Bulk location and source mechanisms of repetitive long-period seismicity at Mount St. Helens in July 2005

Summary The 2004–2008 eruption of Mount St. Helens exemplified the practical challenges of processing high seismic event rates and data volumes. This protracted multi-year eruptive sequence was accompanied by a near-continuous occurrence of long-period (LP, 0.5–5 Hz) seismicity, but network-based picks and locations have previously been performed for only a relatively small subset of the overall seismicity, and full-waveform source-mechanism inversions have previously been performed for just two event types during the eruption. We present a new workflow for the bulk detection, cluster picking, 3-dimensional (3D) location, and full-waveform source-mechanism inversion of repetitive LP volcanic seismicity recorded by a local broadband seismic network. Our emphasis is on methods that: exploit waveform similarity (multiplets) commonly observed for volcanic LP seismicity swarms; can take into account 3D topographic and velocity structure heterogeneity typical of volcanoes; can process high event rates with relatively low analyst burden; and do not require a priori catalog picks or known waveform templates. We develop and illustrate the workflow using data from Mount St. Helens from 29 June 2005 to 31 July 2005, a time-window characterized by high-rate LP seismicity, including numerous lower-amplitude small LPs, and for which relatively dense seismic network coverage is available. Approximately 40% of seismicity (20,561 events out of 52,141 triggers) during 29 June 2005 to 31 July 2005 is repetitive enough to be grouped into 17 multiplets of at least 300 events. We present 3D locations of the 17 multiplets, representing 20,076 events having picks from at least 8 stations, along with full-waveform source mechanism inversions of the 17 separate multiplet stacks. Collectively, our seismic 3D hypocenter distributions for 29 June 2005 to 31 July 2005 are compatible with sources around the margins of the upper conduit section that is associated with plug solidification. All location results indicate compact seismicity clusters, with the 17 multiplets spatially connected and collectively located at shallow depth, approximately ~775 meters depth beneath the 1980s crater floor and directly beneath, or immediately adjacent to, the extruding dacite spine. The full-waveform source-mechanism inversions for the 17 multiplets generally indicate mechanisms dominated by volumetric components consistent with sub-horizontal cracks along with minor contributions from sub-vertical cracks. The overall picture is that of a stack or plexus of subhorizontal cracks located at shallow depth beneath the extruding dacite spines, consistent with subhorizontal hydrofractures extending from the conduit margins into the surrounding wall rock; minor sub-vertical crack components of the mechanisms could possibly arise from surfaces parallel to the conduit wall, such as within the damage halo associated with degassing pathways.