Supplementary Materials to
Investigating lithospheric velocity structures beneath Taiwan region by
nonlinear joint inversion of local and teleseismic P-wave data: Slab
continuity and deflection
Hsin-Hua Huang1,*, Yih-Min Wu2, Xiaodong Song3, Chien-Hsin Chang4, and Hao
Kuo-Chen5
1. Institute of Earth Sciences, Academia Sinica, Taipei 115, Taiwan.
2. Department of Geosciences, National Taiwan University, Taipei 106, Taiwan.
3. Department of Geology, University of Illinois, Urbana, IL 61801, USA.
4. Central Weather Bureau, Taipei 100, Taiwan.
5. Institute of Geophysics, National Central University, Jhongli, Taiwan.
The supplementary materials contain two sections with four figures. The first section
discusses the nonlinear effect of joint inversion, and the second demonstrates the
detailed results of various synthetic model tests.
S.1 Effect of nonlinear joint inversion
To improve the imaging resolution, a nonlinear joint inversion that iteratively
re-traces the rays for both local and teleseimic events with the updated model at each
iteration was adopted in this study. In the inversion the source parameters of local
earthquakes, such as hypocenter location and origin time, are treated as unknowns to
determine with velocity perturbations simultaneously in the inversion, but those of
teleseismic earthquakes are not. Figure S.1 displays the inversion results in a 3-D
view at the first, third, and fifth iteration, respectively. The high similarity among
different iterations suggests the stablility of the inversions in our study. The effect of
nonlinear inversion shows clearly on improving and sharpening some part of images,
especially for the east-subducting EP, although few parts of the streaking anomalies in
the deep depth around the PSP were also enhanced in this way.
S.2 Synthetic tests on various slab models
S.2.1 Continuous slab model
To test the robustness of obtained slab images, we first input three continuous
high-Vp slab-like anomalies with different dipping angle, depth range, and slab
thickness according to the real-inversion images (Figure 3). Figure S.2 and S.3 show
the recovered results in horizontal slices and profiles by local seismic, teleseismic,
and joint inversion, respectively. The Slab 1 can be retrieved by the local seismic
inversion but not by the teleseismic inversion, and the Slab 2, vice versa, can be
imaged by teleseismic inversion mostly but very limited by the local seismic inversion
(especially for central Taiwan), demonstrating the distinctly different sensitivities of
local and teleseismic data. Due to the subvertical path of teleseismic rays, the
teleseismic inversion usually has poor resolution on depth and shows clear smearing
effect both upward and downward in the imaging. However, by combination of local
and teleseismic data, the joint-inversion can improve greatly the imaging of both Slab
1 and 2, and significantly mitigate the smearing phenomena (see especially on the
profile FF’). This test shows not only the characteristics of local and teleseismic data
but also promises illustrating the 3-D geometry of slab model beneath Taiwan region.
It is worth noting that the teleseismic inversion lowers slightly the Vp values of
surrounding areas. This is due to the de-meaning procedure adopted for teleseismic
data, which contain only the information of relative lateral variation. In other words,
when the teleseismic data are involved, one had better interpret the structures in
velocity perturbation rather than in absolute values.
S.2.2 Slab break model
Modified from the continuous slab models (Figure S.3), we also tested the
hypotheses of slab breaks in different dimensions and locations, including models
with small and large breaks with ~40-km and ~80-km gaps, respectively, and the
foundering model in which we only assumed a lower part of the high Vp anomaly as a
dripping structure of the continental eclogitization as proposed by Kuo-Chen et al
[2012]. The results show that the foundering model cannot reproduce the upper part of
east-dipping high-Vp anomaly we observed in real-inversion images (Figure 3) and
may be excluded. For the other two slab break models, the recovered images are
mostly similar to those in the real inversion and suggest a resolution limitation in
distinguishing the existence of a slab break smaller than ca. 80 km. Only the AA’
profile could resolve the slab break as small as the 40 km with the aid of subduction
events in the southern Taiwan.
References
Kuo-Chen, H., F. T. Wu, and S. W. Roecker (2012), Three-dimensional P velocity
structures of the lithosphere beneath Taiwan from the analysis of TAIGER and
related seismic data sets, J. Geophys. Res., 117, B06306,
doi:10.1029/2011JB009108.
Zhao, D., A. Hasegawa, and H. Kanamori (1994), Deep structure of Japan subduction
zone as derived from local, regional, and teleseismic events, J. Geophys. Res.,
99(B11), 22,313-22,329.
Figure captions
Figure S.1 The process of the nonlinear joint inversion on structure imaging at the
first, third, and fifth iteration, respectively.
Figure S.2 Recovery of continuous slab model at different depths. Horizontal slice of
60 km depth shows the locations of profiles for Figure S.3, which are identical to the
profile locations in Figure 3. The geometry and location of Slab 1, 2, 3 (synthetic
slab-like anomalies) are set upon the tomographic results (Figure 3).
Figure S.3 Recovery of continuous slab model in profiles. The profile locations are
denoted on 60 km-depth horizontal slice in Figure S.2. The geometry and location of
Slab 1, 2, 3 (synthetic slab-like anomalies) are set upon the tomographic results
(Figure 3).
Figure S.4 Recovery of different slab break models in profiles. They are the small
break model (~40-km gap), the foundering model, and the large break model (~80-km
gap) from left to right, respectively.