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are different from the observed slip. The relevant stress perturbations from the 2011 hypocenter promoted surface rupture. Left-
focal mechanism derived from waveform earthquake are unlikely to be the cause. The lateral reverse motion on a WNW-ESE–
analysis and the InSAR unwrapped inter- Sparta focal mechanism (SLUEC, 2020; trending fault is consistent with the regional
ferogram indicate oblique-reverse motion. Horton et al., 2021) is similar to the ones stress field, with S Hmax oriented NE-SW to
Geodetic monuments were displaced differ- obtained for the Giles County, Virginia, ENE-WSW. The occurrence of Quaternary
ently on the hanging wall and footwall sup- seismic zone, located ~100 km to the north- activity preceding the 2020 Sparta earth-
porting a left-lateral and reverse motion. In northeast. Across a six-year survey, Munsey quake has yet to be determined.
contrast, the geologic field evidence sug- and Bollinger (1985) calculated several pre- Two moderate earthquakes (2011 M 5.8
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gests predominantly reverse faulting and dominately strike-slip moment tensor solu- Mineral and 2020 M 5.1 Sparta), occurred
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associated folding along a low-angle (~20°) tions where the left-lateral solutions have an in eastern North America during the past
fault plane that roots in a steeper preexistent ESE trend dipping steeply to the south, sim- decade, causing disruption and economic
fabric (~50°). This discrepancy may indi- ilar to the 2020 Sparta earthquake. loss. These occurred along structures not
cate complex strain partitioning or a change In intraplate settings, recognizing active included in any database as potentially
of slip during the earthquake. The initial, structures and quantifying their deformation active, suggesting that the regional seismic
deep rupture may be predominantly strike- rates for seismic hazard can be challenging hazard may be underestimated.
slip, possibly along a steeper structure with when seismicity is diffuse and infrequent,
subsequent up-dip failures on shallower with long periods of quiescence spanning ACKNOWLEDGMENTS
preexisting planar discontinuities in the 10 years (e.g., Clark et al., 2012). The rec- We thank the National Science Foundation for
5–6
bedrock accommodating most of the reverse ognition and documentation of moderate supporting this research (EAR-2102530, support
component. Slip variations during an earth- earthquakes with surface rupture, like that of from Tectonics, Geomorphology and Land Use
quake rupture are recognized from recent the 2020 Sparta M 5.1 earthquake, has Dynamics, and the Geophysics programs) and the
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and historical earthquakes (Kearse and increased recently (e.g., King et al., 2019; people of Alleghany County in North Carolina for
providing access to their lands, particularly Ann
Kaneko, 2020). Further research is needed Ritz et al., 2020), likely due to the increasing Downing and the business owners within the Green-
to understand slip partioning associated knowledge and availability of remote sens- way Drive industrial park. U.S. Geological Survey
with the Sparta earthquake. ing methods. These earthquakes provide research supported by the National Cooperative Geo-
Eastern North American earthquakes com- direct evidence of seismicity that is generally logic Mapping Program and Earthquake Hazards
monly have complex ruptures. According to poorly expressed or misunderstood in the Program. Thanks to Sarah Wells and Ashley Lynn
for field assistance and Daria Khashchevskaya for
Horton et al. (2015), significant earthquakes paleoseismologic record. In the case of the help with graphics, and to Thomas Pratt and
such as the 1988 M 5.9 Saguenay and 1989 Sparta earthquake, the application of the J. Wright Horton, Jr., and two anonymous reviewers
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M 6.0 Ungava, both in Quebec, and the 2011 magnitude-surface deformation empirical for their comments, which greatly improved this
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M 5.8 Mineral, Virginia, event had large relationships (e.g., Wells and Coppersmith, manuscript. Any use of trade, firm, or product names
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local stress drops and complex ruptures that 1994) would support a larger magnitude is for descriptive purposes only and does not imply
endorsement by the U.S. government.
evolved spatially and temporally. The 2011 event. This discrepancy indicates that paleo-
Mineral earthquake had three subevents, with seismology data may be underestimating
most of the seismic moment release occurring seismic hazard for certain cases. Thus, docu- REFERENCES CITED
Bollinger, G.A., Chapman, M.C., Sibol, M.C., and
at a depth of 8 km as the earthquake rupture mentation of surface deformation generated Costain, J.K., 1985, An analysis of the focal
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Horton et al., 2015). The Sparta earthquake only for intraplate settings. https://doi.org/10.1029/GL012i011p00785.
had a shallow centroid depth and moment Chapman, M.C., 2013, On the rupture process of
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The regional S is NE-SW to ENE- The moderate M 5.1 Sparta earthquake v. 103, no. 2a, p. 613–628, https://doi.org/ 10.1785/
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changes; however, none of these apply to tions of the main surface rupture, document Long-term behaviour of Australian stable conti-
the Sparta region. Walsh et al. (2015) ana- predominantly reverse slip, controlled by a nental region (SCR) faults: Tectonophysics,
v. 566–567, p. 1–30, https://doi.org/10.1016/j.tecto
lyzed the Coulomb static stress transfer preexisting planar structure, inferred to .2012.07.004.
during the 2011 Mineral earthquake, and show brittle deformation of probable Crone, A.J., and Wheeler, R.L., 2000, Data for Qua-
except for an area in the range of 10 km Cenozoic age. This structure, now recog- ternary faults, liquefaction features, and possible
from the main event, stress changes are nized and named Little River fault, is possi- tectonic features in the Central and Eastern Unit-
negligible (~1 mbar) and much less than the bly part of a WNW-ESE set of lineaments ed States, east of the Rocky Mountain front: U.S.
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km from the 2011 Mineral seismic area, structural grain. The shallow (~0.6–1.0 km) quake of 1755: A 250th Anniversary Perspective:

10 GSA TODAY | March-April 2022