Improvements to Cross-Station Analysis of Tectonic Tremor

Cross-station analysis of tectonic tremor, as introduced by John Armbruster for long (150 s) windows and extended by Allan Rubin to short (4 s) windows, has afforded the highest precision mapping of tremor locations achieved to date. Cross-station locations have been used to a) reveal tremor epicentral distributions over broad (to ~10⁴ km²) areas that are significantly sparser than those portrayed by more commonly used envelope-correlation methods; b) document a variety of tremor (and by inference slow-slip) propagation modes, and c) place tremor in a structural context leading to insights into its generation and that of slow slip more generally. In particular, our work on c) suggests that tremor occurs within the upper layer of oceanic metabasalt as the expression of disaggregation and comminution associated with underplating. The success of cross-station analysis in tremor characterization to date warrants further investigation and development of the methodology. In this presentation, we detail two improvements.  The first, applicable to both long and short windows, concerns the judicious analysis of traveltime circuits and binomial coefficients ("n choose 3") corresponding to triples of waveforms formed from many (n ≥ 4) stations. This leads to effective quality-control measures for balancing location precision versus accuracy and the leveraging of cross-event information. The second improvement concerns short windows and relies on low frequency earthquake template waveforms that characterize propagation characteristics between a localized tremor source region and surface stations. Past efforts at cross-station tremor detection have relied upon the similarity of tremor waveforms across stations. This condition can be relaxed through the use of phase normalization afforded by template waveforms thereby enabling inclusion of larger station complements and resulting in increased number and quality of detections. We demonstrate these improvements on deep tremor recordings from southern Vancouver Island between 2003-2006 and 2022-2025.