Morphology and thickness of debris avalanche deposits through electrical resistivity tomography

Abstract:

Volcanic Debris Avalanche Deposits (DADs) are common products of catastrophic volcanic edifice collapses that occur at different scales from < 0,1 up to 1000 km³, representing a ubiquitous hazard to nearby cities and populations. DADs form chaotic, heterolithologic bodies characterized by mounds (hummocks) and depressions that fill broad sectors of volcanic ring-plains with different thicknesses depending on topography. In many cases the analyses of DADs are restricted to a few exposed outcrops, investigation of the entire body and thus, the deposit volume, rarely being possible. Here we show for the first time how electrical resistivity tomography (ERT) can be used to get an estimate of volume and extent of such deposits. We applied ERT, using home-built equipment and a dipole-dipole configuration, to get a 2D and 3D view of two partially exposed DAD outcrops close to the city of Ibarra, Ecuador. This in-situ geophysical technique is able to measure physical parameters directly or indirectly linked with the lithological, hydrological and geotechnical characteristics of the terrains related to the deposit. The good agreement between our ERT profiles and the exposed stratigraphy of the outcrop show that ERT can reliably constrain the 3D shape, extent and volume of not well exposed DADs. Moreover, our home-built cost-effective resistivity meter is easy to use and a comparative study with commercial equipment confirmed its reliability. We also develop a multidisciplinary approach combining stratigraphy, ERT and drone photogrammetry to asses DADs at a regional scale. This novel method allows for rapid and efficient estimates of DADs for future hazard assessment.