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Sediment cores extracted from four lakes in Guatemala provide valuable insights into the seismic activity during the devastating 1976 magnitude 7.5 earthquake, as reported by researchers at the Seismological Society of America’s Annual Meeting.
This catastrophic earthquake resulted in the loss of over 23,000 lives and left approximately 1.5 million individuals displaced. It occurred along the Motagua Fault, a critical boundary between the North American and Caribbean tectonic plates.
The intense ground shaking from the earthquake triggered significant landslides and sediment-laden turbidity currents, which have been observed in the lakebed cores. Typically, one might anticipate that lakes situated further from an earthquake’s epicenter would exhibit thinner sediment deposits, as seismic waves generally lose strength with distance.
However, findings from these Guatemalan lakes indicate that the cores with the thickest sediment deposits were located at the end of the fault rupture. Jonathan Obrist-Farner, a geologist from Missouri University of Science and Technology, noted, “Lakes that are closest to the epicenter, positioned just away from the rupture path, show very thin deposits.”
Jeremy Maurer, a geophysicist from the same institution, highlighted that this unusual sediment pattern reflects the directivity of the 1976 earthquake’s shaking. He pointed out that while it’s common for scientists to uncover evidence of past seismic events in lake sediment cores, the geographical relationship of these lakes to the fault has not been extensively examined.
“What hasn’t been done as much is looking at where these lakes are located in relation to the fault,” he stated. “Are they off-axis or on-axis? Does the direction of the rupture influence sediment deposits?”
The U.S. Geological Survey’s analysis following the 1976 earthquake revealed notable discrepancies in structural damages. For instance, adobe houses located 10 kilometers south of the main rupture path remained standing, while those positioned on the fault trace and along the rupture’s propagation direction collapsed. Maurer emphasized that this highlights the significance of rupture directivity and now extends this understanding to sedimentological studies of lakes.
The research team initiated their coring efforts in 2022, aiming to delve deeper into not only the 1976 earthquake but also to uncover the paleoseismic history associated with the plate boundary—an area that remains underexplored. Obrist-Farner, who has roots in Guatemala, expressed enthusiasm for the opportunity: “We saw this as a chance to expand our knowledge of the seismic history of the region.”
Despite a surge of interest from seismologists in the wake of the earthquake, a prolonged civil conflict lasting 36 years and a lack of adequate monitoring have hampered ongoing research in the area. Collecting paleoseismic data, such as the findings from these lake records, is critical in developing a more comprehensive understanding of the region’s seismic risks.
In the previous year, Obrist-Farner’s team successfully obtained their largest sediment cores to date, with lengths that could encapsulate up to four thousand years of lake history. Preliminary analyses have already uncovered signs of an earthquake in 1816, estimated to be at least magnitude 7.5, mostly identified through historical records.
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