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Shallow Faulting and Folding in the Epicentral Area of the 1886 Charleston, South Carolina, Earthquake
Dendritic reidite from the Chesapeake Bay impact horizon, Ocean Drilling Program Site 1073 (offshore northeastern USA): A fingerprint of distal ejecta?
Front Matter
Chesapeake Bay Impact Structure—Development of “Brim” Sedimentation in a Multilayered Marine Target
ABSTRACT The late Eocene Chesapeake Bay impact structure was formed in a multilayered target of seawater underlain sequentially by a sediment layer and a rock layer in a continental-shelf environment. Impact effects in the “brim” (annular trough) surrounding and adjacent to the transient crater, between the transient crater rim and the outer margin, primarily were limited to the target-sediment layer. Analysis of published and new lithostratigraphic, biostratigraphic, sedimentologic, petrologic, and mineralogic studies of three core holes, and published studies of a fourth core hole, provided information for the interpretation of the impact processes, their interactions and relative timing, their resulting products, and sedimentation in the brim. Most studies of marine impact-crater materials have focused on those found in the central crater. There are relatively few large, complex marine craters, of which most display a wide brim around the central crater. However, most have been studied using minimal data sets. The large number of core holes and seismic profiles available for study of the Chesapeake Bay impact structure presents a special opportunity for research. The physical and chronologic records supplied by study of the sediment and rock cores of the Chesapeake Bay impact indicate that the effects of the initial, short-lived contact and compression and excavation stages of the impact event primarily were limited to the transient crater. Only secondary effects of these processes are evident in the brim. The preserved record of the brim was created primarily in the subsequent modification stage. In the brim, the records of early impact processes (e.g., outgoing tsunamis, overturned flap collapse) were modified or removed by later processes. Transported and rotated, large and small clasts of target sediments, and intervals of fluidized sands indicate that seismic shaking fractured and partially fluidized the Cretaceous and Paleogene target sediments, which led to their inward transport by collapse and lateral spreading toward the transient crater. The succeeding inward seawater-resurge flow quickly overtook and interacted with the lateral spreading, further facilitating sediment transport across the brim and into the transient crater. Variations in the cohesion and relative depth of the target sediments controlled their degree of disaggregation and redistribution during these events. Melt clasts and shocked and unshocked rock clasts in the resurge sediments indicate fallout from the ejecta curtain and plume. Basal parautochthonous remnant sections of target Cretaceous sediments in the brim thin toward the collapsed transient crater. Overlying seawater-resurge deposits consist primarily of diamictons that vary laterally in thickness, and vertically and laterally in maximum grain size. After cessation of resurge flow and re-establishment of pre-impact sea level, sandy sediment gravity flows moved from the margin to the center of the partially filled impact structure (shelf basin). The uppermost unit consists of stratified sediments deposited from suspension. Postimpact clayey silts cap the crater fill and record the return to shelf sedimentation at atypically large paleodepths within the shelf basin. An unresolved question involves a section of gravel and sand that overlies Neoproterozoic granite in the inner part of the brim in one core hole. This section may represent previously unrecognized, now parautochthonous Cretaceous sediments lying nonconformably above basement granite, or it may represent target sediments that were moved significant distances by lateral spreading above basement rocks or above a granite megaclast from the overturned flap. The Chesapeake Bay impact structure is perhaps the best documented example of the small group of multilayer, marine-target impacts formed in continental shelves or beneath epeiric seas. The restriction of most impact effects to the target-sediment layer in the area outside the transient cavity, herein called the brim, and the presence of seawater-resurge sediments are characteristic features of this group. Other examples include the Montagnais (offshore Nova Scotia, Canada) and Mjølnir (offshore Norway) impact structures.
The 23 August 2011 M w (moment magnitude) 5.7 ± 0.1, Mineral, Virginia, earthquake was the largest and most damaging in the central and eastern United States since the 1886 M w 6.8–7.0, Charleston, South Carolina, earthquake. Seismic data indicate that the earthquake rupture occurred on a southeast-dipping reverse fault and consisted of three subevents that progressed northeastward and updip. U.S. Geological Survey (USGS) “Did You Feel It?” intensity reports from across the eastern United States and southeastern Canada, rockfalls triggered at distances to 245 km, and regional groundwater-level changes are all consistent with efficient propagation of high-frequency seismic waves (~1 Hz and higher) in eastern North America due to low attenuation. Reported damage included cracked or shifted foundations and broken walls or chimneys, notably in unreinforced masonry, and indicated intensities up to VIII in the epicentral area based on USGS “Did You Feel It?” reports. The earthquake triggered the first automatic shutdown of a U.S. nuclear power plant, located ~23 km northeast of the main shock epicenter. Although shaking exceeded the plant’s design basis earthquake, the actual damage to safety-related structures, systems, and components was superficial. Damage to relatively tall masonry structures 130 km to the northeast in Washington, D.C., was consistent with source directivity, soft-soil ground-motion amplification, and anisotropic wave propagation with lower attenuation parallel to the northeast-trending Appalachian tectonic fabric. The earthquake and aftershocks occurred in crystalline rocks within Paleozoic thrust sheets of the Chopawamsic terrane. The main shock and majority of aftershocks delineated the newly named Quail fault zone in the subsurface, and shallow aftershocks defined outlying faults. The earthquake induced minor liquefaction sand boils, but notably there was no evidence of a surface fault rupture. Recurrence intervals, and evidence for larger earthquakes in the Quaternary in this area, remain important unknowns. This event, along with similar events during historical time, is a reminder that earthquakes of similar or larger magnitude pose a real hazard in eastern North America.
Magnitude, recurrence interval, and near-source ground-motion modeling of the Mineral, Virginia, earthquake of 23 August 2011
The Mineral, Virginia (USA), earthquake occurred at 17:51:3.9 UTC (Coordinated Universal Time) on 23 August 2011; the hypocenter was at 37.905°N, 77.975°W and depth was 8 km. The widely reported moment magnitude (M w ) was 5.7 ± 0.1. The m b (teleseismic short-period) magnitude estimated here using 26 global network stations is 5.77 ± 0.23, in agreement with m b values reported by national and international data centers, and not significantly different from M w . However, the m bLg magnitude of the earthquake determined here using 386 stations in eastern North America is 6.28 ± 0.26. The m bLg magnitude is a short-period magnitude correlated with m b that is based on the amplitude of the Lg phase at regional distances. Lg is a crust-guided phase that represents the largest amplitudes observed on short-period seismograms at regional distances in eastern North America. The m bLg magnitude was the primary magnitude appearing in catalogs of eastern United States earthquakes until superseded recently by M w . The catalog of previous earthquakes in central Virginia is keyed to m bLg , rather than M w . The Mineral shock reveals large regional variations in the Lg phase attenuation in the eastern United States. The expected value for the return period of m bLg 6.3 and larger earthquakes in the Central Virginia seismic zone is 752 yr, with a 95% confidence interval of 385–1471 yr. The Mineral earthquake caused Modified Mercalli Intensity (MMI) VIII damage in the epicentral area, with several instances of partial and total collapse of masonry chimneys and walls. A finite-fault, full wavefield simulation of the motions within 30 km of the epicenter fits the velocity recordings and Fourier spectral amplitudes in the 1–10 Hz frequency band, at the only strong-motion station in that distance range. The strongest motions are predicted to have occurred in two areas offset to the northwest and southeast of the epicenter, within which peak ground accelerations may have approached 2 g, and peak velocities were probably well in excess of 20 cm/s. The only factor mitigating damage in this earthquake was the brief (<3 s) duration of strong shaking.
We characterize shear-wave velocity versus depth (Vs profile) at 16 portable seismograph sites through the epicentral region of the 2011 M w 5.8 Mineral (Virginia, USA) earthquake to investigate ground-motion site effects in the area. We used a multimethod acquisition and analysis approach, where active-source horizontal shear (SH) wave reflection and refraction as well as active-source multichannel analysis of surface waves (MASW) and passive-source refraction microtremor (ReMi) Rayleigh wave dispersion were interpreted separately. The time-averaged shear-wave velocity to a depth of 30 m (Vs30), interpreted bedrock depth, and site resonant frequency were estimated from the best-fit Vs profile of each method at each location for analysis. Using the median Vs30 value (270–715 m/s) as representative of a given site, we estimate that all 16 sites are National Earthquake Hazards Reduction Program (NEHRP) site class C or D. Based on a comparison of simplified mapped surface geology to median Vs30 at our sites, we do not see clear evidence for using surface geologic units as a proxy for Vs30 in the epicentral region, although this may primarily be because the units are similar in age (Paleozoic) and may have similar bulk seismic properties. We compare resonant frequencies calculated from ambient noise horizontal:vertical spectral ratios (HVSR) at available sites to predicted site frequencies (generally between 1.9 and 7.6 Hz) derived from the median bedrock depth and average Vs to bedrock. Robust linear regression of HVSR to both site frequency and Vs30 demonstrate moderate correlation to each, and thus both appear to be generally representative of site response in this region. Based on Kendall tau rank correlation testing, we find that Vs30 and the site frequency calculated from average Vs to median interpreted bedrock depth can both be considered reliable predictors of weak-motion site effects in the epicentral region.
Earthquake damage is often increased due to local ground-motion amplification caused by soft soils, thick basin sediments, topographic effects, and liquefaction. A critical factor contributing to the assessment of seismic hazard is detailed information on local site response. In order to address and quantify the site response at seismograph stations in the eastern United States, we investigate the regional spatial variation of horizontal:vertical spectral ratios (HVSR) using ambient noise recorded at permanent regional and national network stations as well as temporary seismic stations deployed in order to record aftershocks of the 2011 Mineral, Virginia, earthquake. We compare the HVSR peak frequency to surface measurements of the shear-wave seismic velocity to 30 m depth (Vs30) at 21 seismograph stations in the eastern United States and find that HVSR peak frequency increases with increasing Vs30. We use this relationship to estimate the National Earthquake Hazards Reduction Program soil class at 218 ANSS (Advanced National Seismic System), GSN (Global Seismographic Network), and RSN (Regional Seismograph Networks) locations in the eastern United States, and suggest that this seismic station–based HVSR proxy could potentially be used to calibrate other site response characterization methods commonly used to estimate shaking hazard.
Shear-wave velocity structure and attenuation derived from aftershock data of the 2011 Mineral, Virginia, earthquake
A dense seismic array was deployed at a 2 km spacing to record the aftershocks of the M w (moment magnitude) 5.8 Mineral, Virginia (USA), earthquake in 2011. The three-component seismometers, installed on a 60-km-long profile, recorded 40 aftershocks over 9 days of deployment. Based on manual picking of P-wave (primary, compressional) and S-wave (secondary, shear) arrival times of 15 aftershocks, we find that the P-wave propagates with a velocity of 6.15 km/s through the upper crust, and the direct S-wave travels with a velocity of 3.66 km/s within the first 20 km (Vs <20km ) and decreases slightly to 3.54 km/s (Vs >20km ) for distances >20 km. Hence, the aftershock data show a Vp/Vs ratio of 1.68 within the first 20 km of hypocentral distance, and a ratio of 1.73 for distances >20 km. We attribute the small decrease in Vs with increased distance to the complex geologic setting: the recording array was deployed across the geologic boundary between the Quantico Formation and the Ta River Metamorphic Suite. Near-source attenuation of S-waves (amplitude decay with hypocentral distance R) was measured using ~1200 digital seismograms (north-south and east-west components) from 40 aftershocks. The decay of amplitude was extracted using a nonlinear least-squares regression for different frequency bands: 1–2, 2–4, 4–8, and 8–16 Hz. For 1–2 Hz the decay can be described as a function of distance (R) as R −0.8 , for 2–4 Hz as R −0.9 , for 4–8 Hz as R −1.05 , and for 8–16 Hz as R −1.15 . The decay exponents, or b values, increase ~9%–15% from a lower to the next higher analyzed frequency band. These values are valid to a distance of as much as ~45 km from the aftershocks.
Regional seismic-wave propagation from the M5.8 23 August 2011, Mineral, Virginia, earthquake
The M5.8 23 August 2011 Mineral, Virginia, earthquake was felt over nearly the entire eastern United States and was recorded by a wide array of seismic broadband instruments. The earthquake occurred ~200 km southeast of the boundary between two distinct geologic belts, the Piedmont and Blue Ridge terranes to the southeast and the Valley and Ridge Province to the northwest. At a dominant period of 3 s, coherent postcritical P-wave (i.e., direct longitudinal waves trapped in the crustal waveguide) arrivals persist to a much greater distance for propagation paths toward the northwest quadrant than toward other directions; this is probably related to the relatively high crustal thickness beneath and west of the Appalachian Mountains. The seismic surface-wave arrivals comprise two distinct classes: those with weakly dispersed Rayleigh waves and those with strongly dispersed Rayleigh waves. We attribute the character of Rayleigh wave arrivals in the first class to wave propagation through a predominantly crystalline crust (Blue Ridge Mountains and Piedmont terranes) with a relatively thin veneer of sedimentary rock, whereas the temporal extent of the Rayleigh wave arrivals in the second class are well explained as the effect of the thick sedimentary cover of the Valley and Ridge Province and adjacent Appalachian Plateau province to its northwest. Broadband surface-wave ground velocity is amplified along both north-northwest and northeast azimuths from the Mineral, Virginia, source. The former may arise from lateral focusing effects arising from locally thick sedimentary cover in the Appalachian Basin, and the latter may result from directivity effects due to a northeast rupture propagation along the finite fault plane.
Widespread groundwater-level offsets caused by the M w 5.8 Mineral, Virginia, earthquake of 23 August 2011
Groundwater levels were offset in bedrock observation wells, measured by the U.S. Geological Survey or others, as far as 553 km from the M w 5.8 Mineral, Virginia (USA), earthquake on 23 August 2011. Water levels dropped as much as 0.47 m in 34 wells and rose as much as 0.15 m in 12 others. In some wells, which are as much as 213 m deep, the water levels recovered from these deviations in hours to days, but in others the water-level offset may have persisted. The groundwater-level offsets occurred in locations where the earthquake was at least weakly felt, and the maximum water-level excursion increased with felt intensity, independent of epicentral distance. Coseismic static strain from the earthquake was too small and localized to have contributed significantly to the groundwater-level offsets. The relation with intensity is consistent with ground motion from seismic waves leading to the water-level offsets. Examination of the hydrographs indicates that short-period ground motion most likely affected the permeability of the bedrock aquifers monitored by the wells.
Finite element simulation of an intraplate earthquake setting—Implications for the Virginia earthquake of 23 August 2011
The 23 August 2011 M 5.7 intraplate earthquake occurred in the Central Virginia seismic zone near Mineral, Virginia (USA), far from the nearest plate boundaries. I suggest here that this earthquake, as well as others occurring in this region since 1774, was triggered by pore-fluid pressure diffusion associated with groundwater recharge. Using finite element modeling (FEM) estimates are made of the magnitude and timing of pressure diffusion with respect to the time of the earthquake. Two scenarios are considered: (1) the diffusion took place along vertical to near-vertical diffusion paths, or (2) the diffusion was restricted to a hydraulically transmissive fracture zone that was later illuminated by thousands of aftershocks. Both scenarios have merit. The fracture zone may not have been entirely generated by the main shock. For either model a fractured crust is assumed to be recharged at the surface of the Earth by groundwater recharge with subsequent pore-fluid pressure diffusion propagating to the hypocenter. The transient behavior of pore-fluid pressure is examined at the focal depth of 8 km. These results are compared with the duration and timing of base flow (groundwater recharge) as estimated from a hydrograph separation. The delays of the peaks in pore-fluid pressure diffusion as computed by the FEM simulations are found to be consistent with the start and duration of groundwater recharge with respect to the timing of the Virginia earthquake.
Geotechnical aspects in the epicentral region of the 2011 M w 5.8 Mineral, Virginia, earthquake
A reconnaissance team documented the geotechnical and geological aspects in the epicentral region of the M w (moment magnitude) 5.8 Mineral, Virginia (USA), earthquake of 23 August 2011. Tectonically and seismically induced ground deformations, evidence of liquefaction, rock slides, river bank slumps, ground subsidence, performance of earthen dams, damage to public infrastructure and lifelines, and other effects of the earthquake were documented. This moderate earthquake provided the rare opportunity to collect data to help assess current geoengineering practices in the region, as well as to assess seismic performance of the aging infrastructure in the region. Ground failures included two marginal liquefaction sites, a river bank slump, four minor rockfalls, and a ~4-m-wide, ~12-m-long, ~0.3-m-deep subsidence on a residential property. Damage to lifelines included subsidence of the approaches for a bridge and a water main break to a heavily corroded, 5-cm-diameter valve in Mineral, Virginia. Observed damage to dams, landfills, and public-use properties included a small, shallow slide in the temporary (“working”) clay cap of the county landfill, damage to two earthen dams (one in the epicentral region and one further away near Bedford, Virginia), and substantial structural damage to two public school buildings.
Residential property damage in the epicentral area of the Mineral, Virginia, earthquake of 23 August 2011
The Mineral, Virginia (USA), earthquake of 23 August 2011 was an unusually strong seismic event in the eastern United States. It caused widespread structural damage to residential property near the epicenter. An analysis of residential property damage reports, in conjunction with visits to some damaged residences, reveals a 40 km 2 area of concentrated damage centered 11 km south of the town of Louisa. This area is west of the earthquake’s epicenter and may be in the immediate hanging wall of a northeast-striking, moderately southeast dipping causative fault suggested by seismic data. The degree of damage in this area is consistent with a maximum Modified Mercalli Intensity (MMI) of VIII. A surrounding area of ~550 km 2 reported damage that is consistent with an MMI intensity of VII. A statistical analysis of dwelling characteristics confirms that home age and condition were factors that influenced the frequency and severity of reported property damage. The median damage to homes constructed between 1900 and 1973, relative to assessed value, was approximately twice that of homes constructed after 1973 in Louisa County, and three times greater within areas of MMI intensity VI, VII, and VIII.
The 2011 Mineral, Virginia, earthquake is one of the larger recorded seismic events occurring east of the Rocky Mountains since seismic instrumentation was first deployed. The operation of the North Anna nuclear power station (NANPS), located ~22 km northeast of the epicenter, was affected by the earthquake vibration. This moderate event caused the first incident in which a commercial U.S. nuclear power plant experienced a safe shutdown as a result of earthquake ground motion. Post-earthquake investigations confirmed that important safety-related structures, systems, and components (SSCs) at the NANPS did not have any detectable damage. Damage at the NANPS consisted of cracking and spalling of some of the non-safety-related ancillary structures, and the plant was restarted after three months of intensive inspections and reviews. Response spectra developed from the recorded ground motion at the NANPS showed a modest exceedance of the plant seismic design levels for safety-related SSCs, but these SSCs were not damaged and maintained their functionality. The NANPS performance, in combination with other global examples, shows that nuclear power plants have been able to function safely even when earthquake ground motions exceeded the design levels of the SSCs. In this paper we describe the observed earthquake effects at the NANPS and discuss the original geologic and seismic characterization of the plant site. We also discuss the impacts of other earthquakes on the performance of various nuclear power plants, and previous and current seismic hazard and risk evaluations for U.S. nuclear power plants.
Ground shaking and structural response of the Washington Monument during the 2011 Mineral, Virginia, earthquake
The moment magnitude (M w ) 5.8 Mineral, Virginia, earthquake of 23 August 2011, was centered ~130 km south-southwest of Washington, D.C. (USA), and caused minor damage across Virginia and the Washington metropolitan area. The Washington Monument sustained masonry damage; a post-earthquake survey of the monument performed for the National Park Service identified cracking and spalling of the pyramidion (the topmost piece of the obelisk), and cracking, spalling, and lesser damage over the entire length of the monument shaft. A seismic vulnerability assessment of the monument was then performed to evaluate the potential for damage to the monument from future earthquakes. No ground-motion recordings of the Mineral earthquake were available for the monument site; therefore, deterministic and probabilistic seismic hazard analyses were performed to develop site-specific response spectra representing the Mineral earthquake and the maximum considered earthquake (MCE). These spectra were initially developed for a firm rock site condition, and each event was represented by a suite of seven three-component time histories. The rock motions were then modified through site-response analyses to develop time histories and response spectra representing ground motions in the clayey and gravelly soils that support the base of the monument. The results of the site-response analysis show significant amplification at short to intermediate response periods; this amplification is also observed in recordings of the 2011 Mineral earthquake obtained from another site in the general vicinity of the monument. The ground shaking conditions, along with expected foundation load-deflection behavior, were used in detailed structural modeling of the monument to help understand the structure’s response and damage during the 2011 Mineral earthquake and to predict expected performance during future MCE-level ground shaking. The analyses indicate that the period range of the pyramidion’s modes of vibration corresponds closely with the characteristic period range of the monument’s subsurface profile, and is reflected by the peak response of the site-specific spectra analyzed for the Mineral scenario earthquake. This similarity caused amplification of motions experienced by the monument and increased damage to the pyramidion. The nonlinear analysis for the MCE ground motions indicates damage to the pyramidion similar to that from Mineral earthquake effects, but larger lateral displacements of the top of the shaft due largely to the greater soil-bearing stresses associated with a greater first-mode response. The monument was found to generally meet accepted seismic safety criteria without need for strengthening.
Behavior and damage of the Washington Monument during the 2011 Mineral, Virginia, earthquake
This paper investigates the potential causes of the damage to the Washington Monument sustained from the 2011 Mineral, Virginia (USA), earthquake through time-history dynamic analysis. Ambient vibration field test data were obtained and utilized to calibrate a finite element model of the structure and its foundation. The impact of the foundation modeling and the uncertainties associated with the material properties of the stone and iron, in the absence of in situ material testing, were investigated through several parametric studies, in which the material property values are permuted at three (upper, average, and lower) levels to bound the predicted dynamic characteristics of the structure. Because ground-motion data recorded in the Washington, D.C., area during the earthquake are scarce, the ground motion at the Washington Monument site was simulated using an angular transformation of the recorded ground motions in Reston, Virginia, deconvoluted to the bedrock level and upward propagation of the rotated motions to the ground surface based on soil profiles in Reston and the Washington Monument site provided by the U.S. Geological Survey. The finite element model of the Washington Monument shaft subjected to these bidirectional earthquake records showed high acceleration amplification at the observation level, as well as tensile stress concentration at the ~107 m level. These observations correlate with the damage observed in the pyramidion section and upper levels of the Washington Monument shaft following the 2011 Virginia earthquake.
Aftershocks illuminate the 2011 Mineral, Virginia, earthquake causative fault zone and nearby active faults
Deployment of temporary seismic stations after the 2011 Mineral, Virginia (USA), earthquake produced a well-recorded aftershock sequence. The majority of aftershocks are in a tabular cluster that delineates the previously unknown Quail fault zone. Quail fault zone aftershocks range from ~3 to 8 km in depth and are in a 1-km-thick zone striking ~036° and dipping ~50°SE, consistent with a 028°, 50°SE main-shock nodal plane having mostly reverse slip. This cluster extends ~10 km along strike. The Quail fault zone projects to the surface in gneiss of the Ordovician Chopawamsic Formation just southeast of the Ordovician–Silurian Ellisville Granodiorite pluton tail. The following three clusters of shallow (<3 km) aftershocks illuminate other faults. (1) An elongate cluster of early aftershocks, ~10 km east of the Quail fault zone, extends 8 km from Fredericks Hall, strikes ~035°–039°, and appears to be roughly vertical. The Fredericks Hall fault may be a strand or splay of the older Lakeside fault zone, which to the south spans a width of several kilometers. (2) A cluster of later aftershocks ~3 km northeast of Cuckoo delineates a fault near the eastern contact of the Ordovician Quantico Formation. (3) An elongate cluster of late aftershocks ~1 km northwest of the Quail fault zone aftershock cluster delineates the northwest fault (described herein), which is temporally distinct, dips more steeply, and has a more northeastward strike. Some aftershock-illuminated faults coincide with preexisting units or structures evident from radiometric anomalies, suggesting tectonic inheritance or reactivation.
Aftershock imaging using a dense seismometer array (AIDA) after the 2011 Mineral, Virginia, earthquake
The Aftershock Imaging with Dense Arrays (AIDA) project recorded 12 days of high-density seismic array data following the 23 August 2011 Mineral, Virginia (USA), earthquake. AIDA utilized short-period, vertical-component seismographs at 201 locations to record closely spaced data that would reduce spatial aliasing. Interstation correlation enabled a detection threshold between magnitude −1.5 and −2. A joint hypocenter and velocity inversion algorithm was applied to compressional and shear wave arrival times for 300 of the larger events. Traveltime misfits were minimized using a constant velocity of Vp = 6.2–6.25 and Vs = 3.61–3.63. Hypocenter location error estimates for this range of velocities are ~100 m. Little to no three-dimensional variation exists in the seismic velocity of the upper crust, consistent with the aftershock zone being within a single crystalline rock terrane. The hypocenter locations define a 1–2-km-wide cloud with a strike of ~029° and dip of ~53°E, consistent with the focal mechanism of the main shock. The cloud bends ~5° along strike and has a slightly shallower dip angle below ~6 km depth, indicating a broad, complex fault zone with a slightly concave shape. This study shows that seismic arrays comparable to those used in controlled-source seismology can be successfully applied to aftershock sequences, and that dense array data can produce high-resolution information about earthquake rupture zones.