Uncovering Hidden Patterns: Lake Deposits Reveal Directional Shaking During Devastating 1976 Guatemala Earthquake

The recent analysis of lake deposits in Guatemala has provided researchers with a unique insight into the 1976 magnitude 7.5 earthquake that ravaged the country, killing over 23,000 people and leaving about 1.5 million homeless. The study, presented at the Seismological Society of America’s Annual Meeting, reveals the distinct direction that ground shaking traveled during the devastating event.

The researchers, from Missouri University of Science and Technology, drew sediment cores from four lakes in Guatemala, which recorded the severe ground shaking caused by the earthquake. Contrary to expectations, the thickest sediment deposits were found at the end of the fault rupture, rather than in lakes furthest away from the epicenter.

According to Jonathan Obrist-Farner, a geologist and lead researcher on the project, this unusual pattern is believed to have recorded the directivity of the 1976 shaking. Directivity refers to the way seismic waves propagate in different directions depending on the orientation of the fault rupture.

The researchers’ findings are significant because they provide new information about the paleoseismic history of the plate boundary between the North American and Caribbean tectonic plates. This region has been poorly monitored due to a 36-year civil war, which ended with sparse instrumentation being left behind.

Jeremy Maurer, a geophysicist on the team, highlighted that scientists have previously found evidence of past earthquakes in lake sediment cores from various locations around the world, such as New Zealand and Turkey. However, what hasn’t been done as much is looking at where these lakes are located in relationship to the fault, specifically whether they are off-axis or on-axis.

The researchers began recovering and analyzing cores from the lakes in 2022, aiming not just to study the 1976 earthquake but also to learn more about the paleoseismic history of the plate boundary. They retrieved their largest cores yet last year, with lengths of sediment that may represent up to four thousand years of lake history.

Their initial analysis shows evidence of the 1816 earthquake of at least magnitude 7.5, which is known mostly from historical documents. This discovery is crucial for building a more complete picture of Guatemala’s seismic risk, as it highlights the importance of paleoseismic data in understanding the country’s tectonic history.