Supershear Rupture on Multiple Faults for the Mw 8.6 Off Northern Sumatra, Indonesia Earthquake of April 11, 2012 Supplementary material Dun Wang1,2, Jim Mori2, and Takahiko Uchide2 1 Key Laboratory of Earthquake Geodesy, Institute of Seismology, China Earthquake Administration, Wuhan, China 2 Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan Contact information: Dun Wang, Key Laboratory of Earthquake Geodesy, Institute of Seismology, China Earthquake Administration, Wuhan, China; DPRI, Kyoto University, Kyoto, Japan, [email protected] Jim Mori, DPRI, Kyoto University, Kyoto, Japan, [email protected] Takahiko Uchide, DPRI, Kyoto University, Kyoto, Japan, [email protected] 1. Results from back projection of Hi-net data at different frequency bands In order to trace the rupture fronts of the earthquake, we look at the higher frequency bands of the data recorded at Hi-net, Japan (say higher than 0.2 Hz). We carried out the same back projection procedure using data filtered into several high frequency bands [Kiser and Ishii, 2011; Wang and Mori, 2011] to see if there is much difference. The spatial locations for each time step (similar to Figure 2c) are shown for each frequency band in Figure S1. The locations of the rupture front as a function of time (similar to Figure 3b) are shown in Figure S2. The spatial and temporal patterns of the rupture are generally similar to the results using the data band-pass filtered in the frequency band 0.5 to 5.0 Hz that is presented in the main text. The rupture speed is also similar showing an initial value of 2.0 to 2.5 km/s, and then accelerating to about 5 km/s for the remainder of the time. 2. Results from back projection of European data We also carried out the back projection analyses using European seismic stations, which are located at azimuths of 307 to 340 degrees and distances of 80 to 93 degrees. Of 515 available stations, we chose data from 170 to 250 sites based on the station density and waveform similarities evaluated with cross-correlations of the initial 6 seconds of the P wave. The broadband data from Europe covers a wider frequency range than the short-period stations of Hi-net in Japan. We used the same frequency bands to filter the data in order to compare the results to the Hi-net data. The locations of the maximum stack amplitude at each time step, as shown in Figure S3. One can observe that results have consistent pattern with the results calculated from Hi-net (Figure S1). The locations of the high frequency waves are a good representation of the rupture fronts, hence we calculated the rupture speeds for each stage of the rupture as defined in the main text (Figure S4). Since the European data are recorded at a different azimuth from the Hi-net data, there is a directivity effect. We corrected the timing of the four stages according to the results derived from the Hi-net data. The European results also show the very fast rupture speeds of 5.0 to 6.0 km/s for Stage 2 and Stage 3, similar to the results we obtained for the Hi-net data. Because there are fewer stations compared to Hi-net and the stations are located at the surface, compared to the borehole site of Hi-net, the results may not be as high quality compared to the Hi-net data. This is verified in the location test described in the next section. 3. Location test using moderate earthquakes In order to check the location bias for the two data sets due to structure heterogeneity in the source region, we examined the locations of six moderate-sized earthquakes (Mw 5 to 6.2) located in the source region of the mainshock (see Table S1). European data for event no.3 were not available. We used stations corrections that were derived from aligning the initial arrivals of the mainshock, and used the same back projection method to locate the smaller earthquakes. Similar to the analyses described in the main text, both data were band pass filtered in the band of 0.5 to 5.0 Hz using two-pole butterworth filters. An area of 100 by 100 km was used to search for the locations of the moderate size earthquakes. Figure S5 shows the locations of the maximum stack amplitude for the 30 seconds starting from the P wave onset; these results indicate that the centroid locations of the back projection results are within 20 to 40 km of the USGS epicenters for both the Hi-net in Japan and the European data. Although the back projection results give centroid locations, not epicenter locations, the source size for M5 to M6 earhtquakes is be expected to be have dimensions of less than 40 km. As expected, the small events closer to the mainshock show smaller location differences, since there should be smaller travel time differences. The Ninety-East Ridge region to the west of the mainshock may be a structurally heterogeneous region that causes larger travel time differences compared the area near the mainshock epicenter. The locations of the local maximum derived from the European data are more widely scattered, implying a larger uncertainty for locating the sources of the high frequency energy. Fewer stations and the surface observations (compared to borehole Hi-net stations) might account for the relative weaker performance of European stations for the high frequency range. 4. Using different events for the empirical array correction In our analyses of the Hi-net data, we used a moderate earthquake (No.2 in table S1) to correct for the array response, when calculating the spatial distribution of the high frequency energy radiation for the mainshock (Figure 2a). Here we used another event (No.1) to check the effect of using different earthquakes. The results show similar pattern for the high frequency energy radiation (Figure S6), suggesting that this empirical correction for the high frequency energy radiation conducted is fairly stable. References 1. Kiser, E., and M. Ishii (2011), The 2010 Mw 8.8 Chile earthquake: Triggering on multiple segments and frequency‐dependent rupture behavior. Geophys. Res. Lett. 38, doi:10.1029/2011GL- 047140. 2. Wang D. and J. Mori (2011), Frequency-dependent energy radiation and fault coupling for the 2010 Mw8.8 Maule, Chile, and 2011 Mw9.0 Tohoku, Japan, earthquakes. Geophys. Res. Lett. 38, L22308, 1029/2011/GL049652. Figure captions Figure S1. Locations, timings and amplitudes for the stacks with the maximum correlation at each time step (2 s) using data filtered in different frequency bands. Top right shows the scale for the size of normalized amplitude. The red star indicates the mainshock epicenter determined by USGS. Figure S2. Distance as a function of time for the locations of the maximum stack amplitude in each time window for different frequency bands. The distance is measured from the epicenter for the first 100 s and from the reference point (black triangle in Figure 2c) after 100 s. Colored lines show the slope of various rupture velocities for reference. Figure S3. Same as Figure S1 except for the results derived from European data. Figure S4. Same as Figure S2 except for the results derived from European data. Figure S5. Locations, timings and amplitudes for the stacks with the maximum correlation at each time step for the several moderate-sized earthquakes (Table S1) using band pass filtered data (0.5 to 5.0 Hz) from European stations (top) and Hi-net (bottom). Solid stars show the epicenters determined by USGS. Open star indicates the epicenter of the 2012 Mw 8.6 earthquake determined by USGS. Figure S6. Comparison of distributions of High frequency Radiated energies using an empirical correction from two different moderate earthquakes Top: Results using earthquake No.1 in Table S1 as the array response. Bottom: Results using earthquake No.2 in Table S1 as the array response. Tables Table S1. Six small earthquakes used for location tests Date and Latitude Longitude Magnitude Depth Time (°N) (°E) (Mw) (km) No.1 2012/04/12 20:21:52 3.780 92.784 5.1 9.2 No.2 2012/04/11 10:21:15 2.833 92.476 5.4 14.8 No.3 2006/04/19 20:36:46 2.640 93.230 6.2 17.0 No.4 2012/04/11 09:27:56 1.281 91.731 6.0 9.8 No.5 2012/04/11 13:58:06 1.485 90.836 5.5 15.6 No.6 2012/04/15 05:57:38 2.549 90.277 6.2 15.2 Dynamic Content Captions: Animation 1: Rupture propagation of the April 11, 2012 Off Northern Sumatra earthquake as imaged by band pass filtered data (0.5 to 5.0 Hz) of Hi-net, Japan array.