supporting information for “uppermost mantle structure...
Post on 12-Jun-2020
3 Views
Preview:
TRANSCRIPT
GEOPHYSICAL RESEARCH LETTERS
Supporting Information for “Uppermost mantle structurebeneath eastern China and its surroundings from Pn and Sntomography”
Weijia Sun1 and B.L.N. Kennett2
1 Key Laboratory of Earth and Planetary Physics, Institute of Geology and
Geophysics, Chinese Academy of Sciences, Beijing 100029, China.
2 Research School of Earth Sciences, Australian National University, Canberra,
ACT 2601, Australia.
Contents of this file
1. Data selection
2. The FMTOMO procedure
3. Resolution tests
4. The influence of relocation
5. Figure S1 - resolution tests
6. Figure S2 - residual histograms
2 SUN AND KENNETT: MANTLE VP , VS IN EASTERN CHINA
S1. Data selection
As noted in the main paper we have applied strict selection criteria to the bulletin arrivals times for
both P and S.
In the region closest to the source the crustal phases Pg, Sg will be the first arrivals for the particular
wavetypes, but beyond 1.8◦ epicentral distance it is expected that Pn, Sn becomes the first arrival, since
the average Moho depth is about 33 km [Li et al., 2014]. The Pn or Sn diving waves can exit from
the base of thin lithosphere, leading to significant changes in the frequency spectrum for epicentral
distances greater than 15◦ even for thick lithosphere. The thin lithosphere in parts of eastern China
[Hearn et al., 2008; Zhao et al., 2013], means that Pn and Sn are only trapped in the lithosphere out to
12◦. We have therefore limited the epicentral distance for the use of the Pn and Sn travel times to the
range 1.8◦ – 12◦, for the inversions for P and S velocity structure in the uppermost mantle.
The relocation step is also used as a filter on the arrival times to be used in the inversions. After
relocation we only retain events for which the focal depths are shallower than 2 km less than the Moho
depth. We also discard events for which there is a very large epicentral shift on relocation (>1◦). To
avoid erroneous readings we exclude any paths for which Pn or Sn travel-time deviations are larger than
8 s relative to the ak135 model [Kennett et al., 1995] using the relocated hypocentres.
S2. The FMTOMO procedure
The FMTOMO package is designed to invert multiple classes of body wave datasets, including re-
fracted waves, reflected waves, local and regional and teleseismic events, and data from active sources
[Rawlinson and Urvoy, 2006]. The multi-stage fast marching method is employed to solve the forward
problem of travel-time prediction. Assuming local linearity, a subspace inversion scheme is used to
optimise the objective function of model parameters and observed data, which is a iterative non-linear
SUN AND KENNETT: MANTLE VP , VS IN EASTERN CHINA 3
scheme. The model parameters are defined on regular grids in spherical coordinates, which are quite
suitable for regional and global tomography, because of minimising travel-time prediction errors at
these scales.
The FMTOMO method is quite suitable to solve large tomographic problems, and is computationally
efficient and robust for our study of Pn and Sn across the eastern China. The Pn and Sn phases are
represented diving waves in the uppermost mantle returned by interaction with the velocity gradients.
The FMTOMO procedure takes account of the variations of the Moho in the inversion.
Our approach to determine the 3-D wavespeeds can be summarised as:
• Establish a 3-D initial model; in this case we use the CRUST1.0 model [Laske et al., 2013] for the
crust and Moho and the SL2013sv model [Schaeffer and Lebedev, 2013] in the mantle.
• Relocate all sources in the 3-D initial model, and extract Pn and Sn residuals.
• FMTOMO inversion for an inversion domain down to 150 km. The procedure was run for multiple
iterations, and the model with the first minimum in misfit selected.
S3. Resolution tests
The ability of the available data to identify small heterogeneities can be assessed with the aid of a
formal resolution test. A 2◦ × 2◦ pattern of 2% negative and positive velocity perturbations is imposed
on the initial P model and ±4% velocity anomalies are imposed on the S model. To be more realistic,
we set two individual flat layers at depths of 50 km and 60 km for both P and S. Figure S1 shows the
recovered checkerboard resolution test with relative model to the initial 3-D model at depths of 50 km
and 60 km.
For P waves, the checkerboard patterns are generally recovered quite well with the imposed pattern
at 2◦ × 2◦ at both depths of 50 km and 60 km in the continental regions. The exceptions are in eastern
4 SUN AND KENNETT: MANTLE VP , VS IN EASTERN CHINA
Mongolia, due to infrequent events and rather sparse stations, and a small part of Yangtze Craton centred
at (108◦E,26◦N) where only weaker amplitudes can be recovered.
The recovered checkerboard for S wave is quite good, and is similar to that for P at a depth of 50
km. The recovered S velocity anomalies at 60 km depth is generally good, but with a little weaker
amplitudes than those of S at 50 km depth and P waves. This difference is linked to differences in the
ray path for P and S associated with the varying wavespeed distribution, since the P and S velocities
are dominated by different factors. For example, temperature has much more prominent influence on S
velocities in the crust and mantle than P velocities.
As might be expected from the similar pattern of available paths (Figure 2) we are able to achieve
comparable resolution for P and S. Features at a scale of 2◦ × 2◦ or larger should be well represented
for both P and S.
Construction of a direct resolution test for the Vp/Vs ratio is difficult, because the Vp/Vs ratio is ob-
tained by division of Vp by Vs rather than inverted directly. Figure S1e and S1f show indirect resolution
results for the Vp/Vs ratio at depth of 50 km and 60 km for comparison with the direct results for P and
S velocities. The background Vp/Vs is obtained by division of the background Vp by the background
V s, while the perturbed Vp/Vs is given by division of the inverted Vp by Vs. The checkerboard pattern
of Vp/Vs is then obtained by subtracting the background from the perturbation. Since the variation of
Vp/Vs is not proportional to those of Vp and Vs, the procedure cannot completely reflect the resolution
ability of the available data, and so we have small amplitudes of the relative Vp/Vs ratio in Figure S1e
and S1f. For a given Vp = 8.0km/s and Vs = 4.0km/s with perturbations of +0.16km/s for both P
and S, the perturbed Vp/Vs would be −0.027, so that the polarities are reversed. This can be seen by
careful comparison of Figure S1e-f with Figure S1a-d.
SUN AND KENNETT: MANTLE VP , VS IN EASTERN CHINA 5
Although the indirect resolution of the Vp/Vs ratio has been recovered in a good pattern, we still
suggest it is necessary to evaluate the resolution of the Vp/Vs ratio by comparing those for Vp and Vs.
S4. The influence of relocation
The event information for the arrival times we have collected come form location with a global or
local 1-D model. To put all observations on a common basis and to correct for much of the influence of
lateral heterogeneities in the Earth, we carry out a relocation of all events using the initial 3-D model,
before inverting the Pn and Sn travel-times.
This relocation step helps to reduce biases in the travel time distribution and so provide a good
starting point for the FMTOMO inversion. A convenient way of illustrating the effect of the relocation
is to examine the histograms of travel-time residuals. In Figure 2 we illustrate the results for station
GYA (26.46◦N, 106.66◦E) in southern China, an open station at Incorporated Research Institutions for
Seismology (IRIS). This station has a large number of arrivals, with a considerable spread in residuals
associated with paths sampling rather different tectonic provinces. In Figure S2 we show the Pn and Sn
residuals before and after relocation. At first sight, the Pn residuals from the original bulletin (Figure
S2a) seem good, but we note that the residuals are dominantly positive residuals centred at 3 sec.
After relocation with the 3-D initial model, the Pn residual distribution (Figure S2b) is much more
concentrated with a smaller residuals centred at 1 sec. We have similar results for Sn with residuals
centred at 2 sec from the original bulletin (Figure S2c) to around 0 sec after relocation (Figure S2d),
though in this case the spread of residuals does not reduce much. Similar improvements are observed
at most stations, which indicates the value of relocation before travel-time inversion.
6 SUN AND KENNETT: MANTLE VP , VS IN EASTERN CHINA
References
Hearn, T. M., Wang, S. Y., Pei, S. P., Xu, Z. H., Ni, J. F., Yu, Y. X., 2008. Seismic amplitude tomography
for crustal attenuation beneath China. Geophys. J. Int. 174 (1), 223–234.
Kennett, B. L. N., Engdahl, E. R., Buland, R., 1995. Constraints on seismic velocities in the Earth from
travel-times. Geophys. J. Int. 122 (1), 108–124.
Li, Y. H., Gao, M. T., Wu, Q. J., 2014. Crustal thickness map of the Chinese mainland from teleseismic
receiver functions. Tectonophysics 611, 51–60.
Laske, G., Masters, G., Ma, Z., Pasyanos, M., 2013. Update on CRUST1.0 - a 1-degree global model
of Earths crust. In: Geophys. Res. Abstracts. Vol. 15. p. 2658.
Schaeffer, A. J., Lebedev, S., 2013. Global shear speed structure of the upper mantle and transition
zone. Geophys. J. Int., 194 (1), 417–449.
Rawlinson, N., and M. Urvoy (2006), Simultaneous inversion of active and passive source datasets for
3-D seismic structure with application to Tasmania, Geophys. Res. Lett., 33(24), L24313.
Zhao, L. F., Xie, X. B., Wang, W. M., Zhang, J. H., Yao, Z. X., 2013. Crustal Lg attenuation within the
north China craton and its surrounding regions. Geophys. J. Int. 195 (1), 513–531.
SUN AND KENNETT: MANTLE VP , VS IN EASTERN CHINA 7
c)
d)
a)
b)
-0.12 -0.06 0.00 0.06 0.12
P Velocity [km/s]
-0.12 -0.06 0.00 0.06 0.12
S Velocity [km/s]
60.0 km
100
100
110
110
120
120
130
130
140
140
20 20
25 25
30 30
35 35
40 40
45 45
50 50
55 55
60 60
50.0 km
100
100
110
110
120
120
130
130
140
140
20 20
25 25
30 30
35 35
40 40
45 45
50 50
55 55
60 60
60.0 km
100
100
110
110
120
120
130
130
140
140
20 20
25 25
30 30
35 35
40 40
45 45
50 50
55 55
60 60
50.0 km
100
100
110
110
120
120
130
130
140
140
20 20
25 25
30 30
35 35
40 40
45 45
50 50
55 55
60 60
-0.06 -0.03 0.00 0.03 0.06
Vp/Vs
e)
f)
60.0 km
100
100
110
110
120
120
130
130
140
140
20 20
25 25
30 30
35 35
40 40
45 45
50 50
55 55
60 60
50.0 km
100
100
110
110
120
120
130
130
140
140
20 20
25 25
30 30
35 35
40 40
45 45
50 50
55 55
60 60
Figure S1. Results of checkerboard resolution test for Pn (a-b) and Sn (c-d) tomography at depths
of 50 km (top panel) and 60 km (bottom panel). The 2◦ × 2◦ pattern of positive and negative velocity
anomalies is imposed on the initial models for both P and S. The indirect resolution test for Vp/Vs is
shown at depth of 50 km (e) and 60 km (f).
8 SUN AND KENNETT: MANTLE VP , VS IN EASTERN CHINA
a) b)
c) d)
-10 -8 -6 -4 -2 0 2 4 6 8 10
Residuals [s]
0
5
10
15
20
25
30
Co
un
t
-10 -8 -6 -4 -2 0 2 4 6 8 10
Residuals [s]
-10 -8 -6 -4 -2 0 2 4 6 8 10
Residuals [s]
0
5
10
15
20
25
30
Co
un
t
-10 -8 -6 -4 -2 0 2 4 6 8 10
Residuals [s]
P P
S S
Catalog Relocation
Figure S2. Travel-time residuals for the Pn and Sn arrivals at station GYA located at (26.46N ,
106.66E), with the theoretical travel-time predicted with the ak135 model: (a) Pn residuals from the
original CEDC catalog, and (b) Pn residual pattern after relocation with the 3D initial model. The
corresponding Sn residuals before and after the relocation procedure are shown in (c) and (d).
top related