Transient compression fault slip detected within andesitic rocks of the Casma Group, Lima, Peru

A fault slip within the ?a?a tunnel Lima, Peru has been monitored since 2012. The data are recorded using an optical-mechanical 3D extensometer, capable of providing very precise long-term three-dimensional measurements of relative displacement across discontinuities. The ?a?a tunnel has an extremely stable environment and cannot possibly be affected by gravitationally-induced mass movements. The host rock of the tunnel is an aphanitic hornblende-bearing basaltic trachyandesite. Several fault and fracture zones detected in the tunnel represent the major discontinuities of the broader surrounding of the monitored site. The recorded fault slip on the NNW-SSE and E-W striking fracture and fault, with inclinations of 76° to WSW and 78° to N respectively, appoint to compressional event (discontinuity contraction) with a maximum horizontal compression stress axis oriented approximately WSW to ENE corresponding to the direction of the Nazca and South America plates convergence. This event resulted in an aseismic slow fault slip between July 2012 and May 2013. The anticipated compression orientation matches the previously published in-situ stress measurements and fault plane solutions, as well as GPS measurements of the movements of the corresponding part of the Peruvian coast. Nevertheless, the presented monitoring results reflect only short-term fault slip dynamics and need to be considered with caution, even though they correspond to the overall tectonic activity driven by continental subduction.