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Rainwater can help trigger earthquakes

Where it rains, it rumbles. Rainwater and snowmelt help fuel intense earthquakes along a New Zealand tectonic fault, new research suggests.

Tracing the source of water flowing through New Zealand’s Alpine Fault shows that more than 99 percent of it originated from precipitation, researchers report April 19 in Earth and Planetary Science Letters. Scientists knew that underground fluids help trigger quakes, but the origins of these fluids have been uncertain. In this case, the nearby Southern Alps concentrate rainfall and meltwater on top of the Alpine Fault while the fault itself serves as an impermeable dam that traps the water.

The fault “essentially [is] promoting its own large fluid pressures that can lead to earthquakes,” says study coauthor Catriona Menzies, a geologist at the University of Southampton in England. Identifying the fluid source will help scientists better predict the fault’s seismic cycle, she says.

New Zealand sits on the boundary where the Australian and Pacific tectonic plates collide. This collision generates a powerful earthquake along the Alpine Fault around once every 330 years, with the most recent temblor in 1717; it also gradually formed the Southern Alps as the two plates scrunched upward. Moist air condenses on its way up and over the mountains, causing torrential rainfall that typically exceeds 10 meters annually. Menzies and colleagues wondered how much rainwater makes its way to the fault. Fluids within a fault help induce quakes by altering the strength of rock and by counteracting the forces that hold two sides of a fault together (SN: 7/11/15, p. 10).

Water divulges its origins in several ways. The researchers looked at water-deposited minerals in rocks, the relative abundance of helium in nearby hot springs and the various oxygen and hydrogen isotopes that made up the water — all fingerprints of the water’s source. Even though only about 0.02 to 0.05 percent of rainwater makes it to the fault’s depth, the work revealed that more water came from precipitation than from all other sources, such as water released from surrounding rocks and the underlying mantle. The 3-kilometer-tall Southern Alps may even serve as a water tower that boosts water pressure by heightening the stack of groundwater that sits on top of the fault.

While local geography makes the Alpine Fault unique, the new work provides a template for studying fluids in other earthquake-prone areas such as the recently active Japanese fault, says Patrick Fulton, a geophysicist at Texas A&M University in College Station.

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