Scientists have long puzzled over the origin of earthquake swarms: the sudden appearance of many small earthquakes in a small region. They commonly appear before big earthquakes or volcanic eruptions, but these are a tiny fraction of all earthquake swarms. Most swarms happen far from volcanoes and areas of tectonic activity.
Studying an earthquake swarm that hit an oil and gas field in Oklahoma, scientists from ETH Zurich and the University of Texas at Austin suggest a simple explanation in a paper published in Nature Geoscience. Swarms are caused by changes in the balance of groundwater and fossil water from deep layers brought up by oil and gas extraction.
"Swarm quakes are one of the most common types of earthquakes worldwide, but their origin has been a real mystery," says ETH Professor Domenico Giardini. "We wanted to find out why, in most cases, there is nothing more behind them than water".
Hydroquakes caused by human activity
Most of the water that is present naturally in the Earth's crust is locked in tiny pores between mineral grains. However, when rocks are subjected to very high pressure, they may start leaking the absorbed water.
The swarm studied by the scientists in Oklahoma lasted six months and occurred at relatively shallow depths of 1.5 to 6.5 kilometers. As the scientists show in their paper, during this swarm, the level of groundwater in the affected region rose by more than 10 meters. At the same time, the fluid pressure within the Earth's crust increased as well, which triggered the series of earthquakes.
The earthquakes were likely triggered when the rising water reached geological fault zones. Faults are fractured surfaces between rocks, and under normal conditions the surfaces of the opposing rock are sealed by the pressure they are subjected to. However, as pressure in the surrounding rock increases, the fluids in the fault plane become able to dissolve and wear down the rock surfaces, weakening the rocks' ability to resist.
Eventually the forces acting on the fault planes cause the rocks to slip, releasing the built-up energy in the form of an earthquake. "Swarms are like tiny copies of what happens before big tectonic earthquakes, so we can learn from them on a much smaller scale", explains ETH Postdoctoral Researcher Michael Goebel, lead author of the paper.
A chain of events
For some faults the process of weakening happens fairly rapidly, within a matter of months or even hours, and the fluids are able to force the rocks apart quite rapidly, which creates a swarm of small tremors. For other faults the weakening happens more slowly over years, and the resulting movements will produce just a few but much bigger earthquakes.
In their study, the researchers describe a complex interplay between the changes in the groundwater level, which lead to changes in pressure in the rock and the properties of the faults, which lead to changes in the size of earthquakes. "Understanding this interplay, is crucial to understanding the whole process", says Giardini, who is also the Director of the ETH Zurich's Swiss Seismological Service (SED), which operates a network of seismic monitoring stations across the country.
A common phenomenon?
The scientists studied this particular swarm in Oklahoma, but they suspect that the underlying mechanisms are common to other swarms happening in other regions of the world. Swarms frequently occur in connection with oil and gas extraction and also happen in geothermal fields and near dams, where changes in the water budget could trigger changes in crustal pressure.
The results obtained by the researchers are especially relevant for seismic monitoring for safety purposes, for example the monitoring at nuclear power plants or during fluid injections into the ground. Their findings help to separate harmless fluid-driven swarms from other swarms that might be related to volcanic or tectonic activity.
