Which Mantle Interfaces Do Seismologists See?

Peter Shearer*

IGPP/SIO/U.C. San Diego

* With figures from Castle, Deuss, Dueker, Fei, Flanagan, Gao, Gu, Kaneshima, Kato, Kellogg, Kosarev, Kruger, Lebedev, Li, Masters, Niu, Owens, Reif, Tackley, Tseng, van der Hilst, Vidale, Vinnik

Interface Depth vs. Publication Date

Most depths are sampled at least once

Consistency in depths greatest for 220, 410, 520, 660

Note: plot is not complete, especially in last 15 years

Three Types of Mantle Interfaces

Unobserved seismically, hypothesized by geochemists or geodynamicists (e.g., Kellogg et al. lower mantle boundary)

Routinely observed seismically, known mineral physics origin (410, 520, 660)

Intermittently observed seismically, undetermined origin (220, 900, 1200, D”)

SS precursors probe layering near the SS bounce point

Nice for global studies since SS bounce points are widely distributed

figure from Lebedev et al. (2002)

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410 and 660 observations are consistent with mineral physics predictions for olivine phase changes

• Absolute depths agree with expected pressures

• Topography consistent with Clapeyron slopes

• Size of velocity and density jumps are about right

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Flanagan & Shearer (1998) Lebedev et al. (2002)

Global, SS precursors Australia region, Receiver functions

• Correlation between TZ thickness and velocity anomalies• Agrees with mineral physics data for olivine phase changes• Permits calibration of dT/dv and Clapeyron slopes

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Analysis of different discontinuity phases can resolve density, P & S velocity jumps across discontinuities

A puzzle: Where is the 660 reflector?

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Shearer & Flanagan (1999)

SS & PP precursors

Kato & Kawakatsu (2001)

ScS reverberations

Tseng & Chen (2004)

Triplicated waveforms

Estimated S velocity and density jumps across 660 km

Global Study Northwest Pacific Philippine Sea

Dueker & Sheehan (1997)

Snake River Plane

Eastern US, MOMA Array

Li et al. (1998)

Tibet

Tanzania

Kosarev et al. (1999)

Owens et al. (2000)

Southern Africa

Gao et al. (2002)

Earthquake

Station P'P'df P'P'ab

Mantle

OuterCore

InnerCore

Figure 1

P’P’ phase: seen at short periods, good for sharpness constraints

0

0.2

0.4

0.6

0.8

1

-200 -150 -100 -50 0 50 100

Envelope stack:1/19/69 earthquake at LASA

Time relative to P'P'(ab) (sec)

P'P' onset

P'660P' P'410P'

Several minute envelope stack

0

0.01

0.02

0.03

0.04

0.05

-200 -150 -100 -50

Precursors to P'P'

Time relative to P'P' (sec)

P'660P'P'410P'

XXlong-period

reflectionamplitudes

Comparison to long-period reflections

Corrected for attenuation

0.00

0.02

0.04

0.06

0.08

0.10

2200 2240 2280 2320

LASA stacks at two frequencies

0.7 Hz stack1.0 Hz stack1.3 Hz stack

Time Figure 11

"660"

"410"

No visible 410 in P’P’ at higher frequencies

from Fei, Vidale & Earle

Conclusions from Fei, Vidale & Earle P’P’ study

410 is not so sharp — results suggest half is sharp jump, half is spread over 7 km

520 is not seen in short-period reflections — jump must occur over 20 km or more

660 is sharp enough to efficiently reflect 1 Hz P-waves — less than 2-km thick transition

S520S

Seen in global stacks but weaker than 410 and 660 reflectors

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Deuss & Woodhouse (2001)

520-km discontinuity may be intermittent and/or split into two interfaces

SS precursors stacked in bounce point caps

Deuss & Woodhouse propose this may be phase changes in two components, olivine and garnet, whose depths don’t always coincide.

Transectsof the 520

Lateral continuity of

structure

A global map, where there is

coverage

JohnWoodhouse

Intermittently observed seismically, undetermined origin (220, 900, 1200, D”)

Three Types of Mantle Interfaces

Are there regional reflectors hidden in here, such as the 220?

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figure from Deuss & Woodhouse (2002)

SS Precursor Stacks in Different Regions

figure from Deuss & Woodhouse (2002)

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figure from Deuss & Woodhouse (2002)

Discontinuity depths in SS precursor bounce point caps

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figures from Gu, Dziewonski & Ekstrom (2001)

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But another SS precursor study does not find N. American 220

Are there regional reflectors hidden down here, such as at 900 km or 1200 km?

Strongest evidence for mid-mantle discontinuities is from S-to-P conversions from deep earthquakes

to receiver

Slide 44

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figures from Niu & Kawakatsu (1997)

• Interface near 1000 km below Indonesia

• Seen in S-to-P conversions

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figure from Kaneshima & Helffrich (1999)

• Interface near 1500 km below Marianas

• Seen in S-to-P conversions below deep earthquakes

Unobserved seismically, hypothesized by geochemists or geodynamicists (e.g., Kellogg et al. lower mantle boundary)

Three Types of Mantle Interfaces

Hypothesized Undulating Mid-Mantle Discontinuity

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figures from Kellogg, Hager, van der Hilst (1999)

Low density contrast (chemical+thermal) implies large dynamic topography

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figure from van der Hilst & Karason (1999)

Some tomography models may have change in character near 1700 km depth

But this is not direct evidence for an interface,and models have limited resolution and uniqueness….

Tackley (2002) test of Kellogg et al. hypothesized layer

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Interface position Temperature perturbation

• 3-D numerical simulation of mantle convection

• Filtered to match seismic modeling

• Simulations predict peak in heterogeneity near interface depth

• Not seen in real tomography models – argues against hypothesis

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Masters et al. model 10% in dense layer 30% in dense layer

figures from Tackley (2002)

Radial correlation function is measure of vertical continuity of model

Convection simulations with dense lower layer predict significant de-correlation (narrowing) near interface, which is not observed in Masters or Grand tomography models

Convection SimulationsSeismic Inversion

Thermal boundary layer at interface will cause sharp velocity change, which should cause observable triplication in seismic travel time curve*

* Provided source/receiver geometry is just right

figure from Vidale, Schubert & Earle (2001)

figures from Vidale, Schubert & Earle (2001)

• California network data, 10 earthquakes• No triplications or complicated waveforms• But approach could miss:

- interface not at 1770 km, or - dipping interface

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figure from Castle & van der Hilst (2003)

S-to-P conversions can detect discontinuities below earthquake source regions

S1700P phase arrives between P and pP

Useful for finding interfaces below subduction zones from ~800 to ~2000 km depth

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figure from Castle & van der Hilst (2003)

1500 - 1800 km

Systematic search for interfaces below Pacific subduction zones

Nothing

(see also Wicks & Weber, 1996; Kaneshima & Helffrich, 1999)

Nothing

Nothing

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figure adapted from Vinnik, Kato & Kawakatsu (2001)

1850 km

1200 km

900 km

950 & 1050 km

1200 km

Yet Vinnik et al. study finds interfaces in some of same places

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figure from Vinnik, Kato & Kawakatsu (2001)

Quake in Fiji-Tonga, recorded in Japan

S1200Ps410P S1700P

Quake in Marianas, recorded in western US

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figure from Castle & van der Hilst (2003)

What about the lowermost mantle?

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figures from Castle & van der Hilst (2003)QuickTime™ and a

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• Mid-mantle reflectors seen below many subduction zones

• Related to ancient slabs?

• Discontinuity observations lacking in other areas

• Not present? or Not easy to observe?

• Conclusion: Seismic evidence argues against continuous mid-mantle boundary