Source Complexity of the 2015 Mw 4.0 Guthrie, Oklahoma Earthquake

Abstract

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.

Resolution and uncertainties in estimates of earthquake stress drop and energy release

“…Another category of analyses involves modelling of EGF-style spectral ratios obtained from coda waves invariably report that the largest earthquakes have higher stress drops than the smaller ones e.g. in California: Mayeda et al [139], Nevada: Mayeda & Malagnini [142] and Yoo & Mayeda [165], Italy: Malagnini et al [14,166], and Oklahoma: Boyd et al [12], Walter et al [140] and Wu et al [148]. These studies use measurements for the numerator and the dominator events in the spectral ratios which may cause systematic bias (e.g.…”

“…The most detailed studies focus on the best recorded small and moderate-sized earthquakes (e.g. [68,148,168]). They include unilateral rupture processes, and finite fault inversion, and produce results more comparable with those from the best understood larger earthquakes.…”

“…To illustrate the combined effects of a complex earthquake source and recorded with limited bandwidth, I perform a simple test using the complex M4 mainshock of the 2014 Guthrie, Oklahoma, sequence [98,148]. I select this earthquake for analysis as it has a complex spectral shape, and is recorded over a relatively wide frequency range.…”

“…Effects of bandwidth limitation and source complexity, using example of a real M4 earthquake recorded in Guthrie, Oklahoma. (a) Source time functions for the earthquake at two example stations derived using multiple EGF by Wu et al [148] who identified four sub-events. (b) Map showing locations of subevents and associated seismicity (c) fitting the P wave spectral ratio of this earthquake to stacked EGF events, following the approach of Abercrombie et al [133].…”

Cross Validation of Stress Drop Estimates and Interpretations for the 2011 Prague, OK, Earthquake Sequence Using Multiple Methods

“…The use of a variance threshold excludes the spectral ratios most poorly fit by the assumed source model, but some of the remaining ratios exhibit clear evidence of more complex sources (Uchide & Imanishi, 2016; Wu et al., 2019). These are supported by the relative source time functions calculated as part of the inversion (Abercrombie et al., 2017).…”