Insights from Dynamically Triggered and Induced Earthquakes in Oklahoma

Abstract

AbstractIn the last decade, induced seismicity (earthquakes incited by anthropogenic activity) has drastically increased resulting from subsurface waste-water fluid injection and CO2 sequestration. In addition, seismologists have observed large (M ≥ 7) earthquakes that can trigger other earthquakes via (1) changes in static stress and (2) imposed transient dynamic stressing related to seismic waves generated by a mainshock. Although the exact mechanism for dynamic triggering remains uncertain, observations of earthquake triggering may reveal mechanisms that lead to earthquake failure. Given well-documented instances of induced earthquakes and fluid injection in Oklahoma, we investigate the occurrence of dynamic triggering and mechanisms of failure in the region. We analyze 124 M ≥ 7 remote earthquakes across a seven-year period (2010–2016), utilizing seismic data retrieved from EarthScope’s USArray Transportable Array, the Oklahoma Seismic Network, and a template matched earthquake catalog for Oklahoma to identify dynamically triggered earthquakes. We also identify previously uncataloged events through a short-term to long-term average ratio energy detector and analyst inspection of waveforms. We quantify the results using several statistical approaches to identify significant increases in local seismicity rates following the P-wave arrival of each remote mainshock. We identify 26 mainshocks that dynamically trigger either instantaneous or delayed earthquakes in Oklahoma. We conclude that mainshock transient stresses appear to contribute to natural and induced stress states in Oklahoma and can advance the earthquake cycle in the region. Our results emphasize the identification of instantaneous dynamic triggering; however, we also capture delayed triggering (i.e., past the first few hours following a mainshocks wavetrain passes). We find triggered earthquakes correlate well with regions of sustained fluid injection in Oklahoma, suggesting that increased pore fluid pressure may be lowering the effective normal stress across faults in the region and, thus, increasing susceptibility to transient stressing especially by Rayleigh waves.