Source Complexity of the 2015 Mw 4.0 Guthrie, Oklahoma Earthquake

• Published in: Geophysical research letters 2019-05, Vol.46 (9), p.4674-4684
• Main Authors: Wu, Qimin, Chen, Xiaowei, Abercrombie, Rachel E.
• Format: Article
• Language: English
• Subjects: Aftershocks; Analysis; Azimuth; Complexity; Directivity; Earthquakes; Empirical analysis; Empirical Green's function; Fault zones; Geological faults; Green's function; Green's functions; Ground motion; Ground stations; Heterogeneity; Induced seismicity; Material properties; Relative source time function; Rupture; Rupture directivity; Rupturing; Seismic activity; Skewed distributions; Slip; Source complexity; Time functions; Wastewater
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL082690

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. Plain Language Summary To better understand the rupture growth and development of induced earthquakes in Oklahoma, we provide a comprehensive analysis of the rupture process of an Mw 4.0 earthquake from an earthquake sequence near Guthrie, Oklahoma, that was previously found to show correlation with nearby wastewater injection activity. Using advanced techniques in both the time and frequency domains, we show that this earthquake ruptured four successive slip patches located progressively toward the southeast along the NW‐SE‐trending fault plane. The source complexity of this earthquake reflects the heterogeneity of fault zone stress, and/or material properties, which may also have prevented the generation of larger earthquakes in this area. The inferred unilateral rupture model also agrees well with the asymmetric distributions of aftershocks and high‐frequency ground motions caused by directivity effects. Our results suggest that complexity in source processes scales down to low‐magnitude earthquakes for induced earthquakes Oklahoma, implying that geological factors such as geometric or stress heterogeneity in the fault zone might be of primary importance in controlling the rupture growth and development of induced earthquakes. Key Points A time domain iterative forward‐modeling method is applied to obtain reliable relative source time functions Significant source complexity including rupture directivity and multiple subevents i