  Relative relocation methods are commonly used to precisely relocate
earthquake clusters consisting of similar waveforms. Repeating waveforms are
often recorded at volcanoes, where, however, the crust structure is expected to
contain strong heterogeneities and therefore the 1D velocity model assumption
that is made in most location strategies is not likely to describe reality. A
peculiar cluster of repeating low-frequency seismic events was recorded on the
south flank of Katla volcano (Iceland) from 2011. As the hypocentres are
located at the rim of the glacier, the seismicity may be due to volcanic or
glacial processes. Information on the size and shape of the cluster may help
constraining the source process. The extreme similarity of waveforms points to
a very small spatial distribution of hypocentres. In order to extract
meaningful information about size and shape of the cluster, we minimize
uncertainty by optimizing the cross-correlation measurements and
relative-relocation process. With a synthetic test we determine the best
parameters for differential-time measurements and estimate their uncertainties,
specifically for each waveform. We design a relocation strategy to work without
a predefined velocity model, by formulating and inverting the problem to seek
changes in both location and slowness, thus accounting for azimuth, take-off
angles and velocity deviations from a 1D model. We solve the inversion
explicitly in order to propagate data errors through the calculation. With this
approach we are able to resolve a source volume few tens of meters wide on
horizontal directions and around 100 meters in depth. There is no suggestion
that the hypocentres lie on a single fault plane and the depth distribution
indicates that their source is unlikely to be related to glacial processes as
the ice thickness is not expected to exceed few tens of meters in the source
area.
