These slow earthquakes occur adjacent to, but deeper than, ordinary earthquakes and do not produce noticeable ground shaking. In a recent Science Advances article co-authored with his PhD student Jeremy Gosselin and other members of his team, Professor Pascal Audet’s research shed light on this relatively new and exciting type of earthquake.
Previously, Prof. Audet and his team had measured the speed at which seismic waves travel through the slow earthquake source region and found evidence for anomalously high fluid pressures. At these depths, the pressure exerted on the rocks is very high, which normally tends to drive fluids out, similar to squeezing a sponge. However, these fluids are trapped within the rocks and are virtually incompressible. The resulting high fluid pressure may explain why slow earthquakes behave differently than ordinary earthquakes and move so slowly.
In his most recent work, Prof. Audet and his team compiled 25 years of seismic data and successfully detected slight temporal changes in the seismic velocities of the waves where fluid pressures are abnormally high. They interpreted these changes as strong evidence that pore fluid pressures fluctuate during slow earthquakes. This study provides the first direct evidence of this process in subduction zones.
Understanding what controls slow earthquakes can bring experts closer to characterizing the mechanisms that may trigger future large earthquakes. What remains unknown is whether fluid pressure fluctuations are a symptom or a cause of slow earthquakes. In the second hypothesis, these results would imply that large subduction zone faults are extremely sensitive to tiny perturbations in stress, meaning that they are constantly in a critical state, close to failure. This research has spurred the interest of several international groups studying slow earthquakes and has led to new collaborations with scientists from the United States, New Zealand and Japan.
Prof. Audet is currently planning experiments to test these ideas using highly sensitive instruments installed directly on the sea floor, closer to where slow earthquakes happen.
