Precise Correlation-Based Phase Repicking of Pinatubo Pre-Eruption Seismicity Illuminates Families of Repeating Earthquakes and a Refined Image of Precursory Hypocenter Locations.

We applied waveform cross-correlation and hierarchical clustering methods to volcanic seismicity recorded at Mount Pinatubo between 1 May and 14 June, 1991, identifying 155 clusters of up to 50 similar earthquakes each. Relative corrections of initial P-wave picks were as large as 0.563 s within similar event families, although the mean adjustment at our master station was only 0.04 s, with a standard deviation of 0.05 s. Final, absolute picks were obtained by applying an Akaike Information Criteria autopicker to stacks of the aligned waveforms within each cluster. We demonstrate enhanced phase identification for noisy arrivals by application of the cross-correlation, clustering and stacking strategy, and we show refined hypocenter locations following the automatic phase repicking. Station PIE (6 km ENE of the summit) was selected as our master station because of its longevity and high signal quality. Waveforms for brittle-failure events display great similarity at PIE for much of the activity during May, 1991. Over 80% of the picked seismograms at PIE recorded before 3 June fall into clusters originating some 5 km northwest of the crater. Consistent waveforms at this and other stations suggest similarity in mechanism; however, clear reversals in P-wave polarity within these families at station PPO (~5 km NNW of the summit) indicate near-nodal arrivals at this station and my help to constrain focal mechanisms. Families of shallow volcano-tectonic (VT) events beneath the summit begin to dominate on 3 June. Long-period events, mixed with continued shallow VT events beneath the summit, begin to dominate the digital record at PIE from 12 to 14 June, when correlatable signals ceased prior to the station's demise in the 15 June cataclysmic eruption.