Abstract
Volcanic explosions can produce large, ash-rich plumes that pose great hazard to aviation, yet may often have few precursory geophysical signals. Mount Cleveland is one of the most active volcanoes in the Aleutian Arc, Alaska (United States) with at least 65 explosions between December 2011 and June 2020. We characterize the seismo-acoustic signals from explosions at Mount Cleveland over a period of 4 years starting in 2014 when the permanent local instrumentation was installed. While the seismic explosion signals are similar, the acoustic signals vary between explosions. Some explosion acoustic waveforms exhibit a single main compressional phase while other waveforms have multiple compressions. The time lag between seismic and acoustic arrivals varies considerably (up to 2.20 s) at a single station ∼3 km from the vent, suggesting a change in propagation path for the signals between explosions. We apply a variety of methods to explore the potential contributions to this variable time lag from atmospheric conditions, nonlinear propagation, and source depth within the conduit. This variable time lag has been observed elsewhere, but explanations are often unresolved. Our results indicate that while changes in atmospheric conditions can explain some of the variation in acoustic arrival time relative to the seismic signal arrivals, substantial residual time lag variations often still exist. Additionally, nonlinear propagation modeling results do not yield a change in the onset time of the acoustic arrival with source amplitudes comparable to (and larger) than Cleveland explosions. We find that a spectrum of seismic cross-correlation values between events and particle motion dip angles suggests that a varying explosion source depth within the conduit likely plays a dominant role in the observed variations in time lag. Explosion source depths appear to range from very shallow depths down to ∼1.5–2 km. Understanding the seismo-acoustic time lag and the subsequent indication of a variable explosion source depth may help inform explosion source modeling for Mount Cleveland, which remains poorly understood. We show that even with a single co-located seismic and acoustic sensor that does not always remain on scale, it is possible to provide meaningful interpretations of the explosion source depth which may help monitoring agencies understand the volcanic system.
Highlights
Coupled seismic and acoustic analyses can be used to help understand shallow to subaerial explosion sources, including buried chemical explosions (e.g., Arrowsmith et al, 2010; Jones et al, 2015; Blom et al, 2020) and volcanic eruptions (e.g., Johnson and Aster, 2005; Petersen and McNutt, 2007; Wech et al, 2018)
Multi-year studies of volcanic activity provide observations of trends. Deviations from those trends prove useful for volcano monitoring that may not be apparent for studies using temporary deployment data that may only catch a few explosions (e.g., Lamb et al, 2019; Wallace et al, 2020)
We examined the activity at Mount Cleveland from 2014 to 2018 and put it in context of recent work focused on shorter-term, dense observations
Summary
Coupled seismic and acoustic analyses can be used to help understand shallow to subaerial explosion sources, including buried chemical explosions (e.g., Arrowsmith et al, 2010; Jones et al, 2015; Blom et al, 2020) and volcanic eruptions (e.g., Johnson and Aster, 2005; Petersen and McNutt, 2007; Wech et al, 2018). Tameguri et al (2002) describe the “bottom-up” explosion source model of Sakurajima as beginning with an isotropic expansion at a few kilometers depth, whose pressure waves propagate up the conduit, inducing an expansion of the lava cap or plug (viewed as a small increase in pressure on the infrasound sensor) followed by the main acoustic explosion signal. A “top-down” model is sometimes considered, where the explosion initiates near the surface due to the pressurized build-up and failure of the plug or lava dome. This model causes a very long period earthquake (VLP) at depth (Lyons and Waite, 2011)
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