Clusters of long-period (LP) seismic events recorded during 34 days at the Irazu volcano: What are they telling us?

This work analyses seismic signals at the Irazu volcano (83 & DEG;51 & PRIME;09 & DPRIME; W, 9 & DEG;58 & PRIME;45 & DPRIME; N; active stratovolcano in Central America) in a continuous 34-day record starting on 13 April 2013. Since December 2012, the activity of the volcano was increasing and along with it the frequency of occurrence of certain long-period (LP) signals. In this work, 6 families of LP signals have been identified, with 83 total events. These events show energy concentrated mainly in the frequency range between 1 and 3 Hz, significant variations in amplitude, and some locations on the SW flank from the volcano at 1900 m.a.s.l. (corrected for site effects). The LP signals are interpreted as linked to the interaction between the fluids of the active hydrothermal system and a fault system (NE-SW direction) in the west slope of the Irazu volcano, but without being associated with magma ascent. This contribution provides useful information for the evaluation of the volcanic hazard of the Irazu volcano, in terms of the analysis of possible precursors.