Villarrica volcano outgasses through an open lava lake, with bubbles ranging in size from submillimeter to several meters, the largest of which produce strombolian bursting events that are visible from the crater rim. Thousands of shallow strombolian events identified through seismic waveform cross correlation were found to produce discrete and repetitive long-period seismic and infrasonic signals. We identified variations of up to 0.7 s in seismic-acoustic arrival delay times between April and July 2010 at a station~750 m from the vent, which we interpret as due to fluctuations in the level of lava lake. During time periods interpreted as having high lava lake levels, based on reduced time delays, interevent times were also reduced, and average seismic amplitude measurements, seismic and acoustic event energies, and volcano acoustic-seismic ratios were all high as compared to times when the lava lake was lower. The crater is also a source of nearly continuous, monotonic infrasonic tremor. We found that the peak frequency of this infrasonic tremor, typically around 0.5-1.0 Hz, was inversely correlated with seismic-acoustic delay times and therefore an indicator of lava lake level. We use this correlation to propose a new model for infrasonic tremor generation, namely, using crater geometry to approximate a Bessel horn. We interpret the two clearest cycles of elevated seismicity and lava lake level as due to an increase in exsolved gas, resulting from an injection of volatile-rich magma or an overturn in a deeper magma reservoir.

[1] Villarrica volcano is presently characterized by an active lava lake in the summit crater, with mild strombolian style eruptions of basaltic-andesitic products. The lava lake acts as a permeable barrier where gas-rich magma releases volatiles and then descends through the process of conduit convection to a deep storage reservoir. We identified thousands of repetitive long-period (LP) seismic events within a three-year time period using a matched filter; each was discovered to be associated with an infrasonic emission, and hence outgassing from the lava lake. Using this catalog of events for timing, we stacked signals over the three-year deployment from 2010-2012 to produce representative seismic and infrasonic signatures, and performed a full-waveform seismic inversion using nine stations from the network. The data are well represented by a single horizontal force source-time function, oriented N75°E and dipping 7°W from horizontal, with a maximum amplitude of 34.4 MN. The source is stable in time, and can be explained by a force couple system produced by a bursting bubble in the lava lake and the responsive drag force of the lava sloshing to fill in the void produced by the gas emission in an asymmetrical crater. Path effects lengthen the coda of the events with increasing distance from the source, resulting in emergent tremor-like signals at distant seismometers. Since the occurrence of LP events correlates with tremor intensity, we suggest much of the tremor results from the superposition of discrete LPs.

[1] We recorded 126 calving and iceberg breakup events from the terminus of the Bering Glacier during five days in August 2008 using seismometers and three smallaperture arrays of infrasound sensors. The seismic signals were typically emergent, narrow-band, and lower-frequency, similar to records at other glaciers. The acoustic records were characterized by shorter-duration, higher-frequency signals with more impulsive onsets. We demonstrate that triangular infrasound arrays permit improved locations of calving events over seismic arrivals that rely on a relatively complicated, poorly known, velocity model. Twenty-six of 35 well-located events occurred on icebergs in Vitus Lake, rather than the glacier face. While our data do not permit a complete description of the source process, the distinctive frequency contents and durations in the seismic and infrasound data suggest that the two data types record different aspects of the same process. Citation: Richardson,

[1] The complex source processes associated with both glacier calving and the breakup of icebergs, combined with commonly heterogeneous periglacial seismic velocity structure, can result in complicated seismic records. Key features of the waveforms, which are typically characterized by low-amplitude or emergent first-arrivals and long-duration, narrow-band codas, have been attributed to either source processes or propagation path effects. This uncertainty must be addressed in order for seismic data to be effective for studying the calving process as it relates to terminus dynamics. In this study, we use sets of 3-element arrays of 3-component geophones and infrasound sensors to locate calving and iceberg breakup events and isolate path effects in the seismic records obtained near the Bering Glacier terminus in the summer of 2010. Using waveform correlation, we treat each array as an antenna and determine the direction to the source and apparent velocity of the wavefield across the array. The initial few ($3) cycles of P waves recorded from an array, beam formed to identify coherent arrivals for each event, are useful for deriving a propagation azimuth and apparent velocity, allowing for location of events using a small number of similar arrays. We locate 125 calving and iceberg breakup events near the terminus with this method. We also demonstrate that the longer-lived narrow-band coda is not coherent across individual arrays, suggesting that the narrow-band coda observed at the Bering Glacier is attributable to a path effect rather than to the source process. The large number of iceberg breakup events that we located has important implications for other calving glaciers where icebergs are present, and calving rates may be erroneously overestimated from the seismic data if their contribution is not taken into account.

A significant number of microseismic events were detected over 120 days of passive monitoring with a deepwater PRM pilot array offshore Brazil. The array is installed in 1240-1310m water depth and consists of over 700 four-component stations. Recording occurred during two consecutive two-month periods in between active seismic surveys. The passive monitoring detected distinct event swarms that are highly clustered in space and time. These events occur at an estimated depth of about 5 km with moment magnitudes ranging from 0.2 to 1.9. The seismicity occurs in a depth interval near a currently undeveloped deeper reservoir and is possibly of natural origin. The capture of such seismicity is valuable input for longterm risk assessment and development planning of the lower reservoir.