Fault reactivation due to the M7.6 Nicoya earthquake at the Turrialba-Irazu volcanic complex, Costa Rica: Effects of dynamic stress triggering

The M7.6 Nicoya earthquake struck at the interface between the Cocos plate and the Caribbean plate on 5 September 2012 inducing a ground acceleration of 0.5 m s(-2) at the Irazu-Turrialba volcanic complex. We use data from six seismic stations deployed around and atop the Irazu-Turrialba volcanic complex to show the increase of local seismic activity after the M7.6 Nicoya earthquake. The response consists in more than 300 locatable earthquakes occurring in swarm sequences along a fault system that intersects the Irazu-Turrialba volcanic complex. In addition, we point out that major aftershocks are followed by increases of seismic activity in the same region. The weak static stress variation imposed by the main slip of the Nicoya earthquake at the Irazu-Turrialba volcanic complex suggests a dynamic triggering mechanism. We expand this concept suggesting that this behavior may be similar to the one observed in the Chilean and Japanese volcanic arcs during the M8.8 2010 Maule, Chile, and M9.0 2011 Tohoku, Japan, earthquakes. Finally, we highlight that the combined action of dynamic stress and short-lived coseismic relaxation may trigger seismic activity in geological systems in near-critical conditions.