The Lazufre Volcanic System (LVS), on the border of northern Chile and Argentina, is an active complex of two volcanoes, Lastarria to the north and Cordón del Azufre to the south. The LVS is not ...regularly monitored with any scientific equipment despite being recognized as a top ten volcanic hazard in Argentina by the Observatorio Argentino de Vigilancia Volcánica of the Servicio Geológico y Minero Argentino. The system has shown unusual inflation signatures observed in InSAR but the level of seismic activity and its spatial and temporal distribution were unknown due to the lack of a permanent local seismic network. The PLUTONS Project deployed eight broadband seismic stations throughout the LVS between November 2011 and March 2013. This study shows event locations and types from November 2011 through March 2012. We analyze 591 seismic events within 20 km of Lastarria. Most events cluster tightly beneath Lastarria and almost no activity is observed beneath Cordón del Azufre or the primary inflation center. All events are reviewed manually, and located using a velocity model that assimilates prior studies and accounts for hypocenters within the edifice up to 5 km above sea level. More than 90% of the resulting hypocenters are shallower than 10 km below sea level. The waveforms have characteristics similar to those observed at many other volcanoes, suggesting five classes of events: volcano-tectonic (VT), long-period 1 (LP1), long period 2 (LP2), hybrid (HY), and unknown (X). Frequency-magnitude analysis reveals distinct b-values ranging from 1.2 for VT events to 2.5 for LP1 events. Based on the spatial distribution of events and the b-values, we infer that seismic activity is driven mainly by movement of fluids and gases associated with the regional magma zones and inflation centers. The seismic activity is energetic at times, and quieter at others, suggesting the presence of episodic magmatic and/or hydrothermal activity, focused at Lastarria. Our findings indicate that the previously observed inflation signals are indeed volcanic in origin. These results also demonstrate the potential for success of a future seismic monitoring system and provide a framework for interpreting the subsequent observations, both of which are critical to assessing the volcanic risk of the northern Chile-Argentina region.
Duration–amplitude relationships were studied for tremor episodes and earthquake swarms occurring during the 2009 eruption of Redoubt Volcano, Alaska. Duration–amplitude distribution plots were ...generated daily from January 1 to May 31 and fit with both an exponential law and power law. Comparing R2 values of the fit for both laws showed that the exponential law fit better for days in which volcanic tremor and earthquake swarms occurred, while the power law fit better for other days. Fitting segments of seismic data with both an exponential and a power law leads to a metric that has potential for volcano monitoring: R2exp/R2pow, the ratio of the R2 fits using the exponential law and the power law. The ratio R2exp/R2pow tended to be greater than 1 when volcanic activity or precursory seismic activity was occurring, and less than 1 when no volcano-seismic activity was occurring. Duration–amplitude plots were generated for episodes of volcanic tremor that were identified by the R2exp/R2pow≥1 method and compared in an attempt to identify changes that may have occurred during the eruption. Stronger episodes of volcanic tremor showed higher characteristic amplitudes. Maximum heights of the plumes generated by the explosions showed a positive correlation with the characteristic amplitude of the concurrent tremor.
•We generated duration–amplitude plots for the 2009 eruption of Redoubt Volcano.•Periods of volcanic tremor and earthquake swarms were better fit by an exponential law.•Periods without significant tremor or swarms occurring were better fit by a power law.•We propose using the ratio of regression coefficients (R2exp/R2pow) as a monitoring tool.•We found that stronger volcano-seismic episodes had higher characteristic amplitudes.
The explosive phase of the 2009 Redoubt Volcano eruption produced predominantly short duration, high-amplitude infrasound signals recorded up to 4500km away. All 19 numbered explosive events were ...recorded at a local microphone (DFR, 12km), as well as at an infrasound array in Fairbanks, Alaska (I53US, 547km), most with high signal to noise ratios. The local microphone provides an estimate of the source parameters, and comparison between the two datasets allows the unique opportunity to evaluate acoustic source term estimation at a remote array. High waveform similarity between DFR and I53US occurs during much of the explosive phase due to strong stratospheric ducting, permitting accurate source constraints inferred from I53US data. Cross-correlation analysis after applying a Hilbert transform to the I53US data shows how the acoustic energy has passed through a single caustic, as predicted by ray theory. Similar to previous studies, significant low-frequency infrasound from Redoubt recorded at I53US is coincident with high-altitude ash emissions. The largest events also produced considerable energy at greater than 50s periods, likely related to the initial oscillations of the volcanic plume or jet. Many of the explosive events have emergent onsets, somewhat unusual for explosive, short-duration eruptions. Comparison of the satellite-derived SO2 emissions with the relative amount of acoustic energy at I53US shows a very high, statistically significant correlation. This study reiterates the utility of using remote infrasound arrays for detection of hazardous emissions and characterization of large volcanic eruptions, and demonstrates how, under typical meteorological conditions, remote infrasound arrays can provide an accurate representation of the acoustic source.
► 2009 Redoubt eruption produced extensive infrasound. ► Infrasound recorded up to 4500km from volcano. ► High waveform similarity between local and remote stations. ► Extensive ULP infrasound. ► Strong correlation between infrasound energy and SO2.
In January 2006, Augustine Volcano began erupting following an increase in seismicity that was first noted in late April 2005. Thirteen large explosive eruptions of Augustine occurred from January 11 ...to 28, 2006, followed by a continuously erupting phase and then by a dome growth phase in which numerous pyroclastic flows and block-and-ash flows occurred. As a new steep-sided and unstable dome grew in spring 2006, rockfalls and related events, likely block-and-ash flows, dominated the seismic record. Relative amplitudes at pairs of seismic stations for 68 block-and-ash flow events were examined to constrain locations of the flow-events. Higher amplitudes were associated with events closer to a given station. These relations were confirmed by images collected on a low-light camera. Captured images show a correlation between flow direction and seismic amplitude ratios from nearby stations AUE and AUW. Seismic amplitudes and energies of the flow signals, measured in several different ways, were found to correlate with the surface areas and run-out distances of the flows. The ML range of rockfalls was 0.1 to 1.1, and seismic efficiencies were estimated to be much less than 1%. Particle motion analyses showed that the seismic waves contained both body waves and surface waves and demonstrate that the flows were acting as moving sources with velocities of 30–93m/s.
► Joint analyses of seismic and low-light camera data of block-and-ash flows during eruption were performed. ► Ratios of seismic amplitudes between stations correlate well with the block-and-ash flow locations. ► Seismic amplitudes correlate with flow sizes and run-out distances.
In summer 2003, a Chaparral Model 2 microphone was deployed at Shishaldin Volcano, Aleutian Islands, Alaska. The pressure sensor was co-located with a short-period seismometer on the volcano's north ...flank at a distance of 6.62 km from the active summit vent. The seismo-acoustic data exhibit a correlation between impulsive acoustic signals (1-2 Pa) and long-period (LP, 1-2 Hz) earthquakes. Since it last erupted in 1999, Shishaldin has been characterized by sustained seismicity consisting of many hundreds to two thousand LP events per day. The activity is accompanied by up to 200 m high discrete gas puffs exiting the small summit vent, but no significant eruptive activity has been confirmed. The acoustic waveforms possess similarity throughout the data set (July 2003-November 2004) indicating a repetitive source mechanism. The simplicity of the acoustic waveforms, the impulsive onsets with relatively short (10-20 s) gradually decaying codas and the waveform similarities suggest that the acoustic pulses are generated at the fluid-air interface within an open-vent system. SO^sub 2^ measurements have revealed a low SO^sub 2^ flux, suggesting a hydrothermal system with magmatic gases leaking through. This hypothesis is supported by the steady-state nature of Shishaldin's volcanic system since 1999. Time delays between the seismic LP and infrasound onsets were acquired from a representative day of seismo-acoustic data. A simple model was used to estimate source depths. The short seismo-acoustic delay times have revealed that the seismic and acoustic sources are co-located at a depth of 240±200 m below the crater rim. This shallow depth is confirmed by resonance of the upper portion of the open conduit, which produces standing waves with f=0.3 Hz in the acoustic waveform codas. The infrasound data has allowed us to relate Shishaldin's LP earthquakes to degassing explosions, created by gas volume ruptures from a fluid-air interface.PUBLICATION ABSTRACT
Mt. Veniaminof, Alaska Peninsula, is a stratovolcano with a summit ice-filled caldera containing a small intracaldera cone and active vent. From January 2 to February 21, 2005, Mt. Veniaminof ...erupted. The eruption was characterized by numerous small ash emissions (VEI 0 to 1) and accompanied by low-frequency earthquake activity and volcanic tremor. We have performed spectral analyses of the seismic signals in order to characterize them and to constrain their source. Continuous tremor has durations of minutes to hours with dominant energy in the band 0.5-4.0 Hz, and spectra characterized by narrow peaks either irregularly (non-harmonic tremor) or regularly spaced (harmonic tremor). The spectra of non-harmonic tremor resemble those of low-frequency events recorded simultaneously with surface ash explosions, suggesting that the source mechanisms might be similar or related. We propose that non-harmonic tremor at Mt. Veniaminof results from the coalescence of gas bubbles while low-frequency events are related to the disruption of large gas pockets within the conduit. Harmonic tremor, characterized by regular and quasi-sinusoidal waveforms, has duration of hours. Spectra containing up to five harmonics suggest the presence of a resonating source volume that vibrates in a longitudinal acoustic mode. An interesting feature of harmonic tremor is that frequency is observed to change over time; spectral lines move towards higher or lower values while the harmonic nature of the spectra is maintained. Factors controlling the variable characteristics of harmonic tremor include changes in acoustic velocity at the source and variations of the effective size of the resonator.PUBLICATION ABSTRACT
The recent explosive eruptions of Okmok and Kasatochi volcanoes provide an opportunity to use seismic, local infrasound, distant infrasound array, and remote sensing data in concert to better monitor ...volcanoes in the Aleutian Arc and to better understand the source processes. The eruption of Okmok Volcano began on 12 July 2008 and included a seismically active phase that lasted continuously for about 10 h. In contrast, the eruption of Kasatochi which began on 7 August 2008 consisted of five explosive events that lasted from 26 to 68 min each and had a cumulative duration of 3.4 h. Given the event times by local seismic stations, the corresponding infrasound signals were found in the data recorded by local infrasound sensors and by distant infrasound arrays. Signals from the Okmok eruption were detected by three International Monitoring System (IMS) arrays as far away as 4400 km; signals from the Kasatochi eruption were detected at greater distances up to 5200 km away by seven infrasound arrays including the ones that detected the event at Okmok Volcano. Back azimuth propagation and a simple acoustic wave propagation model in unison with known event times were used to confirm that the planar, acoustic signals recorded at the arrays had originated from the eruptions. The infrasound array data reflected the differences in eruption styles between Okmok and Kasatochi as the signals from Kasatochi were of shorter duration, of greater amplitude, and detected over greater distances. The infrasound array data were also able to distinguish between two types of tremor episodes that occurred at Kasatochi Volcano based on atmospheric disturbance.
The frequency‐magnitude distribution of earthquakes, characterized using the b‐value, is examined as a function of space beneath Mount St. Helens (1988–1996), and Mt. Spurr (1991–1995). At Mount St. ...Helens, two volumes of anomalously high b (b > 1.3) can be observed at depths of 2.6–3.6 km below the crater floor and below 6.4 km. These anomalies coincide with (1) the depth of vesiculation of ascending magma, and (2) the suggested location of a magma chamber at Mount St. Helens. Study of Mt. Spurr reveals an area of high b‐value (b ≥ 1.3) at a depth of about 2.3–4.5 km below the crater floor of the active vent Crater Peak. We propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b‐value at shallow depths. Alternatively, interaction of magma with groundwater may have increased pore pressure and lowered the effective stress. The deeper anomaly at Mount St. Helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber. Our results indicate that detailed mapping of the frequency‐magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers.
On November 3, 2002 three segments of the Denali fault in interior Alaska ruptured during a Mw 7.9 earthquake, offering a unique opportunity to study earthquake-volcano interactions. Out of the 24 ...volcanoes that are seismically monitored by the Alaska Volcano Observatory (AVO) only Mt. Wrangell, the closest volcano to the epicenter (247 km), showed a clear response to the shaking in the intermediate-term (weeks to months) time scale. The response was unexpected because it consisted of a decline by at least 50% in the volcano's seismicity rate (mostly low-frequency events) that lasted for five months. Because most well documented previous instances of short-term (minutes to days) responses of volcanic centers to the passing waves of distant earthquakes, have all been seismicity increases, the decline in seismicity at Mt. Wrangell poses a controversial puzzle. By using several independent methods to measure the seismicity rate at the volcano from before to after the main shock, and applying rigorous statistical testing, we conclude that the change in seismicity at the volcano was a real effect of the Denali earthquake. We suggest that a depressurization of the volcanic plumbing system took place either as a result of sudden decompression (static stress changes) or because of creation of new pathways resulting from the strong shaking (dynamic stresses). At present we cannot distinguish between these two possibilities.
From September 1999 through April 2004, Shishaldin Volcano, Aleutian Islands, Alaska, exhibited a continuous and extremely high level of background seismicity. This activity consisted of many ...hundreds to thousands of long-period (LP; 1–2 Hz) earthquakes per day, recorded by a 6-station monitoring network around Shishaldin. The LP events originate beneath the summit at shallow depths (0–3 km). Volcano tectonic events and tremor have rarely been observed in the summit region. Such a high rate of LP events with no eruption suggests that a steady state process has been occurring ever since Shishaldin last erupted in April–May 1999. Following the eruption, the only other signs of volcanic unrest have been occasional weak thermal anomalies and an omnipresent puffing volcanic plume. The LP waveforms are nearly identical for time spans of days to months, but vary over longer time scales. The observations imply that the spatially close source processes are repeating, stable and non-destructive. Event sizes vary, but the rate of occurrence remains roughly constant. The events range from magnitude ∼
0.1 to 1.8, with most events having magnitudes <
1.0. The observations suggest that the conduit system is open and capable of releasing a large amount of energy, approximately equivalent to at least one magnitude 1.8–2.6 earthquake per day. The rate of observed puffs (1 per minute) in the steam plume is similar to the typical seismic rates, suggesting that the LP events are directly related to degassing processes. However, the source mechanism, capable of producing one LP event about every 0.5–5 min, is still poorly understood. Shishaldin's seismicity is unusual in its sustained high rate of LP events without accompanying eruptive activity. Every indication is that the high rate of seismicity will continue without reflecting a hazardous state. Sealing of the conduit and/or change in gas flux, however, would be expected to change Shishaldin's behavior.
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