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SUMMARY Long-period seismic events (LPs) are observed within active volcanoes, hydrothermal systems and hydraulic fracturing. The prevailing model for LP seismic 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 these events are generated from fluid-filled cracks, they have been used to analyse 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 these 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 investigate fundamental factors affecting long-period seismic signals, including crack stiffness, fluid density and viscosity, radiation patterns and triggering location. Our findings are consistent with the theoretical model for Krauklis waves within a fluid-filled crack. In this study, we examine the interplay between fluid properties and characteristics of waves within and radiated from the crack model. Records from a pressure transducer within the crack model have the same frequency characteristics as the surface sensors, indicating that the surface sensors are recording the crack waves. Because the crack stiffness parameters for all the fluids are relatively high, fluid density variations have a larger effect on the crack wave frequency, with higher density fluids yielding lower resonance frequencies. Similarly, the quality factor (Q) decreases with increasing fluid density. We also find that an increase in fluid viscosity along with the increased fluid density results in a decrease in resonance frequency and Q. Trigger locations at the middle of the crack length and width most effectively resonated the first and second transverse modes. Thus, this physical model can offer new horizons in understanding LP seismicity and bridge the gap between theoretical models and observed LP signals. 

Abstract Lahars, or volcanic mudflows, are one of the most devastating natural, volcanic hazards. Deadly lahars, such as the one that occurred after the Nevado del Ruiz, Columbia eruption in 1985, in which at least 23,000 people tragically lost their lives, threaten the safety and well-being of humans, the economy, and the infrastructure of many of the communities living in the vicinity of volcanoes. Due to their complex flow behaviors, lahars remain a major challenge to those studying them. We present an analysis of several rain-triggered lahar events at Volcán Fuego in Guatemala using both seismic and infrasound monitoring to quantify both ground vibrations and low-frequency atmospheric sound waves associated with these mudflows. Geophysical data collected over this field campaign quantifies flow parameters such as velocities, stage and the frequency of these rain-triggered lahars. Time-lapse imagery of lahar flows is compared with filtered seismo-acoustic signal characteristics to ascertain stage predictions and relationship to stage fluxes. Using random forest regression models, we establish moderate correlations (correlation coefficient modes 0.48–0.53) with statistical significance (pvalue = 0.01–0.02) between signal energetics and respective stage. Compiling a catalog of rain-triggered lahar events in Volcán de Fuego’s drainages over a season permits a dataset amenable to statistical analysis. Our goal is the development of new-generation geophysical monitoring tools that will be capable of remote and real-time estimation of flow parameters. 

Abstract Krauklis waves are generated by pressure disturbances in fluid‐filled cavities and travel along the solid‐fluid interface. Their far‐field radiation, observed in seismic data from volcanoes or hydraulic fracturing, is known as long‐period events. Characterized by low velocity and resonance, Krauklis waves help estimate fracture size and discern fluids in saturated fractures. Despite numerous theoretical models analyzing Krauklis waves, the existing paradigms are founded on static flow conditions. However, in geological contexts, the assumption of static flow may not be valid. We developed an experimental apparatus using a tri‐layer model consisting of a pair of aluminum plates to examine the effect of fluid flow on Krauklis waves. We employed an infusion syringe pump to inject fluids into the fracture under different flow rates. We used water, oil, and an aqueous solution of Polyethylene glycol as fracture fluids. We calculated resonant frequency, phase velocity, and quality factor to characterize the Krauklis waves. Our findings reveal that an increase in flow rate leads to a higher phase velocity, higher quality factor, and a shift to higher resonant frequency when the flow is in the direction of initial wave propagation while decreasing amplitude. Additionally, when the flow is in the opposite direction of initial wave propagation, we note higher wave absorption and distortion of the Krauklis waves. Our observations unequivocally affirm that fluid flow leaves strong signatures on the Krauklis waves, providing a robust basis for characterizing fluid dynamics within geological settings through the analysis of Krauklis wave.