Statistical Analysis of the Recent Unrest at Volcán de Colima, Mexico, Monitored With a Doppler Radar

Abstract:

Volcán de Colima was equipped with a Doppler radar monitoring station in 2007 (March to July) and from March 2014 to 11 July 2015, when a pyroclastic flow partly destroyed the instrument. The Doppler radar measures the velocity of ejecta (>0.5 mm) and the reflected energy (a proxy for their cumulative size) within the eruption cloud. These data constrain the dynamics of the eruption cloud between 100-200m above the vent, where the field of view is located. We measure the precise timing of surface activity when tephra is included, while pure gas emissions (white steam clouds and fumaroles) are transparent to the radar.

In 2007 a new dome grew slowly within the crater, accompanied by almost daily tephra emissions. This continued until 2011, when a larger explosion signalled the end of the effusion. After 18 months of quiescence, a new eruptive episode commenced in January 2013 and continues to date (Aug. 2015).

Analysis of the temporal trends of ejecta velocities reveals that tephra emissions are pulsed. This matches observations, where the ash concentration commonly varies between discrete pulses. The statistical analysis of times between pulses best fits a log-logistic distribution. A log-logistic distribution indicates the interplay of two competing processes: for example short duration pressure release within a few seconds through a shallow fracture network vs. long term (tens of seconds) gas supply from deeper regions. Or it could be the competition of processes affecting the magma viscosity: rapid degassing acting to increase it, whilst a temperature increase from latent heat or shear heat generation acts to decrease the viscosity.

The repose-times (in 2007) also follow a log-logistic distribution with different parameters. A single set of parameters fitting both the repose- and inter-pulse-times has not been found.

In the recent data set (2014-2015) the number of events remarkably increased from 29 per month (June 2014) up to 23 per day in February 2015. The Doppler radar measured several pyroclastic flows until it was partly destroyed by a major flow on 11 July 2015. Statistics provide indirect insight into the explosion generation mechanism; the Doppler radar additionally provides direct insight into the dynamics of pyroclastic flows from a (usually) safe distance.