Guatemalan Lake Sediments Reveal Directional Shaking of 1976 Earthquake

[kevinaggner]

Sediment cores extracted from four Guatemalan lakes have provided rare geological evidence of the direction in which ground shaking traveled during the devastating 1976 magnitude 7.5 earthquake, according to findings presented at the Seismological Society of America's Annual Meeting.

The earthquake, which struck along the Motagua Fault—marking the boundary between the North American and Caribbean tectonic plates—claimed over 23,000 lives and left 1.5 million people homeless. Severe ground motion triggered widespread landslides and turbidity currents, which deposited thick layers of sediment on lakebeds. These layers are now helping researchers better understand the dynamics of the quake.

Typically, scientists expect sediment deposits from earthquakes to be thinnest in lakes located farthest from the epicenter, as seismic energy diminishes with distance. However, in this case, the thickest deposits were found at the far end of the fault rupture, not near the epicenter, said Jonathan Obrist-Farner, a geologist at Missouri University of Science and Technology.

“Interestingly, lakes near the epicenter but outside the main rupture path show only thin sediment layers,” he explained.

Jeremy Maurer, a geophysicist also at Missouri University, suggested this pattern reflects the rupture’s directivity—an effect where the shaking intensity is stronger in the direction the rupture propagates. This idea is supported by historical field data from the U.S. Geological Survey, which noted that buildings 10 kilometers south of the fault survived, while those located along and ahead of the rupture direction were destroyed.

The team’s findings show that sediment records can do more than capture earthquake occurrences—they can also preserve information about the direction and behavior of ground motion.

“Sediment cores from lakes around the world, from New Zealand to Turkey, have helped reconstruct earthquake histories,” said Maurer. “But what’s less explored is how the lake’s location relative to the fault—whether it’s directly in the rupture’s path or off to the side—affects the sediment record.”

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The research team began coring the Guatemalan lakes in 2022, hoping to learn more not only about the 1976 quake but also about the region’s broader paleoseismic history. Despite early interest following the disaster, civil unrest and limited instrumentation have left Guatemala’s plate boundary poorly understood.

Originally from Guatemala, Obrist-Farner said the project offers an opportunity to fill critical gaps in seismic knowledge. Last year, the team recovered their longest sediment cores yet—some potentially capturing up to 4,000 years of lake history. Preliminary analysis has already revealed signs of a major earthquake in 1816, believed to have been at least magnitude 7.5, known primarily through historical records.

These findings underscore the value of lake sediment archives in regions where traditional seismic monitoring is sparse, offering new insight into past earthquakes and improving our understanding of future seismic risk.