Very-Long-Period Events Linked to Outgassing at Pacaya Volcano, Guatemala and Villarrica Volcano, Chile

Abstract:

Repetitive long-period (LP: 0.2-2 s) seismicity is common at active and quiescent volcanoes. The repeating signals usually result from pressure changes arising from flow of gas or magma through conduits or from outgassing at the magma free surface. Similarly, very-long-period (VLP: 2-100 s) signals are found at some volcanoes, are typically repetitive, and result from unsteady flow in conduits. Models of VLP signals have been used to map portions of the shallow conduit where changes in geometry may occur. The processes that produce these signals may span the LP and VLP band but, in many cases, LPs occur without VLPs and VLPs may occur without obvious LP components. However, when they are interpreted jointly, they can provide fuller picture of magma dynamics.

At both Pacaya volcano, Guatemala, and Villarrica volcano, Chile we have recorded LP seismicity that is clearly linked to outgassing at the surface. The LP events are similar over timescales of months for Villarrica and dominate the 10-day broadband recording at Pacaya, but these events do not have clear VLP components at either volcano. Here we have taken advantage of the repeating LPs to reveal the subtle VLP signals that occur nearly coincidentally with the LPs. After stacking tens of thousands of events, we filter in the VLP band and find a simple VLP signal visible on all stations at each of these volcanoes. At Villarrica, we used a linear array of 10 broadband sensors deployed down the north side of the edifice. The VLP particle motions point to a location 200-300 m below the summit and below the LP location. In addition to having a deeper location, the dominant N-S particle motion of the Villarrica VLP is distinct from the dominantly E-W particle motion of the repeated LP. At Pacaya, the broadband network consisted of only four stations, one of which operated for just a day, so locating the VLP is more challenging. However, the VLP signal precedes the LP by ~15 seconds suggesting it occurred deeper in the conduit. The recognition of weak VLP signals offers insight into geometry of the upper conduit and highlights the utility of repeating seismic events for modeling gas and magma migration.