Coseismic pore pressure changes linked to local, induced earthquakes

Abstract

An unprecedented increase in seismicity in south-central Kansas (KS), starting in 2013, has been linked to elevated pore fluid pressures in the subsurface resulting from high rate wastewater injections. Improving the understanding of pore pressure communication at local scales (~10s of km) through the shallow basement (upper 10 km in the subsurface) can help mitigate the risk of induced seismicity. Here, I examine whether induced earthquakes, typically small local events, can induce detectable pore fluid pressure changes in the subsurface. Previous research has demonstrated that fluid levels in wells are sensitive to large magnitude, regional, and teleseismic earthquakes. The effect of small, local earthquakes on pore pressures is unknown. I analyze pore pressure measurements obtained every second over a time period of 11 months in a deep borehole ~30 m above basement at Wellington Field in KS, USA. I identify pore pressure signals by adapting an STA/LTA trigger algorithm, analogous to earthquake detection methods. The pressure event triggers produced from this algorithm are compared to earthquake data recorded at Wellington field by a seismometer network surrounding the pressure monitoring borehole.Results from this analysis show that small magnitude local earthquakes can induce detectable pore fluid pressure changes. Two hundred and seventy-nine earthquakes were detected using the pore pressure dataset; 183 of these events were above the background noise level of the data and can be confidently related to changes in downhole pore pressure. The detected pressure events correspond to seismometer identified earthquakes ranging in magnitude from 0.8 to 5.8 at distances of 2 to 180 km from the monitoring well. Out of the full catalog of interest only 20% of known earthquakes relate to changes in downhole pressure. Earthquakes related to pressure have detectable pressure changes at the time of the earthquake that differentiates them from earthquakes that were not related to coseismic pressure changes. Earthquake characteristics like the phase, impulsiveness, magnitude, frequency, phase duration, and distance from the pressure well do not correlate to pressure events. Not all seismometer recorded earthquakes cause detectable pore pressure changes. This can be due to the sparse sampling of the pressure data (1 Hz) compared to the earthquake data (200 Hz) resulting in potentially missing the earthquake pressure signal, pressure signals falling below noise level or below the instrument detection threshold, as well as geologic heterogeneity of the subsurface influencing pressure communication. By using downhole pressure measurements and earthquake data from the field, it was possible to detect coseismic pressure changes related to small magnitude, induced earthquakes. Water level monitors are commonly inexpensive compared to seismic monitoring systems. This investigation offers compelling evidence that pressure monitoring can be a viable, inexpensive method for detecting local earthquakes. However, additional studies are needed in a controlled setting to examine the factors that can influence pressure sensing of earthquakes.