selected seismic observations of upper-mantle...
TRANSCRIPT
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).