Three-dimensional velocity structure of the Galeras volcano (Colombia) from passive local earthquake tomography

• 3D velocity structure of the Galeras volcano was estimated with 14,150 events between 1989 and 2015.
• Inversion of full dataset represents the stacked image of the most permanent features of the volcano.
• Magma and fluids flow up along intersection of the Silvia-Pijao and Buesaco faults.
• 3D analysis highlights role of vertical fracture network through which magma and fluids migrates to surface.

A three-dimensional estimation of the Vp, Vs and Vp/Vs ratio structure at Galeras volcano was conducted by means of passive local earthquake tomography. 14,150 volcano-tectonic events recorded by 58 stations in the seismological network established for monitoring the volcanic activity by the Colombian Geological Survey – Pasto Volcano Observatory between the years 1989 and 2015, were inverted by using the LOTOS code. The seismic events are associated with shear-stress fractures in solid rock as a response to pressure induced by magma flow. Tomography resolution tests suggest a depth of imaging that yield 10 km from the summit of the main crater, illuminating a large portion of the volcanic structure and the interaction of tectonic features like the Buesaco and Silvia-Pijao faults. Full catalog tomographic inversion, that represents the stacked image of the volcanic structure or the most permanent features underneath the volcano, shows vertical structures aligned with seismicity beneath the main crater. We hypothesize that these structures correspond to a system of ducts or fractures through which magma and fluid phases flow up from deeper levels toward the top and related with the intersection of the surface traces of the Silvia-Pijao and Buesaco faults.