John J. Lyons

Explosive dome eruptions modulated by periodic gas‐driven inflation

Volcan Santiaguito (Guatemala) “breathes” with extraordinary regularity as the edifices conduit system accumulates free gas, which periodically vents to the atmosphere. Periodic pressurization controls explosion timing, which nearly always occurs at peak inflation, as detected with tiltmeters. Tilt cycles in January 2012 reveal regular 26 ± 6 min inflation/deflation cycles corresponding to at least ~101 kg/s of gas fluxing the system. Very long period (VLP) earthquakes presage explosions and occur during cycles when inflation rates are most rapid. VLPs locate ~300 m below the vent and indicate mobilization of volatiles, which ascend at ~50 m/s. Rapid gas ascent feeds pyroclast-laden eruptions lasting several minutes and rising to ~1 km. VLPs are not observed during less rapid inflation episodes; instead, gas vents passively through the conduit producing no infrasound and no explosion. These observations intimate that steady gas exsolution and accumulation in shallow reservoirs may drive inflation cycles at open-vent silicic volcanoes.

Distinguishing high surf from volcanic long‐period earthquakes

Repeating long-period (LP) earthquakes are observed at active volcanoes worldwide and are typically attributed to unsteady pressure fluctuations associated with fluid migration through the volcanic plumbing system. Nonvolcanic sources of LP signals include ice movement and glacial outburst floods, and the waveform characteristics and frequency content of these events often make them difficult to distinguish from volcanic LP events. We analyze seismic and infrasound data from an LP swarm recorded at Pagan volcano on 12–14 October 2013 and compare the results to ocean wave data from a nearby buoy. We demonstrate that although the events show strong similarity to volcanic LP signals, the events are not volcanic but due to intense surf generated by a passing typhoon. Seismo-acoustic methods allow for rapid distinction of volcanic LP signals from those generated by large surf and other sources, a critical task for volcano monitoring.

Crustal growth by magmatic overplating in the Galápagos

The isotopic compositions of xenoliths hosted in lavas from Floreana Island indicate that they formed from magmas unlike those at present-day Floreana. Instead, the xenoliths are geochemically more similar to magmas now erupting from Sierra Negra and Cerro Azul volcanoes, at the leading edge of the Galapagos hotspot. This is the first evidence for compositional evolution at a Galapagos volcano and indicates increasing contributions from an iso topically enriched source with time as the volcano is carried away from the focus of the hot-spot. Clinopyroxenes in many of the xenoliths exhibit positive anomalies of Sr and Eu, which are attributed to the breakdown of plagioclase. The growth of clinopyroxene at the expense of plagio clase results from compression as the crust cools. Compression is caused by growth mostly from above, as shallow intrusions and lavas load the middle and upper oceanic crust.

Switching from seismic to seismo-acoustic harmonic tremor at a transition of eruptive activity during the Shinmoe-dake 2011 eruption

This paper reports a sequence of harmonic tremor observed during the 2011 eruption of Shinmoe-dake volcano, Kyushu, Japan. The main eruptive activity started with subplinian eruptions, followed by lava effusion. Harmonic tremor was observed as seismic waves during the final stage of the effusive eruption. The tremor observed at this stage had unclear and fluctuating harmonic modes. In the atmosphere, however, many impulsive acoustic waves indicating small surface explosions were observed. When effusion stopped and explosive degassing began, harmonic tremor was observed as acoustic waves in the air and in the seismic data, and the harmonic modes became clearer and more stable. This transition in the character of the harmonic tremor coincided with rapid deflation of the lava that had accumulated in the crater. Based on these observations, and laboratory experiments reproducing the features of the wave fields, it is concluded that the harmonic tremor sequence at Shinmoe-dake was generated by gas flowing through channels in the gradually solidifying lava. Comparing our results with the few cases of similar transition observed at other volcanoes, we expect that the transition indicates changes in magma rheology and degassing conditions in the crater, and therefore of changes in eruptive activity. Key words: Harmonic tremor, infrasound, lava deflation, degassing, lava viscosity, bubbles.

Long‐period seismicity and very‐long‐period infrasound driven by shallow magmatic degassing at Mount Pagan, Mariana Islands

Long period (LP) seismicity and very long period infrasound (iVLP) were recorded during continuous degassing from Mount Pagan, Mariana Islands, in July 2013 to January 2014. The frequency content of the LP and iVLP events and delay times between the two arrivals were remarkably stable and indicate nearly co-located sources. Using phase-weighted stacking over similar events to dampen noise, we find that the LP source centroid is located 60 m below and 180 m west of the summit vent. The moment tensor reveals a volumetric source modeled as resonance of a subhorizontal sill intersecting a dike. We model the seismoacoustic wavefields with a coupled earth-air 3-D finite difference code. The ratios of pressure to velocity measured at the infrasound arrays are an order of magnitude larger than the synthetic ratios, so the iVLP is not the result of LP energy transmitting into the atmosphere at its epicenter. Based on crater shape and dimensions determined by structure from motion, we model the iVLP as acoustic resonance of an exponential horn. The source of the continuous plume from gas analysis is shallow magmatic degassing, which repeatedly pressurized the dike-sill portion of the conduit over the 7 months of observation. Periodic gas release caused the geologically controlled sill to partially collapse and resonate, while venting of gas at the surface triggered resonance in the crater. LP degassing only accounts for ~12% of total degassing, indicating that most degassing is relatively aseismic and that multiple active pathways exist beneath the vent.

Seismic Envelope‐Based Detection and Location of Ground‐Coupled Airwaves from Volcanoes in Alaska

Abstract Volcanic explosions and other infrasonic sources frequently produce acoustic waves that are recorded by seismometers. Here we explore multiple techniques to detect, locate, and characterize ground‐coupled airwaves (GCA) on volcano seismic networks in Alaska. GCA waveforms are typically incoherent between stations, thus we use envelope‐based techniques in our analyses. For distant sources and planar waves, we use f ‐ k beamforming to estimate back azimuth and trace velocity parameters. For spherical waves originating within the network, we use two related time difference of arrival (TDOA) methods to detect and localize the source. We investigate a modified envelope function to enhance the signal‐to‐noise ratio and emphasize both high energies and energy contrasts within a spectrogram. We apply these methods to recent eruptions from Cleveland, Veniaminof, and Pavlof Volcanoes, Alaska. Array processing of GCA from Cleveland Volcano on 4 May 2013 produces robust detection and wave characterization. Our modified envelopes substantially improve the short‐term average/long‐term average ratios, enhancing explosion detection. We detect GCA within both the Veniaminof and Pavlof networks from the 2007 and 2013–2014 activity, indicating repeated volcanic explosions. Event clustering and forward modeling suggests that high‐resolution localization is possible for GCA on typical volcano seismic networks. These results indicate that GCA can be used to help detect, locate, characterize, and monitor volcanic eruptions, particularly in difficult‐to‐monitor regions. We have implemented these GCA detection algorithms into our operational volcano‐monitoring algorithms at the Alaska Volcano Observatory.

The sounds of submarine volcanoes

When submarine volcanoes erupt, several processes can create sounds in the ocean, mostly at low frequencies <100 Hz. Explosions may occur directly in the water column, while earthquakes and other seismicity may produce seismic waves that convert into hydroacoustic waves or boundary (Scholte) waves. Volcanic sounds can propagate large distances through the SOFAR channel. During its 2014 eruption, Ahyi seamount, Northern Mariana Islands produced repetitive signals for approximately 2 weeks at a high rate. These likely explosions were widely recorded on seismometers throughout the region and on hydrophone arrays as far as Chile, ∼12,000 km distant. Bogoslof volcano, a shallow submarine volcano in the Aleutian Islands, Alaska, began erupting in December 2016. Many of the detected earthquakes associated with this eruption have large amplitude hydroacoustic phases, likely Scholte waves. A few earthquake swarms were recorded as converted hydroacoustic waves by seismometers on Tanaga volcano, ∼700 km away. A few ...

Acoustic waves in the atmosphere and ground generated by volcanic activity

This paper reports an interesting sequence of harmonic tremor observed in the 2011 eruption of Shinmoe-dake volcano, southern Japan. The main eruptive activity started with ashcloud forming explosive eruptions, followed by lava effusion. Harmonic tremor was transmitted into the ground and observed as seismic waves at the last stage of the effusive eruption. The tremor observed at this stage had unclear and fluctuating harmonic modes. In the atmosphere, on the other hand, many impulsive acoustic waves indicating small surface explosions were observed. When the effusion stopped and the erupted lava began explosive degassing, harmonic tremor started to be transmitted also to the atmosphere and observed as acoustic waves. Then the harmonic modes became clearer and more stable. This sequence of harmonic tremor is interpreted as a process in which volcanic degassing generates an open connection between the volcanic conduit and the atmosphere. In order to test this hypothesis, a laboratory experiment was performed and the essential features were successfully reproduced.

Short-Term Forecasting and Detection of Explosions During the 2016–2017 Eruption of Bogoslof Volcano, Alaska

We describe a multidisciplinary approach to forecast, rapidly detect, and characterize explosive events during the 2016–2017 eruption of Bogoslof volcano, a back-arc shallow submarine volcano in Alaska’s Aleutian arc. The eruptive sequence began in December 2016 and included over 60 discrete explosive events. Because the volcano has no local monitoring stations, we used distant stations on the nearest volcanoes, Okmok (54 km) and Makushin (72 km), combined with regional infrasound sensors and lightning detection from the Worldwide Lightning Location Network (WWLLN). Monitoring of activity used a combination of scheduled checks combined with automated alarms. Alarms triggered on real-time data included real-time seismic amplitude measurement (RSAM); infrasound from several arrays, the closest being on Okmok; and lightning strokes detected from WWLLN within a 20-km radius of the volcano. During periods of unrest, a multidisciplinary response team of four people fulfilled specific roles to evaluate geophysical and remote-sensing data, event-specific ash-cloud dispersion modeling, interagency coordination, and development and distribution of formalized warning products. Using this approach, for events that produced ash clouds ≥7.5 km above sea level, AVO called emergency response partners 15 minutes, and issued written notices 30 minutes, after event onset (mean times). Factors that affect timeliness of written warnings include event size and number of data streams available; bigger events and more data both decrease uncertainty and allow for faster warnings. In remote areas where airborne ash is the primary hazard, the approach used at Bogoslof is an effective strategy for hazard mitigation.