SOURCE CHARACTERIZATION OF LP SEISMICITY AT POPOCATEPETL VOLCANO, MEXICO

Seismicity of Popocatepetl is dominated by long-period (LP) and very-long period (VLP) signals associated with hydrothermal processes and degassing. We model the source mechanism of repetitive LP signals in the 0.5 - 2-s band using data recorded with a 15-station broadband network deployed on the upper flanks of the volcano. To improve the signal-to-noise ratio we stack LP waveforms component by component at each station prior to waveform inversion. To determine the source centroid location and source mechanism, we minimize the residual error between the stacked waveforms and synthetics calculated by the finite difference method for a point source embedded in a homogeneous medium that takes topography into account. The observed LP signals are well fitted by a simple point source located under the southern perimeter of the summit crater ~300 m below the crater floor. The inferred seismic source mechanism includes both a volumetric component and a vertical single force component. The volumetric component can be modeled as resonance of a horizontal steam-filled crack. Previous moment tensor analysis of VLP waveforms with periods longer than 15 s related to degassing bursts, pointed to the expansion and compression of a sill located ~1500 m below the crater floor, coupled with smaller components of expansion and compression of a dike, which was interpreted to represent the main conduit in the top 1.5 km below the summit crater. The LP source centroid is located ~100 m from the inferred magma conduit, suggesting LP events may be closely related to the interaction between the magmatic system and a perched hydrothermal system. Heating by magmatic activity may increase the pressure in a steam-filled fracture to a critical threshold, at which point an abrupt opening of a pathway allows steam to escape suddenly, initiating rapid pressure loss, collapse of the fracture, and attendant resonance of the fluid remaining in the fracture. Once pressure is lost, lithostatic pressure seals the escape pathway shut, allowing the next cycle of pressure recharge to begin. The imaged single force contributes about 25% of the amplitudes of the observed LP waveforms and maybe attributed to the movement of the overlying magma column induced during steam escape.