Degassing and hydrothermal activity at Mt. Spurr, Alaska during the summer of 2004 inferred from the complex frequencies of long-period events (original) (raw)
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Geophysical research letters, 2006
1] A swarm of six long-period (LP) events with slowly decaying coda wave amplitudes and durations up to 120 s, was recorded by seismic stations located in the proximity of Mt. Griggs, a fumarolically active volcano in the Katmai National Park, Alaska, during December 8 -21, 2004. Spectral analyses reveal the quasi-monochromatic character of the waveforms, dominated by a 2.5 Hz mode frequently accompanied by a weaker high-frequency onset (6.0 -9.0 Hz). Particle motion azimuths and inclination angles show a dominant WNW-ESE direction of polarization for all the signals, and suggest that seismic energy is radiated by a stable source at shallow depth. Damping coefficients between 0.0014 and 0.0063 are estimated by fitting an exponential decay model to the signal's coda; corresponding quality factors range from 78 to 351. The source of the waveforms is modelled as a resonant cavity filled with a fluid/ gas mixture.
Frontiers in Earth Science, 2021
Detection of the earliest stages of unrest is one of the most challenging and yet critically needed aspects of volcano monitoring. We investigate a sequence of five unusual long-period (LP) earthquakes that occurred in the days prior to the onset of a months-long volcano-tectonic (VT) earthquake swarm beneath Little Sitkin volcano in the Aleutian Islands during late 2012. The long-period earthquakes had two distinctive characteristics: their signals were dominated by a monochromatic spectral peak at approximately 0.57 Hz and they had impulsive P and S-wave arrivals on a seismometer located on Amchitka Island 80 km to the southeast of the volcano. In each case, the monochromatic earthquakes ended with a higher-frequency event after approximately 2 min of duration. We find evidence that the five monochromatic LP earthquakes resulted from the resonance of a tabular magma body at middle crustal depths (15 km) on the western side of Little Sitkin. Based on the resonant frequency and qual...
Bulletin of Volcanology, 2007
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 2 measurements have revealed a low SO 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.
New insights into the 1999 eruption of Shishaldin volcano, Alaska, based on acoustic data
Bulletin of Volcanology, 2003
Data collected by a pressure sensor provide new insights into the 1999 eruption of Shishaldin volcano, Unimak Island, Alaska. On 19 April 1999, after 3 months of unrest and an extended period of low-level Strombolian activity, Shishaldin experienced a Subplinian eruption (ash plume to >16 km), followed by several episodes of strong Strombolian explosions. Acoustic data from the pressure sensor allow us to investigate the details of an eruption which was instrumentally well recorded, but with few visual observations. In the 12 h prior to the Subplinian phase, the pressure sensor detected a series of small, repeated pulses with a constant spectral peak at 2-3 Hz. The amplitude and occurrence rate of the pulses both grew such that the signal became a nearly continuous hum just before the Subplinian eruption. This humming signal may represent gas release from rising magma. The main Subplinian phase was heralded by (1) the abrupt end of the humming signal, (2) several pulses of lowfrequency sound interpreted as ash bursts, and (3) a dramatic increase in seismic tremor amplitude. The change in acoustic signature at this time allows us to precisely time the start of the Subplinian eruption, previously approximated as the time of strongest tremor increase. The 50-min Subplinian phase actually contained several bursts of signal, each of which may represent a discrete volume of magma passing through the system. Following the Subplinian event, the pressure sensor recorded four discrete episodes of Strombolian gas explosions on 19-20 April and another on 22-23 April. Four of the five episodes were accompanied by strong seismic tremor; the fifth has not been previously recognized and was not associated with anomalous tremor amplitudes. In time series these events are similar to explosions recorded at other volcanoes but in general they are much larger, with maximum amplitudes of >65 Pa at 6.5 km from the vent, and they have low (0.7-1.5 Hz) peak frequencies. These large explosions occurred at rates of 3-20 per minute for 1-5 h in each episode. The explosions were accompanied by a small (<5 km above sea level) ash plume and only minor amounts of ejecta were produced. Thus, the explosion activity was dominated by gas release.
2010
1] We use shear wave splitting (SWS) analysis and double-difference relocation to examine temporal variations in seismic properties prior to and accompanying magmatic activity associated with the 2008 eruption of Okmok volcano, Alaska. Using bispectrum cross-correlation, a multiplet of 25 earthquakes is identified spanning five years leading up to the eruption, each event having first motions compatible with a normal fault striking NE-SW. Cross-correlation differential times are used to relocate earthquakes occurring between January 2003 and February 2009. The bulk of the seismicity prior to the onset of the eruption on 12 July 2008 occurred southwest of the caldera beneath a geothermal field. Earthquakes associated with the onset of the eruption occurred beneath the northern portion of the caldera and started as deep as 13 km. Subsequent earthquakes occurred predominantly at 3 km depth, coinciding with the depth at which the magma body has been modeled using geodetic data. Automated SWS analysis of the Okmok catalog reveals radial polarization outside the caldera and a northwest-southeast polarization within. We interpret these polarizations in terms of a magma reservoir near the center of the caldera, which we model with a Mogi point source. SWS analysis using the same input processing parameters for each event in the multiplet reveals no temporal changes in anisotropy over the duration of the multiplet, suggesting either a short-term or small increase in stress just before the eruption that was not detected by GPS, or eruption triggering by a mechanism other than a change of stress in the system.
Sustained long-period seismicity at Shishaldin Volcano, Alaska
Journal of Volcanology and Geothermal Research, 2006
From September 1999 through April 2004, Shishaldin Volcano, Aleutian Islands, Alaska, exhibited a continuous and extremely 10 high level of background seismicity. This activity consisted of many hundreds to thousands of long-period (LP; 1-2 Hz) 11 earthquakes per day, recorded by a 6-station monitoring network around Shishaldin. The LP events originate beneath the summit 12 at shallow depths (0-3 km). Volcano tectonic events and tremor have rarely been observed in the summit region. Such a high rate of 13 LP events with no eruption suggests that a steady state process has been occurring ever since Shishaldin last erupted in April-May 14 1999. Following the eruption, the only other signs of volcanic unrest have been occasional weak thermal anomalies and an 15 omnipresent puffing volcanic plume. The LP waveforms are nearly identical for time spans of days to months, but vary over longer 16 time scales. The observations imply that the spatially close source processes are repeating, stable and non-destructive. Event sizes 17 vary, but the rate of occurrence remains roughly constant. The events range from magnitude~0.1 to 1.8, with most events having 18 magnitudes b 1.0. The observations suggest that the conduit system is open and capable of releasing a large amount of energy, 19 approximately equivalent to at least one magnitude 1.8-2.6 earthquake per day. The rate of observed puffs (1 per minute) in the 20 steam plume is similar to the typical seismic rates, suggesting that the LP events are directly related to degassing processes. 21 However, the source mechanism, capable of producing one LP event about every 0.5-5 min, is still poorly understood. Shishaldin's 22 seismicity is unusual in its sustained high rate of LP events without accompanying eruptive activity. Every indication is that the 23 high rate of seismicity will continue without reflecting a hazardous state. Sealing of the conduit and/or change in gas flux, however, 24 would be expected to change Shishaldin's behavior. 25 D 2005 Elsevier B.V. All rights reserved. 26
2003
The M w 7.9 Denali fault earthquake ruptured segments of the Susitna Glacier, Denali, and Totschunda faults in central Alaska, providing a unique opportunity to look for intermediate-term (weeks to months) responses of active volcanoes to shaking from a large earthquake. The Alaska Volcano Observatory (AVO) monitors 24 volcanoes with seismograph networks. We examined one station per volcano. Digitally-filtered data for the period four weeks before to four weeks after the mainshock were plotted at a standard scale. Mt. Wrangell, the closest volcano to the epicenter (247 km), had a background rate of 16 events/day. For the following 30 days, however, its seismicity rate dropped by 50%. Mt. Veniaminof (1400 km from the epicenter) had a rate of 8 seismic events/day, but suffered a drop in seismicity by 80% after the maishock; this may have lasted for 15 days. The seismicity at both volcanoes is dominated by long-period seismic events. With the exception of Martin and Novarupta volcanoes, the other 20 volcanoes showed no changes in seismicity attributable to the Denali fault earthquake. We conclude that the changes in seismicity observed are real, and are related to the Denali fault earthquake. These seismicity drops are in strong contrast to cases of short-term triggered seismicity increases observed at other volcanic systems such as Martin-Novarupta, Mt. Rainier, Yellowstone, Mammoth Mountain, and The Geysers, Coso and Cerro Prieto (Mexico) geothermal fields. This suggests that fundamentally different mechanisms may be acting to modify seismicity at volcanoes.
Bulletin of Volcanology, 2020
Bogoslof volcano, in the central Aleutian arc, experienced a major eruption between December 2016 and August 2017 that was characterized by explosive activity (Volcanic Explosivity Index 2 to 3) and the extrusion of lava domes. The Alaska Volcano Observatory tracked the activity in real time using seismicity observed on distant stations as well as infrasound, lightning, satellite data, and occasional visual observations. In this study, we measure the duration of seismic signals associated with individual explosive events to track their progression during the two explosive phases of the eruption. Seismic recordings of Bogoslof explosions show complex waveforms that suggest both individual explosive events and sequences of several explosions separated by lower amplitude tremor. The lack of local seismic monitoring (stations at distances of 1 km to 15 km distance) unfortunately limits our ability to closely observe seismicity and to interpret changing conditions at the vent such as position, the presence of a lava dome or plug, and the role of seawater associated with the eruption. We use the rate of explosive activity, seismic waveform character, and repose time between explosions to infer the conditions within the conduit.
Geophysical Journal International, 1995
The scaling of pulse duration to seismic moment is estimated for earthquakes along an interplate thrust zone, from digital waveforms recorded by short-period and broad-band instruments of the East Aleutian (Shumagin) Seismic Network. We measure pulse duration using an empirical Green's function technique based on damped time-domain deconvolution. From several thousand events, 22 earthquakes with magnitudes 3.0–7.0 and depths 23-56 km are found to give reliable estimates of pulse duration. Durations are also determined directly from one-parameter nonlinear inversions, for a variety of simple functional forms of source time functions. Symmetric source pulses (boxcar or triangle shapes) fit waveforms better than an asymmetric model [t exp (-2t/D)] for most (62 per cent) of the waveform pairs, while the asymmetric model fits best for only 8 per cent of the data. Pulse duration increases with the size of events, from 0.1 to 10s over the seismic moment (M0) range of 1014 to 3 × 1019 Nm. When normalized by the cube root of seismic moment, pulse durations show ∼8 x variation; comparable static stress drop estimates range from 0.2 to 135 MPa. Contrary to predictions of some laboratory and theoretical studies, earthquakes at the deepest part of the thrust zone do not show significantly higher stress drops than do shallower events. Rupture properties, however, show a strong dependence on earthquake size. the three largest events (M0>5 × 1018Nm) have the three longest normalized durations, on average 3.8 times longer than those for smaller events. the durations require smaller events to have 10–100 x larger static stress drops, or ∼4 x faster rupture velocities, or some combination of the two. Possibly, the largest events rupture both strong and weak patches while smaller events just rupture strong patches on the fault surface. the characteristic dimension that separates large from small events, 3-15 km, is comparable to characteristic wavelengths of Pacific basin bathymetry and may reflect the influence of the subducted sea-floor upon fault-zone heterogeneity.
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. RESUMEN En Noviembre 3 de 2002 durante el terremoto de 7.9 Mw, se quebraron tres segmentos de la falla Denali en Alaska, ofreciendo una única oportunidad de estudiar los terremotos por interacción volcánica. Aparte de los 24 volcanes monitoreados sismicamente por el Observatorio Volcanológico de Alaska (AVO), sólo el Mt. Wrangell, el volcán más cercano al epicentro (247 km), mostró una respuesta clara ante el movimiento en un término de escala de tiempo intermedio de semanas a meses. La respuesta fue inesperada porque consiste en un declive de por lo menos 50% en la velocidad sismológica del volcán (frecuencia de eventos cada vez más lenta) que duró hasta cinco meses. Puesto que muchos casos de corto término (minutos a días), muy bien documentados previamente, responden a olas de distantes terremotos centros volcánicos, todos han tenido incrementos en la sismisidad; por tanto, el declive sísmico en el volcán tuvo un efecto real sobre el terremoto Denalie de Mt. Wrangell que ocasionó