The determination of seismic event locations with sparse networks or
single-borehole systems remains a significant challenge in observational
seismology. Leveraging the advantages of the location approach HADES, which was
initially developed for locating clustered seismicity recorded at two stations,
we present here an improved version of the methodology: HADES-R. Where HADES
previously needed a minimum of 4 absolutely located master events, HADES-R
solves a least-squares problem to find the relative inter-event distances in
the cluster, and uses only a single master event to find the locations of all
events, and subsequently applies rotational optimiser to find the cluster
orientation. It can leverage iterative station combinations if multiple
receivers are available, to describe the cluster shape and orientation
uncertainty with a bootstrap approach. The improved method requires P- and
S-phase arrival picks, a homogeneous velocity model, a single master event with
a known location, and an estimate of the cluster width. The approach is
benchmarked on the 2019 Ridgecrest sequence recorded at two stations, and
applied to two seismic clusters at the FORGE geothermal test site, including a
microseismic monitoring scenario with a DAS in a vertical borehole. Traditional
procedures struggle in these settings due to the ill-posed network
configuration. The azimuthal ambiguity in this scenario is partially overcome
by assuming that all events belong to the same cluster around the master event
and a cluster width estimate. We find the cluster shape in both cases, although
the orientation remains uncertain. The method's ability to constrain the
cluster shape and location with only one well-located event offers promising
implications, especially for environments where limited or specialised
instrumentation is in use.Comment: 33 pages, 15 figures. Manuscript submitted to Geophysical Journal
Internationa