Seismometer Trigger mechanism: brittle failure at conduit walls QiQi R (reflection coefficient) T (transmission coefficient) Q r -1 Total amplitude decay is a combination of these contributions: ff ff ss ss Aki, K., Magma intrusion during the Mammoth Lakes earthquake. JGR, 89, pp Collier, L. & Neuberg, J., 2006, Incorporating seismic observations into 2D conduit flow modelling. J. Volcanol. Geotherm.,, 152, pp Collier, L., Neuberg, J., Lensky, N. & Lyakhovsky, V., 2006, Attenuation in gas-charged magma. J. Volcanol. Geotherm., 153, pp Jousset, P., Neuberg, J. & Jolly, A., 2004, Modelling low-frequency volcanic earthquakes in a viscoelastic medium with topography. J.GI.., 159, pp 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, pp Patrick Smith's Ph.D. is funded by NERC grant NER/S/A/2006/ The data collection and archiving by staff of the Montserrat Volcano Observatory is fully acknowledged. Using the seismic amplitude decay of low-frequency events to constrain magma properties. AGU Fall Meeting, San Francisco, December Session & Poster number: V51D-0782 P. J. Smith & J. Neuberg School of Earth and Environment, University of Leeds., UK. 1. Background i. Soufrière Hills Volcano, Montserrat. ii. Low-frequency seismicity. iii. Seismic trigger mechanism: brittle fracturing of the magma 5. Wavefield modelling Domain Boundary Solid medium (elastic) Fluid magma (viscoelastic) Variable Q Damped Zone Free surface Seismometers Source Signal: 1Hz Küpper wavelet (explosive source) ρ = 2600 kgm -3 α = 3000 ms -1 β = 1725 ms -1 Characteristics of low-frequency events Similar waveforms Repeatable source mechanism Tight clusters of source locations Swarms precede dome collapse Amplitude spectra of synthetic low-frequency signals for a 30m wide and 50m wide conduit. i.Observations and modelling ii. Implications. 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) 30m conduit 50 m 30m 50m conduit Comparison of a 30m and 50m wide conduit: illustrating the change in frequency content with widening conduit 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. Increased conduit width is an important observation and may mark a significant change in the volcano’s behaviour. 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. Results of numerical modelling verify change in frequency of resonance with width. 4. Conduit Widening i.Finite-difference model ii. Calculation of apparent Q iii. Data Analysis 6. Summary 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. 2-D O(Δt 2,Δx 4 ) scheme based on Jousset et al. (2004). Volcanic conduit modelled as a viscoelastic fluid-filled body embedded in homogenous elastic medium. To include anelastic ‘intrinsic’ attenuation, the rheology of the material is parameterized by an array of Standard Linear Solids (SLS). 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. filtered signals is used to determine a set of apparent Q values. Example low-frequency event from April Band-pass filtered traces with apparent Q values Event amplitude spectrum 3. Factors determining the seismic amplitude Apparent (coda) Intrinsic (anelastic) Radiative (parameter contrast) true damping amplitude decay (Aki, 1984) Conduit resonance: energy generated by a seismic source is trapped by the impedance contrast between fluid and solid and travels as interface waves. Swarm of low-frequency events merging into tremor before a dome collapse. Different types of volcanic seismicity. Low frequency events are characterized by their harmonic coda and spectral content. i. Source mechanism Brittle fracturing on ring-fault as seismic source: View of the lava dome of Soufrière Hills Volcano, Montserrat, from the MVO in April Photograph by P. Smith. Cylindrical shear fracturing at the edge of the conduit as seismic triggering mechanism The island of Montserrat and its location within the Lesser Antilles volcanic island chain. The amplitude decay of the Geometry and parameters chosen to produce monochromatic smoothly decaying synthetic signals ‘Peaked’ amplitude spectrum used to choose frequencies for band-pass filters 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. 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, volcano- tectonic earthquakes and low-frequency events. Link model for source mechanism to cycles of deformation and seismicity. ii. Components of amplitude loss Need to conduct more analysis of the apparent Q of data from Montserrat. Want to examine any azimuthal variation for single events, variation with distance from conduit and changes over time. Further develop magma flow meter idea. Need full moment tensor inversion to get seismic moment and hence determine the amount of slip per event. Effects of including bubble growth by diffusion and mass flux on the seismic attenuation. Collier et al. (2006) 2. Seismic attenuation in bubbly magma ii. Magma viscosities are derived from flow modelling: i. Q and magma properties Q a -1 =Q i -1 +Q r -1 Q a -1 Seismic attenuation is quantified through the quality factor Q, the inverse of the attenuation. 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. 7. Acknowledgements Gas diffusion No seismicity Pressure increasing 4 ττ No seismicity Magma slowing Gas diffusion 3 ττ Diffusion lags behind Gas loss (Neuberg et al, 2006 ) 1 2 Q a -1 = Q i -1 + Q r -1 M 0 = µAu Unfiltered data Time [cycles] log(Amplitude) Apparent Q value based on synthetic signal envelope Gradient of line = Q value from gradient = Linear Fit Data Amplitude Synthetic trace Time [number of cycles] A = area of fault rupture u = average slip μ = shear modulus or rigidity Gives seismic moment: Generation of interface waves at the conduit walls. ττ Seismicity Pressure decreasing Stress threshold: slip plug flow gas loss parabolic flow Collier & Neuberg, 2006; Neuberg et al., 2006 depth of brittle failure Collier & Neuberg, 2006; ii. Magma viscosities are derived from flow modelling: ηbηb ηmηm Melt viscosity Magma viscosity 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. Match/use as input 1.Determine 2-D distribution of intrinsic Q values in a volcanic conduit through magma flow modelling, including the effects of bubble growth by diffusion. 2.Transfer into finite-difference models of the seismic wavefield by fitting an array of Standard Linear Solids (SLS) to model the 2-D intrinsic Q distributions. 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. 5.By comparison of modelling results with data analysis link the Q values back to the magma properties and gain information about the system. Discussion and further work Want to determine amount of slip, u, from the seismic moment, so we then get Amount of slip × Event rate → Magma ascent rate But requires full moment tensor inversion of a non-couple source mechanism. Need understanding of amplitude losses. (P. Jousset) Most of the energy remains within the conduit Events are recorded by seismometer as surface waves Interface waves