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

Francesca Martini,Ivan Lokmer,Gerardo J. Soto,Christopher J. Bean,Mauricio M. Mora,Javier F. Pacheco,Louis De Barros,Thomas S. Eyre

doi:10.1002/2014jb011108

Abstract

AbstractA 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.

Highlights

Turrialba volcano (3340 m.a.s.l.) is a stratovolcano located at the eastern edge of the Cordillera Volcánica Central, Costa Rica, Central America, 35km ENE of the capital San José
A previous study was carried out into the source processes of long period (LP) seismic events recorded at Turrialba volcano in 2009 (Eyre et al, 2013), where LPs were inverted for their mechanism using full-waveformmoment tensor inversion
As the LP waveforms on Turrialba volcano are non-resonating, by analyzing the waveforms, source locations, source mechanisms and the scaling laws of these events, this study aims to determine if the slow failure model can reasonably explain the LP 112 events observed on Turrialba volcano

Summary

Introduction

Turrialba volcano (3340 m.a.s.l.) is a stratovolcano located at the eastern edge of the Cordillera Volcánica Central, Costa Rica, Central America, 35km ENE of the capital San José (figure 1a). Activity at the volcano has increased in recent years, with intensifying fumarolic gas discharges, especially in the active Southwest Crater; summit inflation detected from 2005 to 2007; increases in seismicity; and recent minor phreatic explosions (Campion et al, 2012; Martini et al, 2010; Soto and Mora, 2013; Tassi et al, 2004; Vaselli et al, 2010). Monitoring the volcano is extremely important, as its close proximity to San José means that a large eruption has the potential to adversely affect 1.5 million people (Soto et al, 2010). A previous study was carried out into the source processes of long period (LP) seismic events recorded at Turrialba volcano in 2009 (Eyre et al, 2013), where LPs were inverted for their mechanism using full-waveformmoment tensor inversion. Similar to studies conducted at other volcanoes, a crack source mechanism was obtained, in the case of Turrialba dipping shallowly towards the southwest. For the 2009 dataset this was interpreted, drawing on the Chouet (1986)

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