P AND S VELOCITY STRUCTURE IN THE NEVADO DEL HUILA VOLCANO (COLOMBIA)

The Nevado del Huila Volcanic Complex (NHVC) is the highest active volcanic structure in the Colombian Andes (5,364 masl). It is located on the axis of the Central Cordillera and morphologically divided into three main peaks: North, Central, and South, and one secondary peak, La Cresta. The Geological Survey of Colombia reported its reactivation on February 19th, 2007, and the subsequent activity on April 18th, 2007 and November 20th, 2008 with phreatomagmatic eruptions that led to the formation of large cracks on the glacier cap. This last episode is related to small emissions of ash that promoted lahar events and a magmatic episode linked to the extrusion of coalescent domes in the SW flank of the Central peak in 2008 and 2009.

We used a 3D seismic tomography for imaging the Vp, Vs and Vp/Vs structure of the NHVC using P and S first arrivals from 12916 local events recorded by ten stations of the GSC and monitored from the Volcanological and Seismological Observatory of Popayan. We used an iterative simultaneous velocity and source parameters inversion, focusing on the period 2007 - 2016, which corresponds to the phreatomagmatic reactivation and its relaxation stage. Events are broadly distributed in the volcanic structure from the summit to >12 km depth, and most of them guarantee good azimuthal.

Our results show vertical structures aligned with seismicity beneath the Central peak and the new dome formed during the phreatomagmatic eruption in 2008. This new dome correlates with high Vp/Vs ratios from the surface to 9 km depth. This feature may represent a fracture zone that served as a conduit for the expulsion of material in the new crater formed in 2008. We hypothesize that these structures were formed by the interaction of the magma source with the main trend of the Moras fault system.