The aftershocks of the 23 August 2011 M w 5.7 Mineral, Virginia, earthquake were recorded by 36 temporary stations installed by several institutions. We located 3960 aftershocks from 25 August 2011 through 31 December 2011. A subset of 1666 aftershocks resolves details of the hypocenter distribution. We determined 393 focal mechanism solutions. Aftershocks near the mainshock define a previously recognized tabular cluster with orientation similar to a mainshock nodal plane; other aftershocks occurred 10-20 km to the northeast. A large percentage of the aftershocks occurred in regions of positive Coulomb static stress change, and ∼80% of the focal mechanism nodal planes were brought closer to failure. However, the aftershock distribution near the mainshock appears to have been influenced strongly by rupture directivity. Aftershocks at depths less than 4 km exhibit reverse mechanisms with north-northwest-trending nodal planes. Most focal mechanisms at depths greater than 6 km are similar to the mainshock, with north-northeast-trending nodal planes. A concentration of aftershocks in the 4-6 km depth range near the mainshock are mostly of reverse type but display a 90°range of nodal-plane trend. Those events appear to outline the periphery of mainshock rupture, where positive Coulomb stress transfer is largest. The focal mechanisms of aftershocks at depths less than 4 km and those greater than 6 km, along with the mainshock, point to the possibility of a depthdependent stress field prior to the occurrence of the mainshock.Analysis of earthquake occurrence using a new magnitude scale (M D ) indicates a Gutenberg-Richer law b-value of 0.864 and an Omori law p-value of 1.085, indicative of a typical aftershock sequence.Online Material: Catalogs of aftershock location, magnitude, and focal mechanisms.

No abstract

Earthquake source parameters provide important insight into the physical source processes occurring during earthquake rupture. The increase in seismicity rate within the central United States related to wastewater disposal from oil and gas production (Langenbruch & Zoback, 2016; Zhai et al., 2019) has led to a number of studies attempting to determine whether the source parameters of these induced earthquakes differ from those of tectonic earthquakes. Such a finding could allow for the identification of induced events and improve ground motion predictions for them. The 2011 Mw 5.7 Prague, Oklahoma (OK), earthquake was the first large earthquake linked to an increase in wastewater injection in Oklahoma (Keranen et al., 2013). Consequently, this earthquake sequence has been the subject of numerous source parameter studies aimed at resolving possible differences between induced and tectonic earthquake sources.

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.