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
Advice on Seismic Data Crunching 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!
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 90 Time (minutes) 60 30 0 0 90 180 270 360 Distance (degrees) from Shearer (1991)
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) Topside reflections PP P S wave (transverse) SS 660-km discontinuity 410-km discontinuity S No global 220-km discontinuity
CD-ROM stack: SS precursors SS-wave stack (transverse) Time (minutes) 4 2 0-2 -4-6 SS SS S660S 410-km discontinuity 520 660-8 80 100 120 140 160 180 Range (degrees) S diff from Shearer (1991)
No coherent reflectors above 410 or below 660
SS precursors are ideal for global mantle discontinuity studies Source Bounce point Receiver Good global distribution of bounce points from Flanagan & Shearer (1998)
Depression in 660 in NW Pacific from Shearer (1991)
660 topography from SS precursors Shearer & Masters (1992) Flanagan & Shearer (1998) Gu et al. (2002) red = elevated blue = depressed (~10 20 km)
CD-ROM stacks (1991) P-wave stack (radial) SV/P discontinuity conversions (Faber & Muller, 1984) PcS diff P/SV discontinuity conversions (Vinnik, 1977)
Receiver functions at GSN stations Shearer (1991) Lawrence & Shearer (2005)
Transition Zone Thickness Models SS precursors Receiver functions Gu et al. (1998) Lawrence & Shearer (2005) Flanagan & Shearer (1998)
Slabs in the transition zone P-wave tomography 660 topography Flanagan & Shearer (1998) from Karson and van der Hilst (2000)
Slabs in the transition zone Response of 660-km discontinuity to slab: 50 100 km deflection in vicinity of slab Lesser deflection in large region beneath slab
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 figure from Lebedev et al. (2002)
Correlation between TZ thickness and velocity anomalies Agrees with mineral physics data for olivine phase changes Permits calibration of dt/dv and Clapeyron slopes Global, SS precursors Australia region, Receiver functions Flanagan & Shearer (1998) Lebedev et al. (2002)
Analysis of different discontinuity phases can resolve density, P & S velocity jumps across discontinuities A puzzle: Where is the 660 reflector?
Estimated S velocity and density jumps across 660 km Global Study Northwest Pacific Philippine Sea Shearer & Flanagan (1999) SS & PP precursors Kato & Kawakatsu (2001) ScS reverberations Tseng & Chen (2004) Triplicated waveforms
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)
P P phase: seen at short periods, good for sharpness constraints Earthquake Station P'P'df P'P'ab Outer Core Inner Core Mantle
Relative amplitude 1 0.8 0.6 0.4 0.2 Envelope stack: 1/19/69 earthquake at LASA P'660P' P'410P' P'P' onset 0-200 -150-100 -50 0 50 100 Time relative to P'P'(ab) (sec) from Xu et al. (2003)
Comparison to long-period reflections Amplitude relative to P'P' 0.05 0.04 0.03 0.02 0.01 Precursors to P'P' P'660P' X long-period reflection amplitudes P'410P' X Corrected for attenuation 0-200 -150-100 -50 Time relative to P'P' (sec)
No visible 410 in P P at higher frequencies 0.10 LASA stacks at two frequencies Amplitude relative to P'P' 0.08 0.06 0.04 0.02 "660" "410" 0.7 Hz stack 1.0 Hz stack 1.3 Hz stack 0.00 2200 2240 2280 2320 Time Figure 11 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
Snake River Plane Eastern US, MOMA Array Dueker & Sheehan (1997) Li et al. (1998)
Tibet Tanzania Owens et al. (2000) Kosarev et al. (1999)
Southern Africa Gao et al. (2002)
410 P-to-S conversion points from Niu et al. (2005)
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).