FIBERGLASS PIPE, GALVANIZED CONDUIT or PVC-COATED CONDUIT ONLY
Conduit length Conduit top Tilt: 17 µrad due to pressure 50 MPa 100 MPa P Tilt Pressure Conduit...
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Transcript of Conduit length Conduit top Tilt: 17 µrad due to pressure 50 MPa 100 MPa P Tilt Pressure Conduit...
Conduit length
Con
du
it t
op
Tilt: 17 µraddue to pressure
50 MPa
100 MPa
P
Tilt Pressure
Conduit length
Con
du
it t
op
τ
Tilt Traction Strain rate Magma velocity
(Green et al., 2006)
gUUUPdt
dU T
b
Continuity equation for compressible fluids: t 0
U
reference
Dome collapseJuly 12, 2003Photo: J. Neuberg
Generation of interface waves:
--30m--
const(ρc=2680 kg
m-3
χc= 30 %)
const(~1,5 10-5 Pa s)
bmgcggcc
Magma = Melt + Crystals + Gas
ρg ηgρm ηm
f(Cm, P)(Hess &
Dingwell,1996)
const(~ 2300 kg m-3)
b ηb ρc χc
mP/RT
ηbηm
Melt viscosity Magma viscosity
(Collier & Neuberg, 2006)
Reaction to volume change
Reaction toshear stress
= 0
with λ = K – 2/3 ηb
K: volume viscosity ηb : magma shear (dynamic) viscosity
Modelling: Compressible Navier Stokes equation
• melt• gas• crystals
30m conduit
50m
30m
Observations & Modelin summary:
τ τ
Gas diffusion
No seismicity
Pressure increasing
1
2
Seismicity
Pressuredecreasing
4
τ τ
No seismicity
Magma slowing
Gas diffusion
(Green et al., 2006)
3
τ τ
Diffusion lagsbehind
Gas loss
Each instrument is a filter
Seismometer:• differentiation• bandpass limitation
Lee’s Yard
St George’s Hill
Lee's Yard
0 2 4 6Time (s)
Nor
mal
ised
Am
p.
Data Analysis.
From seismograms to magma: interpreting broadband
seismic signals in terms of magmatic processesEGU, Vienna, April 2007.
J. Neuberg1, P. Smith1, D. Green1, M. Collombet1, L. Collier1, C. Hammer2 & J. Key1.1. School of Earth and Environment, University of Leeds. UK. ([email protected]) 2. Institute of Geosciences, University of Potsdam, Germany. ([email protected])
Seismic wavefield modelling. Magma flow modelling.
50m conduit
Ultra-long period signals.
Trigger mechanism and seismic moment tensor analysis. Deformation.
90% CLVDCompensated linear vector dipole
Ring dyke
Near field terms & single forces
→ Source mechanism that is non-destructive, repeatable and with a stationary source location.
Characteristics of low-frequency events• Similar waveforms
• Repetitive
• Tight clusters of source locations
• Swarms precede dome collapse
Family members:normalised & stacked
Overlain, Normalized Traces
Link seismicity rate with magma
movement at depth. Pre-
cursor to dome collapse
Conduit filled with melt, gas & crystals
Montserrat topography
Onset contains information on trigger mechanism
Conduit resonance
Comparison of a 30m and 50m wide conduit: illustrating the change in frequency content with widening conduit
2-D Finite-Difference Model:
• Fluid-filled conduit (rather than crack)• Physical properties are depth & time dependent• Viscoelastic model includes effects of intrinsic attenuation and damping
● Seismicity correlates with tilt/deformation
• Sources are dependent on pressure
Models for seismicity & tilt:
Modelling parameters: conduit top & pressure
3-D detail - cut out to
see tilt source
(vertically exaggerated
x 3)
References and Acknowledgements.Collier, L. & Neuberg, J., 2006, Incorporating seismic observations into 2D conduit flow modelling. J. Volcanol. Geotherm., 152, pp331-346
Green, D., Neuberg, J., & Cayol, V., 2006, Shear stress along the conduit wall as plausible source of tilt at Soufrière Hills Volcano, Montserrat. GRL., 33, L10306.
Jousset, P., Neuberg, J. & Jolly, A., 2004, Modelling low-frequency volcanic earthquakes in a viscoelastic medium with topography. J. Geophys. Int., 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. Volc. Geotherm., 153, pp37-50.
(Jousset et al., 2004)(Neuberg et al, 2006)
Determine: pressure, density, temperature, viscosity, gas volume %, magma velocity, velocity gradient (= strain rate)
Employ:Navier Stokes equation for compressible flow, gas loss – permeability, temperature loss & friction, water solubility, viscosity
Magma velocity profiles for 30m and 50m wide conduit
Moment tensor inversion with corresponding radiation pattern
Cylindrical shear fracturing at the edge of the conduit as triggering mechanism?
The resulting particle motion pattern should be symmetrical, centred around the conduit location, with downward motions in the inner ring and upward motions in the outer ring
Only upward particle motionsOnly upward particle motions
Particle motions compatible with a " shallow " event
Fit two stations: use ratio of displacement for ground deformation modelling
Models of shear stress for conduits with depth dependent geometries – using a constant magma viscosity
2-D conduit model:
narrowing from 60m
to 30m
3-D conduit model: 30m wide pipes joined by 10m deep sill
xu
tizikxl
Sxl
zz
tizikxlz
xlcx
eeelekiu
eeeikelu
zSP
zSP
xu zuand : displacement components
: wavenumber
and : potentials of the P- and S-waves
zk pzP Vkfl ,,: SzS Vkfl ,,:
Trigger mechanism
velocityprofile
Magma ruptures if
Pa7
at constant depth
Conduit resonance whereviscosity is low
Shear stress
Tilt: 17 µraddue to traction
0.5 MPa
1 MPa