On the relation between infrasound, seismicity, and small pyroclastic explosions at Karymsky Volcano

Abstract

Ground‐ and atmospheric‐propagating elastic waves are integrated with digitized video to reconstruct important parameters during the onset of eruptions at Karymsky Volcano in 1999. Muzzle velocities, which range from 10 to 55 m/s for 34 analyzed explosive events, are found to be very well correlated with eruption velocities synthesized from infrasonic trace data. Acoustic modeling proposed here invokes a linear acoustic source consistent with a 3‐m piston that accelerates upwards. Timing of acoustic arrivals relative to video records of material emission is used to constrain fragmentation sources depths, which are assumed to be coincident with infrasound generation. These sources are generally shallow, with an observed vertical range of about 15 m. Systematic changes in source depth are observed during two nights of data collection and are attributed to variations in the depth of the magma free surface. Relative timing of acoustic and seismic arrivals is used to identify the presence of seismicity prior to fragmentation. This seismicity is emergent and low intensity, but significant because it precedes the presumed fragmentation events by variable amounts up to 4 s. The primary short‐period seismic phases appear coincident with the initiation of fragmentation and infrasound generation. Because of the complexity of the seismic Green's function, it is not possible to correlate amplitude from a specific seismic pulse with observed eruption intensity.