In the aftermath of magnitude 5.5 or larger earthquakes, the U.S. Geological Survey’s National Earthquake Information Center (NEIC) creates and distributes disaster response guides to decision-makers, search and rescue operations, and other groups. It also creates and circulates these products for smaller-magnitude but “societally important” events, such as quakes that cause fatalities or property damage, as well as ones that are scientifically interesting or show promise for informing future response efforts, said William Barnhart, an Earth scientist at the University of Iowa in Iowa City.
Recently, researchers studied the role of geodetic observations—especially interferometric synthetic aperture radar (InSAR) and satellite optical imagery—in earthquake response efforts. They explored how those observations inform and validate seismically derived source models, independently constrain earthquake impact products, and more, Barnhart and his colleagues noted in a study published in Remote Sensing on 6 June 2019. They also studied how geodetic observations improve ShakeMap, the Prompt Assessment of Global Earthquakes for Response (PAGER) system, and other NEIC earthquake response products.
Traditionally, NEIC products have relied on data from seismic networks because “seismometers are highly sensitive and provide data very rapidly,” equipping them to detect smaller events more quickly than if geodetic methods are used, Jessica Murray, the geodesy topical coordinator for the Earthquake Hazards Program in Menlo Park, Calif., wrote in an email to Eos. Murray wasn’t involved with the recent study, but she collaborated with Barnhart on another study of one of the earthquakes discussed in the paper.
However, geodetic measurements record the “permanent offset of the ground due to the fault motion without ‘clipping’ (loss of information that occurs when the ground shaking amplitude exceeds the range that the seismometer can measure),” Murray wrote. Therefore, geodetic data provide more accurate magnitude estimates for some large earthquakes, she added.
The recent article includes case studies of four earthquakes to show a wide range of utility for geodetic observations in earthquake response, says Barnhart.
Earthquake Case Studies
In 2013 the 7.7 moment magnitude earthquake in Baluchistan, Pakistan, killed hundreds. It occurred in a region with sparse seismic observations, “which contributed to an initial event mislocation that biased subsequent response products,” the researchers wrote in the study.
Spatially complex and unusual—“rupturing a non-planar fault bilaterally” over about 200 kilometers “at relatively shallow depths for a large crustal earthquake”—the quake’s characteristics were challenging to capture using teleseismic fault models, the researchers noted. However, “the geodetic observations provided the most direct constraint on the spatial characteristics of the Baluchistan earthquake without having to undertake any fault source inversions,” the researchers wrote.
The 6.0 moment magnitude quake in Napa, Calif., in 2014 was, at the time, the largest quake in the San Francisco Bay in more than 25 years. InSAR and GPS observations “were critical to imaging the spatially complex distribution of fault slip. Although this capability contributed little new information in densely instrumented portions of California, similar observations and modeling efforts applied to [other] moderate magnitude earthquakes…could prove critical in characterizing the rupture details of the earthquake,” the researchers wrote.
The 7.8 moment magnitude earthquake in Gorkha, Nepal, in 2015 killed approximately 9,000 people, injured thousands more, and damaged or destroyed more than 600,000 structures. Here the geodetic data refined and verified “an already well-constrained earthquake source model,” the researchers wrote. “The spatial constraints from InSAR were used to re-parameterize the teleseismic finite fault model that was then ingested into ShakeMap,” condensing the region with the most shaking and constraining fatality estimates to the 1,000 to 10,000 range, which “ultimately bore true,” they added.
Finally, the 7.5 moment magnitude earthquake in Palu, Indonesia, triggered a tsunami, liquefaction, and landslides, the combination of which killed at least 2,077 people, injured at least 4,438 more, and caused an estimated $911 million worth of damage. As in the Gorkha quake, the Palu quake’s hypocenter was correctly identified using the NEIC data. However, the alert level and impacts were underestimated until “pixel tracking results from Landsat-8 and Sentinel-2 imagery” revealed the earthquake’s approximately 130-kilometer extent, the researchers wrote. This tracking raised the PAGER alert level from yellow to red because of Palu’s expected high shaking exposure.
In the future, researchers hope to automate geodetic image processing and analysis at the NEIC, Barnhart said.
—Rachel Crowell (@writesRCrowell), Science Journalist