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Using the seismic amplitude decay of low-frequency events to constrain magma properties. AGU Fall Meeting, San Francisco, December 2007. Session & Poster number: V51D-0782 P. J. Smith & J. Neuberg School of Earth and Environment, University of Leeds., UK. ([email protected]) The island of Montserrat and its location within the Lesser Antilles volcanic island chain. 1. Background i. Soufrière Hills Volcano, Montserrat. ii. Low-frequency seismicity. Characteristics of low-frequency events 3 • Repeatable source mechanism μ = shear modulus or rigidity moment: Magma ascent rate Apparent Q Apparent Q value based on synthetic signal envelope (explosive source) Damped Zone Domain Boundary To include anelastic ‘intrinsic’ attenuation, the rheology of the material is parameterized by an array of Standard Linear Solids (SLS). Apparent Q greater than intrinsic Q: log(Amplitude) 70 60 40 1 e Apparent Q less than intrinsic Q: Radiative energy loss dominates Resonance dominates 50 30 m 20 For a fixed parameter contrast 10 2 SLS in array 0 -23.4 0 10 20 30 40 50 60 70 80 90 100 Intrinsic Q Apparent Q vs. intrinsic Q for the synthetic signals, showing two regimes. Gradient of line =-0.10496 Q value from gradient = 31.5287 0 2 4 6 Time [cycles] 8 10 Synthetic trace 12 Produce synthetic seismograms from which an apparent Q is determined via the gradient of log(Amplitude) against time. We then see to what extent the apparent Q is determined by the intrinsic Q given to the model. Amplitude 1Hz Küpper wavelet Fluid magma (viscoelastic) Variable Q Linear Fit Data -23.6 Source Signal: Intrinsic Q vs. Apparent Q Intrinsic=Apparent 90 80 -24 Trigger mechanism: brittle failure at conduit walls -1 Qa = -1 Qi + Time [number of cycles] Geometry and parameters chosen to produce monochromatic smoothly decaying synthetic signals Preliminary apparent Q analysis of the waveforms of low-frequency events from Montserrat. The ‘peaked’ amplitude spectra are used to create a series of narrow band-pass filters for the data. -1 Qr parabolic flow τ Gas diffusion Collier & Neuberg, 2006; Neuberg et al., 2006 i. Observations and modelling Photographic evidence from an extruded spine suggests a widening of the conduit from 30m to 50m. Seismic observations: frequency shift of low-frequency events over extended period of time. Results of numerical modelling verify change in frequency of resonance with width. Comparison of a 30m and 50m wide conduit: illustrating the change in frequency content with widening conduit ii. Implications. Changes overall flow behaviour, particularly velocity profiles, mass flux and ascent rate and therefore also the velocity and shear stresses. This will impact on the occurrence and location of brittle fracturing, and also degassing processes. Suggests flow behaviour and seismicity may be controlled by shallow processes rather than the magma chamber. 6. Summary Example low-frequency event from April 1997. 1 2 The amplitude decay of the filtered 2 ‘Peaked’ 3 amplitude signals is 1 spectrum used used to to choose frequencies for determine a 4 band-pass filters set of apparent Q values. 10 Pa Increased conduit width is an important observation and may mark a significant change in the volcano’s behaviour. iii. Data Analysis 100 -22.6 ρ = 2600 kgm-3 α = 3000 ms-1 β = 1725 ms-1 Qa-1=Qi-1+Qr-1 Intrinsic Q vs. Apparent (coda) Q ii. Calculation of apparent Q Seismometers rf af Intrinsic Radiative (parameter But requires full moment tensor inversion Apparent (coda) (anelastic) contrast) of a non-couple source mechanism. Need understanding of amplitude losses. (Aki, 1984) true damping amplitude decay Match/use as input -23.2 Qi Want to determine amount of slip, u, from the seismic moment, so we then get Both the viscosities and Q are highly dependent on the gas-phase. Particularly the gas-volume fraction, bubble number density and also bubble size and shape. Q 4. Conduit Widening Amplitude spectra of synthetic low-frequency signals for a 30m wide and 50m wide conduit. Total amplitude decay is a combination of these contributions: Magma slowing Link model for source mechanism to cycles of deformation and seismicity. T (transmission coefficient) Seismometer rs R (reflection coefficient) a s M0 = µAu Gives seismic τ Diffusion lags behind Qa-1 Qr-1 Collier & Neuberg, 2006; τ ii. Components of amplitude loss u = average slip Amount of slip × Event rate → Unfiltered data τ A = area of fault rupture ii. Magma viscosities are ηb ηm derived from flow modelling: ii. Magma viscosities are derived from flow modelling: -23 Generation of interface waves at the conduit walls. Stress threshold: 7 depth of brittle failure No seismicity Gas loss τ Brittle fracturing on ring-fault as seismic source: Magma viscosity -22.8 τ 3. Factors determining the seismic amplitude i. Source mechanism slip Melt viscosity Solid medium (elastic) Pressure increasing Most of the energy remains within the conduit Conduit resonance: energy generated by a Different types of volcanic seismicity. Low seismic source is trapped by the impedance frequency events are characterized by their contrast between fluid and solid and travels harmonic coda and spectral content. as interface waves. plug flow gas loss 1 Pressure decreasing ii. Magma viscosities are derived from flow modelling: -23.8 Gas diffusion slip (P. Jousset) This intrinsic Q is highly dependent on the properties of the magma. Q is quantified using the phase-lag between stress and strain for a sinusoidal pressure wave – equivalent to using the material properties (viscosities). The method includes the effects of bubble growth by diffusion. Free surface Swarm of low-frequency events merging into tremor before a dome collapse. Cylindrical shear fracturing at the edge of the conduit as seismic triggering mechanism 2 Seismicity Seismic attenuation is quantified through the quality factor Q, the inverse of the attenuation. 2-D O(Δt2,Δx4) scheme based on Jousset et al. (2004). Volcanic conduit modelled as a viscoelastic fluid-filled body embedded in homogenous elastic medium. Events are recorded by seismometer as surface waves Interface waves i. Q and magma properties i. Finite-difference model No seismicity • Swarms precede dome collapse 2. Seismic attenuation in bubbly magma 5. Wavefield modelling 4 • Tight clusters of source locations The volcano has been well monitored throughout this period of activity and in particular several types of volcanic seismic signals have been observed including, rockfalls, volcanotectonic earthquakes and low-frequency events. Effects of including bubble growth by diffusion and mass flux on the seismic attenuation. Collier et al. (2006) (Neuberg et al, 2006) • Similar waveforms Soufrière Hills Volcano is an andesitic stratovolcano situated on the island of Montserrat at the northern end of the Lesser Antilles volcanic arc, formed by the subduction of Atlantic oceanic lithosphere beneath the Caribbean plate. The current phase of eruptive activity has been ongoing since 1995, beginning with phreatic activity, and has since been characterized by cycles of lava dome growth followed by subsequent dome collapses. View of the lava dome of Soufrière Hills Volcano, Montserrat, from the MVO in April 2007. Photograph by P. Smith. iii. Seismic trigger mechanism: brittle fracturing of the magma 3. Synthesise seismic wave propagation in such a conduit and determine apparent Q from amplitude decay of the signals produced. 4. Compare results with analysis of data from Montserrat. Event amplitude spectrum Band-pass filtered traces with apparent Q values 50 m ux 30m Magma velocity profiles for 30m and 50m wide conduits, derived using a 2-D finite-element model of three-phase magma flow in a conduit. (by M. Collombet) Discussion and further work 1. Determine 2-D distribution of intrinsic Q values in Need to conduct more analysis of the apparent Q a volcanic conduit through magma flow modelling, of data from Montserrat. Want to examine any including the effects of bubble growth by diffusion. azimuthal variation for single events, variation with distance from conduit and changes over time. 2. Transfer into finite-difference models of the seismic wavefield by fitting an array of Standard Further develop magma flow meter idea. Need full Linear Solids (SLS) to model the 2-D intrinsic Q moment tensor inversion to get seismic moment distributions. and hence determine the amount of slip per event. 3 4 50m conduit 30m conduit 5. By comparison of modelling results with data analysis link the Q values back to the magma properties and gain information about the system. 7. Acknowledgements Aki, K., 1984. Magma intrusion during the Mammoth Lakes earthquake. JGR, 89, pp7689-7696. Collier, L. & Neuberg, J., 2006, Incorporating seismic observations into 2D conduit flow modelling. J. Volcanol. Geotherm.,, 152, pp331-346 Collier, L., Neuberg, J., Lensky, N. & Lyakhovsky, V., 2006, Attenuation in gas-charged magma. J. Volcanol. Geotherm., 153, pp21-36. Jousset, P., Neuberg, J. & Jolly, A., 2004, Modelling low-frequency volcanic earthquakes in a viscoelastic medium with topography. J.GI.., 159, pp776-802. Neuberg, J., Tuffen, H., Collier, L., Green, D., Powell T. & Dingwell D., 2006, The trigger mechanism of low-frequency earthquakes on Montserrat. J. Volcanol. Geotherm., 153, pp37-50. Patrick Smith's Ph.D. is funded by NERC grant NER/S/A/2006/14150. The data collection and archiving by staff of the Montserrat Volcano Observatory is fully acknowledged.