bga 2007 different aspects of seismic amplitude decay in viscous magma patrick smith supervisor:...
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BGA 2007
Different aspects of seismic amplitude decay in viscous magma
Patrick SmithSupervisor: 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 energyConduit 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 MeltBGA 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: I1NORMALDISPERSION
SLOW MODE: I2INVERSEDISPERSION
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 energystayswithin the conduit
Propagation of seismic energy
BGA 2007
Most of energystayswithin the conduit
Propagation of seismic energy
BGA 2007
Most of energystayswithin the conduit
Propagation of seismic energy
BGA 2007
Most of energystayswithin 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
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(Δt2,Δx4) 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
0 1 2 3 4
0
Time [number of cycles]A
mpl
itude-A0
A0
A1
A2
A3
Take ratio of successive peaks,
e.g.A1
A2
= Q
Q =A2
A1 – A2
BGA 2007
Calculation of coda QCalculating Q using logarithms
Gradient of the line given by:
Unfiltered data
Hence Q is given by:
0 2 4 6 8 10 12-24
-23.8
-23.6
-23.4
-23.2
-23
-22.8
-22.6
Time [cycles]
log(
Am
plitu
de)
Q value based on envelope maxima
Gradient of line =-0.10496
Q value from gradient = 31.5287
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
0 10 20 30 40 50 60 70 80 90 1000
10
20
30
40
50
60
70
80
90
100
Intrinsic Q
Ap
pa
ren
t Q
Intrinsic Q vs. Apparent (coda) Q
2 SLS in array
0 10 20 30 40 50 60 70 80 90 1000
10
20
30
40
50
60
70
80
90
100
Intrinsic Q
Ap
pa
ren
t Q
Intrinsic Q vs. Apparent (coda) Q
2 SLS in arrayFor 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