Temporal evolution of the magmatic system at Nevado del Ruiz Volcano (Colombia) inferred from long-period seismic events in 2003–2020

Abstract

Long-period (LP) seismic events have been broadly assumed to be generated by oscillations of crack fluid-filled resonators. We investigated the waveforms and spectral characteristics of LP seismic events recorded by five stations at Nevado del Ruiz Volcano, Colombia. The observed event types of LP events exhibited decaying harmonic oscillations with either monochromatic or polychromatic spectral features (NLP events) or events characterized by non-decaying harmonic oscillations (BLP events). The NLP events occurred over four periods: September 2003 (period I), January 2004–October 2004 (period II), October 2010–March 2012 (period III), and October 2019–April 2020 (period IV). Their source locations were determined to be within the first kilometer of the conduit placed below the Arenas crater. Their locations involved a distinct resonator for each of the four periods but a single resonator for all the events during a single period. Assuming a crack geometry at their sources and based on their locations, we considered a dusty gas (mixture of ash and water vapor) filling a crack resonator. We estimated the crack model parameters to investigate the temporal processes generating variations in frequency and quality factor Q. Then, the estimated parameters indicate that NLP events were triggered by magma fragmentation. We found that events from period III preceded phreatomagmatic eruptions, increased ash and gas emissions and SO2 flux, and increased overall seismic activity. Three years after this period III, a dome was emplaced at the base of the Arenas crater. Besides, events from period IV also preceded by ~3 years a significant increase in the overall seismic activity, which led to a change in alert activity at the volcano. We argue that BLP events were driven by resonances provoked by the unstable degassing through the uppermost magma plug's open cracks (tuffisite veins).In contrast, NLP events occurred by overpressure released through fractures along the conduit's walls (shear fractures). The shear fractures at the conduit wall were in a region of high strain rate where a decrease in confining pressure triggered fragmentation, resulting in dusty gas and, subsequently, a resonance of the crack. NLP and BLP events were in a narrow relationship with the ascent of vesiculated magma to the surface. Our results show that the LP events help diagnose the state of magmatic fluids and understand the eruption processes of Nevado del Ruiz Volcano. LP events are not always followed by explosive eruptions, but they do indicate significant overpressure developing in the shallow conduit.