Crustal seismicity associated to rapid surface uplift at Laguna del Maule Volcanic Complex, Southern Volcanic Zone of the Andes

Abstract:

We present the first detailed characterization of the seismic activity at Laguna del Maule Volcanic Complex and integrate it with structural data acquired in the field in order to illuminate the possible connection between the ongoing process of surface uplift and the activation of crustal faults. Our main finding is the recognition of repetitive volcano-tectonic (VT) seismic swarms that occur periodically between 2011 and 2016 near the SW corner of the sill modeled by InSAR studies. A cross-correlation analysis of the waveforms recorded for these VT events allows identifying four different seismic families. Families F1 and F3 share some common features in the stacked waveform and its locations, which markedly differ from those of family F2. Swarms belonging to this latter family are more energetic and its energy was increasing since 2011 to a peak in January 2013, which coincide with maximum vertical velocities detected by local GPS stations. This point to a common process relating both phenomena. The area where F2 swarms seems to occur is characterized by the intersection of a NE-SW lineament with a WNW-ESE lineament. The former is called Troncoso fault and shows clear field evidences of dextral strike-slip that are fully consistent with one nodal plane of focal mechanism for well-recorded F2 events. The conjugate nodal plane of these focal mechanisms could coincide with the WNW-ESE lineament (parallel to Laguna Fea lake), for which our field reconnaissance suggests a dominant normal motion. Events belonging to families F1 and F3 are clustered along this latter lineament westward to its intersection with the Troncoso fault and their focal mechanism are also dominantly strike-slip but with some mixture with thrust and normal components. Our results suggest a complex mechanical interaction between the inflating source and the arrangement of crustal faults forming the structural framework on which the magmatic plumbing system of LMVC is emplaced. By means of coulomb stress modeling, taking as source structure an inflating sill and as receiver structure SW-NE faults, we suggest that the inflation process could affect the local stress field, promoting the failure on neighbor faults. Some aspects as the geometry, stress state, location and disposition of both sources could play a relevant role to explain how this process work.