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

Date:
February 9, 2022
Source:
Seismological Society of America
Summary:
Field surveys conducted in the days after the 7 January 2020 Puerto
Rico earthquake documented more than 300 landslides and severe
liquefaction in southern coastal regions, according to a new study.



FULL STORY ========================================================================== Field surveys conducted in the days after the 7 January 2020 Puerto Rico earthquake documented more than 300 landslides and severe liquefaction
in southern coastal regions, according to a new study by U.S. Geological
Survey and University of Puerto Rico Mayagu"ez researchers.


==========================================================================
The ground failure that resulted from the magnitude 6.4 earthquake
wasn't exceptional, especially compared to the catastrophic ground
failure in recent events such as the August 2021 Haiti earthquake. But
the ground failure survey conducted by USGS research geophysicist Kate
Allstadt and colleagues provides a valuable resource for Puerto Rico,
which has no island-wide hazard map for earthquake-triggered landslides
or liquefaction.

In their paper published in Seismological Research Letters, Allstadt and
her colleagues discuss how they can use the new survey data to refine situational awareness tools such as the USGS' Ground Failure product,
which quickly estimates landslide and liquefaction hazards and population exposure after an earthquake.

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

"When the shaking's really strong, you lose the ability to understand
the limitations of ground failure," she explained. "And that's one of
the hardest things to model, because often most of the attention is
given to the really dramatic ground failure events." In the week after
the earthquake, USGS and UPR Mayagu"ez scientists went into the field
to map ground failure, guided by the USGS Ground Failure product and
satellite imagery. Social media and news reports, along with tips from emergency managers and citizens, helped Allstadt and colleagues track
down landslides and liquefaction.



==========================================================================
The landslides were mostly concentrated in areas where peak ground
acceleration exceeded 30% g--a measure equivalent to a person feeling
"very strong" shaking.

Liquefaction mostly occurred in coastal areas where peak ground
acceleration was larger than 50% g (shaking that feels "severe"),
but some of the most damaging liquefaction took place in Ponce, where
shaking estimates were as low as 20% g.

Near Ponce, "there was a stretch of houses along a creek where people
had to evacuate because their homes were so damaged by liquefaction,"
Allstadt said.

"It was heartbreaking to see." The researchers noted that landslides
mostly occurred in limestone rock along the coasts, a pattern very similar
to the landslides that occurred during Puerto Rico's infamous 1918 San
Fermin earthquake, one of the island's most deadly and economically
devastating seismic events. The description of landslides in a 1919
report, they note, "could very well describe the 2020 earthquake if
the place names were changed." "It does suggest that this is the characteristic type of behavior for these types of rock when they are
shaken," said Allstadt.

The earthquake surprised people in Puerto Rico, who told Allstadt and
her colleagues that they are more used to hurricanes which come almost
every year.

Few Puerto Ricans were alive the last time a major earthquake affected
the island.



==========================================================================
K. Stephen Hughes of UPR Mayagu"ez, a co-author on the SRL paper, and his colleagues previously mapped where rainfall-induced landslides are mostly likely in Puerto Rico. Only about 25% of the 2020 earthquake landslides occurred "where we mapped the rainfall-induced susceptibility as high,
very high, or extremely high," Hughes noted.

"Obviously steep slopes can be susceptible for all kinds of landsliding,
but there are many, many other factors that influence how and when a
slope fails-- soil type, bedrock type, and land use, just to name a few,"
he said.

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

Detailed information like that collected in the Puerto Rico reconnaissance survey can be used to develop regional versions of the Ground Failure
product, Allstadt said. For the moment, she and her colleagues are working
on a regional version for Alaska, prompted by the damaging 2018 Anchorage earthquake. "We are using the global model as the initial guess and
then updating it with regional information," she said. "We'll likely
do something like that for Puerto Rico eventually with these data."
The paper is part of an upcoming special focus section in SRL on Puerto
Rico seismicity, tectonics, and the 2020 earthquake sequence.

========================================================================== Story Source: Materials provided by
Seismological_Society_of_America. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Kate E. Allstadt, Eric M. Thompson, Desiree' Bayouth Garci'a, Edwin
Irizarry Brugman, K. Stephen Hughes, Robert G. Schmitt. Ground
Failure Triggered by the 7 January 2020 M 6.4 Puerto Rico
Earthquake.

Seismological Research Letters, 2022; DOI: 10.1785/0220210235 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220209154948.htm

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