Our group uses seismic and infrasonic waves to investigate how volcanoes work. Infrasound is atmospheric sound with frequencies below 20 Hz, the lower frequency limit of human hearing. With seismology, we study magmatic, hydrothermal, and faulting processes occurring within and around volcanoes. With infrasound, we study the mechanisms and dynamics of explosive eruptions and shallow volcanic degassing. This work is focused on understanding the geophysical signatures of volcanic unrest and eruption, with application in monitoring and mitigating volcanic hazards.

(above) Strombolian explosions from Yasur volcano, Tanna Island, Vanuatu, July 2016. Movie credits: Nick Key, Ben Kennedy (UAV camera); David Fee (crater camera).

(1) Volcano seismology

Volcano seismology involves the analysis, interpretation, and modeling of seismic signals generated inside and around active volcanoes, as well as the application of seismic techniques to image internal volcanic structure. Volcano seismology is advancing dramatically as a result of improvements in instrumentation, new powerful analysis and computational techniques, integration of seismic with other geophysical and geological methods, and focused numerical and laboratory experiments. Our group approaches volcano seismology with the philosophy that we should: (1) fully exploit the potential of existing seismic networks and data, and (2) augment these observations with carefully designed experiments to test specific hypotheses about volcano-seismic source processes and internal volcanic structure.

(left) Seismic long-period (LP) events recorded on a broadband seismometer at Mount St. Helens in July 2005. LP events, closely related to volcanic tremor, are thought to result from fluid movement and oscillation in magmatic and hydrothermal systems. These signals are used routinely by volcano monitoring scientists to assess and forecast unrest and eruption despite an incomplete understanding of their origin [Matoza and Chouet, 2010; Matoza et al., 2015].

(2) Volcano acoustics

Our previous work has demonstrated the utility of infrasound for capturing local and remote explosive volcanism and for understanding the dynamics of shallow volcanic degassing and eruption columns. We have shown that infrasound is useful for studying volcanic eruptions for two main reasons: (1) Shallow and subaerial volcanic processes radiate sound directly into the atmosphere; sampling this sound complements seismic data, which record subsurface sources from mantle depths all the way to the surface, and (2) Infrasound propagates long distances in the atmosphere and is routinely detected on sparse ground-based infrasound networks. For these two reasons, infrasound is sometimes the only ground-based technology to record an explosive eruption.

(right) 5-day infrasound waveforms from the June 2009 eruption of Sarychev Peak, Kuriles recorded 640 km away on IMS station IS44. No seismic data were available for this eruption, whereas these remote infrasonic detections can be used to detect, locate, and provide a detailed explosion chronology for the eruption to complement satellite data [Matoza et al., 2011].

(3) Infrasound and seismo-acoustics

More generally, we are interested in all aspects of infrasound and seismo-acoustic research, including numerous natural and man-made sources as well as infrasound propagation through the atmosphere. In addition to volcanoes, sources of infrasound include earthquakes, mass-wasting events, bolides, ocean wave-wave interactions, lightning, hurricanes, tornadoes, wind flow over mountains, glacier calving, auroras, and many other sources.

(left) The planned 60-station IMS infrasound network (inverted triangles) and the world's potentially active volcanoes (red triangles) [Smithsonian].

(above) An explosive eruption at Sakurajima Volcano, Japan [image: R.S. Matoza] and example infrasound arrays from the International Monitoring System (IMS) infrasound network (CTBTO images).