Additional file 1 of Shear wave splitting and seismic velocity structure in the focal area of the earthquake swarm and their relation with earthquake swarm activity in the Noto Peninsula, central Japan
Creators: name: Okada, Tomomi
name: Savage, Martha K.
name: Sakai, Shin’ichi
name: Yoshida, Keisuke
name: Uchida, Naoki
name: Takagi, Ryota
name: Kimura, Shuutoku
name: Hirahara, Satoshi
name: Tagami, Ayaka
name: Fujimura, Ryotaro
name: Matsuzawa, Toru
name: Kurashimo, Eiji
name: Hiramatsu, Yoshihiro
Publisher: name: figshare
Year: 2024
Subjects: subject: Geology
subject: FOS: Earth and related environmental sciences
subject: Geophysics

description: Additional file 1: Fig. S1. Spatio-temporal distribution of the earthquake swarm. We used the Japan Meteorological Agency (JMA) catalog. The color denotes the occurrence time. (a) Map view. (b) Time–latitude plot. Large and small white stars are the 2023 M6.5 and the 2022 M5.4 earthquakes, respectively. Open and solid red squares are the permanent station the temporary stations, respectively. Fig. S2. Active tectonics in northern Honshu: seismic tectonic sketch map showing epicenters of recent reverse fault ruptures and basin boundary faults, exposed areas of pre-Neogene basement, actively growing regional anticlines, Quaternary volcanoes, and volcanic front, relationship to subducting plate boundaries in the Japan Trench, and estimated maximum horizontal stress trajectories (modified from Sibson, 2009, original mainly following Sato, 1994). Red thin square denotes the area of Fig. S1. Fig. S3. Result of the checkerboard resolution test. In the checkerboard resolution test, we adopted the three-dimensional seismic velocity model of the checkerboard pattern with +-5% for P-wave and -+5% (negative) for S-wave, 1.57 or 1.91 for Vp/Vs, respectively. We calculated the travel time for each ray path of the data and added the random noise of 0.18s for P-wave and 0.20s for S-wave, respectively. Thus we did the same procedure as the real data for the synthetic travel time data to obtain the result of the checkerboard resolution test. Hypocenter location error was defined as the difference between the true (applied) location and the relocated location by the checkerboard resolution test. The result of the checkerboard resolution is shown instead of the actual Vp image in (d) and (e) in Fig. 2. Fig. S4. Result of the reconstruction resolution test. In the reconstruction resolution test, we adopted the obtained three-dimensional seismic velocity structure as the model. We calculated the travel time for each ray path of the data and added the random noise of 0.18s for P- wave and 0.20s for S-wave, respectively. Thus we did the same procedure as the real data for the synthetic travel time data to obtain the result of the reconstruction resolution test. The result of the reconstruction resolution is shown instead of the checkerboard resolution e in (d) and (e) in Fig. S3. Fig. S5. Examples of high-quality A-grade measurements of regional events recorded at station TU.KKMS. (a) Filtered waveforms of the eastern (e), northern (n), and vertical (z) components. The vertical solid line is the S arrival time. The dashed lines are the minimum start time (1) and maximum end time (4) of the window used for processing. (b) For the window shown in gray, the original filtered waveform (top two curves) and the waveform corrected with dt determined by SC91 (bottom two curves) are shown rotated in the early S-wave polarization direction (p), and its vertical direction (p⊥) determined by SC91. The vertical red solid line is the S arrival time. The two dashed lines on either side of the vertical line indicate the range of start windows (1 and 2) and end windows (3 and 4) allowed in the SC91 measurements. (c) Values of φ and dt determined for each measurement window as a function of the window number shown on the abscissa. (d) For all clusters of five or more measurements, the large cross is the cluster finally chosen as the optimal value. (e) The (top) waveform and (bottom) particle motion of the (left) original and (right) corrected waveforms obtained by the finally selected window. (f) Contours of the smallest eigenvalue of the covariance matrix of the finally selected measurement with SC91. Gray boxes in a, b, and e represent the time window for the final measurement. Fig. S6. Result of the time window and filter automatedly determined by MFAST. An example at the station N.SUZH is shown. (a) Histogram of the length of the time window. (b) Histogram of the starting time of the window from S-wave arrival. (c) The lower cut-off frequency of the filter. (d) The higher cut-off frequency of the filter. Fig. S7. Shear wave splitting: temporal change at station N.SUZH with decimal years on the horizontal axis. Delay time between two split shear waves divided by the distance (b) Distance between the hypocenter of the earthquake and the observed station, (c) the orientation of anisotropy, (d) delay time. Fig. S8. Median values of the delay time in 7-day moving windows at station N.SUZH. (a) Cumulative value of median of delay time normalized by the path length. The horizontal axis is the number of the window. (b) The median of delay time is normalized by the length of the path. The horizontal axis is the number of windows. (c) The median of delay time is normalized by the length of the path. The horizontal axis is the year. (d) The delay time, which is not normalized by the length of the path. The horizontal axis is the year. Fig. S9. Shear wave splitting: temporal change: map plots for five time windows. The observation at station N.SUZH is shown. The direction of anisotropy is the direction of the bar. The delay time is shown as the color of a small circle at the center of the bar. They are plotted at the epicenter location. Delay time versus hypocenter depth is also shown. The white star in (d) denotes the M5.4 earthquake on June 19, 2022. The white star in (e) denotes the M6.5 earthquake on May 5, 2023. Fig. S10. (a) Delay time and (b) fast shear wave oscillation direction (the orientation of anisotropy) versus hypocenter depth at station N.SUZH. (c) The direction of anisotropy is the direction of the bar. The delay time is shown as the color of a small circle at the center of the bar. They are plotted at the epicenter location. Fig. S11. Focal mechanisms and stress tensors. They are shown in the lower hemisphere projection. The left figure (a) shows the focal mechanism at each epicenter. We obtained 93 mechanisms from May 2021 to May 8, 2023. The right figure (b) shows the stress tensors in sub-areas with a 0.0625 x 0.0625 degrees size. Grey and white dots denote the P- and T-axis, respectively. Fig. S12. Gravitational-induced stress. This figure shows, from left to right, the topography, the gravity-induced stress field averaged from the surface to a depth of 25 km, and the depth of the Moho surface. In the gravitational stress map, red and blue arrows denote the maximum and minimum compressional stress axis, respectively. Fig. S13. The spatial average of the polarization of a fast shear wave from shear wave splitting is calculated by weighting inversely proportional to the square of the distance from the station. The red rose diagram is normalized, and the yellow bars are the average polarization. Rose diagrams are plotted at the center of each block; blocks with fewer data than the threshold are not plotted. Map view of Vp/Vs at a depth of 18km is presented as background. The ellipse with broken line shows the approximate location of the inherited caldera suggested by the gravity anomaly in Fig 3a. Table S1. List of the temporary stations.