Source Spectral Properties of Small to Moderate Earthquakes in Southern Kansas

Abstract

The source spectral properties of injection‐induced earthquakes give insight into their nucleation, rupture processes, and influence on ground motion. Here we apply a spectral decomposition approach to analyze P wave spectra and estimate Brune‐type stress drop for more than 2,000 ML1.5–5.2 earthquakes occurring in southern Kansas from 2014 to 2016. We find that these earthquakes are characterized by low stress drop values (median ∼0.4 MPa) compared to natural seismicity in California. We observe a significant increase in stress drop as a function of depth, but the shallow depth distribution of these events is not by itself sufficient to explain their lower stress drop. Stress drop increases with magnitude from M1.5 to M3.5, but this scaling trend may weaken above M4 and also depends on the assumed source model. Although we observe a nonstationary, sequence‐specific temporal evolution in stress drop, we find no clear systematic relation with the activity of nearby injection wells.

Spatial and Temporal Variations in Earthquake Stress Drop on Gofar Transform Fault, East Pacific Rise: Implications for Fault Strength

“…Stress drops for aftershocks also decreased, consistent with stress drop observations from other tectonic earthquake sequences (e.g., Abercrombie, 2014;Kim et al, 2016;Trugman & Shearer, 2018) and induced seismicity (e.g., Trugman et al, 2017) and may be indicative of a temporary reduction in fault strength within the mainshock rupture zone. Similarly, stress drops for aftershocks in the 2007 M W 6.2 rupture asperity also decreased, suggesting these earthquakes occurred in the region of strong ground motion from the M W 6.0 mainshock.…”

“…In induced-seismicity studies, time-dependent stress drops have been correlated with changes in effective normal stress due to fluid pressure variations as a result of nearby water injection (e.g., Goertz-Allmann et al, 2011;Lengliné et al, 2014;Lin et al, 2016). These studies found lower stress drop for aftershocks than for mainshocks (e.g., Sumy et al, 2017;Trugman et al, 2017), while other studies found changes in stress drop with distance from the injection well (e.g., Goertz-Allmann et al, 2011;Kwiatek et al, 2014).…”

Earthquake Declustering Using the Nearest‐Neighbor Approach in Space‐Time‐Magnitude Domain

“…The other mode is located considerably closer in time and space to the origin; it corresponds to clustered events. This bimodality has been documented in multiple regions and on multiple scales (Gentili et al, , ; Gu et al, ; Kossobokov & Nekrasova, ; Moradpour et al, ; Peresan & Gentili, ; Ruhl et al, ; Schoenball et al, ; Trugman et al, ; Vasylkivska & Huerta, ; Zaliapin & Ben‐Zion, , , , ). The bimodality of observed seismicity facilitates cluster detection and declustering.…”

“…The problem of detecting earthquake clusters (which is different from although closely related to declustering) was shown recently to be effectively addressed by a nearest‐neighbor analysis in space‐time‐magnitude domain (Zaliapin et al, ; Zaliapin and Ben‐Zion, , , , , ). This cluster identification methodology is effective in diverse settings, including tectonic seismicity (Gentili et al, , ; Gu et al, ; Kossobokov & Nekrasova, ; Moradpour et al, ; Peresan & Gentili, ; Reverso et al, ; Ruhl et al, ; Trugman et al, ; Zaliapin et al, ; Zaliapin & Ben‐Zion, , , ), induced earthquakes (Goebel et al, ; Martínez‐Garzón et al, ; Schoenball et al, ; Schoenball & Ellsworth, ; Teng & Baker, ; Vasylkivska & Huerta, ; Zaliapin & Ben‐Zion, ), synthetic seismicity in Epidemic Type Aftershock Sequence (ETAS) models (Gu et al, ; Zaliapin et al, ; Zaliapin & Ben‐Zion, ), and laboratory rock fracture experiments (Davidsen et al, ).…”

Illuminating the Rupturing of Microseismic Sources in an Injection‐Induced Earthquake Experiment

“…Fluid pressure is usually considered as the major driving force in the generation of induced earthquakes when injection wells are close to the fault (Raleigh et al, ), whereas stress loading due to poroelastic deformation can take over when injection wells are more distant (Goebel & Brodsky, ; Segall & Lu, ). Geomechanical models show that the current injection wells in Oklahoma (USA) can induce fluid pressure and stress loading of the order of 0.1 MPa (Keranen et al, ), which is much smaller than the stress drop estimates of moderate‐magnitude‐induced earthquakes that have a median stress drop of about 1–20 MPa (Boyd et al, ; Huang et al, ; Trugman et al, ; Wu et al, ). The small ratio of fluid pressure perturbation and earthquake stress drop suggests that faults are almost critical before injection takes place (Townend & Zoback, ) and even a small fluid pressure or stress perturbation can make the faults reach favorable conditions for earthquake nucleation.…”