Selected Seismic Observations of Upper-Mantle Discontinuities

Peter Shearer IGPP/SIO/U.C. San Diego

August 31, 2009 Earthquake Research Institute

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

•  Analyze entire dataset whenever possible

•  Use simple methods to get sense of data before doing complicated inversions

•  Consider reflection seismology methods like stacking and back-projection

•  Avoid any hand-processing of seismograms!

Advice on Seismic Data Crunching

Global Stacking using Automatic Gain Control (AGC) •  Calculate average absolute value in 5 s bins •  Divide each bin by average of previous 24 bins.

This normalizes the amplitude of each trace. •  Stack in 0.5˚ distance bins

AGC Stack: Long-period vertical

from Shearer (1991) Distance (degrees)

Tim

e (m

inut

es)

90

60

30

0 0 90 180 270 360

Stacking using a reference phase Unaligned SH waves Aligned SH waves

1 minute Stack

Reference pulse stacks for 20 different range bins

CD-ROM stacks (1991) P wave (vertical)

S wave (transverse)

P

PP

410-km discontinuity

660-km discontinuity

No global 220-km discontinuity

SS

S

Topside reflections

CD-ROM stack: SS precursors SS-wave stack (transverse)

SS

S660S

660

410-km discontinuity 520

Sdiff

SS

from Shearer (1991)

80 100 120 140 160 180

4

2

Range (degrees)

Tim

e (m

inut

es)

0

-2

-4

-6

-8

No coherent reflectors above 410 or below 660

SS precursors are ideal for global mantle discontinuity studies

Source Receiver Bounce point

Good global distribution of bounce points

from Flanagan & Shearer (1998)

Depression in ‘660’ in NW Pacific

from Shearer (1991)

Gu et al. (2002)

Shearer & Masters (1992)

Flanagan & Shearer (1998)

‘660’ topography from SS precursors

blue = depressed (~10–20 km) red = elevated

CD-ROM stacks (1991) P-wave stack (radial)

P/SV discontinuity conversions (Vinnik, 1977)

SV/P discontinuity conversions (Faber & Muller, 1984)

PcSdiff

Receiver functions at GSN stations

Shearer (1991) Lawrence & Shearer (2005)

Transition Zone Thickness Models SS precursors Receiver functions

Gu et al. (1998)

Flanagan & Shearer (1998)

Lawrence & Shearer (2005)

Slabs in the transition zone

from Karson and van der Hilst (2000)

Flanagan & Shearer (1998)

660 topography

P-wave tomography

Slabs in the transition zone

Lesser deflection in large region beneath slab

50–100 km deflection in vicinity of slab

Response of 660-km discontinuity to slab:

figure from Lebedev et al. (2002)

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

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

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

A puzzle: Where is the 660 reflector?

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

Computing simple ray theoretical synthetics

Solve for best-fitting model using niching genetic algorithm

• 660-km discontinuity has small contrasts in density & P velocity • Largest change at 520 km is in density • 410-km discontinuity is thicker than 660-km discontinuity • 410 seems to fit pyrolite model, 660 is more complicated, may be

double discontinuity with more than one phase change

From Lawrence & Shearer (2006)

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

Rela

tive

ampl

itude

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

P'P' onset

P'660P' P'410P'

from Xu et al. (2003)

0

0.01

0.02

0.03

0.04

0.05

-200 -150 -100 -50

Precursors to P'P'

Am

plitu

de r

elat

ive

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

Am

plitu

de r

elat

ive

to P

'P'

Time Figure 11

"660"

"410"

No visible 410 in P’P’ at higher frequencies

from Xu et al. (2003)

Conclusions from Xu et al. 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

Regional constraints on discontinuity topography

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)

from Niu et al. (2005)

410 P-to-S conversion points

Future of upper-mantle discontinuity studies

• Continued high-resolution regional analyses using seismic arrays and migration processing methods (USArray, Japan)

• More detailed comparisons to mineral physics (temperature, composition, water content, possible multiple phase changes)

• Analyses of hard-to-image interfaces between the Moho and the 410, e.g., the lithosphere-asthenosphere boundary (LAB).