Combining magma flow models with seismic signals Patrick Smith M.Res. Physics of the Earth and Atmosphere (2005/6) Supervisor: Prof. Jürgen Neuberg School of Earth and Environment, The University of Leeds

Outline of Presentation Background: low frequency seismicity Project split into 3 sections: Incorporating flow model data (main focus) Conduit geometry and stiffness factor Comparison of a 30m and 50m wide conduit Motivation & Aims → Method → Results Conclusions

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

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

Trigger Mechanism = Brittle Failure of Melt Conduit Resonance Propagation of seismic energy Energy travels as interface waves along conduit walls at velocity controlled by magma properties Top and bottom of the conduit act as reflectors and secondary sources of seismic waves Fundamentally different process from harmonic standing waves in the conduit Source Trigger Mechanism = Brittle Failure of Melt

Propagation of seismic energy

Propagation of seismic energy S-wave P-wave

Propagation of seismic energy Interface waves S-wave P-wave

Propagation of seismic energy Interface waves

Propagation of seismic energy Interface waves

Propagation of seismic energy Interface waves

Propagation of seismic energy Interface waves

Propagation of seismic energy

Propagation of seismic energy reflections

Propagation of seismic energy reflections

Propagation of seismic energy

Propagation of seismic energy FAST MODE: I1 NORMAL DISPERSION Low frequencies Acoustic velocity of fluid High frequencies High frequencies SLOW MODE: I2 INVERSE DISPERSION Low frequencies

Propagation of seismic energy

Propagation of seismic energy

Propagation of seismic energy

Propagation of seismic energy

Propagation of seismic energy ‘Secondary source’

Propagation of seismic energy Surface-wave ‘Secondary source’

Propagation of seismic energy Surface-wave

Propagation of seismic energy I1R1

Propagation of seismic energy I1R1

Propagation of seismic energy I1R1 I2

Propagation of seismic energy ‘Secondary source’

Propagation of seismic energy ‘Secondary source’

Propagation of seismic energy

Propagation of seismic energy

Propagation of seismic energy

Propagation of seismic energy Most of energy stays within the conduit

Propagation of seismic energy Most of energy stays within the conduit

Propagation of seismic energy Most of energy stays within the conduit

Propagation of seismic energy Most of energy stays within the conduit

Propagation of seismic energy

Propagation of seismic energy

Propagation of seismic energy Events are recorded by seismometers as surface waves R2

Incorporating flow model data Motivation Properties of the magma Seismic parameters Signal characteristics Incorporate flow model data into wavefield models Magma properties (internal) seismic signals (surface)

Incorporating flow model data Aims & Methodology Flow model data Derive seismic parameters Use in wavefield models

Magma flow models A 2-D finite-element model for magma flow has recently been developed (Collier & Neuberg, 2006) Magma properties resolved at all depths and lateral positions within a volcanic conduit

Incorporating flow model data Seismic (acoustic) velocity Weighted average of components from the three different phases Acoustic velocity → calculated from gas volume fraction, pressure & bulk density

Incorporating flow model data Seismic (acoustic) velocity Acoustic velocity profiles calculated for models with: 30m wide conduit two different exsolved gas contents ______ - - - - - - Lower gas content Higher gas content

Wavefield Models (horizontal component) System of differential equations (elasto-dynamic equations) (horizontal component) Solved numerically using a finite-difference method

Finite-Difference Method Domain Boundary Solid medium Liquid magma Damped Zone Free surface Seismometers Source Signal: 1Hz Küpper wavelet 100m below top of conduit ρ = 2600 kgm-3 α = 3000 ms-1 β = 1725 ms-1

Results Introducing flow models based parameters produces: more noise and higher frequencies horizontal components Magma model Average less regular spacing of sub-events vertical components large amplitude of sub-events transmitted from the bottom of the conduit

Results Impedance Contrast Flow Model: High contrast at the top, low at the bottom more energy transmitted from bottom of conduit Magma model Average

Constant averaged values Results Constant averaged values

Flow model derived values Results Flow model derived values

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

Adjusting acoustic velocity Adjusting density Method Varied parameter contrast part by: Adjusting acoustic velocity Adjusting density μ B

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

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 shift to higher frequencies in the amplitude spectra with increasing width. Aim: to see if this prediction is validated by results of numerical modelling

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

Results Amplitude Spectra horizontal component ______ - - - - - - Show a clear shift to higher frequency peaks with increasing width 30m 50m vertical component ______ - - - - - - 30m 50m

Incorporating flow model data Conclusions Incorporating flow model data More complex seismograms Steep gradients in impedance contrast → more energy transmitted from bottom of conduit Less regular spacing of sub-events in time and frequency domains

Conduit geometry and stiffness Conclusions Conduit geometry and stiffness Stiffness factor does not fully define the resonance characteristics → using a single parameter contrast B/μ within the stiffness factor is not justified. Better approach to consider effects of component ratios individually

Conclusions Widening of conduit Results show expected shift to higher frequencies Provides further evidence and validation for widening of upper conduit Larger width implies reduced rise rate of magma → more time for gas to escape → reduced likelihood of explosions (M.V.O., 2006)

References and Acknowledgements I would like to thank my supervisor Professor Jürgen Neuberg for his help and guidance throughout this research. I would also like to acknowledge the support of Dr. Marielle Collombet and thank her for providing the magma flow model data that was used in this project. This M.Res. was funded by a NERC studentship. 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. Collier, L. & Neuberg, J., 2006, Incorporating seismic observations into 2D conduit flow modelling. J. Volcanol. Geotherm., 152, pp331-346. 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. 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.