Long-duration seismicity and their relation to Copahue volcano unrest

Ivan Melchor,Sergey Samsonov,Javier Almendros,Enzo Martínez,Alberto Caselli,Dominique Derauw,Marcia Hantusch

doi:10.1186/s40623-021-01561-7

Abstract

Understanding seismic tremor wavefields can shed light on the complex functioning of a volcanic system and, thus, improve volcano monitoring systems. Usually, several seismic stations are required to detect, characterize, and locate volcanic tremors, which can be difficult in remote areas or low-income countries. In these cases, alternative techniques have to be used. Here, we apply a data-reduction approach based on the analysis of three-component seismic data from two co-located stations operating in different times to detect and analyze long-duration tremors. We characterize the spectral content and the polarization of 355 long-duration tremors recorded by a seismic sensor located 9.5 km SE from the active vent of Copahue volcano in the period 2012–2016 and 2018–2019. We classified them as narrow- (NB) and broad-band (BB) tremors according to their spectral content. Several parameters describe the characteristic peaks composing each NB episode: polarization degree, rectilinearity, horizontal azimuth, vertical incidence. Moreover, we propose two coefficients C_P and C_L for describing to what extent the wavefield is polarized. For BB episodes, we extend these attributes and express them as a function of frequency. We compare the occurrence of NB and BB episodes with the volcanic activity (including the level of the crater lake, deformation, temperature, and explosive activity) to get insights into their mechanisms. This comparison suggests that the wavefield of NB tremors becomes more linearly polarized during eruptive episodes, but does not provide any specific relationship between the tremor frequency and volcanic activity. On the other hand, BB tremors show a seasonal behavior that would be related to the activity of the shallow hydrothermal system.Graphical

Highlights

Volcanic tremors are sustained long-period seismic signals related to the internal flow dynamics of fluids feeding volcanic and hydrothermal systems (Ferrick et al 1982; Julian 1994)
We found a slight trend of increasing the frequency as Linearly polarization coefficient of a characteristic peak (CL) and CP jointly increase (Fig. 4a)
We considered this minimum as the spectral onset of the tremor; Table 2 Periods of time on which BB tremor emerge in swarms and volcanic activity observed in the crater (Additional files 3, 4)

Summary

Introduction

Volcanic tremors are sustained long-period seismic signals related to the internal flow dynamics of fluids feeding volcanic and hydrothermal systems (Ferrick et al 1982; Julian 1994). Apart from being a continuous signal, volcanic tremors can be composed of short-duration events in a temporally dense swarm (Almendros et al 1997) or emerging as regular, cyclic increases of amplitude (Cannata et al 2010). This latter case is referred to as “banded” tremor and is usually associated with a hydrothermal origin (Fujita 2008). A wide variety of mechanisms have been proposed to explain volcanic tremors: transient hydraulic pressure pulses (Seidl et al 1981); resonant scattering by fluid-filled cavities (McMechan 1982), acoustic resonance in fluid-filled conduits (Chouet 1986), hydrothermal boiling (Leet 1988); non-linear flow effects in magmatic conduits (Julian 1994); gas coalescence in the crater vent (Ripepe and Gordeev 1999); pressure perturbations in a bubble-rich magma (Neuberg and O’Gorman 2002); hydrothermal two-phase flow instabilities (Fujita 2008); or periodic pressure perturbations triggered by permeable gas flow (Girona et al 2019), among others (Montegrossi et al 2019; Gestrich et al 2020; Takeo 2021)

Results

Discussion

Conclusion