Relationship between volcanic ash fallouts and seismic tremor: quantitative assessment of the 2015 eruptive period at Cotopaxi volcano, Ecuador

Abstract

International audienceUnderstanding the relationships between geophysical signals and volcanic products is critical to improving real-time volcanic hazard assessment. Thanks to high-frequency sampling campaigns of ash fallouts (15 campaigns, 461 samples), the 2015 Cotopaxi eruption is an outstanding candidate for quantitatively comparing the amplitude of seismic tremor with the amount of ash emitted. This eruption emitted a total of ~1.2E + 9 kg of ash (~8.6E + 5 m3) during four distinct phases, with masses ranging from 3.5E + 7 to 7.7E + 8 kg of ash. We compare the ash fallout mass and the corresponding cumulative quadratic median amplitude of the seismic tremor and find excellent correlations when the dataset is divided by eruptive phase. We use scaling factors based on the individual correlations to reconstruct the eruptive process and to extract synthetic Eruption Source Parameters (daily mass of ash, mass eruption rate, and column height) from the seismic records. We hypothesize that the change in scaling factor through time, associated with a decrease in seismic amplitudes compared to ash emissions, is the result of a more efficient fragmentation and transport process. These results open the possibility of feeding numerical models with continuous geophysical data, after adequate calibration, in order to better characterize volcanic hazards during explosive eruptions

Evolution of the 2015 Cotopaxi Eruption Revealed by Combined Geochemical and Seismic Observations

“…These events were followed by eruptive tremor with a strong increase of the MSA on 21 August (Figure a) and a progressive decay of the number of detected significant events (red curve in Figure b) until 11 September when their number stabilized. MSA sharply dropped also on 11 September, corresponding to the waning of the second phase of ash emission (Bernard et al, ). Subsequently, there was an increase in the number per day of small detected events (black curve in Figure b).…”

“…At the correlation threshold of 0.80 we identify five families of VTs that are active on 13 and 14 August (030.80, 140.80, 268.80, and 395.80 in Figure ). On 13 August, these families start their activity around 22h00 UTC, several hours before the first explosion at 09h02 on 14 August (Bernard et al, ). Their activity vanishes at 19h00, just after the third explosion mentioned by Bernard et al () on this same day.…”

“…On 13 August, these families start their activity around 22h00 UTC, several hours before the first explosion at 09h02 on 14 August (Bernard et al, ). Their activity vanishes at 19h00, just after the third explosion mentioned by Bernard et al () on this same day. During this time, a total of 73 events were detected by matched filtering at a threshold of 0.70 and 144 at a threshold of 0.60.…”

Periodicity in the BrO/SO<sub>2</sub> molar ratios in the volcanic gas plume of Cotopaxi and its correlation with the Earth tides during the eruption in 2015

“…The authors explain the deformation by an inclined sheet intrusion located a few km southwest of the summit. Bernard et al (2016) and Gaunt et al (2016) agree in the division of the post-explosion dynamics in four phases: August 14-15 (phase 1), August 15 to October 2 (phase 2), October 2 to November 4 (phase 3), and November 4-11 (phase 4). In phase 1, several phreatomagmatic explosions occurred followed by ash clouds reaching altitudes of 9.3 km above the crater and strong gas emissions.…”

“…In this manuscript we retrieve the variation of the BrO/SO 2 20 molar ratios recorded by NOVAC during the 2015 period of unrest at Cotopaxi volcano (e.g. Bernard et al (2016), Gaunt et al (2016), Morales Rivera et al (2017) and examine a correlation between its variation and variations in the meteorological and geological conditions, including the calculated variations of the Earth tide-induced ground surface displacements.…”