Seismic activity during the 2013–2015 intereruptive phase at Lascar volcano, Chile

Abstract

SUMMARY In addition to enabling the physical processes of volcanic systems to be better understood, seismology has been also used to infer the complexity of magma pathways and plumbing systems in steep-sided andesitic and stratovolcanoes. However, in these volcanic environments, the application of seismic location methods is particularly challenging and systematic comparisons of common methods are lacking. Furthermore, little is known about the characteristic seismicity and deep structure of Lascar volcano, one of the most historically active volcanoes in northern Chile known to produce VEI-4 eruptions. To better understand the inner processes and deep structure of Lascar, the local broad-band seismic monitoring network was densified during a temporal installation in 2014–2015. Herein, we focus on the local seismicity during the 2014–2015 unrest episode, during which we recorded numerous seismic events mainly classified as long-period (LP) type, but also denote volcano-tectonic (VT) activity. Specifically, a long-lasting phase of LP activity is observed over a period of ∼14 months that starts in tandem with a pulse of VT activity. The LP rate and amplitude are modulated over time; they are lower in the initial phase, rise during the intermediate period from October 2014 to July 2015, and finally slowly decay while approaching the eruption time. The location of LPs is challenging due to the typical lack of clear seismic onsets. We thus encompass this problem by comparing a broad range of different standard and novel location techniques to map the source region of LPs by fitting the amplitude decay, polarization patterns, coherence of characteristic functions and cross-correlation differential times. As a result, we principally constrain LP locations within the first 5 km depth below the summit extending downward along a narrow, conduit-like path. We identify different regions of complexity: VTs dominate at depth, both VTs and LPs cluster in an intermediate depth region (down to 1.5 km), suggesting a change in the plumbing system geometry, and LPs dominate the shallowest region. Based on these results, we infer the presence of a subvertical conduit extending down to a depth of ∼5 km, and a region of path divergence, possibly accommodating a magma plumbing system, at a depth of ∼3 km beneath the volcano summit. Identifying the locations of complexities in the magma pathways at Lascar may help identify future unrest. The results are compared with independent observations, demonstrating the strength of the location method used herein that will be tested at volcanoes elsewhere.

The February 2018 seismic swarm in São Miguel, Azores

“…Previous works show that seismicity in volcanic environments is often used to track the propagation of fluids within the plumbing system, with recent outstanding cases involving the migration over tens of kilometers and even across the whole crust (Cesca et al, 2020;Cesca et al, 2022). The spatial distribution of volcano seismicity with different characteristics, such as a different frequency content of their signals, is often interpreted to understand the plumbing system and different processes along complex paths for fluid migration (del Fresno et al, 2023;Gaete et al, 2019;Galluzzo et al, 2023). A very visible difference between the earthquake families resides in the frequency content of their waveforms.…”

Seiscloud, a tool for density-based seismicity clustering and visualization

“…5 focal mechanisms, and to track microseismicity patterns induced by anthropogenic operations, such as fluid injection, mining or water reservoir operations. Preliminary versions of the algorithms were applied in the last years for different studies in tectonic, volcanic, and induced seismicity environments, illustrating a number of potential applications: identification of spatial clusters at diffuse plate margins (Custódio et al 2016), reconstruction of complex rupture faults from aftershock distributions (Cesca et al 2017), detection and growth of fractures in mining regions upon fluid injection (López-Comino et al 2017) and thermal perturbations (Maghsoudi et al 2014), identification and characterization of clusters of earthquakes with similar focal mechanisms for tectonic Custódio et al 2016) and anthropogenic seismicity, and also identification of seismic sources radiating similar seismic signals at volcanoes (Gaete et al 2019;Cesca et al 2020). Seiscloud is open source a n d c a n b e f o u n d a t h t t p s : / / g i t .…”

Processes culminating in the 2015 phreatic explosion at Lascar volcano, Chile, evidenced by multiparametric data

“…1); the mini-DOAS station yielded one complete scan across the gas plume every 5-15 min depending on the light conditions. SO 2 slant column densities (SCDs) along the viewing direction of the mini-DOAS scanner were retrieved in the wavelength range of 310-325 nm by means of DOAS (Galle et al, 2010;Platt and Stutz, 2008); the spectra measured inside the gas plume were compared with a gas-free spectrum and the pixel-wavelength-calibrated SO 2 absorption spectrum from Vandaele et al (1994), which was convolved with the slit function of the spectrometer. Furthermore, we incorporated an O 3 absorption spectrum (Voigt et al, 2001) and a ring spectrum in the DOAS fit to avoid interference with absorption by ozone and scattering effects.…”