Microseismicity and Temporal Changes in Seismic Velocity Reveal Crustal Response to Dynamic Stress
Abstract:Earthquake occurrence is both a driver of and the result of complexity in the stress field and elastic properties of the crust. If we understand how static and dynamic stresses perturb the crust, we can use these observations to better understand the current state of the crust. Here we focus on how the crust responds to dynamic stresses outside the region where static stresses are important. A tool to understand the current state of the crust has broad applications in seismic hazards, induced seismicity, and a general understanding of earthquake physics.
As dynamic strains outside of the near field of an earthquake are small, often no more than a micro strain, we need sensitive measures to detect any resulting perturbations to the properties of the crust. Two methods we currently use are (i) measuring temporal changes in seismic velocities applying ambient noise, and (ii) measuring the spatiotemporal change in microseismic activity. A strength of using ambient noise to detect changes in seismic velocities is the ability to stack over space and time to reveal very small changes. We use microseismicity because there are many more small earthquakes than large ones, so more robust statistics can be applied to small earthquakes—applying microseismicity improves the detection of changes within the crust.
A striking observation applying these methods is widespread increased seismicity and a transient increase in seismic velocities in Japan after the passage of surface waves from the 2012 M8.6 Indian Ocean earthquake. The velocity observations are obtained by stacking 113 Hinet stations and using 10-day windows. The velocity change extends to at least 5 km depth based on the spatial wavelengths employed (0.1-5 Hz). Based on our field observations and those of others, as well as our laboratory studies one may expect a decrease in the crustal velocity from wave dynamics. The observed increase in velocity could be an indirect effect such that frictional contacts stiffen under changing quasi-static stress conditions in the presence of an increase in microseismicity. Alternatively, an increase in velocity may suggest that weak motions strengthen frictional contacts within the crust, whereas those contacts are slightly destabilized under ambient conditions.