Locating tremor using stacked products of correlations

We introduce a back-projection method to locate tremor sources using products of
cross-correlation envelopes of time series between seismic stations. For a given subset of n stations, we
calculate the (n − 1)th-order product of cross-correlation envelopes and we stack the back-projected
products over combinations of station subsets.We show that compared to existing correlation methods and
for realistic signal and noise characteristics, this way of combining information can significantly reduce the
effects of correlated (spurious or irrelevant signals) and uncorrelated noise. Each back-projected product
constitutes an individual localized estimate of the source locations, as opposed to a hyperbola for the
existing correlation techniques, assuming a uniform velocity in two dimensions. We demonstrate the
method with synthetic examples and a real-data example from tremor at Katla Volcano, Iceland, in July 2011.
Despite very complex near-surface structure, including strong topography and thick ice cover, the method
appears to produce robust estimates of tremor location.