Seismic Imaging of Mantle Plumes - University of British Columbiamjelline/453website/eosc453/... ·...
Transcript of Seismic Imaging of Mantle Plumes - University of British Columbiamjelline/453website/eosc453/... ·...
Seismic ImagingSeismic Imagingof Mantle Plumesof Mantle Plumes
PaperPaper
MotivationIs the plume hypothesis consistent with seismic observations?
… and in particular, at:
•Hawaii
•Iceland
•Yellowstone
•Massif Central
•Bowie
Overview hotspot
Upper mantle
Transition zone
Lower mantle
D``
ΔT ~ 300KΦ ~ 150km
Φ ~ 500 km ΔT ~ 500K
z
ΔT = Tplume - Tambient
Φ ~ diameter
Δvs = vplume - vambient
410km
670km
CMB~1500km?
Φ ~ 1000km
ΔT ~ 600K
Δvs, Δvp ~ -2.5%
How to locate the plumes?
Source Signald
v
In one dimensionVelocity
DistanceSingle layer
Time
Multi layers
⎟⎠⎞
⎜⎝⎛∗=v
dt 1
Slowness
sd ∗=
d1 d2
Source Signal 2
2
1
1
vd
vdt +=
[ ]⎡ ⎤⎣ ⎦2
121
ssddt =
Know
Data = G Model
In two dimension
S1
S4 S5 S6
S7 S8 S9
S3S2
Data = G Model
Discretize the Earth
Small plume, small mesh
Big plume, big mesh
Fine resolution
Coarse resolution
1. Collect earthquake data around the world
2. Find the region close to the plumes and ignore all the unrelated wave path
Plume
3. Set up the equation d=Gm
4. Find plume structure and it’s location by inversion
Prior information: eg. The wave velocity of the plumes and it’s approximate location
5. If there is no prior information, we need to inverseall the seismic data around the world!!!
Aside: Fresnel Zone
What is the effect of multiple rays?
→ depends on phase difference
If L’-L < λ/4 → constructive interference
→ cannot easily resolve signals smaller than w
λfv =
velocity wavelength frequency
V increases with depth
f decreases with depth
→ λ increases with depth
→ Fresnel zone increases with depth. Big problem, for example, in detecting plume in lower mantle.
Upper Mantle – Continental Hotspots
Yellowstone (Iyer et al 1981)
•Array of 50 seismographs
•Aperture (spacing) ~ 200km
•5% slow vp anomaly: depth 100km, Φ ~ 50km
•2% slow vp anomaly: to depth 350km
Massif Central (Granet et al 1995)
•Array of 80 seismographs•Aperture ~ 300km
•2.5% slow vp anomaly: to depth ~ 600km, Φ ~ 200km
GOOD agreement •Anomaly strength,•Φ below “pancake”
BAD agreement •pancake shape missing
Upper Mantle – Oceanic Hotspots
Hawaii•1) Tomographic study: (Ellsworth 1977, Tilmann 1999)
•Small island => Small aperture => depth < 150km
•3% slow vp anomaly to depth ~ 80km
•2) Dispersion study: (Priestley & Tilmann 1999)
•5% slow vs anomaly: to depth 200km
Iceland•1) Tomography (Tryggvason et al 1983)
•Array of 39 seismographs•2.5% slow vp anomaly: to depth ~ 350km, Φ ~
200km
Vs
Upper Mantle – Oceanic Hotspots
GOOD agreement •Anomaly strength
BAD agreement •‘pancake’ shape missing
Limitations •Velocity anomalies only relative•Limited depth (Hawaii)
Iceland
2) Tomography (Wolfe et al 1997)
•2% slow vp anomaly, 4% slow vsanomaly : to depth ~ 400km, Φ ~ 300km
•Forward model => ΔT = 250K
• Global vs inversion (Grand et al 1997)
• Resolution ~ 1000km >> Φ• Some hotspots show Δvs<0
Transition Zone – Global Study
• Look for change in discontinuity height (410km, 670km) using “receiver function” technique:→ P waves convert to S waves at 410km
or 670km→ Travel-time difference gives thickness
of transition zone
Transition Zone – Regional studies
Iceland (Bjarnason et al 1996)
•Zone ~ 230km thick = 20km thinner
Hawaii (Li et al 1999)
•Zone ~ 30-40km thinner beneath Hawaii than Oahu
GOOD agreement
•Discontinuities deflected
Limitations •Few studies•Signals weak
Imaging plumes in the lower mantle (700-2700 KM)
Problem: Fresnel zone > plume diameter
Solutions: 1. select a region and geometry to lower Fresnel zone2. Use diffraction tomography
Bowie:
Hawaii:
Bowie lies between Alaskan subduction zone.Nataf and VanDecar thinks Bowie is at about 700km. incorrect because Fresnel > plume diameter.
Global tomography cannot resolved below 2000km
Iceland:
Further study is needed High level of scattered energy•Imply a larger scattering anomaly than modeled•Strong anomalies cold be produced by partial melt•in the stem of the plume
Diffraction and seismic tomography
Amplitude was much greater than Predicted for a simple plume.
Inconclusive
Imaging plumes in the lowermost mantle (2700-2900 KM)
Hotspot plums originate from the base of the mantle.
Hawaii: Kanasewich claims a velocity anomaly in the lowermost mantle (NE of Hawaii).
Disputed by Wright in 1975
Russell (1998) finds evidence for S wave velocity 1000 km SE of Hawaii. The anomaly is far away from Hawaii.
Conclusion: Unclear about the evidence
hotspot
Upper mantleTransition zone
Lower mantle
ΔT ~ 300K
ΔT ~ 500K
z
410km
670km
~1500km?
ΔT ~ 600K
Lower mostMantle
2700km
Conclusion1. How to locate plumes
by inversion
2. Fresnel zone
3. Velocity anomalies in upper mantle, transition zone, lower mantle and lower most mantle