1. Constructing synthetics from deep earth tomographic models
Sidao Ni, Xiaoming Ding and Don V. Helmberger, 2000
Group 4: Zhang Wenqiang, Zhao Yifan, Zang Nan

2. Outline
Introduction
Method
Application
Discussion and Conclusions

3. Introduction lower mantle structure heterogeneity Triplications
Carribean
Eastern Asia
Triplications
mid-Pacific
Recent high-resolution global seismic tomography models for P-waveand S-wave velocity heterogeneity near depths (Z) of 1,350 km and 2,750 km (Lay, 1998)

4. Triplications Not global (Wysession et al. 1998) SLHE: Asia
SLHO: Alaska
SLHA: central America
derived from
the Sea of Okhotsk data
(Lay & Hermberger, 1983)

5. Triplications
Synthetic long period tangential component S-waves for a 580 km deep source.
(Lay & Hermberger, 1983)

6. Triplications
Observed (left) and synthetic (right) profiles of long period seismograms for the event of 1967 December 1. Source depth is 136 km.
(Lay & Hermberger, 1983)

7. Method —The WKM approximation
source function
the product of the transmission coefficients
WKBJ
use rays overshoot or undershoot the receiver
Geometric Ray Paths

8. Generalized Ray Theory
the product of the reflection and transmission coefficients
sum the primary rays only
Generalized Ray Paths

9. Generalized Ray Theory
(Hermberger et al, 2000)
WKM
Approximation when the structure is smooth:
(Wiggins & Mardrid, 1974)
We name this approximation WKM to distinguish WKBJ method because they have similar solution but different ray paths.

10. Local stretching approximation
Map di, hi, βi into di’, hi’, βi’
Dipping structure flat-layered model
(Hong & Hermberger, 1978)

11. Time and Path correction
Time correction：
Find di, hi, βi for the 1-D reference model
Overlay a new velocity structure and obtain di, hi, βi ’
Path correction：
Overlay the velocities structure
Recomputed the path di’, hi, βi ’

12. Time and
Path correction

13. Treatment of the vertical block boundaries
Approximate
Exact algorithm

14. Treatment of the vertical block boundaries
Exact
Approximate

15. The GCV method
minimize
fit
smooth

16. Variation about triplication beneath America
Application of WKM
Variation about triplication beneath America
(Lay & Helmberger, 1983)

17. High velocity anomaly beneath the Caribbean

18. Grand's model

20. Compare synthetics with real data
The data (SH) were recorded for the Argentine event (1994 May 10)
1-D synthetics assume a 1-D model with a 120 km transitional zone 280 km above the CMB (Sidorin & Gurnis 1998).
2-D synthetics are based on the enhanced model .

24. EDM
FBC

26. Discussion and conclusion
The earth is 3-D, but judiciously chosen 2-D sections should prove useful in mapping these structures.
Sharp features is a limitation for this WKM method, but it can also be solved.
The application shows this method can sucessfully explain some of the Scd behavior observed for the D'' structure. This technique is useful.

27. Thank you!
Questions are welcome