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Quakes shake water from soil

New explanation for why streams flow faster after earthquakes.
19 March 2003

TOM CLARKE

The way streams respond to earthquakes has baffled geologists.
Alamy

Earthquakes shake water out of sodden soils, new research suggests, possibly explaining why streams flow more quickly after a big tremor.

Since the San Francisco earthquake of 1906, the observation that streams seem to carry the equivalent of a few extra millimetres of rain in the days and weeks following a quake has baffled geologists. "It's one of those curiosities of nature that has preoccupied people for years," says geologist Stuart Rojstaczer of Duke University in Durham, North Carolina.

Some experts propose that earthquakes compress the water-bearing rocks that naturally feed streams, wringing more water from them. Others maintain that quake movements expand these aquifers, riddling them with cracks through which can water seep out.

Neither explanation is true, conclude Michael Manga at the University of California, Berkeley, and colleagues. Their experiments indicate that surface soil, not rock, gives up its water and that it does so in response to shaking, rather than to squashing or stretching1.

"When you shake soil, pores containing water become compact and you squeeze out a little bit of water," explains team member Emily Brodsky, a seismologist at the University of California, Los Angeles.

"It's a cute idea," says Rojstaczer. He agrees that shaking is the answer, but doubts that the entire riddle is now solved as surface soils don't contain enough water to account for the increase in larger streams' flow following earthquakes. He argues that deeper aquifers are to blame: "Water tables drop significantly following large earthquakes".

Water tables drop significantly flowing large earthquakes
Stuart Rojstaczer
Duke University
North Carolina

Manga's team studied Sespe Creek in southern California, which is in one of the America's most seismically active regions. The creek has been rocked by 48 large earthquakes since geologists began monitoring its flow in 1928.

The researchers also shook soaked soils in the lab. Soils relinquished their water only in response to powerful fake quakes. This tallies with evidence from Sespe Creek and other streams: only earthquakes measuring 6 or 7 on the Richter Scale increase flow in nature.

References
  1. Manga, M., Brodsky, E. E. & Boone, M. Response of streamflow to multiple earthquakes. Geophysical Research Letters, 30, 1214 - 1217, (2003). |Article|


Nature News Service / Macmillan Magazines Ltd 2003

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science books

Earthshaking Science: What We Know, and Don't Know, About Earthquakes
$17.47
UK readers buy at amazon.co.uk

The Mechanics of Earthquakes and Faulting
$48.00
UK readers buy at amazon.co.uk