Geophysical Research Letters
Supporting Information for
Faulting and hydration of the upper crust of the SW Okinawa trough during
continental rifting: Evidence from seafloor compliance inversion
Ban-Yuan Kuo, Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
Wayne C. Crawford, Institut de Physique du Globe, Paris, France
Spahr C. Webb, Lamont-Doherty Earth Observatory, New York, U. S. A.
Ching-Ren Lin, and Dai-Chieh Yu, Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
Liwen Chen, Institute of Oceanography, National Taiwan University, Taipei, Taiwan.
Contents of this file
Figures S1 to S9
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Figure S1. Observed (a) coherence and (b) compliance for the 3 OBSs. Red for EOS1, green for
S002, and blue for S005. The coherence for S002 and S005 were corrected for the tilt noise at low
frequencies (see Figure S4). The compliance data were slightly smoothed and resampled at equal
space in log frequency (dots) for the purpose of inversion. The number of data is chosen to strike a
balance between redundant sampling and aliasing. The frequency ranges used in the inversion are:
3.02 10-3 – 5.25 10-2 Hz for EOS1; 5.25 10-3 – 2.29 10-2 Hz for S002; and 5.75 10-3 – 2.75 10-2 Hz for
S005.
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Figure S2. Example of removing gravitational attraction effect of the mass perturbation at the sea
surface. (Left) The orange curve is the vertical displacement spectrum of EOS1 observed in the third
hour of Julian day 339 of 2011. The wave height at the sea surface can be inferred from the pressure
recorded by the OBS on the seafloor. This mass perturbation is then downward continued to obtain
the corresponding displacement (black). Note the gravitational effect can reach 1/10 of the total
displacement at frequencies approaching 0.005 Hz for EOS1 which is installed on a depth of only
300 m. (Right) Compliance measurements without (orange) and with (red) the correction for the
gravitational attraction effect. The correction becomes significant and necessary at low frequencies.
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Figure S3. Coherence between the 3 components of (a) EOS1, (b) S002, and (c) S005. Amplitude
and phase are on the first and second row, respectively, for each OBS. Coherence is calculated for the
2 components denoted as 1/Z for horizontal component 1 and vertical (left panel), 2/Z for horizontal
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component 2 and vertical (mid-panel), and 1/2 for horizontal components 1 and 2 (right panel). Note
the high coherence at low frequencies for S002 and S005 on the 1/Z pair, implying the presence of
the tilt noise. The corresponding transfer functions were applied to remove the tilt noise identified in
the 1/Z pair from the vertical data of S002 and S005. The 2 vertical bars crossing the 1/Z coherence
of S002 and S005 mark the range of the cosine taper of the low pass filter applied to the transfer
function. See Figure S4.
Figure S4. The transfer function to remove the tilt noise for S005 and the coherence of the
compliance after the tilt noise is removed. (a) Amplitude (blue) and phase (light blue) of the original
transfer function for the 1/Z pair, and the low-pass filtered transfer function (thin black). These will
be applied to remove the tilt noise on the vertical data induced from horizontal component 1. (b) The
coherence of the compliance without the tilt noise correction (thick blue), with the tilt noise
correction using original transfer function (grey), and with the tilt noise correction using the low-pass
filtered transfer function (thin black). A low-pass filtered transfer function increases the coherence of
the compliance measurements at low frequencies and leaves the coherence intact at higher
frequencies.
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Figure S5. Example of convergence in VS towards the best 5-layer model in two steps for EOS1. (a)
The first search. The dashed line denotes the initial model; dotted lines denote the limits of grid
search, i.e., ±1.4 km/s from the initial value of each layer. The thick solid line represents the best
model after the first search. The search interval is 0.2 km/s as marked by short bars. (b) The second
search. The thick solid line is the best model of the first search and is taken as the initial model.
Dotted lines denote the limits of search, i.e., ±0.5 km/s from the initial model. The red line
represents the best model. The search interval is 0.1 km/s as marked by short bars. The other 2
parameters are searched in the similar style. The range of search given above is the maximum range.
In practice, the search begins from the top layer and the range of search narrows towards deeper
layers. To keep a monotonic increase in model parameters, the lower limit for a parameter at layer i
is set to be the value at trial for that parameter at layer i-1.
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Figure S6. All parameters, (a)VP (b), and VS (c) , determined in the 5-layer model for EOS1 (red),
S002 (green), and S005 (blue).
Figure S7. A test for the effect of layering. (a) A 5-layer models with layers 3 and 4 of thickness of 5
and 5 km, respectively. Red for EOS1, green for S002, and blue for S005. The fits to the data are
poorer than the 5-layer model with thinner layers, but the decreasing VS in the upper crust from
EOS1 to S005 remains. (b) The smooth models inverted from the starting models in (a). The
decrease from EOS1 to S005 retains in a shallower depth range than that in Figure 3b.
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Figure S8. Comparison between the 5-layer and the smooth models for (a) EOS1, (b) S002, and (c)
S005. The triangles and inverse triangles are the same as in Figure 3.
Figure S9. VP / VS resolved in the 5-layer model for EOS1 (red), S002 (green), and S005 (blue). The
ratio is fixed at 1.73 in layer 5, but free in layers above it. Note in layer 3 (crust 1) the ratio increases
from EOS1 to S005 eastward into the matured rifting region of OT, suggesting increasing hydration
of the shallow crust. The variations in the sediment layers may indicate different composition or
change in porosity. Open symbols and dotted lines are for EOS1 without high frequency data.
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