Cave formations yield seismic clues

From London, at the Environmental Catastrophes and Recovery in the Holocene conference

SHAKEDOWN. The ages of a fallen cave formation’s growth rings and the mineral layers that later covered the formation can help pin down when a major quake occurred. Kagan/Geological Survey of Israel

Analyses of toppled stalagmites and other fallen rock formations in two Israeli caves may provide hints about the rate of ancient earthquakes in the area.

Stalactites, stalagmites, and other speleothems–or cave formations–grow as water travels through rocks, dissolves minerals, and then seeps into caves and redeposits those substances. Scientists can find out the age of layers within many such formations by determining the ratios of various chemical isotopes within the layers and comparing the numbers with those from nearby rocks of known ages, says Elisa J. Kagan of the Geological Survey of Israel in Jerusalem. Furthermore, by dating the youngest material in a fallen speleothem and the oldest material that accumulated on the formation after it fell, scientists can bracket the period when the breakage occurred.

Two small caves about 15 kilometers west of Jerusalem are littered with fallen speleothems. Both caves were discovered during quarrying operations in recent years, and neither had natural entrances. Therefore, no people or large animals could have caused the damage, says Kagan. She adds that the area isn’t plagued by subsidence, or settling of land, and there’s no evidence that an underground river ever breached the caves.

Kagan concludes that earthquakes probably caused the speleothems to fall. Most of the formations that have fallen onto flat surfaces point in directions that align with ground movements typically produced by a major fault about 60 km to the east of the caves, she notes.

The fallen speleothems probably reflect only major temblors, says Kagan. The 28 broken formations that could be dated in the two caves suggest there have been at least 15 significant earthquakes in the area at irregular intervals during the past 185,000 years. The dates of the rockfalls that occurred in the caves in the past 6,000 years coincide with quakes that have been documented in the historical record or dated by archaeological excavations. Likewise, all four groups of formations that fell between 70,000 and 20,000 years ago correlate with layers of sediment in the Dead Sea disturbed during quakes.

In eight groups of rockfalls more than 75,000 years old, broken speleothems are the only record of the prehistoric seismicity. Other scientists are now analyzing new samples of Dead Sea sediments that may confirm the dates of these ancient temblors.


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