Site effect determination using seismic noise from Tungurahua volcano (Ecuador): implications for seismo-acoustic analysis

Abstract

University of Bristol -Explore Bristol Research General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. S U M M A R YScattering and refractions that occur in the heterogenous near-surface beneath seismic stations can strongly affect the relative amplitudes recorded by three-component seismometers. Using data from Tungurahua volcano we have developed a procedure to correct these 'site effects'. We show that seismic noise signals store site information, and then use their normalized spectral amplitudes as site frequency response functions. The process does not require a reference station (as per the S-wave and coda methods) or assume that the vertical amplitude is constant (the H/V component ratio method). Correcting the site effects has three consequences on data analysis: (1) improvement of the seismic source location and its energy estimation; (2) identification of a strong influence on the volcanic acoustic seismic ratio (VASR) and (3) decoupling the air wave impact on the ground caused by explosions or eruption jets. We show how site effect corrections improve the analysis of an eruption jet on 2006 July 14-15, appearing two periods of strong acoustic energy release and a progressive increase of the seismic energy, reaching the maximum before finishing the eruption.

Seismic-acoustic energy partitioning during a paroxysmal eruptive phase of Tungurahua volcano, Ecuador

“…Ichihara et al 2012) that is superimposed on the seismic waves travelling through the volcanic edifice. This phenomenon was previously observed in the eruption considered in this study in the high frequency range (Palacios et al 2015).…”

“…Palacios et al (2015) discussed in details these effects for the eruption here studied, demonstrating that coupling effects are most significant at frequencies higher than 4-5 Hz. Due to restrictions at low frequency of the site correction method (Palacios et al 2015), caused by the presence of micro-seismic noise, we will analyse the signal filtered in the range 0.4-4 Hz. This range includes almost all seismic energy before and after site corrections (Fig.…”

“…Crosscorrelograms constructed with signals filtered in a wide range (for instance 0.4-4 Hz, see also fig. 17c in Palacios et al 2015) could include couplings in a more restricted frequency band (for instance, 2-3.5 Hz at BRUN) showing low correlation values. After identifying the coupling frequency band, couplings are highlighted in the cross-correlogram constructed for that band.…”

“…This is the case of BRUN station for 2-3.5 Hz, where even several explosions before and after the paroxysm have been clearly identified. Coupling frequency bands are not the same for all stations, which may depend on site properties (Palacios et al 2015) and topographic conditions. Due to the lack of overlapping between the frequency band 0.4-2 Hz of the seismic tremor caused by internal volcanic sources, and coupling frequency band 2-3.5 Hz at BRUN station, we consider that below 2 Hz the signal is decoupled.…”

“…In our case the presence of coupled components are minimized after correcting the site effects (see the frequency response functions of BRUN, fig. 5 in Palacios et al 2015). A decoupling model still is required and may be considered in further studies.…”