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2 Observations of volcanic tremor during the January-February 2005 eruption of Mt.
3 Veniaminof, Alaska.
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5 Silvio De Angelis and Stephen R. McNutt
6 Alaska Volcano Observatory – Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Drive PO
7 BOX 757320, Fairbanks, Alaska, 99775-7320, USA.
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11 Prepared for submittal to Bullettin of Volcanology
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13 Contact author: Silvio De Angelis
14 e-mail:[email protected];
15 phone +1-907-474-7234
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1 25 Abstract
26 Mt. Veniaminof, Alaska Peninsula, is a strato-volcano with a summit ice-filled caldera
27 containing a small intracaldera cone and active vent. From January 2 to February 21, 2005,
28 Mt. Veniaminof erupted. The eruption was characterized by numerous small ash emissions
29 (VEI 0 to 1) and accompanied by low-frequency earthquake activity and volcanic tremor. We
30 have performed spectral analyses of the seismic signals in order to characterize them and to
31 constrain their source. Continuous tremor has durations of minutes to hours with dominant
32 energy in the band 0.5-4.0 Hz, and spectra characterized by narrow peaks either irregularly
33 (non-harmonic tremor) or regularly spaced (harmonic tremor). The spectra of non-harmonic
34 tremor resemble those of low-frequency events recorded simoultaneously to surface ash
35 explosions, suggesting that the source mechanisms might be similar or related. We propose
36 that non-harmonic tremor at Mt. Veniaminof results from the coalescence of gas bubbles and
37 low-frequency events are related to the disruption of large gas pockets within the conduit.
38 Harmonic tremor, that is characterized by regular and quasi-sinusoidal waveforms, has
39 duration of hours. Spectra, containing up to five harmonics, suggest the presence of a
40 resonating source volume that vibrates in a longitudinal acoustic mode. An interesting feature
41 of harmonic tremor is that frequency is observed to change over time: spectral lines move
42 towards higher or lower values while the harmonic nature of the spectra is maintained. Factors
43 controlling the variable characteristics of harmonic tremor include changes in acoustic
44 velocity at the source and variations of the effective size of the resonator.
45 Keywords: Volcanic tremor, harmonic tremor, low-frequency events, conduit resonance,
46 ash eruptions, Mt. Veniaminof, volcanic seismology
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48 Introduction
2 49 Over the past decade the observation of low-frequency (LF) earthquake activity at volcanoes
50 has become increasingly important in monitoring and forecasting eruptions. LF signals with
51 durations of minutes to days or longer, frequently observed near active volcanoes, are usually
52 referred to as volcanic tremor (Aki and Koyanagi, 1981; Fujita et al., 1995; Hellweg, 2000;
53 McNutt, 2002); tremor has been documented at 160 world volcanic centers (McNutt, 1994)
54 and its detection is an important part of most volcano monitoring programs.
55 The most general appearance of a tremor waveform is that of a continuous signal with
56 emergent onset, smoothly varying amplitudes and energy confined in the band 1.0-5.0 Hz
57 (Julian, 1994; Fujita et al., 1995; Neuberg et al., 2000; McNutt, 2002). Tremor spectra can be
58 either broadband without pronounced peaks, or characterized by a variable number of
59 regularly spaced peaks. If the spectrum contains either a single peak, or a variable number of
60 regularly spaced peaks, the signal is called harmonic tremor.
61 Unlike tectonic earthquakes, that involve mechanisms of shear failure of rock at the source,
62 tremor originates from complex fluid-rock interactions within volcanoes. While volcanic
63 tremor is a common precursor and accompanies most volcanic eruptions, its characteristics
64 including depth, duration and amplitude, can vary considerably. The broad range of tremor
65 properties suggests that multiple mechanisms may be responsible for its generation, even at
66 the same volcano; several models have been proposed in order to account for tremor
67 generation including free oscillations of fluid filled cavities (Sassa, 1936; Crosson and Bame,
68 1985; Fujita et al., 1995), jerky crack propagation (Aki et al., 1977), flow-induced oscillations
69 of volcanic conduits (Julian, 1994; 2000; Hellweg, 2000), and the resonance of fluid filled
70 cracks and conduits (Chouet, 1987; Benoit and McNutt, 1997; Garces, 1997; Garces and
71 McNutt, 1997). Earlier models of tremor, based on the free oscillations of magma chambers,
72 were able to reproduce peaked harmonic spectra but often relied on unrealistic dimensions of
3 73 the resonating volumes. Most recently, the study of tremor has received increasing attention
74 by volcano-seismologists because of its potential as a monitoring and forecasting tool for
75 unrest at volcanoes, and more refined models have been proposed. Hellweg (2000) suggested
76 that the presence of numerous overtones in harmonic spectra, and their exact relationship to a
77 fundamental frequency, is the result of non-linear flow conditions in pipe-like conduits;
78 turbulence in conduit flows with high Reynolds numbers, may generate periodic pressure
79 disturbances and produce regularly peaked spectra characteristic of harmonic tremor. Julian
80 (1994; 2000) proposed that tremor results from the oscillations of slot-like channels with
81 movable elastic (damped) walls, induced by the flow of a viscous incompressible fluid. This
82 model is described by a 3rd order system of non-linear differential equations whose solutions
83 are controlled by the fluid flow pressure; increasing values of this parameter will account for
84 steady flow without oscillations, short-lasting oscillations, sustained oscillations, period-
85 doubling cascades, and chaotic oscillations controlled by non-linear attractors. Gordeev (1993)
86 and Schlindwein et al. (1995), showed that peaked harmonic spectra can be reproduced by the
87 convolution of a series of equi-spaced spikes, i.e., a Dirac comb funcion with a source
88 wavelet. The convolution of an arbitrary function with a Dirac comb yields a series of replicas
89 of the original function with period ΔT, equal to the spacing of the teeth of the comb. The
90 theoretical spectrum of the signal consists of a fundamental frequency (defined by 1/ΔT) along
91 with a number of integer overtones, and is modulated by the spectrum of the source function.
92 The generation of volcanic tremor by resonating fluid filled fractures has been extensively
93 treated in the literature, as well. Aki et al. (1977) proposed that tremor is generated by the
94 pressure driven motion of fluids through a chain of cracks connected by narrow channels; the
95 characteristics of tremor are controlled by parameters such as the length of the cracks and the
4 96 fluid pressure. Chouet (1987) suggested that the resonance modes of a fluid-filled rectangular
97 crack, triggered by a localized pressure disturbance (acting on the crack walls), correspond to
98 the peaks observed in the spectra of long period earthquakes and volcanic tremor. The
99 predicted wavefield depends on parameters that include the crack dimensions, the position and
100 intensity of the pressure disturbance and the impedance contrast between the fluid and the
101 surrounding rocks. McNutt (1986) and Benoit and McNutt (1997) modeled the source of
102 harmonic tremor as a 1D vertical resonating conduit filled with gas-charged magma, each of
103 the observed spectral peaks representing an eigen-mode of vibration of the oscillator. Mori et
104 al. (1989) explained the observations of harmonic tremor at Langila volcano, Papua New
105 Guinea in terms of a gas filled resonating volume. The resonance modes of 1D oscillators are
106 controlled by the length of the resonator, the acoustic properties of the fluid, and a set of
107 specified boundary conditions.
108 In this paper we will present observations and spectral analyses of volcanic tremor recorded
109 during the January-February 2005 eruption of Mt. Veniaminof, Alaska. This eruption, has
110 been well documented with seismic, satellite and web camera observations, and provides the
111 best characterization to date of the eruption style of Mt. Veniaminof volcano.
112 Background
113 Mt. Veniaminof is a large stratovolcano on the Alaska Peninsula (56.2° N, 159.4° W,
114 elevation: 2507 m), 35 km wide at the base, truncated by a steep-walled caldera 8x11 km in
115 diameter that formed about 3700 years B.P. The caldera is filled by an ice field that ranges in
116 elevation from 1750 to 2000 m; an intra-caldera cone is located in the western part of the
117 caldera with a small summit crater. The cone has an elevation of 2156 m, about 330 m above
118 the surrounding ice field (Miller et al., 1998) and is the site of all historical eruptions (Simkin
5 119 and Siebert, 1994). A belt of Quaternary cinder cones (Detterman et al., 1981a,b) extends in
120 the SE-NW direction from the main volcanic edifice to the Bering Sea coast.
121 The recent activity includes moderate Strombolian eruptions in 1983, 1993 and 1994 from the
122 intra-caldera cone accompanied by lava flows and, mild explosive activity in 2002 and 2004.
123 Ash and steam explosions are a characteristic feature of the eruptive activity at Mt.
124 Veniaminof.
125 In the summer of 2001, the Alaska Volcano Obsevatory (AVO) installed a network of 8
126 vertical component, short-period (1s) seismometers (Mark Products L4-C) at Mt. Veniaminof
127 (Figure 1); continuous analog data are transmitted via radio telemetry and phone lines to AVO
128 offices where they are digitally recorded at a sample rate of 100 Hz (Thompson et al., 2002).
129 Archiving of data began in February 2002 and, since then, different types of signals have been
130 recorded including LF earthquakes, volcano tectonic (VT) earthquakes, and volcanic tremor.
131 While low rates of VT activity (< 1 event/day) constitutes the seismic background at Mt
132 Veniaminof during quiescent periods, the occurrence of a large number of LF events and
133 volcanic tremor characterized the eruptions in 2002 and 2004 (Sanchez, 2005; Sanchez et al.
134 2005). During the eruptions in January-February 2005, up to 20-30 LF events/hour were
135 detected. In addition to the seismic network, a web camera located in the town of Perryville,
136 about 35 km from the volcano, provides a photographic record of the surface activity
137 (photographs taken every 5 minutes). Images from the Advanced Very High Resolution
138 Radiometer (AVHRR) sensor on the NOAA-12 and NOAA-14 satellites are also used to
139 detect volcanic eruption clouds and thermal anomalies.
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144 Eruption chronology
145 After a relatively long period of quiescence of about 6 years, Mt. Veniaminof first showed
146 signs of unrest in 2002-2004. During September 2002, pulses of volcanic tremor were detected
147 and bursts of steam (possibly containing small amounts of ash) were observed rising above the
148 intra-caldera cone; intermittent steaming activity accompanied by variable seismicity
149 continued until April 2003 (Sanchez, 2005). In the period February-October 2004, AVO
150 recorded several episodes of low amplitude volcanic tremor and small low-frequency volcanic
151 earthquakes accompanying ash and steam emissions from the intra-caldera cone. Short
152 duration episodes of harmonic tremor were recorded on October 12 and November 2, 2004.
153 During November and December 2004 no significant surface activity was observed and the
154 seismicity remained at background levels. On January 1, 2005, AVO started to record weak
155 tremor that lasted for about two days. On January 4, the signal character changed to that of
156 numerous low-frequency earthquakes (about 1-2 per minute), and the level-of-concern color
157 code (Table 1) was upgraded from green to yellow. Ash outbursts were observed rising to
158 heights of few hundreds meters above the intra-caldera cone starting January 3. The following
159 week was characterized by elevated levels of seismicity, and the episodic surface activity
160 evolved into more continuous emissions forming ash plumes rising up to about 1300 m above
161 the vent (Figure 2). Starting January 7, satellite images showed a persistent thermal anomaly
162 in the vicinity of the active cone; both seismic and surface activity exceeded the levels
163 observed during 2002-2004. On January 10, the level-of-concern color code was upgraded to
164 orange. Amplitude and occurrence of volcanic tremor and LF events increased over the month
165 of January, and the activity at the surface consisted of emissions forming ash clouds and ash
166 fall reaching outside the caldera boundary. On February 4, incandescence was clearly visible
7 167 on the night-time web camera images, indicating strombolian activity with ejection of hot
168 blocks from the intra-caldera cone. This activity was accompanied by an increase in the
169 amplitude and occurrence rate of LF earthquakes and continuous low amplitude volcanic
170 tremor. From February 5 onward, the seismicity was dominated by volcanic tremor; the
171 activity ranged from tremor bursts with durations of few tens of seconds to continuous tremor
172 lasting hours. Harmonic tremor was mostly observed between February 17 and February 21.
173 The pattern of seismicity was consistent with mild explosive activity from the intra-caldera
174 cone, although Mt. Veniaminof was not visible in the web camera images for almost the entire
175 month of February due to cloudy weather. On February 21, harmonic tremor was sporadically
176 observed along with non-harmonic tremor and LF events; earthquake activity abruptly ended
177 late in the evening the same day, marking the end of the eruption. The level-of-concern color
178 code was downgraded to yellow on February 25 and to green on March 4, 2005. Table 1
179 summarizes the observations during the course of the eruption.
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181 Data analyses and results
182 The seismic activity recorded at Mt. Veniaminof during January-February 2005 included LF
183 earthquakes, non-harmonic and harmonic tremor. The occurrence of many LF earthquakes
184 overlying a low level non-harmonic tremor signal, characterized the eruption during January
185 and at the beginning of February 2005. Moderate to strong pulses of tremor (durations of few
186 tens of seconds to minutes) superimposed on a low-amplitude continuous signal, as well as
187 strong harmonic tremor lasting up to hours, were mostly observed during the month of
188 February. We performed spectral analyses of about two months of tremor data in order to
189 characterize the properties of the signal over the duration of the eruption and to constrain its
8 190 source. All data were band-pass filtered between 0.4 and 10.0 Hz in order to reduce the effects
191 of ocean microseisms and incoherent higher frequency noise.
192 Seismograms and amplitude spectra of non-harmonic tremor recorded at station VNSS (5.4
193 km from the vent) are shown in Figure 3; energy is confined to the band 0.5-5.0 Hz and
194 spectra are characterized by narrow and irregularly distributed peaks. It is worth to note that
195 the seismic sensors installed at Mt. Veniaminof have a natural period of 1 second, and they
196 may not allow to recognize spectral peaks at very low frequencies. For each trace, the
197 amplitude spectrum was averaged by use of the direct segment method (Bath, 1974), stacking
198 the amplitude spectra of six adjacent windows of data (10 seconds duration each). The use of
199 this technique enhances the part of the spectra common to the entire waveform while reducing
200 the contributions of incoherent noise. The stacked spectra (fig. 3a-c), exhibit an overall broad
201 triangular shape peaked between 1.0 and 2.0 Hz. We compared the frequency content of non-
202 harmonic tremor with that of low-frequency earthquakes recorded during periods of ash
203 explosions activity at the volcano. Figure 4 is an example of velocity seismogram and
204 amplitude spectrum of a low frequency event recorded at station VNNS; energy appears
205 concentrated in the 1.0-4.0 Hz band with dominant peaks between 1.0 and 2.0 Hz.
206 Figure 5 shows six consecutive hours of harmonic tremor recorded at station VNSS on
207 February 18, 2005. Beneath each waveform the corresponding spectrogram is shown. The
208 spectrograms were generated moving a 1024 sample (10.24 s at 100 Hz sampling rate) sliding
209 window along the waveform, and calculating its periodogram for 512-sample overlapping
210 positions of the window. Energy is spread over the interval 0.5-5.0 Hz; the fundamental
211 frequency, in the band 0.5-2.0 Hz, is accompanied by one to three integer harmonics. Figure 6
212 shows velocity seismograms and the amplitude spectra of one-minute samples of harmonic
9 213 tremor recorded at station VNSS; the spectra are characterized by sets of integer harmonics
214 which are multiples of the 1.1 Hz fundamental frequency.
215 More complicated spectra, containing up to five harmonics, were occasionally observed. For
216 instance, figure 7 shows 1 minute of harmonic tremor recorded at station VNSS and its
217 amplitude spectrum; a set of 5 regularly spaced peaks that are integer multiples of 0.6 Hz, is
218 clearly visible.
219 The frequency content of harmonic tremor was checked for consistency across the seismic
220 network. In Figure 8 we show the velocity seismograms and power spectral density of 1
221 minute data samples recorded at four different sites; the fundamental mode and the first
222 harmonic appear clearly in the spectra at all stations suggesting that it may reflect some source
223 characteristic. On the other hand, as frequency increases, spectral peaks become less visible at
224 the stations located at greater distances from the active vent; this is, probably, the result of
225 increased attenuation that affects higher frequency waves as they travel away from the source.
226 Indeed, seismic data at Mt. Veniaminof are recorded by instruments located at distances
227 between 10.8 and 20.4 km from the active cone, the only exception being station VNSS that is
228 located 5.4 km from the vent.
229 An interesting feature of harmonic tremor at Mt. Veniaminof is that frequencies are observed
230 to change over time; spectral lines systematically move towards lower (converging lines) or
231 higher (diverging lines) values while the harmonic structure of the signal is maintained. This
232 time dependent feature of tremor, known as spectral gliding, has been observed at a number of
233 other world volcanoes including Mt. Semeru, Indonesia (Schlindwein et al, 1995), Arenal,
234 Costa Rica (Benoit and McNutt, 1997; Hagerty et al,, 2000; Julian, 2000), Lascar, Chile
235 (Hellweg, 2000), and Montserrat, West Indies (Neuberg et al., 2000). Gliding episodes at Mt.
236 Veniaminof include frequency changes between 30% and 75 % with respect to the initial value
10 237 over periods of 2-3 minutes to 10-15 minutes. Figure 9 shows about 50 minutes of harmonic
238 tremor with variable harmonic frequencies; within a time interval of about 25 minutes, three
239 gliding episodes (t1, t2, t3) can be distinguished. During period t1, the fundamental frequency
240 decreases from 1.8 Hz to 0.9 Hz in about 300 s; period t2 shows an increase from 1.0 Hz to 1.6
241 Hz in 800 s; during t3 there is a decline from 1.7 Hz to 1.1 Hz in 340 s.
242
243 Discussion
244 We have reviewed data for about two months of seismic activity at Mt. Veniaminof and
245 performed spectral analyses of volcanic tremor and LF earthquakes occurring during the
246 January-February 2005 eruption. Two types of volcanic tremor were identified: broadband
247 non-harmonic and harmonic. The spectral analyses of non-harmonic tremor and low-
248 frequency earthquakes recorded during periods of mild Strombolian activity demonstrate a
249 notable similarity between the two types of seismicity, suggesting that they may share similar
250 sources. In analogy to the mechanism proposed by Ripepe and Gordeev, (1999), we suggest
251 that non-harmonic tremor is produced by coalescence (free or forced) of gas bubbles from a
252 layer of smaller bubbles, through the surface of a magma column. LF events, accompanying
253 the explosive emission of ash at the surface, are likely related to the explosive disruption of
254 individual large gas pockets within the magma in the volcanic conduit.
255 Harmonic tremor mostly occurred during the last stages of the eruption; durations of minutes
256 to hours and spectra consisting of one up to five harmonics in the band 0.5-5.0 Hz were
257 typical. Harmonic frequencies were observed to change over time, by up to 75% with respect
258 to the initial value in 5 minutes. While the irregularly peaked spectra of non-harmonic tremor
259 and low-frequency events are attributed to mechanisms that involve gas buble coalescence and
260 gas explosions, the simple and regular spectral structure of harmonic tremor seem to require a
11 261 different interpretation in terms of its source mechanisms. In the introduction section of the
262 manuscript we discussed various theoretical models potentially able to account for the
263 generation and features of volcanic tremor. They included models based on the flow-induced
264 oscillation of elastic volcanic conduits and on the resonance of gas or fluid filled cavities.
265 Models based on flow-induced oscillations generally require long and narrow conduits and
266 high flow velocities. Hellweg (2000) showed that the generation of harmonic tremor by
267 turbulence in conduit flow, would require conduit length to diameter ratios greater than 50,
268 and flow velocities on the order of 100 m/s for andesitic magma; such magma flow velocities
269 over time periods as long as hours or days, are unlikely at a volcano that is not undergoing a
270 continuous and vigorous explosive eruption. Julian (1994, 2000) proposed a model of
271 incompressible Newtonian fluid flow through a slot-like fissure with elastic damped walls.
272 This mechanism involves flow pressures on the order of 10-15 Mpa and flow velocities of 50-
273 100 m/s, to produce frequencies on the higher end of tremor observed at Mt. Veniaminof
274 (about 5 Hz). The flow of an incompressible fluid with high flow velocities, seems not
275 appropriate to our case, although it may be suitable when more continuous and vigorous
276 eruptive activity is observed.
277 Models that involve the resonance of fluid filled cavities appear to be more suitable candidates
278 to explain the characteristics of the harmonic tremor observed at Mt. Veniaminof. A pressure
279 disturbance applied at the walls of a fluid filled fracture (Chouet, 1988) of length L and width
280 W, for example, can generate standing wave vibrations in the crack; two sets of resonance
281 modes, longitudinal and lateral (2L/n and 2W/n (n=1,2,3,…)) would be observed. We don’t
282 have evidence of 2D resonance modes, although these are likely to appear at frequencies
283 higher than those observed in the spectra at Mt. Veniaminof, that may have been attenuated
284 travelling from the source region to the distant receivers.
12 285 We infer that the features of harmonic tremor at Mt. Veniaminof are explained by a 1D
286 resonant source. In 1D resonating conduits, acoustic wavelengths excited by pressure
287 disturbances propagate along a gas or fluid column, and upon reflection in correspondence
288 with specific boundaries, interfere with the initial wave. For certain characteristic frequencies
289 of the traveling waves, resonance can be established and energy is radiated into the ground in
290 the form of seismic waves through coupling with the surrounding rocks. Specific boundary
291 conditions control the reflection in correspondence of the terminations of the resonant
292 structure, and its modes of vibration. A pipe-like conduit that is an open-open or closed-closed
293 system (matched boundary conditions) has nλ /2 (λ =wavelength, n=1,2,3,…) waves as
294 longitudinal resonance modes whereas a system with one open and one closed end (unmatched
295 boundary conditions) has (2n −1)λ / 4 (λ =wavelength, n=1,2,3,…) waves. In terms of
296 frequency, pipes with matched boundary conditions are characterized by spectra with evenly
297 spaced peaks consisting of the fundamental mode, f0 , and a set of integer harmonics which
298 are multiples of f0 ( f01,fff== 2 02 , 3f0, ... , fnfnNn ==0 ; 1,.., ); spectra that have equally
299 spaced peaks and contain only odd harmonics
300 ( f01,ffff== 3 02 , 5 0 , ... , fn =−= (2 nfn 1)0 ; 1,.., N) characterize resonant systems with
301 unmatched boundary conditions. A practical realization of this model is a conduit filled with
302 gas or bubbly magma bounded at the bottom by more viscous and incompressible magma. The
303 interface between the two phases is likely to correspond to the nucleation level within the
304 conduit. Because of the strong impedance contrast between the source fluid and the underlying
305 non vesciculated magma, this termination acts like a closed termination. The upper end of the
306 conduit can be either open to the atmosphere and act as an open termination, or obstructed by
307 a relatively viscous plug at the vent acting as a closed boundary. However, as noted by Garces
13 308 and McNutt (1997), the observation that the conduit is plugged at the vent does not necessary
309 imply that it is an acoustically closed boundary and, similarly, an open vent does not necessary
310 behave as an open termination. In fact, a conduit that is plugged may enlarge at the vent,
311 causing this boundary to act as an open termination; on the other end an open conduit may act
312 as a closed boundary if it’s constricted at the vent.
313 As opposed to other volcanoes, we do not have evidence of significant emissions of fluid lava
314 at the surface neither continuous nor episodic, and the strength of tremor, measured through
315 reduced displacement, is small. Benoit and McNutt (1997), suggested that reduced
316 displacement of the order of 20 cm2 is typical for sources processes that involve gas charged
317 magma, while lower values, on the order of 5 cm2 or less, have been measured for sources
318 involving pure gas or hydrothermal fluids. Daily averages of reduced displacement at station
319 VNSS (the closest to the active vent) rarely exceeded 1.5 cm2 during the eruptive period.
320 On the basis of the available observation, we believe that the resonance of a volume filled with
321 gas bounded at its bottom by column of magma, may account for the features of harmonic
322 tremor at Mt. Veniaminof. Degassing from the underlying magma may set up standing waves
323 in the gas chamber, hence producing harmonic spectra. The spacing of spectral peaks observed
324 in Figure 6 suggests that the system has matched boundary conditions. Schlindwein et al.
325 (1995), also noted that the greater homogeneity of gases with respect to magma, allows the
326 development of long lasting oscillations without disturbing the harmonic nature of the signal
327 even when frequency changes may occur.
328 The frequency of the signal can be measured from its spectrum; by assuming specific acoustic
329 properties of the source mixture, the length of the resonator, L, can be estimated using the
330 following equation (Hagerty et al., 2000):
14 v L = 331 2 f0 (1.1)
332 where v is the acoustic velocity of the gas/fluid at the source and f0 , is the fundamental
333 frequency measured in Hz. Mori et al. (1989) suggested that an upper limit for the acoustic
334 velocity of gas in volcanic conduits at shallow depths is about 300 m/s. Schlindwein et al.
335 (1995) have used a wave velocity in hot air of 500 m/s from Bergmann and Schaffer (1990).
336 The sound speed of pure gas mixtures can be calculated in the most simple way through the
337 formula:
338 cRTgas =⋅⋅()γ gas /M (1.2)
339 where γ is the is the adiabatic constant for the gas, T is the absolute temperature, Rgas is the
340 universal gas constant, and M is the molecular mass of the gas species. For gas compounds of
341 H2O, CO2, and SO2 , within a reasonable range of temperatures for volcanic systems (300-800
2 342 K), this formula produces estimates of sound velocities on the order of 10 meters per second.
343 The topic of the sound speed of fluids and gas mixtures, however, is complex; while single-
344 phase mixtures are relatively easy to treat, the sound velocity of multi-phase compounds, as
345 found in actual volcanic environments, is dramatically different from the that of either pure
346 component (De Angelis, 2006). In liquid-gas mixtures, that have the density of liquids but the
347 compressibility of gases, even very small variations of the volume fraction of gas can greatly
348 reduce the sound speed from a few hundred to a few meters per second (Kieffer, 1977).
349 Kumagai and Chouet (2000), have studied the acoustic properties of ash-gas mixtures; they
350 found that sound velocity for SO2–ash mixtures varied from 100 to 500 m/s depending on the
351 temperature, the size of solid particles, the pressure and the composition of the mixture. For
352 the purpose of a first order estimate of the linear dimension of the resonant conduit, we
15 353 assume a velocity of 300 m/s. Using this value, and considering the 1.1 Hz fundamental mode
354 in the spectra of Figure 6, a trivial mathematical exercise would give a conduit length of about
355 136 m according to equation (1.1). Although, we can’t constrain the sound velocity of the gas
356 mixture at the source with such a degree of accuracy, based on values widely used in literature
357 (300-500 m/s), the linear dimension of the resonant conduit can be constrained to be on the
358 order of 102 m. While, we favour a gas phase at the source, it still is not possible to discard
359 entirely the hypothesis that the source fluid is bubbly magma. If this was the case, the wave
360 velocity of volcanic fluids may vary from 2500 m/s (Murase and McBirney, 1973) to as low as
361 300 m/s (Aki et al., 1978) according to different flow conditions and magma properties (for
362 example, the more rich in gas the magma, the lower the wave velocity).
363 Spectra like those of figure 7 showing 5 well defined peaks, were occasionally observed. One
364 possible interpretation is a resonant conduit with matched boundary conditions; in this case, a
365 velocity of 300 m/s would require a 250 m-long resonating conduit to account for the observed
366 0.6 Hz standing wave vibration. Another option may be that the conduit at Mt. Veniaminof
367 generates resonant modes corresponding to both matched and unmatched conditions. Similar
368 observations have been already reported at Mt. Spurr by Garces and McNutt (1997). For short
369 periods of time, one of the conduit terminations may act as a partially open and partially
370 closed boundary (i.e. partial reflection and partial transmission); as an example, the vent may
371 be partly obstructed and partly open to the atmosphere. This model is attractive because 0.6
372 Hz is roughly half of the most commonly observed 1.1 Hz fundamental peak. Thus the main
373 structure may remain intact, and only the boundary conditions need to change to produce the
374 two sets of peaks.
375 The obvious gliding of spectral lines in figure 9 can be attributed to either variations of the
376 acoustic properties at the source or changes in the effective length of the resonator. Changes of
16 377 the acoustic velocity may results from a handful of factors such as mixing of different volatile
378 species (e.g. H2O, CO2, SO2) in variable proportions, or development of multi-phase mixtures
379 due to the presence of water and steam at shallow depths in the volcanic conduit. Variations in
380 the effective length of the resonator, instead, involve rising or falling of the bubbly magma
381 within the volcanic conduit. For instance, the gliding shown in figure 9 would require the
382 magma below the gas chamber to rise (upward gliding) or fall (downward gliding) at
383 velocities of about 0.2 m/s (t1), 0.08 m/s (t2) and 0.15 m/s (t3). These velocities are not
384 unreasonable for the flow of a bubbly magma within a volcano. Closer inspection of the
385 spectrogram in figure 9, reveals the presence of bumps and wiggles of the fundamental
386 spectral line, superimposed on the overall upward gliding. Frequency changes by about 0.1-0.2
387 Hz, roughly corresponding to 10-20 % of the most common fundamental frequency, over time
388 periods of few seconds. We think that this short term oscillatory behavior may result from
389 transient instabilities such as changes in density and composition of the volatile phases;
390 alternatively, it may be attributed to small and relatively rapid variations of the magma column
391 height around a position of unstable equilibrium caused, for instance, by quasi-periodic
392 collapse of gas bubbles (this would result in short term variations of the the effective length of
393 the resonator). Similar observations have been previously reported at Arenal volcano, Costa
394 Rica (Hagerty et al., 2000).
395 It is important to point out that resonance is not a “source mechanism” itself. Resonant
396 conduits define “source regions” that, through the propagation of standing and interface waves
397 excited by a “source trigger”, can generate harmonic spectra. The source trigger is a
398 mechanism that releases the elastic energy necessary to kickstart resonance. In the specific
399 case of Mt. Veniaminof we infer that the trigger mechanism is represented by degassing pulses
400 at the lower boundary of the resonator. The period of the signal is then stabilized by a
17 401 feedback mechanism related to the resonance itself. The long duration of the harmonic signals
402 suggests that the trigger should be continuous; this may be obtained through pressure
403 oscillations in the resonator that, once initiated by a degassing pulse from the underlying
404 magma, in turn control further degassing (Schlindwein et al., 1995).
405 We suggest that the seismic activity during November-December 2005 and early January 2006
406 at Mt. Veniaminof was related to the re-activation of the system and ascent of new magma
407 reaching up the surface. Non-harmonic volcanic tremor and low-frequency earthquake activity
408 in early January 2006, reflected degassing through the surface of a magma column that had
409 reached shallow depths. Larger ash explosions and Strombolian activity during the second half
410 of January and early February 2006 were accompanyied by swarms of LF events with larger
411 amplitudes than previously. We have suggested that these LF earthquake are related to the
412 explosive fragmentation of magma within the conduit. Between early and mid-February,2006
413 the column of magma may have slowly dropped at a lower level in the conduit, leaving a
414 “plug” of viscous material at the vent. Under these conditions harmonic tremor, may have
415 been generated around mid-February, 2006. On February 21, 2006 the earthquake activity at
416 Mt. Veniaminof abruptly ended. Since then only minor and episodic ash explosions have been
417 observed. It can be considered that the volcano has returned to its normal state of persistent
418 low-level seismic and surface activity. The cartoon in Figure 10, illustrates the evolution of
419 activity at Mt. Veniaminof volcano between late 2004 and March 2005.
420 Even though a large variety of models exist accounting for the occurrence and features of LF
421 seismic activity and volcanic tremor, their source mechanisms and propagation are still not
422 fully understood, and unanimous consensus has not been reached on these topics. Theoretical
423 models often rely on a number of assumptions. We are aware, for example, of the limitations
424 of the simple model of a fluid filled resonant cavity and that it can not ultimately explain the
18 425 manifold complexities of harmonic tremor. The intent of this manuscript, is to present
426 observations of seismic activity from an interesting case study and to point out the role that a
427 resonating source may have in the generation of harmonic tremor at Mt Veniaminof. Many
428 aspects need to be refined in the formulation of tremor models. Particularly relevant are the
429 determination of the stress conditions at fluid-solid boundaries within volcanic systems,
430 leading to a comprehesive understanding of the conditions under which acoustic energy and
431 pressure disturbances within conduits are transformed into ground vibration.
432
433 Concluding remarks
434 We performed spectral analyses of volcanic tremor and LF earthquake activity recorded
435 during the January-February 2005 eruption of Mt. Veniaminof, Alaska. Tremor had durations
436 of minutes to hours; we observed energy in the band 0.5-5.0 Hz, and spectra characterized by
437 sharp peaks irregularly or regularly spaced. Sustained non-harmonic tremor was interpreted as
438 the result of coalescence of large gas bubbles from a layer of smaller bubbles through the
439 surface of a magma column. We have suggested that the resonance of a gas-filled resonating
440 cavity may account for the occurrence and spectral features of harmonic tremor at Mt.
441 Veniaminof. Within this framework, the variable acoustic properties at the source or changes
442 in the effective length of the resonator, are possible causes for the observed time varying
443 characteristics of tremor such as spectral gliding.
444 Although the quality of present data at Mt Veniaminof is intrinsically limited by
445 instrumentation, there are several avenues for future research. The installation of new
446 instruments, such as broadband seismic sensors closer to the acive vent, may help to further
447 understand the source mechanisms of seismic signals and their propagation. Neverthless, the
19 448 January-February 2005 eruption of Mt. Veniaminof remains the best studied eruption of the
449 volcano to date, and provides a benchmark for comparing future and past eruptive activity.
450
451 Acknowledgements
452 The authors are grateful to the staff of the Alaska Volcano Observatory for their efforts
453 during the course of the eruption. We also thank two anonymous reviewers for insightful
454 comments that largely improved the manuscript. This work was supported by the Alaska
455 Volcano Observatory and the U.S.Geological Survey, as a part of their Volcano Hazards
456 Program, and by additional funds from the State of Alaska.
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24 Date Color Code (*) Seismic observations Visual observations LF earthquakes; 4 Jan Upgraded to Yellow Ash explosions Non-harmonic tremor LF earthquakes; Ash plumes (up to 7 Jan Yellow Non-harmonic tremor 3500 ft a.s.l.) 8-10 Jan Upgraded to Orange Seismic activity increases; Ash plumes (Jan 10) LF earthquakes 10 Jan -3 Feb Orange Continuous non harmonic Ash plumes tremor; Tremor bursts; LF earthquakes 4 Feb Orange Non harmonic tremor; LF Incandescence from the earthquakes intra-caldera cone (Feb 04, 05:32 UT) 5-16 Feb Orange Continuous non harmonic Persistent ash tremor; Tremor bursts emissions 17 Feb Orange Tremor bursts; Harmonic Cloudy weather tremor prevent the view of the intra-caldera cone 18 Feb Orange Continuous harmonic tremor Cloudy
19 Feb Orange Continuous tremor ends at Cloudy 02:35 AST 20 Feb Orange No seismic activity until Cloudy 11:00 AST when harmonic tremor picks up again 21 Feb Orange LF earthquakes; Tremor Cloudy bursts; Harmonic tremor. Tremor abruptly ends at 14:00 AST 25 Feb Downgraded to No relevant seismic activity Clear weather Yellow No activity observed 4 Mar Downgraded to Green No relevant seismic activity Clear weather No activity observed
(*) Level-of-concern color code definitions. To concisely describe the level of concern about possible eruptive activity at volcanoes, the Alaska Volcano Observatory has developed a color-code classification system (http://www.avo.alaska.edu/activity.php). Green: No eruption anticipated. Volcano is in a quiet, "dormant" state. Yellow: An eruption is possible in the next few weeks and may occurr with little or no additional warning. Small earthquakes detected locally and (or) increased levels of volcanic gas emissions. Orange: Explosive eruption is possible within a few days and may occurr with little or no warning. Ash plumes not expected to 25000 feet a.s.l. Increased numbers of local earthquakes. Extrusion of a lava dome or lava flows (non explosive eruptions) may be occurring. Red: Major explosive eruption expected within 24 hours. Large ash plume(s) expected to reach at least 25000 feet a.s.l. Strong earthquake activity detected even at distant monitoring stations. Explosive eruption may be in progress.
567
568 Table 1 Summary of seismic and visual observations during the January-February 2005 eruption of
569 Mt. Veniaminof, Alaska.
25 570
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578
579
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582
583
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591
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593
26 594 Figure 1. Seismograph stations (white squares) at Mt. Veniaminof on a shaded relief image (courtesy
595 NASA/JPL). The active intra-caldera cone is marked by a black triangle. In the bottom right corner,
596 an index map of Alaska shows the location of Mt. Veniaminof on the Alaska Peninsula.
597 Figure 2 Images from the January-February 2005 eruption at Mt. Veniaminof, Alaska. (a) Mt.
598 Veniaminof intra-caldera cone looking N-NW. Ash cloud drifting to NE. Photo taken at ~13,000 ft
599 from a Piper Navajo aircraft (Security Aviation) during an observational overflight (Photo taken on
600 January 11, 2005. Image Creator: K. L. Wallace, Image courtesy U.S. Geological Survey); (b) Mt.
601 Veniaminof intracaldera cinder cone. Ash plume drifting to NE. Photo source same as (a); (c),(d) and
602 (e) Ash plumes at Mt. Veniaminof. Images taken on January 09, 2005 from the AVO webcam located
603 in the town of Perryville (35 km SE of the active vent); c) 16:36:44 UT, d) 16:41:44 UT, e) 16:46:43
604 UT.
605 Figure 3. Seismograms (left) and amplitude spectra (right) of non-harmonic volcanic tremor recorded
606 during January 2005 at station VNSS. Average spectra (a), (b) and (c), were calculated by stacking
607 the Fourier spectra of six 10-second adjacent windows of data.
608 Figure 4. Seismogram (top) and spectrum (bottom) of a low-frequency event recorded on February
609 04, 2005, 06:06:15 UT, at station VNSS, during a period of Strombolian activity. Inset at lower right
610 is a stack of three 10-s windows of data.
611 Figure 5. Seismograms and velocity spectrograms of six hours of harmonic tremor, one hour per
612 panel, recorded at station VNSS on February 18, 2005. Spectrograms were generated moving a 1024
613 sample sliding window over the entire waveform and calculating the periodogram for 512 sample
614 overlapping positions of the window.
615
616
617
27 618 Figure 6. Seismograms (left) and amplitude spectra (right) of harmonic tremor recorded on February
619 18-19, 2005 at station VNSS. Average spectra (a), (b) and (c), were calculated by stacking the Fourier
620 spectra of six 10-second adjacent windows of data. Spectra are characterized by narrow and regularly
621 spaced peaks, which are integer multiples of 1.1 Hz.
622 Figure 7. Seismogram (top) and amplitude spectrum (bottom) of one minute of harmonic tremor
623 recorded at station VNSS on February 18, 2005. Five harmonics are clearly visible; inter-peak and
624 origin-to-first peak spacing is 0.6 Hz. Two additional peaks may be visible at 3.6 and 4.2 Hertz,
625 although their significance is lower.
626 Figure 8. One minute of harmonic tremor recorded on February 18, 2005 at four different stations
627 across the Mt. Veniaminof seismic network, and its power spectral density (PSD). PSD represents the
628 power content of a signal in an infinitesimal frequency band. Peaks at 1.1 and 2.2 Hz are well visible
629 at all stations; peaks at higher frequencies are attenuated at stations more distant from the active vent.
630 Figure 9. Seismogram and velocity spectrogram of harmonic tremor recorded at station VNSS on
631 February 18, 2005 showing gliding . t1, t2, t3 refer to time intervals discussed in the text.
632 Figure 10. Cartoon showing the evolution of activity at Mt. Veniaminof between late 2004 and
633 March 2005: a) variable conduit configuration between late 2004 and March 2005; b) free and c)
634 forced bubble coalescence; d) bubble bursting through the surface of the magma column; d) model of
635 resonant gas chamber. Representative waveforms associated with the different processes are shown
636 (non-harmonic tremor, low frequency event, harmonic tremor from the left to the right).
637
638
639
640
641
28 642
643
644
645
646 Figure 1. Seismograph stations (white squares) at Mt. Veniaminof on a shaded relief image
647 (courtesy NASA/JPL). The active intra-caldera cone is marked by a black triangle. In the
648 bottom right corner, an index map of Alaska shows the location of Mt. Veniaminof on the
649 Alaska Peninsula.
650 651 652 653 654 655 656 657 658
29 659 660 661 Figure 2 Images from the January-February 2005 eruption at Mt. Veniaminof, Alaska. (a) Mt.
662 Veniaminof intra-caldera cone looking N-NW. Ash cloud drifting to NE. Photo taken at
663 ~13,000 ft from a Piper Navajo aircraft (Security Aviation) during an observational overflight
664 (Photo taken on January 11, 2005. Image Creator: K. L. Wallace, Image courtesy U.S.
665 Geological Survey); (b) Mt. Veniaminof intracaldera cinder cone. Ash plume drifting to NE.
666 Photo source same as (a); (c),(d) and (e) Ash plumes at Mt. Veniaminof. Images taken on
667 January 09, 2005 from the AVO webcam located in the town of Perryville (35 km SE of the
668 active vent); c) 16:36:44 UT, d) 16:41:44 UT, e) 16:46:43 UT.
669 670 671 672 673 674 675 676
30 677 678 679 680 681 Figure 3. Seismograms (left) and amplitude spectra (right) of non-harmonic volcanic tremor
682 recorded during January 2005 at station VNSS. Average spectra (a), (b) and (c), were
683 calculated by stacking the Fourier spectra of six 10-second adjacent windows of data.
684 685 686 687 688 689 690 691 692 693 694 695 696 697
31 698 699 700 701
702 703 704 Figure 4. Seismogram (top) and spectrum (bottom) of a low-frequency event recorded on
705 February 04, 2005, 06:06:15 UT, at station VNSS, during a period of Strombolian activity.
706 Inset at lower right is a stack of three 10-s windows of data.
707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723
32 724
725 726 727 728 Figure 5. Seismograms and velocity spectrograms of six hours of harmonic tremor, one hour
729 per panel, recorded at station VNSS on February 18, 2005. Spectrograms were generated
730 moving a 1024 sample sliding window over the entire waveform and calculating the
731 periodogram for 512 sample overlapping positions of the window.
732
33 733
734
735
736
737
738 739 740 Figure 6. Seismograms (left) and amplitude spectra (right) of harmonic tremor recorded on
741 February 18-19, 2005 at station VNSS. Average spectra (a), (b) and (c), were calculated by
742 stacking the Fourier spectra of six 10-second adjacent windows of data. Spectra are
743 characterized by narrow and regularly spaced peaks, which are integer multiples of 1.1 Hz.
744 745 746 747 748 749 750 751
34 752 753 754 755 756 757 758 759 760 761 762 763 764
765 766 767 768 Figure 7. Seismogram (top) and amplitude spectrum (bottom) of one minute of harmonic
769 tremor recorded at station VNSS on February 18, 2005. Five harmonics are clearly visible;
770 inter-peak and origin-to-first peak spacing is 0.6 Hz. Two additional peaks may be visible at
771 3.6 and 4.2 Hertz, although their significance is lower.
772 773 774 775 776 777 778 779 780 781
35 782 783 784 785 786 787 788 789 790 791 792 793 794 795
796 797 798 Figure 8. One minute of harmonic tremor recorded on February 18, 2005 at four different
799 stations across the Mt. Veniaminof seismic network, and its power spectral density (PSD).
800 PSD represents the power content of a signal in an infinitesimal frequency band. Peaks at 1.1
801 and 2.2 Hz are well visible at all stations; peaks at higher frequencies are attenuated at stations
802 more distant from the active vent.
803 804 805 806 807
36 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825
826 827 828 Figure 9. Seismogram and velocity spectrogram of harmonic tremor recorded at station VNSS
829 on February 18, 2005 showing gliding . t1, t2, t3 refer to time intervals discussed in the text.
830 831 832 833 834 835 836 837 838 839 840
37 841 842 843 844
845 846 847 Figure 10. Cartoon showing activity at Mt. Veniaminof between late 2004 and March 2005
848 and : a) variable conduit configuration between late 2004 and March 2005; b) free and c)
849 forced bubble coalescence; d) bubble bursting through the surface of the magma column; d)
850 model of resonant gas chamber. Representative waveforms associated with the different
851 processes are shown (non-harmonic tremor, low frequency event, harmonic tremor from the
852 left to the right).
853 854 855 856
38