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Relative relocation methods are commonly used to precisely relocateearthquake clusters consisting of similar waveforms. Repeating waveforms areoften recorded at volcanoes, where, however, the crust structure is expected tocontain strong heterogeneities and therefore the 1D velocity model assumptionthat is made in most location strategies is not likely to describe reality. Apeculiar cluster of repeating low-frequency seismic events was recorded on thesouth flank of Katla volcano (Iceland) from 2011. As the hypocentres arelocated at the rim of the glacier, the seismicity may be due to volcanic orglacial processes. Information on the size and shape of the cluster may helpconstraining the source process. The extreme similarity of waveforms points toa very small spatial distribution of hypocentres. In order to extractmeaningful information about size and shape of the cluster, we minimizeuncertainty by optimizing the cross-correlation measurements andrelative-relocation process. With a synthetic test we determine the bestparameters for differential-time measurements and estimate their uncertainties,specifically for each waveform. We design a relocation strategy to work withouta predefined velocity model, by formulating and inverting the problem to seekchanges in both location and slowness, thus accounting for azimuth, take-offangles and velocity deviations from a 1D model. We solve the inversionexplicitly in order to propagate data errors through the calculation. With thisapproach we are able to resolve a source volume few tens of meters wide onhorizontal directions and around 100 meters in depth. There is no suggestionthat the hypocentres lie on a single fault plane and the depth distributionindicates that their source is unlikely to be related to glacial processes asthe ice thickness is not expected to exceed few tens of meters in the sourcearea.

geophysical journal international

Relative relocation of earthquakes without a predefined velocity model:  an example from a peculiar seismic cluster on Katla volcano's south-flank  (Iceland)