Seismic evidence for high pore pressures in the oceanic crust:
Implications for fluid-related embrittlement
Auxiliary Materials
Takahiro Shiina, Junichi Nakajima, and Toru Matsuzawa
Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School
of Science, Tohoku University, Sendai 980-8578, Japan
shina@aob.gp.tohoku.ac.jp
Text S1: Method of inversion
We used a travel-time tomography technique [Zhao et al., 1992] to estimate the
spatial variation in P-wave velocity in the subducting crust. A 3-D grid net was set in the
model space and grid nodes were spaced at 0.25° in the horizontal direction and at
10–30 km in the vertical direction (Figure S1). In the subducting Pacific slab, four grid
layers were set parallel to the plate interface, and the shallowest layer was located in the
subducting crust. In the inversion, we considered three velocity discontinuities: the
continental Moho [Katsumata, 2010], the upper boundary of the Pacific slab [Nakajima
et al., 2009], and the oceanic Moho. The thickness of the subducting crust was assumed
to be 10 km. A 1-D velocity model [JMA2001; Ueno et al., 2002] was adopted as the
initial velocity model, and P- and S-wave velocities within the Pacific slab were
assigned to be 5% faster than in the mantle wedge.
We used an iterative approach to correct for the effects of 3-D heterogeneous
structures above the Pacific slab on travel times of PS-converted waves and on the
locations of hypocenters. In step 1, we estimated 3-D P- and S-wave velocity structures
using arrival-time data of 130,150 P waves and 102,169 S waves from 5198 earthquakes
(Figure S2), and relocated the hypocenters with the obtained 3-D models. In step 2, we
estimated P-wave velocity variation in the subducting crust using arrival-time data of
PS-converted waves. P-wave velocity was estimated only at grid nodes located in the
subducting crust, and the 3-D P- and S-wave velocity structures outside the subducting
crust and the hypocenters were fixed with those determined in step 1. In step 3, we
replaced P- and S-wave velocities in the subducting crust with the P-wave velocity
estimated in step 2, and relocated the hypocenters with the updated velocity model.
Because we do not have any information on S-wave velocity in the subducting crust, we
assumed a Vp/Vs value of 1.90 [e.g., Tsuji et al., 2008] to calculate S-wave velocities in
the subducting crust from P-wave velocities. We repeated steps 1 through 3 twice to
obtain the final 3-D P- and S-wave velocity model for the crust and mantle beneath
northeastern (NE) Japan. Travel-time residuals of PS-converted waves were reduced
from 1.6 s in the initial model to 0.6 s in the final 3D velocity model, while those of the
direct P and S waves were reduced from 0.3 s and 0.5 s to 0.2 s and 0.3 s, respectively.
We obtained P- and S-wave velocity structures in the crust and mantle in the
overlying plate, and they are similar to those obtained in previous studies [e.g., Zhao et
al., 1992, 2011; Nakajima et al., 2001]. A prominent feature is the existence of an
inclined P- and S-wave low-velocity zone at the core of the mantle wedge, and the
low-velocity zone is interpreted as mantle upwelling induced by the subduction of the
Pacific slab [e.g., Hasegawa et al., 2005]. In the crust, low-velocity anomalies are
located beneath active volcanoes, and they appear to connect with the low-velocity zone
in the uppermost mantle. Although these features are important to understand ongoing
arc magmatism in NE Japan, we focus on the seismic properties of the subducting crust
in the main text because this paper is the first study that uses arrival-time data of
PS-converted waves in the tomographic inversion in NE Japan and imaged crustal
structure with much higher resolution than in the previous studies.
Text S2: Method and result of resolution tests
To assess the reliability of the obtained velocity structure, we carried out
checkerboard resolution test (CRT) and reconstruction test (RT). In CRT, we set up a
synthetic model in the subducting crust by assigning velocity anomaly of 10%
alternatively to every 0.50○ degree (about 50 km) (Figure S3a), while in RT. we
introduced a sharp velocity change in the crust by 10 % beneath the volcanic front
(Figure S3c). In the both tests, synthetic travel times of PS-converted waves were
calculated for the same source-receiver geometry as the observations, with random
noises corresponding to picking errors of PS-converted waves (standard deviation of 0.2
sec).
The obtained results of CRT (Figure S3b) show that checkerboard patterns are well
recovered in the eastern side of volcanic front. On the other hand, the patterns are
recovered in the back arc only in the central and southern parts of NE Japan. The results
of RT show that a clear velocity change is resolved in the central and southern parts of
NE Japan (Figure S3d). These tests demonstrate that the data set used in this study can
resolve characteristic velocity anomalies in the subducting crust.
References
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Figure captions
Figure S1
(a) Horizontal configurations of grid nodes adopted in the inversion. (b) Vertical
configurations of grid nodes along the latitude of 39° N. Solid lines denote the
continental Moho and the upper boundary of the Pacific slab. Dached line denotes the
oceanic Moho.
Figure S2
Distribution of hypocenters used in this study to estimate 3-D P- and S-wave velocity
structure beneath NE Japan.
Figure S3
Result of synthetic tests. (a) An input model and (b) inversion results for checkerboard
resolution test. (c) An input model and inversion results for reconstruction test.