Seismicity during the recent activity (2009–2020) of Turrialba volcano, Costa Rica

Turrialba is a stratovolcano located at the easternmost part of the Costa Rican volcanic front. After remaining quiescent for more than a century, in 1996 it started to show signs of unrest, until a first phreatomagmatic explosion occurred on January, 2010. Since then, the activity evolved from phreatic to magmatic, in a series of distinct eruptive phases. In this paper, we investigate the seismic records that span the whole eruptive process (2010-present), in order to identify precursory signals and characterize the volcanic evolution. A long-term analysis was carried out based on the continuous records, as well as seismic catalogs (volcano-tectonic seismicity, harmonic tremor, etc.). In addition, the gradual character of the evolution of this eruption allowed for the analysis of independent precursory stages. Thus, we inspected in detail the most important of those periods, particularly, prior to the first 2010 phreatomagmatic eruption, and prior to the 2016 transition to an open vent system. Temporary tremor amplitude decreases were found to precede most of the eruptive phases. In total, 9 pre-eruptive tremor abatement periods were identified spanning several days (5-44), which often concurred with a decrease in the SO2 flux. The analysis of the volcano-tectonic seismicity highlights the migration of magma from a deep (6-10 km) reservoir beneath the neighboring Irazú volcano towards Turrialba volcano, especially between the years 2015 and 2016. This activity peaked on December 2016 when a Mw 5.5 earthquake took place between both volcanoes. Harmonic tremor episodes thrived in the later phase when the system finally opened (2017-2018). In the short-term, compounded tonal seismic signals were identified as precursor events, such as long-period events followed by harmonic tremor or by a multichromatic coda similar to tornillo-type events. The co-occurrence of tremor amplitude decreases and tonal seismic signals is interpreted to be caused by a sealing of the hydrothermal system, which blocked the circulation of fluids and permitted the resonances in the inner cavities. This process leaded to pressure accumulation and the consequent eruptions. Thus, trough a series of cycles of sealing and rupture the system of conduits gradually opened. The seismic characterization of this eruption constitutes insightful knowledge useful for monitoring and risk assessment purposes.Universidad de Costa Rica, Vicerrectoría de Investigación, Proyecto de investigación Red en Sismología Computacional para el Estudio de los Volcanes Activos en Costa Rica.Universidad de Costa Rica, Vicerrectoría de Investigación, Proyecto de investigación Implementación de un sistema de monitoreo automático en tiempo real para los volcanes activos de Costa RicaUniversidad Autónoma Nacional de Costa Rica, Proyecto de investigación Vigilancia de los volcanes de Costa Rica por medio de su actividad sísmicaCentro Nacional de Alta Tecnología//CeNAT/Costa RicaUCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela Centroamericana de Geologí