Do Injection‐Induced Earthquakes Rupture Away from Injection Wells due to Fluid Pressure Change?

The 2016 M5.8 Pawnee, Oklahoma, earthquake is the largest earthquake to have been induced by wastewater disposal. We infer the coseismic slip history from analysis of apparent source time functions and inversion of regional and teleseismic P waveforms, using aftershocks as empirical Green's functions. The earthquake nucleated on the shallow part of the fault, initially rupturing toward the surface, followed shortly thereafter by slip deeper on the fault. Deeper slip occurred below the aftershocks and at greater depths than most induced seismicity in the region, suggesting that small‐ to moderate‐sized earthquakes may not occur on deeper parts of faults in Oklahoma because they are further from failure than shallower fault sections. Comparisons with models of pore pressure perturbations further suggest that the earthquake may have initiated within a region of higher pore pressure perturbation but was not confined to this zone. These observations inform source physics and understanding of maximum magnitudes.

Section: Discussionmentioning

confidence: 79%

Rupture Model of the M5.8 Pawnee, Oklahoma, Earthquake From Regional and Teleseismic Waveforms

Moschetti

Hartzell

Herrmann

2019

“…In addition, this event appears to have ruptured away from the injection well (“Berg,” Figure S7) that contributed most to the Guthrie sequence (Chen et al, ), which seems to favor the numerical model in which injection does not occur directly onto a fault (Dempsey & Suckale, ). However, previous studies reported various rupture styles with respect to the location of injection wells (e.g., Folesky et al, ; López‐Comino & Cesca, ; Lui & Huang, ). It is difficult to conclude from analysis of only one event whether the observed rupture directivity is representative of earthquakes in this region.…”

Section: Discussionmentioning

confidence: 99%

Source Complexity of the 2015 Mw 4.0 Guthrie, Oklahoma Earthquake

Chen

Abercrombie

2019

We demonstrate the complex source process of a potentially induced Mw 4.0 earthquake near Guthrie, Oklahoma. Relative source time functions (RSTFs), retrieved using a time domain empirical Green's function (EGF) deconvolution method, show clear evidence of rupture complexity with the presence of multiple pulses and systematic azimuthal variations. Directivity analysis reveals a well‐defined rupture propagation at ~120° toward the southeast. Detailed modeling of the RSTFs results in a four‐subevent model spanning ~1 s in time and ~1.5 km in space, in which the earthquake ruptured four successive slip episodes unilaterally to the southeast. Multiple EGF spectral ratio analysis also shows significant complexity as evidenced by azimuth‐dependent deviations from the omega‐square (Brune) model. The observations of the concentration of early aftershocks at the periphery of the inferred rupture model along the rupture direction and amplified ground motions at stations to the southeast can both be well explained by the directivity effect.

“…The inferred rupture model indicates directivity toward the injection well (Figures 1 and 5c), that is, "backward" rupture, following the terminology of Lui and Huang (2019). According to the 1-D numerical models of Dempsey and Suckale (2016), this would imply a high pressurization/criticality ratio (i.e., the ratio between injection pressure and initial fault stress) for the St. Gallen geothermal site.…”

Section: Discussionmentioning

confidence: 90%

Rupture Process of the M_w 3.3 Earthquake in the St. Gallen 2013 Geothermal Reservoir, Switzerland

Király-Proag

Satriano

Bernard

et al. 2019

We analyze slip distribution and rupture kinematics of a Mw3.3 induced event that occurred in the St. Gallen geothermal reservoir (NE Switzerland) in 2013. We carry out a two‐step procedure: (1) path effects are deconvolved from the seismograms using an empirical Green's function, resulting in relative source time functions at all seismic stations; (2) the relative source time functions are back‐projected to the corresponding isochrones on the fault plane. Results reveal that the mainshock rupture propagates toward NNE from the hypocenter with an average velocity of 2,000 m/s. Spatiotemporal organization of foreshocks and aftershocks shows that the mainshock broke a previously less active portion of the fault and suggests that the aftershock sequence could be mainly driven by stress transfer. Applying this method in an operational environment could enable fast retrieval of seismic slip, allowing assessment of fault asperities and structures involved in the reservoir creation process.