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.

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The study models the response of near-surface materials in Charleston, South Carolina, and the adjacent area. Geotechnical investigations at 281 locations were made available by local engineering firms. The data used for dynamic siteresponse analysis were derived from shear-wave velocity measurements at 52 locations. Site response was quantified as the ratio of surface motion to hypothetical hard-rock basement outcrop motion. Scenario earthquake motions were developed with the stochastic model. Acceleration response ratios for 5% critical oscillator damping were computed for 12 frequencies ranging from 0.1 to 30 Hz and for peak ground acceleration.Two features determine the general nature of site response in the study area: the impedance contrast between Mesozoic basement and Cretaceous sediments, and the shallow impedance contrast between Quaternary and Tertiary sediments. Average Swave velocities in the Quaternary are relatively uniform and range from 150 to 250 m/sec. They are not strongly correlated with surface geology. The velocities of the immediately underlying Tertiary sediments range from 300 to 500 m/sec. Because of the uniformity of velocity in the Quaternary, depth to the Quaternary-Tertiary contact appears to be the most important variable leading to differences in calculated site response. This surface is irregular, and varies in depth from near surface at inland sites to approximately 30 m at sites near the coast. As a consequence, estimated site response in the frequency band 1-10 Hz varies by as much as a factor of 3. Site response at frequencies less than 1 Hz is dominated by the first few resonant harmonics of the entire sedimentary section, with fundamental frequency near 0.2 Hz.

The study focuses on evidence of Cenozoic faulting in the epicentral area of the 1886 Charleston, South Carolina, earthquake and its connection with Mesozoic structure. The seismic data consist of several reflection profiles collected near Summerville, South Carolina, in the period 1975. Reprocessing of the data reveals an extensive early Mesozoic extensional basin, approximately 20 km in width, between Summerville and Charleston. The basin is delineated by the geometry of reflections that image early Mesozoic volcanic and sedimentary rocks and by positive magnetic and gravity anomalies. Cenozoic compressional reactivation of Mesozoic extensional faults is imaged in the interior of the basin. The northwestern boundary of the basin is marked by a sharp gradient in the magnetic field. Folded Cretaceous and Tertiary Atlantic Coastal Plain sediments in association with diffractions and truncated reflections from the early Mesozoic section at four locations along this magnetic gradient indicate that the northwestern basin boundary is faulted. Instrumentally located earthquakes are clustered at the location of the faults imaged in the interior of the basin and in proximity to the northwestern basin margin. Modeling of magnetic and gravity data indicates that the upper crust beneath the seismically imaged structural basin is composed largely of mafic rocks to a depth of at least 4 km. We propose that the Charleston earthquake occurred due to compressional reactivation of a Mesozoic fault in a localized zone of intense early Mesozoic continental rifting.Online Material: Images of the CMP-stacked record sections and corresponding SEGY files and zipped ESRI Shapefiles containing the "shot" locations for each of the surveys.

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