Greg McLaskey, Assistant Professor at Cornell U., presents "Laboratory generated earthquakes from meter-scale to mm-scale with heterogeneous fault properties."
"Laboratory generated earthquakes and acoustic emissions range from M -2, for meter-scale ruptures, down to M -10. On the larger scale, I describe dynamic rupture events that terminate within a 3 m long granite sample due to a nonuniform stress state along the simulated fault. On a smaller scale, I describe the deformation and microseismicity of a 76 mm-diameter Westerly granite cylinder that contains both a sawcut fault and a 21 mm diameter cylinder (a pin) of intact Westerly granite that crosses the sawcut fault. This “pin” sample was loading to 80 MPa in a triaxial machine at the USGS in Menlo Park. Dynamic slip events (stick-slip events) were either stopped by the pin or ruptured through it and were preceded by clusters of foreshocks, all located on the fault plane. The spatial extent of the foreshock clusters is roughly consistent with our estimate of a critical nucleation dimension h*. The microseismic (M -6 to M -8) events generated by this “pin” sample separate into three categories: slip on the sawcut fault, fracture of the intact rock pin, and off-fault seismicity associated with pin-related rock joints. From focal mechanism analysis, we found that pin fracture events are exclusively implosive (anticrack). The distinct locations, timing, and focal mechanisms of the different categories of microseismic events provide a unique opportunity to test the robustness of moment tensor analysis for M -6 to M -8 acoustic emissions."