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recognition. We identified two small al., 2020). Trench T1 was excavated three RELATION TO TECTONIC
N110°- trending fissures in the south days after the mainshock at the Greenway FRAMEWORK AND
river bank, with 2–3 cm of reverse verti- Drive industrial park (section A) at the tip of GEOMORPHOLOGY
cal offset. Minor rockfall on the southern a rupture segment (Figs. 2A and 3A–3D). A The focal mechanism, InSAR interfero-
slope and a small liquefaction feature in ~5-m-long and ~1.2-m-deep trench exposed grams, field observations, and aftershock
a sand bar on the northern riverbank weathered Neoproterozoic to Ordovician sequence provide evidence supporting a
were also documented. metasedimentary bedrock of the Ashe SSW-dipping seismogenic fault. The surface
3. Rivers Edge Road (section C): An Metamorphic Suite overlain by northward- ruptures are ~N110°, similar to the strike of
~8-cm-high rupture scarp, trending thickening horizontal layers of clay and the south-dipping nodal plane for the first
~N90°, crossed Rivers Edge Road, causing sand construction fill. A thrust fault is rec- motion moment tensor solutions and the
buckling of the road and breakage of a bur- ognized in the upper few dm of the trench InSAR unwrapped interferograms. None of
ied water pipe (Figs. 2E–2F). Eastward, displacing surficial fill deposits ~10 cm these matches mapped tectonic structures in
the surface rupture crosses a north-facing along a fault plane (N100°, 19° S) with 4-cm the Sparta area (Rankin et al., 1972; NCGS,
slope in open fields and forest patches, vertical displacement of the southern hang- 1985; Merschat et al., 2020). However, the
striking ~N110° with a linear and continu- ing wall, forming a small, very well- Little River fault is subparallel to the Boone
ous trace. The topographic step is single or preserved scarp. The low-angle fault roots and Mills Gap faults, located 50 and 150 km
complex, with heights varying between 5 into weathered bedrock ~20 cm into a to the southwest, respectively (Wooten et al.,
and ~25 cm. It was trackable for ~1,100 m steeper preexisting plane (N115°, 45° S) 2010; Hill, 2018). Hill (2018) argued that
until the scarp and fissures stopped being interpreted to be associated with the WNW lineaments in North Carolina are
detectable due to dense vegetation. Paleozoic fabric. Primarily dip-slip slicken- likely brittle Cenozoic structures. The sur-
4. Chestnut Grove Church Road (section lines were identified in the low-angle and face rupture is located along a subtle ~10-km-
D): The surface rupture is subdued as steeper fault surfaces, although oblique long WNW-ESE–trending topographic lin-
it continues from Duncan Drive to slickenlines were observed near the surface eament visible in digital elevation models
Chestnut Grove Church Road. Small fis- in folded materials of the scarp (Hill et al., and centered on the fault zone (Fig. 1A).
sures and WNW-trending 3–4-cm-high 2020). Small excavations within the indus-
steps occur in a cattle path close to trial park across minor scarps corroborate DISCUSSION
Duncan Drive. Following the rupture reverse faulting and cumulative co-seismic The Sparta earthquake is unusual for
strike N110° to Chestnut Grove Church vertical displacements of up to 10 cm. eastern North America and worldwide
Road, extensional co-seismic fissures Moreover, markers in the pavement across because 4.5 < M < 5.5 earthquakes rarely
w
broke the road into several decameter the fault trace were not laterally displaced. produce surface ruptures. It was also unex-
blocks, which have a similar geometry to Older fault gouge with manganese-coated pected, occurring along an unknown struc-
older cracks in the asphalt. These co- surfaces and breccia in the weathered bed- ture trending oblique to the regional NE-SW
seismic fissures suggests that preexist- rock indicates brittle deformation, with structural trend and triggered at shallower
ing mass wasting–related features were dip-slip slickenlines. The age of the brittle depths than regional seismicity.
activated during the earthquake. We deformation is unknown. The fault identified in trench T1 (N115°,
interpret these as minor deformation Trench T2 (~1.3-m deep and ~10-m long) 45°S) was interpreted as reactivation of a
along the eastward rupture termination. was opened along the side of Rivers Edge foliation plane (Fig. 2A). We argue that the
South of U.S. Route 21 and southwest of Road across an ~8-cm-high scarp next to a foliation parallel to the active fault in T1
Sparta, co-seismic deformation not related to buckled road and broken water pipe (Figs. could be locally rotated due to brittle defor-
the main rupture trace, expressed by ground 2A and 3E–3F). The excavation exposed two mation, similar to anomalous WNW rotated
fissures in less consolidated material in road clay-rich colluvial units (2 and 3) and sapro- foliation observed in the Mills Gap fault
cuts and riverbanks, resulted from ground lite (4), which were not displaced by a fault zone (Wooten et al., 2010). Trench T2 is
shaking or translational sliding (Fig. 2A). despite being positioned across the scarp. placed at a compressional step-over without
Near Little River, riverbank fissures are sub- The upper colluvium (1) was gently warped; evidence of faulting. Nonetheless, GPR pro-
parallel with cm-dm–long and mm-cm–wide however, this folding was not observed in the files acquired 20 m to the east of T2 and
openings. At an industrial parking lot (3238 lower colluvium or saprolite. The absence of along an ~600-m segment consistently
U.S. Route 21, Glade Valley, North Carolina), faulting despite the evident compression was show a low-angle south-dipping reflector
co-seismic fissures extend for several m with corroborated by a 250 MHz GPR profile par- (Fig. 3G). This reflector projects to the sur-
mm-to-cm openings, exhibiting shortening allel to T2 interpreted as recording minimal- face a few meters north of the co-seismic
and extensional displacement. These features to-no stratigraphic disturbance in the upper scarp, where ground deformation was not
are likely due to co-seismic deformation along 4 m. We argue that T2 is located in a com- recognized. We suggest that this reflector
several preexisting pavement discontinuities pressional step-over. may be (1) related to an older earthquake
formed by earlier downslope processes and There was no clear evidence for cumula- structure or (2) the result of deformation
differential compaction in artificial fills. tive Quaternary deformation prior to the partitioning on an unidentified complex set
2020 earthquake due to a lack of additional of structures during the 2020 earthquake.
FAULT TRENCHING AND Quaternary markers at depth in T1. None- The strike of the seismogenic structure
SUBSURFACE ANALYSIS theless, both trenches unequivocally dem- inferred from seismology and remote sens-
We excavated two trenches to investigate onstrate surface deformation caused by an ing is consistent with field observations.
the earthquake deformation (Figueiredo et active fault. However, and interestingly, the kinematics

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