Soil may supply surging streams after earthquakes

by Julia Rosen
Tuesday, June 16, 2015

Earthquakes don’t just rattle buildings and psyches — they shake up the hydrological cycle too. Scientists have long noticed spikes in streamflow following big quakes, but they have yet to pinpoint the causes of these surges. While most proposed explanations conjure different ways of uncorking groundwater reservoirs, a new study suggests that at least some of this extra water may actually get shaken out of the soil.

The biggest obstacle to studying earthquake hydrology is that researchers never know where to conduct studies or when to start paying attention; earthquakes are nothing if not hard to predict. “You can’t plan it; you are lucky or you are not lucky,” says Christian Mohr, an ecohydrologist at the University of Potsdam in Germany and lead author of the new study, published in Geology.

Mohr’s team had the good fortune of having a hydrologic network in place when the otherwise unfortunate magnitude-8.8 Maule earthquake rocked Chile from Concepción to Santiago on Feb. 27, 2010. The quake buckled buildings, triggered a tsunami and claimed more than 700 lives. But in the weeks that followed, it also afforded the scientists a valuable opportunity to investigate the hydrologic changes that the earthquake set in motion.

Using a network of streamgages positioned throughout small catchments in Chile’s coastal mountains, the researchers documented several clear responses to the temblor: an increase in overall streamflow; an amplification of the daily variation in streamflow; and, in the smallest tributaries, temporary drops in flow rates that preceded sustained surges.

In searching for the causes of these changes, the researchers first considered previously suggested mechanisms. For instance, one study suggested that shaking alters soil permeability, potentially allowing groundwater to flow more easily. But this did not fit with Mohr’s observations. He found that the catchment responded to rainfall the same way before and after the event, suggesting the permeability remained largely unchanged.

Others have suggested that earthquake water might be displaced from pores and cavities in the saturated zone (the zone below the water table) as the sediment settles into a denser configuration, like flour in a baker’s measuring cup, or that tectonic jolts rupture aquifers. But Mohr’s team found no evidence to support either of these mechanisms either.

So Mohr and his colleagues developed a new hypothesis. As part of their hydrologic monitoring network, the team installed sensors to measure soil moisture in the catchment. They discovered that the unsaturated layers of sediment (the zone above the water table) actually held a significant volume of water before the earthquake. Mohr figured that if you shook the soil hard enough to overcome the forces that bound water molecules to particles of dirt, the unsaturated zone might unleash a small flood.

To test the idea, the researchers used a numerical model to estimate what would have happened as a result of the 2010 earthquake. They found that water shaken loose from the soil could have raised the height of the water table across the catchment by as much as 20 millimeters. As it turned out, this closely matched the amount of excess water Mohr saw coursing through the streamgages following the quake, after he adjusted for the effects of plant evapotranspiration.

“It’s a hypothesis,” Mohr says, “but the most compelling suggestion that this might be a correct way to explain earthquake hydrology is that it quantitatively fits very well with the observations.”

Emily Brodsky, a seismologist at the University of California at Santa Cruz, agrees that Mohr’s data implicates the unsaturated zone, at least in this situation. The next step, she says, is to gather “a suite of streamflow measurements in a suite of environments” with varying soil moisture capacities and sediment types, and which are influenced differently by the underlying bedrock. After all, she says, “one stream does not a generalization make.”

Regardless of where the water came from, Mohr’s team suggests that while earthquakes are clearly hard on people, they may sometimes nurture plants. The researchers interpreted the strong daily variations in streamflow after the event as a sign that plants were consuming more water during the day. The quake struck during Chile’s dry season, and the vegetation in the catchment quickly capitalized on the windfall.

Mohr’s team is now investigating whether this boost in water uptake translated into healthier plants and increased landscape stability. “Tree growth is always related to root growth,” Mohr says, “and there could be positive effects.

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