Transcript Body wave tomography: an overview

Surface wave tomography：
1. dispersion or phase based approaches
(part A)
Huajian Yao
USTC
April 19, 2013
From IRIS
Surface waves
 Surface wave propagates along the surface of the
earth, mainly sensitive to the crust and upper mantle
(Vs) structure
Love and Rayleigh waves
Generated by constructive interference between postcritically reflected body waves
Surface waves: evanescent waves
Decreasing wave amplitudes as depth increases
Wave displacement patterns in a layer over half space
Wavelength
increases
Generally, wavespeed increases as the depth
increases. Therefore, longer period (wavelength)
surface waves tend to propagate faster.
Surface wave dispersion: frequency-dependent
propagation speed
(phase or group speed)
Group V:
Energy
propagation
speed
Phase or group velocity dispersion curves
(PREM model)
Phase or group velocity depth sensitivity kernels
Usually 80-90% importance
is the 1-D depth sensitivity kernel
Phase or group velocity depth sensitivity kernels
fundamental mode
Rayleigh wave
Love wave
dU/dVSV
dc/dVSV
dc/dVSH
Rayleigh wave phase velocity depth sensitivity kernels at
shorter periods: also quite sensitive to Vp and density at
shallow depth
(A) 0.15 Hz,
(B) 0.225 Hz,
(C) 0.3 Hz.
Rayleigh wave phase velocity depth sensitivity kernels: An image view
Surface wave tomography from dispersion data:
a two-step approach
 1. Construct period-dependent 2-D phase/group
velocity maps from many dispersion measurements
 2. Point-wise (iterative) inversion of dispersion data at
each grid point for 1-D Vs model; combine all the 1-D
Vs models to build up the final 3-D Vs model
Now the global search approaches are widely used for this step
due to very non-linear situation of this problem.
Popular approaches for surface wave tomography (Step 1)
(1). Single-station group velocity approach
(event  station)
(2). Two-station phase velocity approach
(event  station1  station 2)
(3). Single-station phase velocity approach
(1) U = D/tg
(2) c = (D2 – D1)/Δt
(1). Single-station group velocity approach
frequency-time analysis (matched filter technique) to
measure group velocity dispersion curves
Widely used in regional
surface wave tomography
Possible errors:
(1) off great-circle effect, (2)
mislocations of earthquake
epicenters, (3) source origin
time errors and (4) the finite
dimension and duration of
source process.
(2 – 4): source term errors
Ritzwoller and Levshin, 1998
Eurasia surface wave
group velocity
tomography
Ritzwoller and Levshin, 1998
(2). Two-station phase velocity approach (very useful for
regional array surface wave tomography)
Teleseismic surface waves
Narrow bandpass filtered waveform
cross-correlation  travel time
differences between stations almost
along the same great circle path
(circle skipping problem!)
Advantage: can almost remove “source
term errors”
CTS (20 – 120 s)
Yao et al., 2006,GJI
SW China Rayleigh
wave phase velocity
tomography from the
two-station method
Yao et al., 2006,GJI
(3). Single-station phase velocity approach
Observed Seismogram:
Theoretical reference Seismogram from a spherical Earth model
Propagation
phase
Perturbation Theory
circle skipping problem at
shorter periods!
Spherical harmonics
representation of the 3-D model
Ekstrom et al, 1997
Example: Global phase velocity
tomography (Ekstrom et al., 1997)
Inversion of Vs from point-wise dispersion curves (Step 2)
 Iterative linearize inversion
 2. non-linear inversion or global searching methods
Simulated annealing, Genetic algorithm
Monte Carlo method, Neighborhood algorithm
Iterative linearize inversion: example
The results may depend on the initial velocity model. Better to
give appropriate prior constraints, e.g., Moho depth.
Nonlinear inversion: example using neighborhood
algorithm (Yao et al. 2008)
Neighborhood search
http://rses.anu.edu.au/~malcolm/na/na.html
(Sambridge, 1999a, b)
Bayesian Analysis of
the model ensemble
Posterior mean:
1-D marginal PPDF
2-D marginal PPDF
1-D PPDF: resolution & standard
error of model parameter;
2-D PPDF: correlation between
two model parameters