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