The Aftershock Sequence of the 2011 Mineral, Virginia, Earthquake: Temporal and Spatial Distribution, Focal Mechanisms, Regional Stress, and the Role of Coulomb Stress Transfer

Abstract

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.

Source Complexity of the 2015 Mw 4.0 Guthrie, Oklahoma Earthquake

“…Almost all the early aftershocks (within 3 days) of the Guthrie event are located at the periphery of the inferred rupture model (Figure ), indicating a possible causal dynamic and/or static triggering effects. The absence of early aftershocks in our modeled rupture area suggests an anticorrelation of aftershock productivity with the slip distribution, which was also observed for earthquakes in various tectonic settings (e.g., Mendoza & Hartzell, ; Ross et al, ; Wetzler et al, ; Wu et al, ). The framework that combines stress transfer and source directivity (e.g., Convertito et al, ) may be further improved by incorporating pore pressure increase and poroelastic stress changes due to injection to better explain triggering mechanisms of induced earthquakes.…”

Reflection imaging with earthquake sources and dense arrays

“…Figure a shows the epicenters and the receivers that recorded the events. Focal mechanisms indicate reverse motion with a large assortment of fault plane orientations [ Wu et al ., ]. In Figures b–e we show records of four events with their corresponding depths, magnitudes, and most prominent reflectors specified.…”

“…(a) Map showing the magnitude, focal mechanisms, and epicenters [ Wu et al ., ] of 13 aftershocks recorded with the AIDA stations (black dots). The event identifier (white font) and the event magnitude (black font) is displayed next to the corresponding focal mechanism.…”

“…By applying the processing described above to the subset of 13 events in the Wu et al [] catalog, we produce a stacked depth image in the vicinity of the array. Figure shows the result of the VSP‐CRP mapping and stacking process when applied to the subset of 13 events.…”

“…Prior investigations of the CVSZ have been unable to conclusively tie earthquakes to a particular fault, but aftershocks for this event clearly delineate a ~10 km long, southeast dipping zone [Horton and Williams, ; Davenport et al ., ; McNamara et al ., ]. Thorough analysis of the events shows that their focal mechanisms have a wide range of orientations, implying they occurred in a zone of positive Coulomb stress transfer [ Wu et al ., ].…”