BGA 2007 Different aspects of seismic amplitude decay in viscous magma Patrick Smith Supervisor: Jürgen Neuberg School of Earth and Environment, The University of Leeds. Photo : R Herd, MVO

BGA 2007 Outline of Presentation Background: low-frequency seismicity, seismic attenuation in gas-charged magma Methodology: Viscoelastic finite-difference model & Coda Q analysis Results and Implications: plus some discussion of future work

Low frequency seismicity High frequency onset Coda: harmonic, slowly decaying low frequencies (0-5 Hz) → Are a result of interface waves originating at the boundary between solid rock and fluid magma What are low-frequency earthquakes? Specific to volcanic environments BGA 2007

Source Propagation of seismic energy Conduit Resonance 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 Trigger Mechanism = Brittle Failure of Melt BGA 2007

Propagation of seismic energy BGA 2007

P-wave S-wave Propagation of seismic energy BGA 2007

Interface waves P-wave S-wave Propagation of seismic energy BGA 2007

Interface waves Propagation of seismic energy BGA 2007

Interface waves Propagation of seismic energy BGA 2007

Interface waves Propagation of seismic energy BGA 2007

Interface waves Propagation of seismic energy BGA 2007

Propagation of seismic energy BGA 2007

reflections Propagation of seismic energy BGA 2007

reflections Propagation of seismic energy BGA 2007

Propagation of seismic energy BGA 2007

Low frequencies High frequencies FAST MODE: I1 NORMAL DISPERSION SLOW MODE: I2 INVERSE DISPERSION Low frequencies High frequencies Acoustic velocity of fluid Propagation of seismic energy BGA 2007

I1 I2 Propagation of seismic energy BGA 2007

I1 I2 S Propagation of seismic energy BGA 2007

S I1 I2 Propagation of seismic energy BGA 2007

S I1 I2 Propagation of seismic energy BGA 2007

‘Secondary source’ I2 Propagation of seismic energy BGA 2007

Surface-wave ‘Secondary source’ Propagation of seismic energy BGA 2007

Surface-wave Propagation of seismic energy BGA 2007

I1R1 Propagation of seismic energy BGA 2007

I1R1 Propagation of seismic energy BGA 2007

I2 I1R1 Propagation of seismic energy BGA 2007

I2 ‘Secondary source’ Propagation of seismic energy BGA 2007

‘Secondary source’ Propagation of seismic energy BGA 2007

Propagation of seismic energy BGA 2007

Propagation of seismic energy BGA 2007

Propagation of seismic energy BGA 2007

Most of energy stays within the conduit Propagation of seismic energy BGA 2007

Most of energy stays within the conduit Propagation of seismic energy BGA 2007

Most of energy stays within the conduit Propagation of seismic energy BGA 2007

Most of energy stays within the conduit Propagation of seismic energy BGA 2007

Propagation of seismic energy BGA 2007

R2 Propagation of seismic energy BGA 2007

R2 Events are recorded by seismometers as surface waves Propagation of seismic energy BGA 2007

Why are low frequency earthquakes important? Have preceded most 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 BGA 2007

Conduit Properties seismic signals (surface) Magma properties (internal) Seismic parameters Signal characteristics Context: combining magma flow modelling & seismicity Conduit geometry + Properties of the magma Attenuation via Q

BGA 2007 Seismic attenuation in magma Provides information about magma properties Why is attenuation important? Definitions: Apparent (coda) Intrinsic (anelastic) Radiative (parameter contrast, geometric spreading) true damping amplitude decay

BGA 2007 Modelling Intrinsic Q To include anelastic ‘intrinsic’ attenuation – the finite-difference code uses a viscoelastic medium: stress depends on both strain and strain rate. Parameterize material using Standard Linear Solid (SLS): viscoelastic rheological model whose mechanical analogue is as shown: Intrinsic Q is dependent on the properties of the magma: Viscosity (of melt & magma) Gas content Diffusivity Use in finite-difference code to model frequency dependent Q

BGA 2007 Finite-Difference Method 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 2-D O(Δt 2,Δx 4 ) scheme based on Jousset, Neuberg & Jolly (2004) Volcanic conduit modelled as a viscoelastic fluid-filled body embedded in homogenous elastic medium

BGA 2007 Determining apparent (coda) Q Coda Q methodology : Decays by factor (1­  Q) each cycle Aki & Richards (2003) Model produces harmonic, monochromatic synthetic signals Time [number of cycles] Amplitude -A 0 A0A0 A1A1 A2A2 A3A3 Take ratio of successive peaks, e.g. A1A1 A2A2  =  Q Q =  A 2 A 1 – A 2

BGA 2007 Calculation of coda Q Calculating Q using logarithms Gradient of the line given by: Unfiltered data Hence Q is given by: Time [cycles] log(Amplitude) Q value based on envelope maxima Gradient of line = Q value from gradient = Linear Fit Data

Results BGA 2007 Apparent (coda) Intrinsic (anelastic) An amplitude battle: competing effects Radiative (parameter contrast, geometric spreading) High intrinsic attenuation overcome by resonance effect – but need better understanding of how energy of interface waves is trapped Determines behaviour at high intrinsic Q – shifts the curve vertically For a fixed parameter contrast Apparent Q greater than intrinsic Q: Resonance dominates Apparent Q less than intrinsic Q: Radiative energy loss dominates

BGA 2007 Future Work and developments Compare attenuation of acoustic waves with interface waves, both intrinsic & radiative – aim to understand the different components of amplitude loss. Relate amplitudes at surface to slip at source → ‘magma flow meter’ idea Use flow magma models to derive viscosities – examine impact on seismic amplitude decay Link observables, e.g. coda decay & frequency content to magma properties such as the viscosity, gas content & pressure