Volcano observatories worldwide use seismicity to monitor volcanoes and set alert levels to protect life and property, but do not fully understand how the seismic signals are generated. This research will better elucidate seismic source processes during volcanic unrest and eruption, potentially improving capability in volcanic hazard monitoring and mitigation. Long-period (LP) seismicity, a particular type of volcanic seismicity, is used routinely by volcano monitoring scientists to forecast and assess eruptions and mitigate hazards, but its source origin remains controversial. Sequences of LP events and more continuous volcanic tremor signals (collectively called LP seismicity) commonly appear on seismograms before eruptions, providing a warning, but not a guarantee, of impending eruption. This project will perform detailed investigations into the origin of an intriguing and largely overlooked additional type of volcanic seismicity: numerous tiny-amplitude LP subevents that accompany the regular LP events. LP subevents could be a useful new signal for monitoring volcanoes. This research will lead to improved descriptions and catalogs of seismicity for multiple volcanoes, generating a public database of information suitable for other researchers in their studies of volcanoes and seismicity. These products will be of interest to the geophysics, geology, seismology, and volcanology communities. This project initiates collaboration between UC Santa Barbara, UC San Diego, and the USGS. It supports the educational program at UC Santa Barbara by providing funds for graduate student support. The volcano-seismic event cataloging code will be made publicly available and will be useful to volcano seismologists for research and monitoring.
This project will utilize novel computationally intensive processing methods adapted from studying regional seismicity in Southern California and Hawaii. Tiny LP subevents have apparently been recorded at multiple volcanoes worldwide, but their origin remains mysterious. Millions of tiny LP subevents were exceptionally well recorded by a dense seismic network during the 2004-2008 eruption of Mount St. Helens (MSH), but were not cataloged or analyzed. These LP subevents contain rich, unexploited information that has the potential to better elucidate the processes generating volcanic seismicity. This research will map the spatiotemporal distribution and source mechanisms of millions of tiny LP subevents to high precision and determine their relation to other volcanic seismicity and eruptive activity. The primary dataset is from MSH, but additional datasets from Mammoth Mountain, CA, Kilauea Volcano, HI, and other volcanoes will be exploited for comparative analyses and hypothesis testing across multiple volcanic systems. This project will systematically detect and locate tiny LP subevents using a match-and-locate template matching approach, perform high-precision relocation and cluster analysis, stack groups of similar events (multiplets), and perform full-waveform inversion to reveal the moment-tensor and single-force equivalent point-sources. Integrating these results will help to resolve the relationships between seismicity and magmatic, hydrothermal, and solid extrusion conduit processes. The results will address the following questions: (1) Relocated seismicity will likely cluster into compact volumes, or align along fault, magma conduit, or hydrothermal structures: what do these structures reveal about the underlying volcano-seismic processes? (2) Do source mechanisms and locations of subevents reveal a dendritic plexus of cracks in the volcano groundwater system, or solid extrusion processes at the conduit margin? (3) What is the relationship between LPs and subevents? (4) Do LPs and subevents trigger each other? (5) Are these phenomena at MSH unique or is similar behavior observed at other volcanoes? (6) What is the underlying physical process causing this seismicity? (7) Can subevents be used to improve eruption forecasting?
