettersAllstadt and his colleagues discuss how they can use new survey data to refine situational awareness tools such as the USGS’s Ground Failure product, which quickly estimates landslide and liquefaction risk and population exposure after an earthquake.

The product uses statistically-based global models to make its estimates, but the data that goes into the models is biased toward extremely damaging events at the expense of more common and moderate events, Allstadt said.

“When the shaking is very strong, you lose the ability to understand the limits of a ground failure,” she explained. “And that’s one of the hardest things to model, because often the most attention is paid to really dramatic ground breaking events.”

In the week following the earthquake, USGS and UPR Mayagüez scientists went into the field to map ground failures, guided by the USGS Ground Failure product and satellite imagery. Social media and news reports, along with advice from emergency officials and citizens, helped Allstadt and his colleagues track down landslides and liquefaction.

The landslides were mostly concentrated in areas where peak ground acceleration exceeded 30% g – a measurement equivalent to someone feeling “very strong” shaking. Liquefaction mainly occurred in coastal areas where peak ground acceleration was greater than 50% g (shakes which appear “severe”), but some of the most damaging liquefaction occurred in Ponce, where estimates of jerks were as low as 20% g.

Near Ponce, “there was a stretch of houses along a creek where people had to evacuate because their houses were so badly damaged by liquefaction,” Allstadt said. “It was heartbreaking to watch.”

The researchers noted that the landslides occurred primarily in limestone rock along coastlines, a pattern very similar to landslides that occurred during the infamous 1918 San Fermin earthquake in Puerto Rico, one of the most deadly and economically devastating seismic events on the island. The description of the landslides in a 1919 report, they note, “could very well describe the 2020 earthquake if the place names were changed”.

“This suggests that this is the type of behavior characteristic of these types of rocks when shaken,” Allstadt said.

The quake surprised residents of Puerto Rico, who told Allstadt and his colleagues that they were more used to hurricanes that occur almost every year. Few Puerto Ricans were alive the last time a major earthquake hit the island.

K. Stephen Hughes of UPR Mayagüez, co-author of the SRL paper, and his colleagues previously mapped where precipitation-induced landslides are most likely in Puerto Rico. Only about 25% of landslides from the 2020 earthquake occurred “where we mapped precipitation-induced susceptibility as high, very high, or extremely high,” Hughes noted.

“Obviously, steep slopes can be susceptible to all kinds of landslides, but there are many other factors that influence how and when a slope fails – soil type, bedrock type and land use, for n ‘to name a few,’ he said.

Hughes and his colleagues are using the datasets they collected after Hurricane Maria and the 2020 earthquake to help them predict future landslides in Puerto Rico, especially when seismic and storm hazards coincide. .

Detailed information like that collected during the Puerto Rico reconnaissance survey can be used to develop regional versions of the Ground Failure product, Allstadt said. At the moment, she and her colleagues are working on a regional version for Alaska, caused by the devastating Anchorage earthquake in 2018. “We use the global model as an initial estimate, then update it with regional information. “, she said. “We’ll probably do something like that for Puerto Rico eventually with that data.”

The article is part of an upcoming special section in SRL on seismicity, tectonics and the sequence of the 2020 Puerto Rico earthquake.