Long-period events (LPs) are observed in active volcanoes, hydrothermal systems, and hydraulic fracturing. The prevailing model for LP events suggests that they result from pressure disturbances in fluid-filled cracks that generate slow, dispersive waves known as Krauklis waves. These waves oscillate within the crack, causing it to act as a seismic resonator whose far-field radiations are known as LP events. Since LP events are generated from fluid-filled cracks, they have been used to analyze fluid transport and fracturing in geological settings. Additionally, they are deemed precursors to volcanic eruptions. However, other mechanisms have been proposed to explain LP seismicity. Thus, a robust interpretation of LP events requires understanding all parameters contributing to LP seismicity. To achieve this, for the first time, we have developed a physical model to investigate LP seismicity under controlled-source conditions. The physical model consists of a 30 cm × 15 cm × 0.2 cm crack embedded within a concrete slab with dimensions of 3 m × 3 m × 0.24 m. Using this apparatus, we extensively investigate fundamental factors affecting LP signals, including crack stiffness, fluid viscosity, radiation patterns, and triggering location. Our findings are consistent with the theoretical model for Krauklis waves within a fluid-filled crack. For instance, a reduction in stiffness leads to an increase in resonance frequency, whereas an increase in fluid viscosity results in a decrease in resonance frequency. Thus, this physical model can offer new horizons in understanding LP seismicity and bridge the gap between theoretical models and observed LP signals.

Fuego volcano in Guatemala began its current eruptive episode in 1999. From 2008-2015 we observed repeating and near-repeating seismic events in the long period (LP: 0.5-5 Hz) and very long period (VLP: 100-10s) bandwidths. Two separate types of repeating VLP events indicate pressurization within the shallow conduit prior to explosions with different surficial expressions, including emissions from two separate vents from at least 2008-2012. Between explosions, repeating LP events which do not have associated visible emissions provide a mechanism for small magnitude degassing. The seismic amplitudes of the LP events are 1-3 orders of magnitude lower than the amplitudes of the VLP events. The coefficient of variation of the interevent times for these repeating LP events in 2012 were all above 1.5, which suggests a renewal process driven by interactions of more than one factor. Based on the at least eight-year stability of both the LP and VLP signals and coupled with various other visual datasets, we present an updated model of the shallow conduit dynamics controlling explosive events. In this model, the VLP source acts as a possible constriction point allowing for crystal and volatiles to form local concentrations out of an otherwise steady supply of magma. High water content leads to undercooled magma and promotes rapid crystallization and the formation of partial seals within the conduit. Pressurization due to and breaking of these seals results in the modeled VLP source. Strain along the conduit margins promotes the formation of fracture networks which facilitate degassing, the opening and closing of which are sources for the LP signals. Small fluctuations in magma ascent rates therefore have drastic effects on changes in shallow magma rheology and eruption style. These small fluctuations average out over the intermediate term (week to month) to maintain observed, stable, long-term (year to multi-year) degassing rates from the volcano.

The 2018 eruption of Kīlauea Volcano included caldera collapse at the summit of the volcano that was well recorded by a network of both permanent and temporary seismometers and infrasound microphones. Volcanic activity prior to the start of the eruption was elevated, including high lava lake levels and increased seismicity. Deflation of the summit began shortly after the eruption in the lower East Rift Zone commenced and was accompanied by a drop in the summit lava lake, which eventually disappeared from view after lowering by several hundred meters. Continued volume loss from beneath the summit eventually led to caldera collapse, which was accompanied by increases in earthquake rates and tremor amplitudes. Infrasonic tremor that originally rose from spattering at the surface of the lava lake was replaced by discrete infrasonic events arising from rockfalls and slumps. On May 16, the first of tens of collapse events occurred, beginning a cycle of increasing earthquake rates and energy release in the presence of deflation. After an initial increase in rate, seismicity rates remained constant for several hours prior to the next collapse event. Earthquakes during these events were typically part of repeating earthquake families and occurred on one of several circumferential cracks that marked active collapsing blocks. Through time, earthquake activity has mirrored the morphology of the collapse, migrating primarily north and east as caldera down drop extended in those directions. Collapse-related infrasound arrivals were initially down, suggesting downdropping of the surface, followed several seconds later by higher frequency infrasound, which we interpret to reflect the explosive or expulsion phase reaching the surface. Infrasound signals were 2-3 times more energetic below 0.5 Hz than above. Especially after May 29, when caldera collapse became much larger in surface area, infrasound signals were highly repetitive with a strong downward first motion. The collapse events were also deficient at seismic frequencies > 0.5 Hz, compared to typical tectonic earthquake sources. Despite similarities in waveforms at low frequencies (<0.1 Hz) and in infrasound, the seismic waveforms at high frequencies were not similar, reflecting either a unique source in each collapse event or a change in location.

Blasting experiments were performed that investigate multiple explosions that occur in quick succession in the ground and their effects on host material and atmosphere. Such processes are known to occur during volcanic eruptions at various depths, lateral locations, and energies. The experiments follow a multi-instrument approach in order to observe phenomena in the atmosphere and in the ground, and measure the respective energy partitioning. The experiments show significant coupling of atmospheric (acoustic)- and ground (seismic) signal over a large range of (scaled) distances (30–330 m, 1–10 mJ^-1/3). The distribution of ejected material strongly depends on the sequence of how the explosions occur. The overall crater sizes are in the expected range of a maximum size for many explosions and a minimum for one explosion at a given lateral location. The experiments also show that peak atmospheric over-pressure decays exponentially with scaled depth at a rate of d0 = 6.47×10-4 mJ-1/3; at a scaled explosion depth of 4×10-3 mJ-1/3 ca. 1% of the blast energy is responsible for the formation of the atmospheric pressure pulse; at a more shallow scaled depth of 2.75×10-3 mJ-1/3 this ratio lies at ca. 5.5–7.5%. A first order consideration of seismic energy estimates the sum of radiated airborne and seismic energy to be up to 20% of blast energy.