Stress Evolution and Driving Mechanism of an Induced Earthquake Sequence near Caldwell, Kansas

ABSTRACT We use the Park et al. (2022) earthquake catalog to explore the driving mechanism(s) responsible for a sequence of injection-induced earthquakes in Sumner County in southern Kansas. We use precise differential travel-time measurements to develop precise earthquake centroid locations, which we use to identify repeating earthquakes, and to model the evolution of stress. We use the stress evolution to evaluate the possible roles of earthquake-to-earthquake triggering, aseismic slip, and pore fluid pressure in the evolution of the swarm. We find some pairs of repeating earthquakes, but they do not form persistent families that have been used as indicators of fault-spanning aseismic slip driving earthquakes on creeping faults. Our stress evolution modeling suggests that many earthquakes occur in areas where there would have been an increase in stress greater than 1 MPa from previous earthquakes in the sequence, which suggests a role for earthquake-to-earthquake triggering in this sequence. These earthquakes are an expression of the expansion of the faulted area; however, the fact that the earthquakes do not completely cover the fault, suggests that static stress changes are not the only active process. Stress modeling also indicates that some earthquakes occur for which the driving stress decreased due to earlier earthquakes in the sequence. This could be attributed either to localized loading from aseismic slip or continued weakening of portions of the fault in the presence of continuing increases in pore fluid pressure. It is impossible to conclusively rule out either of these processes with the available data. Our analysis shows that there are likely contributions from multiple physical processes to the spatial and temporal behavior of earthquakes in this sequence and suggests that further study of additional sequences could provide useful constraints on the mechanisms behind induced seismicity.