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Observations of Volcanic Tremor During the January-February 2005 Eruption of Mt

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Observations of Volcanic Tremor During the January-February 2005 Eruption of Mt 1 2 Observations of volcanic tremor during the January-February 2005 eruption of Mt. 3 Veniaminof, Alaska. 4 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. 8 9 10 11 Prepared for submittal to Bullettin of Volcanology 12 13 Contact author: Silvio De Angelis 14 e-mail:[email protected]; 15 phone +1-907-474-7234 16 17 18 19 20 21 22 23 24 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 47 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.
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