The Sparta cluster is named for the city of Sparta in northwestern North
Carolina. It covers a rather broad area and includes events in eastern
Tennessee, western Virginia and southernmost West Virginia. The cluster
is based on a 5.1 Mw earthquake on August 9, 2020 very close to Sparta.
The Sparta earthquake is remarkable for its shallow depth (with various
estimates in the 2-4 km range) and the observation of surface faulting.
It has a depth of 4 km in the calibrated relocation, based on arrivals
at local distance, but a shallower depth cannot be ruled out. The
cluster as a whole is at quite shallow depths, with only a single event
as deep as 10 km. It is quite likely that many of the events are mining
blasts. Most events have magnitudes in the 2-3 range. The distribution
of seismic stations is very good and the location calibration is very
strong, with an uncertainty of ~500 m for the hypocentroid at the 90%
confidence level. However the calibrated epicenter, like most other
location-based estimates that were reported, is a few km south of the
surface faulting. This could be a consequence of lateral heterogeneity
in the region. Seismograph stations to the southeast are on the coastal
plain, while the rest are in the Appalachian Mountains.
