Velocity Models for the Crust Hosting the Main Aftershock Cluster of the 2011 Mineral, Virginia, Earthquake

Three-dimensional P- and S-wave velocity (VP and VS) models are determined for the crust containing the main aftershock cluster of the 2011 Mineral, Virginia, earthquake using local earthquake tomography. The inversion uses a total of 5125 arrivals (2465 P- and 2660 S-wave arrivals) for 324 aftershocks recorded by 12 stations. The inversion volume (22 × 20 × 16 km) is completely contained within the Piedmont Chopawamsic metavolcanic terrane. The models are well resolved in the central portion of the inversion volume in the depth range 1–5 km; good resolution does not extend to the hypocenter depth of the mainshock. Most aftershocks are located within a northeast-trending, southeast-dipping region containing negative VP anomalies, positive VS anomalies, and VP/VS ratios as low as 1.53. These velocity results strongly argue for the presence of quartz-rich rocks, which we attribute to either the presence of a giant quartz vein system or metamorphosed orthoquarzite sandstones originally deposited on the Laurentian passive margin and subsequently incorporated into the Chopawamsic thrust sheets during island arc collision in the Taconic orogeny.

Ten Years on from the Quake That Shook the Nation’s Capital

A decade of study into the Virginia earthquake that damaged D.C. and reverberated up and down the Atlantic coast in 2011 has shed light on rare, but risk-laden, seismicity in eastern North America.

The Liquefaction Record of Past Earthquakes in the Central Virginia Seismic Zone, Eastern United States

Following the 2011 moment magnitude, M 5.7 Mineral, Virginia, earthquake, we conducted a search for paleoliquefaction features and found 41 sand dikes, sand sills, and soft-sediment deformation features at 24 sites exposed in cutbanks along several rivers: (1) the South Anna River, where paleoliquefaction features were found in the epicentral area of the Mineral earthquake and farther downstream to the southeast; (2) the Mattaponi and Pamunkey Rivers east of the Fall Line, where liquefiable sediments are more common than in the epicentral area; and (3) the James River and Rivanna River–Stigger Creek, where a few sand dikes were found in the 1990s. Liquefaction features are grouped into two age categories based on dating of host sediment in which they occur and weathering characteristics of the features. A younger generation of features that formed during the past 350 yr are small, few in number, and appear to be limited to the James and Pamunkey Rivers. Though there are large uncertainties in their locations and magnitudes, one or more preinstrumental earthquakes, including the 1758, 1774, and 1875 events, likely caused these features. An older generation of liquefaction features that formed between 350 and 2800 yr ago are larger, more numerous, and more broadly distributed than the younger generation of features. Several earthquakes could account for the regional distribution of paleoliquefaction features, including one event of M 6.25–6.5 near Holly Grove, or two events of M 6.0 near Mineral and M 6.25 near Ashland. Amplification of ground motions in Coastal Plain sediment might have contributed to liquefaction along the Mattaponi and Pamunkey Rivers.

Method for Determination of Focal Mechanisms of Magnitude 2.5–4.0 Earthquakes Recorded by a Sparse Regional Seismic Network

ABSTRACT Focal mechanisms of earthquakes with magnitudes Mw 4.0 and less recorded by a sparse seismic network are usually poorly constrained due to the lack of an appropriate method applicable to finding these parameters with a sparse set of observations. We present a new method that can accurately determine focal mechanisms of earthquakes with Mw (3.70–3.04) using data from a few regional seismic stations. We filter the observed seismograms as well as synthetic seismograms through a frequency band of 1.5–2.5 Hz, which has a good signal-to-noise ratio for small earthquakes of the magnitudes with which we are working. The waveforms are processed to their envelopes to make the waveforms relatively simple for modeling. To find the optimal focal mechanism for an event, a nonlinear moment tensor inversion in addition to a coarse grid search over the possible dip, rake, and strike angles at a fixed value of focal depth and a fixed value of scalar moment is performed. We tested the method on 18 aftershocks of Mw (3.70–2.60) of the 2011 Mw 5.7 Mineral, Virginia, earthquake and on five aftershocks of Mw (3.62–2.63) of the 2013 Mw 4.5 Ladysmith, Quebec, earthquake. Our method obtains accurate focal mechanisms for 16 out of the 21 events that have previously reported focal mechanisms. Tests of our method for different crustal models show that event focal mechanism determinations vary with an average Kagan angle of 30° with the different crustal models. This means that the event focal mechanism determinations are only somewhat sensitive to the uncertainties in the crustal models tested. This study confirms that our method of modeling envelopes of seismic waveforms can be used to extract accurate focal mechanisms of earthquakes with short-time functions (Mw<4.0) using at least three regional seismic network stations at epicentral distances of 60–350 km.

Seismic Fragility Analysis of the Smithsonian Institute Museum Support Center

This paper presents the seismic fragility assessment of the Smithsonian Institute Museum Support Center (MSC), which sustained appreciable damage during the 2011 Virginia earthquake. A three-dimensional (3-D) finite element model (FEM) for the building was created and validated using measured dynamic characteristics determined from field vibration test data. Two suites of bidirectional ground motions at different hazard levels were applied to the FEM to generate fragility curves for structural as well as nonstructural (storage cabinets) damage. The effect of brace yielding strength on structural and nonstructural damage is also investigated to provide recommendations for future retrofit. The fragility curves show that the spectral acceleration to cause structural damage to the building is not high. Due to low seismicity, however, the probability for the structure to be damaged at the design basis earthquake is small. Nevertheless, the probability for nonstructural damage is considerable, which is an important issue related to the seismic performance of the building.