  We show the capabilities of a downhole Distributed Acoustic Sensing (DAS)
array in detecting, locating and characterizing low-magnitude earthquakes
occurring in the vicinity of the Frontier Observatory for Research in
Geothermal Energy (FORGE) site in Utah. 10.5 days of continuous data were
acquired in a monitoring well at the FORGE geothermal site during the initial
stimulation of an Enhanced Geothermal System in April-May 2019. Earthquake
activity beneath Mineral Mountains, Utah also occurred within 10 km of the
FORGE monitoring well. During the experiment, four events from those areas were
cataloged by the University of Utah Seismograph Stations. Our processing of DAS
data, including template matching, finds 82 earthquakes during that period, of
which 16 are visible on the regional network. The magnitude of completeness
obtained by DAS processing is better by at least M=0.5 than the dense surface
array around the FORGE site. While a single vertical DAS array is limited in
terms of event location due to its azimuthal ambiguity, multiple DAS wells or a
combination of a downhole array with surface stations or near-surface
horizontal DAS could jointly resolve locations. All detected events probably
originated from the two active source areas and can be clustered into several
distinct families.
