EGU 2007

1. EGU 2007 Linking low-frequency events to conduit properties Patrick Smith and Jürgen Neuberg School of Earth and Environment, The University of Leeds.

2. EGU 2007 Outline of Presentation Background: low-frequency seismicity Conduit geometry and stiffness factor Comparison of a 30m and 50m wide conduit

3. Weak high frequency onset • Coda: • harmonic, slowly decaying • low frequencies (1-5 Hz) EGU 2007 Low frequency seismicity What are low-frequency earthquakes? Specific to volcanic environments →Are a result of interface waves originating at the boundary between solid rock and fluid magma

4. EGU 2007 Why are low frequency earthquakes important? • Have preceded several major eruptions in the past • Correlated with the deformation and tilt - implies a close relationship with pressurisation processes (Green & Neuberg, 2006) • Provide direct link between surface observations and internal magma processes

5. Model: Trigger Mechanism = Brittle Failure of Melt (Neuberg et al. 2006) viscosity x strain rate > shear strength of melt EGU 2007 How are low-frequency earthquakes generated?

6. Properties of the magma Conduit geometry Incorporate flow model data into wavefield models + EGU 2007 Combining magma flow modelling and seismicity Motivation for PhD Project Conduit Properties Seismic parameters Signal characteristics Magma properties (internal) seismic signals (surface)

7. Finite-Difference Method EGU 2007 • 2-D O(Δt2,Δx4) scheme based on work of Virieux (1986) and Levander (1988) • Staggered grid: stress & velocities • Volcanic conduit modelled as a fluid-filled body embedded in homogenous elastic medium Free surface Solid medium Seismometers ρ = 2600 kgm-3 α = 3000 ms-1 β = 1725 ms-1 Source Signal: 1Hz Küpper wavelet 100m below top of conduit Liquid magma Damped Zone Domain Boundary

8. Conduit Geometry and Stiffness factor EGU 2007 Resonance characteristics depend on: • Contrast in physical properties of fluid and solid 2. Geometry of conduit μ B L h (Aki et al. 1977)

9. Method μ B EGU 2007 Varied parameter contrast term by: • Adjusting acoustic velocity • Adjusting • density B = ρVp2

10. Increasing the stiffness factor by increasing the acoustic velocity produces a shift to higher frequencies • Increasing the stiffness factor by increasing the density produces a shift to lower frequencies EGU 2007 Results Both increase stiffness → but opposite behaviour!

11. EGU 2007 Adapted from Fig. 2 of Ferrazini & Aki (1987) • Phase velocity of interface waves key parameter in controlling resonant frequencies. • For fixed aspect ratio: ratio B/µ controls the ratio of phase velocity to acoustic velocity

12. EGU 2007 Comparison of a 30m and 50m wide conduit Motivation and Aims Recent evidence for widening of conduit from 30m to 50m (M.V.O., 2006) Expect to see shift to higher frequencies in the amplitude spectra with increasing width. Aim: to see if this prediction is validated by results of numerical modelling

13. Finite-Difference Method 50 m EGU 2007 • Employ same grid setup as previously • Simple comparison between conduit of • 30m width • and of • 50m width Free surface Solid medium Seismometers ρ = 2600 kgm-3 α = 3000 ms-1 β = 1725 ms-1 Source Signal: 1Hz Küpper wavelet 100m below top of conduit Liquid magma 30 m Damped Zone Domain Boundary

14. Results 30m wide 50m wide EGU 2007 Synthetic Seismograms Vertical component seismograms Faster decay of amplitude for wider conduit

15. Results horizontal component EGU 2007 ______ - - - - - - 30m Amplitude Spectra 50m Show a clear shift to higher frequency peaks with increasing width vertical component ______ - - - - - - 30m 50m vertical components Spectrograms Illustrate the changes in frequency content of synthetic signals

16. Widening of conduit EGU 2007 • Results show expected shift to higher frequencies • Provides further evidence and validation for widening of upper conduit → fed directly into SAC risk assessment • Larger width implies reduced rise rate of magma → more time for gas to escape → reduced likelihood of explosions (M.V.O., 2006)

17. horizontalcomponent vertical component EGU 2007 Conduit Geometry: Summary Conduit Length Conduit width + changes in acoustic velocity Shift in resonant frequencies Changes in dispersion properties of waveforms (Sturton & Neuberg, 2006)

18. EGU 2007 References • Aki, K., Fehler, M. & Das, S., 1977, Source mechanism of volcanic tremor: fluid-driven crack models and their application to the 1963 Kilauea eruption. J. Volcanol. Geotherm., 2, pp259-287. • Ferrazzini V. & Aki K., 1987, Slow waves trapped in a fluid-filled infinite crack: implications for volcanic tremor. J. Geophys. Res., 92,pp9215-9223. • Green, D. N. & Neuberg, J., 2006, Waveform classification of volcanic low-frequency earthquake swarms and its implication at Soufrière Hills Volcano, Montserrat. J. Volcanol. Geotherm., 153, pp51-63. • Levander, A.R., 1988, Fourth-order finite-difference P-SV seismograms. Geophysics, 53,pp1425-1436. • M.V.O. (Montserrat Volcano Observatory), 2006, Assessment of Hazard and Risks Associated with Soufrière Hills Volcano, Montserrat. Sixth Report of the Scientific Advisory Committee, March 2006. Part Two - Technical Report (available at http://www.mvo.ms) • 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. • Sturton, S. & Neuberg, J., 2006, The effects of conduit length and acoustic velocity on conduit resonance: implications for low frequency events. J. Volcanol. Geotherm., 151, pp319-339. • Virieux J., 1986, P-SV wave-propagation in heterogeneous media: velocity-stress finite-difference method. Geophysics, 51, pp889-901.