Long-Period Seismicity and Very-Long-Period Infrasound Driven by Shallow Magmatic Degassing at Mount Pagan, Mariana Islands

Abstract:

Mount Pagan is the currently active vent on the north end of Pagan Volcano, Mariana Islands. A persistent degassing plume, LP seismicity, and VLP infrasound (iVLP) have dominated activity since at least 2013, when ground-based geophysical sensors were first installed. Direct gas sampling with a Multi-GAS sensor indicates a plume generated by a shallow magmatic system. Here we present an analysis of coupled LP and iVLP events in order to reveal the details of their source processes and how the signals are related to shallow magmatic degassing.

The LP and iVLP waveform characteristics were highly stable from July 2013 – January 2014. Both events have durations of 5-20 s, occur every 1-2 minutes, and have emergent onsets. The LP events have a dominant frequency of 0.54 Hz, while the dominant frequency of the iVLP is 0.32 Hz. The delay times between the LP and iVLP arrivals show little variation during the 7-month study, indicating a stable, shallow, and nearly co-located source. Full waveform inversion of a master LP event reveals a volumetric source 60 m below and 180 m west of the summit vent. Inverting Green’s functions of different geometric combinations results in a 2-crack model dominated by a subhorizontal crack intersecting a NW-SE trending dike. The extension of the modeled crack intersects the surface near the vent location. The nearly horizontal orientation of the dominant crack is likely controlled by the orientation of lava flows and pyroclastic deposits that are observed in the western wall of the cone at the LP depth. We propose that the LP seismicity is crack resonance triggered by collapse of the gas-charged upper conduit system following periodic venting. Measured and modeled pressure-velocity (P/V_z) ratios for the seismoacoustic events indicate that elastodynamic processes associated with the seismic LP cannot generate the iVLP. Thus, we model the iVLP as volume resonance of an exponential horn, based on the shape of the crater and the wavelength of the iVLP. We find that periodic injection of warm magmatic gas into the crater causes the iVLP resonance.