|Shake, Rattle, and Flow|
|Contact: Dan Krotz, email@example.com|
To better understand the complex dynamics that play out just beneath the earth's surface, a growing number of researchers are taking a closer look at how streams and wells respond to earthquakes.
Among them is Michael Manga of Berkeley Lab's Earth Sciences Division and the University of California at Berkeley's Department of Earth and Planetary Science. He believes that surges in stream flow and fluctuations in wells following earthquakes offer a window into how both hydrological systems and earthquakes work.
"The two are connected, and we can exploit their link to learn more about both," Manga says. In the June 27 issue of Science, Manga and David Montgomery of the University of Washington compile observations from four decades of research on the impact of tremors on streams and wells. They conclude that earthquakes offer a unique way to observe how hydrological systems behave, from small watersheds to vast aquifers.
For centuries, people have witnessed changes in streams and wells following earthquakes. California's 1989 Loma Prieta quake triggered several Bay Area streams to swell to 15 times their normal capacity. And the great Alaska earthquake of 1964 caused water levels in Florida wells to oscillate by six meters. But these phenomena have been largely dismissed as oddities -- interesting, but not important to a better understanding of hydrological systems and earthquakes.
After poring over dozens of studies, however, Manga and Montgomery find that the influence of earthquakes on groundwater, long considered a geologic sideshow, may be as essential as seismographs and stream gauges in learning how an earthquake's energy ripples across a continent, and how streams are fed by water trapped in soil.
"People have traditionally viewed how wells and streams respond to earthquakes as a scientific curiosity. But these responses are actually very important," says Manga.
Consider wells. Thousands of kilometers from the epicenter of a strong earthquake, water levels in wells sometimes rise and dip in a wavelike pattern that mirrors the quake's seismographic readout. The stronger the quake, the further away this effect is seen. More importantly, a well's water level may remain permanently changed after the quake subsides, indicating a fundamental shift in underground fluid pressure.
Precisely why this occurs remains unclear, but it seems to depend on the magnitude and direction of the seismic waves, and the structure of the aquifer in which the well is located. For example, the water level in a well drilled into bedrock ridges may drop as water seeps into newly formed rock fractures. And wells drilled into unconsolidated, valley-bottom deposits may rise as the loose ground becomes more compact -- like shaking a bowl of flour and watching it settle. This consolidation shrinks aquifers and squeezes water toward the surface.
Earthquakes can also transform lazy streams into rushing torrents. But unlike wells, which can respond to seismic waves generated from earthquakes several thousand kilometers away, only streams within a few hundred kilometers of an earthquake are affected. The difference lies in how water gets to streams, and how earthquakes influence this water source.
In some cases, streams are fed by shallow groundwater, not the deep aquifers that feed wells. The palpable, up-and-down waves that emanate from an earthquake's epicenter travel near the earth's surface, through this soil-trapped groundwater. When the soil is shaken, it becomes more compact, and its water is squeezed into a stream. This explains why some streams surge for only a few days after a quake -- once the groundwater is exhausted, stream flow subsides.
This process doesn't account for all earthquake-induced stream flow surges, however. The greatly increased stream flow that persisted for months after the Loma Prieta earthquake suggests a deeper water source than surface groundwater. But it does explain many stream surges, and it also helps researchers better understand streams and predict their response to earthquakes.
"By studying how streams respond to earthquakes, we learn how water gets into streams in the first place," Manga says. "We can also take what we know about how materials respond to dynamic strain, apply it to stream-scale hydrological systems, and predict a stream's response to a quake."
Such research may also enable scientists to predict where and how much a watershed will subside following an earthquake. By measuring a stream's surge and determining the soil consolidation required to wring out this excess water, they can determine the extent to which the ground deforms -- often by sinking several centimeters.
Another link between quakes and streams is found in the way seismic waves travel from an earthquake's epicenter. The waves, which cause dynamic strain, fade rapidly, which explains why surges in stream flow occur hundreds instead of thousands of kilometers from an earthquake. Seismic waves also cause liquefaction, the destructive phenomenon in which dynamic strain makes loosely consolidated, water-saturated soil behave like liquid.
The two phenomena often go hand in hand. The stronger the earthquake, the greater the distance from the epicenter that both liquefaction and increased stream flow occur. A magnitude 9 tremor, for example, can trigger liquefaction and stream surges up to 600 kilometers away. This doesn't mean increased stream flow is caused by liquefaction. Rather, it illustrates that both phenomena respond to the same process, dynamic strain. And, more importantly, it reveals yet another tie between quakes and streams.
"We want to understand the connections between how hydrological
systems work, and earthquakes work," Manga says. "By studying
their relationship, we learn how earthquakes change the landscape, and
we learn more about the long-term evolution of hydrological systems."