Explosive Activity at Tungurahua Volcano: Analysis of Seismic and Infrasonic Data from 2006 - 2011
Abstract
Tungurahua is an active, steep-sided andesitic stratovolcano, located in central Ecuador. Historic eruptions are characterized by strong explosions, lava and pyroclastic flows, lahars and tephra fallout. After 75 years of quiescence, a renewed phase of explosive activity began in October 1999. Since, Tungurahua has experienced a series of eruptive cycles, with almost continuous activity separated by only short periods (months) of repose. We apply several statistical techniques to a continuous catalog of over 4500 volcanic explosions, recorded between July 2006 and May 2011. Reduced amplitudes and energies are calculated for each event using four collocated broadband seismic and infrasound sensors. An initial time series analysis isolates 8 phases of activity: Jul-Aug 2006, Feb-Apr 2007, Jul 2007-Feb 2008, Mar 2008-Jun 2009, Jan-Mar 2010, May-Jul 2010, Nov-Dec 2010 and Apr-May 2011. Small temporal changes in the volcanic eruption mechanism across successive episodes are identified by a lack of dependency in event rate auto-correlation and a continuous fluctuation in the proxy b-value of moving-window, frequency-amplitude distributions. We highlight the May-July 2010 episode because it is statistically distinct from the other periods of explosive activity. Peak explosion event rate during this time is approximately six times that of any other episode across the observation period (max ~ 242 events; 31 May 2010), while cumulative daily seismo-acoustic explosion energies are at least an order of magnitude greater. The coefficient of variation (Cv = σ/μ, where; σ is the standard deviation; and μ is the mean repose time of explosions) is used to show a strong clustering of events with time (episodes 1-5 & 7-8 = Cv ~ 2-5) and not representative of a Poisson controlled process. A Cv ~ 13.7 in May-July 2010 (episode 6) further highlights the anomalous nature of activity during this period. The volcano acoustic-seismic ratio (VASR, or η), the ratio of elastic energy propagated through the atmosphere and into the earth (η = acoustic energy/seismic energy), calculated from discrete explosions, is used to infer temporal changes in shallow conduit conditions across and between phases of activity. Typically, values in explosion VASR during the 5 years display relatively high acoustic partitioning (η ~ 1-1000), which we relate to a moderately open vent system. However, we attribute lower VASR during the early stages of May-July 2010 to an initially, partially choked conduit, an alteration in volcanic conditions that is reflected within the anomalous results of other statistical tests applied to this period of activity. Through continuous explosive degassing, we envision that the plug is eventually cleared, as an increase in explosion VASR is observed towards the latter stages of the active phase. Several statistical techniques applied to the 2006-2011 explosion dataset at Tungurahua were useful for revealing patterns in eruptive behavior, with statistical differences between periods of activity identifying a temporal evolution in eruption mechanism. Continued monitoring and a comparison of Tungurahua data with similar explosive records at other volcanoes, will lead to improved tools for analyzing sequences of explosive eruptions and more robust forecasting methods.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V33D2898S
- Keywords:
-
- 8414 VOLCANOLOGY / Eruption mechanisms and flow emplacement;
- 8419 VOLCANOLOGY / Volcano monitoring;
- 8428 VOLCANOLOGY / Explosive volcanism