Low‐Frequency Seismicity at Villarrica Volcano: Source Location and Seismic Velocities

Abstract

Villarrica Volcano (Chile) is one of the most active volcanoes in South America. Its low-frequency (≤5 Hz) seismicity consists of a continuous tremor, overlain by impulsive transient events of higher amplitude in 60-s intervals. This signal was recorded in March 2012 by an extensive local network, comprising 75 stations and including 6 subarrays. It allowed us to apply and compare three techniques to locate the origin of the seismicity: intersection of propagation directions determined by array analysis, mapping amplitudes, and modeling of amplitude decay. All methods yield almost identical, temporally stable, epicenters inside the summit crater, which confirms earlier attributions of the seismicity to volcanic activity inside the conduit. The discrete transients and the interevent tremor share the same source location. From the dominance of surface waves and the obvious scattering, we infer a source near the surface. For two arrays at the northern and western flank, a dispersion relation was derived, which allowed for the determination of S wave velocity-depth functions. At both locations, the velocity structure can be modeled by three layers with interfaces at 100 and 400m depths. The velocities (300 to 3,000 m/s) correspond to pyroclastic material at different states of consolidation. The modeling of the amplitude decay reveals a quality factor around 50.

Comparative interevent time statistics of degassing and seismic activity at Villarrica Volcano (Chile)

“…The descriptive term "long-period" on the other hand is inappropriate for these waveforms if recorded close to the source since it commonly implies an upper frequency limit around 5 Hz (Chouet, 1996). Therefore, and to be consistent with a previous publication by Lehr et al (2019), we prefer the neutral term "transient".…”

“…The seismic signal throughout the network was dominated by a persistent unrest with overlain transient increases in amplitude which originated from the crater region (Lehr et al, 2019). At the crater rim, these transients last from a few seconds to several tens of seconds and contain frequencies up to 16 Hz.…”

Evolution of Seismicity During a Stalled Episode of Reawakening at Cayambe Volcano, Ecuador

“…The characteristics of a seismic signal can be indicative of the source process that generated it (Zobin, 2012;Chouet and Matoza, 2013;McNutt and Roman, 2015). If recorded across multiple stations then the locations of the earthquakes can be used to infer the source mechanism (Woods et al, 2018;Lehr et al, 2019). The rates at which earthquakes occur can also indicate whether a source mechanism is constant and stable or evolving.…”

Magnitude and Interevent Time Statistics of Strombolian Activity of Villarrica Volcano and Inference Regarding the Flow Regime

“…The data were acquired by a dense local seismic network (Figure 2), installed from March 2 to 14, 2012 (Lehr et al., 2019; Mora‐Stock, 2015; Rabbel & Thorwart, 2019). The stations were equipped with 3‐component or 1‐component 4.5‐Hz geophones, and DSS‐cubes, sampling at 100 Hz.…”

“…The seismic signal throughout the network was dominated by a persistent unrest, which originated from the crater region (Lehr et al., 2019). The signal at the crater is characterized by transient increases in amplitude in ∼1‐min intervals (Figure 2).…”