Shaking Water Out of Sands: An Experimental Study

Strong earthquakes can cause different kinds of hydrological responses, and several mechanisms have been suggested to explain them. Verification of these mechanisms, however, is often lacking. Here we test some hypotheses with a laboratory experiment, in particular the hypothesis that dynamic strain mobilizes trapped water in the unsaturated zone. We construct a sand chamber, partially saturated with water, and subject it to “seismic” shaking of controlled energy. Pore pressure in the saturated and unsaturated zones is monitored before and after shaking. We identify three distinct mechanisms: consolidation of sediments in the saturated zone, release of capillary water from the capillary fringe, and mobilization of isolated pore water in the unsaturated zone. Each mechanism may cause pore pressure in the saturated zone to suddenly increase with shaking, and each may offer new insights to understand the source of the extra water and other shallow hydrological responses that appear after earthquakes. Plain Language Summary Earthquakes interact with water in a variety of interesting ways. For instance, during earthquakes, streamflow and groundwater level may increase, as occurred following the recent earthquake swarm in Puerto Rico. The mechanisms for such changes, however, are still unclear. In our paper, we measure the response of groundwater pressure to seismic shaking to test the hypothesis that earthquakes can cause shallow groundwater to flow toward streams. To accomplish this, we built a tall sand column, filled it with water, and instrumented it with sensors that measure how the pressure of water between sand grains (pore pressure) responds when the chamber is shaken by an impact. We found that “trapped” water in the shallow subsurface, which is held in place by adhesive forces between sand and water, can be released to the water table by seismic shaking, where it can then flow toward a stream. Key Points We test mechanisms for groundwater response to earthquakes by performing laboratory experiments We observe consolidation of sediments, mobilization of pore water, and release of capillary water from the capillary fringe These mechanisms can help explain coseismic increases in groundwater level and streamflow when there are no other apparent water sources