Seismometer

Trigger mechanism: brittle failure at conduit walls

Qi

R (reflection coefficient)

T (transmission coefficient)Qr

-1

Total amplitude decay is a combination of these contributions:

ff

ss

Aki, K., 1984. Magma intrusion during the Mammoth Lakes earthquake. JGR, 89, pp7689-7696.Collier, L. & Neuberg, J., 2006, Incorporating seismic observations into 2D conduit flow modelling. J. Volcanol. Geotherm.,, 152, pp331-346 Collier, L., Neuberg, J., Lensky, N. & Lyakhovsky, V., 2006, Attenuation in gas-charged magma. J. Volcanol. Geotherm., 153, pp21-36.Jousset, P., Neuberg, J. & Jolly, A., 2004, Modelling low-frequency volcanic earthquakes in a viscoelastic medium with topography. J.GI.., 159, pp776-802.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.

Patrick Smith's Ph.D. is funded by NERC grant NER/S/A/2006/14150. The data collection and archiving bystaff 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 2007. Session & Poster number: V51D-0782

P. J. Smith & J. NeubergSchool of Earth and Environment, University of Leeds., UK. (p.j.smith@see.leeds.ac.uk)

1. Backgroundi. 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 conduit50m

30m

50m conduit

xu

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(Δt2,Δx4) 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 1997.

Band-pass filtered traces with apparent Q valuesEvent 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 mechanismBrittle fracturing on ring-fault as seismic source:

View of the lava dome of Soufrière Hills Volcano, Montserrat, from the MVO in April 2007.

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

0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

80

90

100

Intrinsic Q

Intrinsic Q vs. Apparent (coda) Q

For a fixed parameter contrast

2 SLS in array

Apparent Q vs. intrinsic Q for the synthetic signals, showing two regimes.

Apparent Q greater than intrinsic Q:

Resonance dominates

Intrinsic Q vs. Apparent QIntrinsic=Apparent

Apparent Q less than intrinsic Q:Radiative energy loss dominates

Ap

pa

ren

t Q

‘Peaked’ amplitude spectrum used to choose frequencies for band-pass filters

1

1

2

2

3

4

3

4

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

Qa-1=Qi

-1+Qr-1

Qa-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

τ τ

No seismicity

Magma slowing

Gas diffusion

3

τ τ

Diffusion lagsbehind

Gas loss

(Neuberg et al, 2006)

1

2

Qa-1 = Qi

-1 + Qr-1

M0 = µAu

Unfiltered data

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)

Apparent Q value based on synthetic signal envelope

Gradient of line =-0.10496Q value from gradient = 31.5287

Linear FitData

1 mQ

e

Am

plitu

de

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.

τ τSeismicity

Pressuredecreasing

Stress threshold:slip

slipplug flow

gas loss

parabolic flow

Collier & Neuberg, 2006; Neuberg et al., 2006

depth of brittle failure

710 Pa

Collier & Neuberg, 2006;

ii. Magma viscosities are derived from flow modelling:

ii. Magma viscosities are derived from flow modelling:

ηbηm

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 energyremainswithin the conduit

Events are recorded by seismometer

as surface waves

Interface waves