Thomas S. Eyre

A brittle failure model for long‐period seismic events recorded at Turrialba Volcano, Costa Rica

A temporary seismic network, consisting of 23 broadband and six short-period stations, was installed in a dense network at Turrialba Volcano, Costa Rica, between 8 March and 4 May 2011. During this time 513 long-period (LP) events were observed. Due to their pulse-like waveforms, the hypothesis that the events are generated by a slow-failure mechanism, based on a recent new model by Bean et al. (2014), is tested. A significant number (107) of the LPs are jointly inverted for their source locations and mechanisms, using full-waveform moment tensor inversion. The locations are mostly shallow, with depths < 800 m below the active Southwest Crater. The results of the decompositions of the obtained moment tensor solutions show complex source mechanisms, composed of high proportions of isotropic and low, but seemingly significant, proportions of compensated linear vector dipole and double-couple components. It is demonstrated that this can be explained as mode I tensile fracturing with a strong shear component. The source mechanism is further investigated by exploring scaling laws within the data. The LPs recorded follow relationships very similar to those of conventional earthquakes, exhibiting frequency-magnitude and corner frequency versus magnitude relationships that can be explained by brittle failure. All of these observations indicate that a slow-failure source model can successfully describe the generation of short-duration LP events at Turrialba Volcano.

Relocation of Long Period (LP) seismic events reveals en echelon fractures in the upper edifice of Turrialba volcano, Costa Rica

Swarms of long-period (LP) events were recorded on Turrialba volcano, Costa Rica, during a seismic field experiment in 2009. Families of LP events were previously identified and located using a joint inversion for source location and mechanism; however the spatial resolution of the obtained locations was not sufficient for imaging the structures on which they occur. Using a waveform similarity-based location method, we take advantage of the joint location-mechanism inversion by relocating events around the obtained familial location. The location method is successfully tested on a synthetic data set, and is then applied to the Turrialba LP data set. The relocated events are jointly interpreted with their source mechanisms, and reveal an en echelon structure within the upper-edifice of the volcano. This can be interpreted as a response of a shearing band with high fluid pressure inducing tensile fractures at unconsolidated rock layer interfaces within the upper-edifice of the volcano.