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 low-frequency 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.PUBLICATION ABSTRACT
Duration-amplitude distribution of volcanic tremor Benoit, John P.; McNutt, Stephen R.; Barboza, Vilma
Journal of Geophysical Research - Solid Earth,
March 2003, Letnik:
108, Številka:
B3
Journal Article
Recenzirano
Odprti dostop
The duration‐amplitude distribution of volcanic tremor was examined in eight volcanoes and one geothermal area. An exponential model, implying a scale‐bound source process, is found to be a better ...fit to the data than a power law (scale invariant) model. The exponential model well describes tremor associated with magmatic and phreatic eruptions, shallow and deep source regions, and geothermal sources. We tested the exponential model described by: , where d is the duration of tremor greater than or equal to a particular amplitude DR, dt is the total duration of tremor, and the inverse of λ is the characteristic amplitude of the distribution. λ−1 takes on values between 0.003 and 7.7 cm2. Our results show that the characteristic amplitude for eruptive tremor is greater than for noneruptive tremor, that for deep tremor is greater than for shallow tremor, and that for tremor associated with magmatic eruptions is greater than for tremor associated with phreatic eruptions. The exponential scaling of tremor suggests that tremor is not simply composed of a series of low‐frequency events closely spaced in time. Further, the exponential scaling requires the source to be scale bound; the amplitude variations of tremor are distributed about a constant characteristic amplitude. We propose that exponential scaling of tremor amplitude is caused by fixed source geometry driven by variable excess pressures. The exponential scaling of tremor demonstrates that tremor source processes are fundamentally different from those for earthquakes.
Westdahl volcano, located at the west end of Unimak Island in the central Aleutian volcanic arc, Alaska, is a broad shield that produced moderate‐sized eruptions in 1964, 1978–79, and 1991–92. ...Satellite radar interferometry detected about 17 cm of volcano‐wide inflation from September 1993 to October 1998. Multiple independent interferograms reveal that the deformation rate has not been steady; more inflation occurred from 1993 to 1995 than from 1995 to 1998. Numerical modeling indicates that a source located about 9 km beneath the center of the volcano inflated by about 0.05 km³ from 1993 to 1998. On the basis of the timing and volume of recent eruptions at Westdahl and the fact that it has been inflating for more than 5 years, the next eruption can be expected within the next several years.
Explosive sources triggered inside a magma conduit may excite the conduit into acoustic resonance. The acoustic field in the conduit can propagate into the atmosphere through an open vent and ...ensonify the overlying atmosphere. The character of the airborne sound field is determined by a combination of propagation and source effects: the resonance of the magmatic conduit and the diffraction of the sound field at the volcanic vent are acoustic propagation effects, whereas the explosion pressure signature and the firing rate of the explosions define the source characteristics. For wavelengths larger than the conduit radius, only the longitudinal resonances of the magmatic conduit are relevant, and the open vent radiates like a piston surrounded by an infinite baffle. In this case, the fluid particle velocities are directed along the axis of the conduit and the sound field may propagate into the surrounding bedrock through the conduit wall displacement induced by the fluid overpressure. This coupling may produce seismic signals with banded spectra, such as volcanic tremor and long-period events. The airborne pressure field retains the modal structure of the sound field in the magmatic conduit, which contains information on the conduit geometry and geo-acoustic properties of the magma. The seismic wavefield is driven by the acoustic field in the magma and also contains this information, although it may be filtered by propagation effects in the bedrock. The theoretical sound field in the magma conduit is used to interpret seismic tremor signals recorded at Mt. Spurr Volcano, Alaska. Rapid variations in the acoustic impedance of the magma conduit terminations can create systematic changes in the tremor spectra, which can be used to monitor changes in the magmatic system. The results of the modelling illustrate the difference between source effects and conduit resonance, as well as the value of seismoacoustic measurements in volcanic environments.
The origin of electrical activity accompanying volcanic ash plumes is an area of heightened interest in volcanology. However, it is unclear how intense an eruption needs to be to produce lightning ...flashes as opposed to “vent discharges,” which represent the smallest scale of electrical activity. This study targets 97 carefully monitored plumes <3 km high from Sakurajima volcano in Japan, from June 1 to 7, 2015. We use multiparametric measurements from sensors including a nine‐station lightning mapping array and an infrared camera to characterize plume ascent. Findings demonstrate that the impulsive, high velocity plumes (>55 m/s) were most likely to create vent discharges, whereas lightning flashes occurred in plumes with high volume flux. We identified conditions where volcanic lightning occurred without detectable vent discharges, highlighting their independent source mechanisms. Our results imply that plume dynamics govern the charging for volcanic lightning, while the characteristics of the source explosion control vent discharges.
Plain Language Summary
There are different types of electrical activity that occur within volcanic ash plumes. One well‐known type is volcanic lightning, which creates the familiar flashes of visible light. Another, lesser‐known type is called “vent discharges,” which are tiny and invisible but create a peculiar signal known as continual radio frequency. Vent discharges are important because they sometimes occur very early in an eruption and may provide a way to give early warning of volcanic hazards. For this study, we used a specialized instrument called a lightning mapping array to measure the electrical activity during small eruptions at Sakurajima volcano. We explored when lightning flashes and vent discharges occurred during 97 different explosions. First, we found that the two types did not always happen in the same explosion, showing they are caused by different processes. Second, we found that vent discharges were more likely to occur when the explosive plume rose quickly, possibly because rock breakup and particles rubbing together create a static charge close to the vent. In contrast, lightning flashes occurred when the plume had high volume flux. These findings give clues about how to link electrical signals to eruptive processes for volcano monitoring.
Key Points
Eruptive activity from Sakurajima volcano, 2015 showed that volcanic lightning was a clear indicator of an ash‐bearing volcanic eruption cloud
Vent discharges were linked to impulsive, high‐velocity plumes, whereas lightning flashes were linked to plumes with high volume flux
We infer that the charging for vent discharges is tied to the source explosion while volcanic lightning is governed by volcanic plume processes
Sakurajima volcano in Japan is known for frequent eruptions containing prolific volcanic lightning. Previous studies from eruptions at Redoubt have shown preliminary correlations between seismic, ...infrasound, and radio frequency signals. This study uses field data collected at Sakurajima from 28 May–7 June 2015 and multivariable statistical modeling to quantify these relationships. We build regression equations to examine each of the following parameters of electrical activity: (1) the presence of electrical activity, (2) the presence of the radio frequency signal called continual radio frequency impulses (CRF), (3) the presence of lightning, (4) the overall duration of electrical activity, and (5) the total number of radio frequency sources located by a lightning mapping array. We model these response variables against: (1) seismic energy, (2) infrasound energy, (3) seismic duration, (4) infrasound duration, and (5) the volcano acoustic seismic ratio. Our final regression equations show that each parameter of electrical activity is best defined by a separate set of response parameters, but overall events with greater explosivity correlate with higher amounts of electrical activity. Specifically, (1) the probability of CRF occurring, and the overall number of located radio frequency sources are likely related to deeper fragmentation depths; (2) the probability of electrical activity occurring at all, and specifically the probability of lightning being generated are correlated with high infrasound energies indicating that the gas thrust phase of plume formation plays an important role in charge generation; and (3) the longer an eruption (as determined by the duration of the infrasound signal) the longer we can expect to see radio frequency signals generated.
•Overall volcanic electrical activity is correlated with high infrasound energies•Continual radio frequency impulses, produced at the vent, accompany magmatic fragmentation•Volcanic lightning generation is influenced by plume dynamics
Volcanic lightning detection has become a useful resource for monitoring remote, under-instrumented volcanoes. Previous studies have shown that the behavior of volcanic plume electrification responds ...to changes in the eruptive processes and products. However, there has not yet been a study to quantify the links between ash textures and plume electrification during an actively monitored eruption. In this study, we examine a sequence of vulcanian eruptions from Sakurajima Volcano in Japan to compare ash textural properties (grain size, shape, componentry, and groundmass crystallinity) to plume electrification using a lightning mapping array and other monitoring data. We show that the presence of the continual radio frequency (CRF) signal is more likely to occur during eruptions that produce large seismic amplitudes (>7 μm) and glass-rich volcanic ash with more equant particle shapes. We show that CRF is generated during energetic, impulsive eruptions, where charge buildup is enhanced by secondary fragmentation (milling) as particles travel out of the conduit and into the gas-thrust region of the plume. We show that the CRF signal is influenced by a different electrification process than later volcanic lightning. By using volcanic CRF and lightning to better understand the eruptive event and its products these key observations will help the monitoring community better utilize volcanic electrification as a method for monitoring and understanding ongoing explosive eruptions.
•Field collected volcanic ash samples are analyzed for correlation with volcanic lightning measurements.•Continual Radio Frequency is related to explosive eruptions of juvenile magmatic material.•Continual Radio Frequency is related to increased milling in the conduit as determined by ash shape.•Continual Radio Frequency is related to discrete, high amplitude explosive seismic signals.•Continual Radio Frequency is influenced by a separate electrification process then volcanic lightning.
Local seismic and infrasound data are used to characterize the March–April 2009 eruptions of Redoubt volcano, Alaska. Data are from a network of ten seismic stations ranging from 2.6 to 21km from the ...vent and a single infrasound sensor 12.2km from the vent. Measurements were made for more than 30 explosive eruptions including the 19 numbered events that were identified during the eruption response. More than 30 smaller explosions were also identified. Measured parameters include onset time, duration, reduced displacement (DR), pressure, seismic energy, and acoustic energy. Results were compared with complementary gas and lightning data. Based on ratios of pressure to DR and acoustic to seismic energy we find that the initial group of explosive eruptions on 23–24 March was relatively stronger seismically and included the four highest DR values. We infer that these represent vent enlargement or stronger coupling between the magma and wall rocks. One event, at 12:31 UT on 23 March had an extremely high pressure of 250Pa for the initial pulse and was likely weakly seismically coupled. A second group of explosive eruptions on 27–29 March was relatively stronger acoustically. Deposits were finer grained and of different lithology, with less scoria and more dense clasts. These two groups may represent different modes of gas release. Cumulative acoustic energy as a function of time correlates well with comparable lightning and SO2 data, suggesting that all three provide insight regarding different factors of gas release for the eruption as a whole. The number of events identified depends on criteria measured, and depends on the goals of the investigations. Overall the eruptions were substantially stronger than recent eruptions in Alaska, with some durations exceeding 1h and pressures an order of magnitude higher.
•Seismic and infrasound data for explosions•Two groups based on energy ratios•Very strong explosions, up to 250Pa at 12km distance•Correlation of seismic, infrasound, lightning and gas data
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