Abstract
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, > 30 s) that were coincident with eruptive activity centered on the active El Caliente vent. These signals lasted 2–5 min and corresponded to depressurization of a volume beneath the dome. Coincident shorter-period seismic signals (> 0.5 Hz) 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 1.1 km 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 150 m 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 200 m west of and 250 m beneath the centre of the vent with the peak volume changes accompanying eruptions ranging from 125 to 1825 m 3. 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.
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