Quakes shake water from soil
New explanation for why streams flow faster after
19 March 2003
|The way streams respond to earthquakes has baffled
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
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.