Ultra-long period seismic signals and cyclic deflation coincident with eruptions at Santiaguito volcano, Guatemala

Broadband seismic observations made near the active vent at Santiaguito provide constraints on changing pressures inside the edifice. During January 2009 we recorded episodic ultra-long period signals (ULPs, >30s) that were coincident with eruptive activity centered on the active El Caliente vent. These signals lasted 2-5min and corresponded to depressurization of a volume beneath the dome. Coincident shorter-period seismic signals (>0.5Hz) appeared to be associated with shallow level pyroclastic emissions and dome surface motions. These shorter period signals varied significantly from event to event, unlike the ULP waveforms, which were self-similar. As opposed to the short period signals, which were recorded across the network, the ULPs were only recorded on the horizontal channels of 3 stations that were within 1.1km of the vent. This and other factors suggest that they were a result of long period rotational motion rather than horizontal translation. ULP source locations derived from horizontal particle motions point to the northwestern part of El Caliente 150m from the vent axis, the general locus of activity from 1967 until flank collapses in 1989/1990. We modeled the ULP signal as a Mogi source and solved for three dimensional location and equivalent source strength. Through analysis of 78 events, we identify a mean Mogi source that is 200m west of and 250m beneath the centre of the vent with the peak volume changes accompanying eruptions ranging from 125 to 1825m³. These inferred volume losses coincide with the onset of gas venting and/or explosive eruptions at the surface of Caliente. Based upon these observations we develop a model to describe the pre-eruptive, eruptive and inter-eruptive physical processes occurring at the surface and inside Santiaguito.