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Auxiliary Material for
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Changes in the stress field after the 2008 M7.2 Iwate-Miyagi Nairiku earthquake
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in northeastern Japan
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Keisuke Yoshida (National Research Institute for Earth Science and Disaster Prevention) ,
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Akira Hasegawa, Tomomi Okada (Research Center for Prediction of Earthquakes and Volcanic
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Eruptions, Graduate School of Science, Tohoku University) and Takeshi Iinuma (International
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Research Institute of Disaster Sciences, Tohoku University),
Journal of Geophysical Research, Solid Earth, 2014
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Introduction
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This auxiliary material contains six figures.
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Captions to supplementary figures
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Figure S1. (a) Seismic velocity structures by Hasegawa et al. [1978] (black lines) and
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by Ueno et al. [2002] (gray lines). Solid and dashed lines show P- and
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S-wave velocities, respectively. (b) Frequency distribution of differences of
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calculated focal depths due to the different seismic velocity models in (a).
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Figure S2. Distribution of hypocenters relocated in the present study. The figure on the
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left shows a map view. The five figures on the right show along-fault
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vertical cross-sections along lines E, F, G, H and I in the map view. Blue
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circles represent the relocated hypocenters. Others are the same as in Fig.2.
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Figure S3. Examples of focal mechanism solutions determined by the present study. (a)
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focal mechanisms evaluated as rank A and (b) those as rank B by the
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criteria of Hardebeck and Shearer [2002]. (c) and (e) show the frequency
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distributions of the number of polarity data used for determinations of focal
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mechanisms whose ranks are A and B, respectively. (d) and (f) show the
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maximum azimuthal gap of polarity data used for determinations of focal
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mechanisms whose ranks are A and B, respectively.
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Figure S4. Comparison of focal mechanisms determined in the present study with those
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determined by different seismic velocity models and hypocenter locations.
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Differences are shown as frequency distributions of angular differences of
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P-axis. (a) Difference with those determined by using the velocity model of
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Hasegawa et al. [1978] and hypocenter locations in the JMA catalogue, (b)
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with those determined by using the velocity model of Ueno et al. [2002]
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and hypocenter locations in the JMA catalogue, and (c) with those
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determined by using the velocity model of Ueno et al. [2002] and
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hypocenter locations relocated in the present study.
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Figure S5. Distribution of focal mechanism solutions before the mainshock, determined
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in this study. “Beach balls” represent the focal mechanisms. The figure on
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the left shows a map view. The four figures on the right show across-fault
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vertical cross-sections along A-A’, B-B’, C-C’, and D-D’ in the map view.
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Other details are the same as in Fig. 4.
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Figure S6. Distribution of focal mechanism solutions after the mainshock, determined in
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this study. “Beach balls” represent the focal mechanisms. The figure on the
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left shows a map view. The four figures on the right show across-fault
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vertical cross-sections along A-A’, B-B’, C-C’, and D-D’ in the map view.
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Other details are the same as in Fig. 4.
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Figure S7. Distribution of P-axis orientations of focal mechanisms determined in this
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study from (a) before and (b) after the mainshock. Length and color (scale
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is shown at the bottom) of bar corresponds to the plunge and azimuth of the
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P-axes, respectively. Gray contour indicates the coseismic slip model of
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Iinuma et al. [2009].
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Figure S8. Distribution of T-axis orientations of focal mechanisms determined in this
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study from (a) before and (b) after the mainshock. Length and color (scale
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is shown at the bottom) of bar corresponds to the plunge and azimuth of the
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T-axes, respectively. Gray contour indicates the coseismic slip model of
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Iinuma et al. [2009].
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Figure S9. Rose diagrams showing the azimuth of 𝜎1 axis. In (a) and (b), azimuths of
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𝜎1 axis in the entire area before and after the mainshock are shown,
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respectively. In (c), azimuths of 𝜎1 axis in the large slip area (enclosed in
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Fig. 7b) after the mainshock are shown.
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Figure S10. Comparison of principal stress orientations before and after the mainshock.
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The stress orientations after the mainshock were estimated at the same
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locations before the mainshock as in Figure 6. In (a) and (b), orientations of
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the best-fit 𝜎1 axes are shown by bars before and after the mainshock,
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respectively. They are colored by depth. In (c), 95% confidence limits of
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the 𝜎1 and 𝜎3 axes before and after the mainshock are plotted by using
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different color contours. 95% confidence limits of the 𝜎1 and 𝜎3 axes
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before the mainshock are shown by orange and light blue contours,
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respectively, on lower focal hemispheres. Those after the mainshock are
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shown by red and blue contours, respectively. Best orientations of the 𝜎1
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and 𝜎3 axes are shown by circles.
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Figure S11 Comparison of static stress changes calculated using different values of
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Poisson’s ratio. In (a) and (b), gray circles indicate differential stresses
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calculated by assuming Poisson’s Ratio as 0.18 and 0.31, respectively,
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normalized by those assuming Poisson’s Ratio as 0.25 at the same locations.
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In (c) and (d), gray circles indicate angular differences of 𝜎1 axis between
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static stress changes calculated by assuming Poisson’s Ratio as 0.25 and
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those assuming as (c) 0.18 and (d) 0.31 at the same locations. Solid curves
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show average values.
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Figure S12. Estimated distributions of 𝜎1 axis orientations after the mainshock given
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by calculating the sum of stress tensors before the mainshock and the static
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stress change. The static stress change was based on the slip model moved
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horizontally without rotation from the model of Iinuma et al. [2009], in
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order to be consistent with the aftershock hypocenters. They are plotted
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separately into three depth ranges, 0–3 km, 3–5 km, and 5–8 km, so that
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they do not overlap. Assumed differential stresses before the mainshock are
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(a) 100 MPa, (b) 50 MPa, (c) 30 MPa, (d) 20 MPa, (e) 10 MPa, and (f) 5
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MPa. Other details are the same as in Fig. 6.
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Figure S13. Estimated distributions of 𝜎1 axis orientations after the mainshock given
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by calculating the sum of stress tensors before the mainshock and the static
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stress change. The static stress change was based on the slip model moved
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horizontally without rotation from the model of Hikima and Koketsu (2013)
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in order to be consistent with the aftershock hypocenters. They are plotted
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separately into three depth ranges, 0–3 km, 3–5 km, and 5–8 km, so that
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they do not overlap. Assumed differential stresses before the mainshock are
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(a) 100 MPa, (b) 50 MPa, (c) 30 MPa, (d) 20 MPa, (e) 10 MPa, and (f) 5
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MPa. Other details are the same as in Fig. 6.
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