Earthquake waveform similarity and evolution at Augustine Volcano from 1993 to 2006: Chapter 5 in The 2006 eruption of Augustine Volcano, Alaska

Temporal changes in waveform characteristics and earthquake locations associated with the 2006 Augustine eruption and preeruptive seismicity provide constraints on eruptive processes within the edifice. Volcano-tectonic earthquakes occur within the upper 1 to 2 km at Augustine between and during eruptive cycles, and we use the Alaska Volcano Observatory hypocenter and waveform catalog from 1993 to 2006 to constrain changes in event similarity and location over time. Waveform crosscorrelation with bispectrum verification improves the pick accuracy of the catalog data to yield better locations and allows for identification of families of similar earthquakes. Event waveform similarity is low at Augustine, with ~60 to 70 percent of events failing to form event families of more than 10 events. The remaining earthquakes form event families over multiple time scales. Events prior to the 2006 eruption exhibit a high degree of similarity over multiple years. Earthquakes recorded during the precursory and explosive phases of the 2006 eruption form swarms of similar earthquakes over periods of days or hours. Seismicity rate and event similarity decrease rapidly during the explosive and effusive eruption phases. The largest recorded swarms accompany reports of increased steaming and explosive eruptions at the summit. Relative relocation of some event families indicates upward migration of activity over time, consistent with magma transport by way of an ascending dike. Multiple regions of the edifice generate seismicity simultaneously, however, suggesting the edifice contains a network of fractures and/or dikes.