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Monday, November 28, 2022

Ancient zircons offer insight into past earthquakes

Earthquakes have rocked the planet for eons. Studying ancient earthquakes could help scientists better understand modern tremors, but the tools to do that job are in short supply.

Enter zircons. The researchers used the gemstones to determine the temperatures reached within a fault during earthquakes millions of years ago. The method provides insight into the intensity of earthquakes long ago and could improve understanding of how current tremors release energy, the researchers report in the April issue. Geochemistry, Geophysics, Geosystems.

“The more we understand about the past, the more we can understand what might happen in the future,” says Emma Armstrong, a thermochronologist at Utah State University in Logan.

Armstrong and his colleagues focused on California’s Punchbowl Fault. That now-quiet portion of the larger San Andreas fault was probably active between 1 million and 10 million years ago, Armstrong says.

Frictional heat is generated on a fault when it slips and triggers an earthquake. Previous analyzes of preserved organic material suggested that temperatures within the Punchbowl Fault peaked between 465° Celsius and 1065°C. The researchers suspected that zircons in the fault rocks might narrow that wide window.

Zircons often contain the radioactive chemical elements uranium and thorium, which decay into helium at a predictable rate (Serial number: 2/5/22). That helium then accumulates in the crystals. But when a zircon is heated beyond a threshold temperature, the magnitude of which depends on the composition of the zircon, the accumulated helium escapes.

Measurement of the amounts of the three elements in the fault’s zircons suggests that the most intense earthquake generated temperatures below 800 °C. That roughly halves the previously reported range. The finding provides clues about the amount of heat earthquakes release, something difficult to measure for modern tremors because they often occur at great depths.

Armstrong plans to continue studying zircons, hoping to find more ways to exploit them for details about ancient earthquakes.

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