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1 doi: /nature11492 Figure S1 Short-period Seismic Energy Release Pattern Imaged by F-net. (a) Locations of broadband seismograph stations in Japanese F-net used for the Hz P wave back-projection of the M w 8.7 and M w 8.2 Indo-Australia earthquakes. Stations were selected for high P wave similarity. (b) Local beam power maxima during the back-projections, with color indicating time after the nominal origin time. Symbol size is proportional to beam power. (c) Time integrated beam power for the M w 8.7 backprojection. The circles are early NEIC aftershocks with symbol size proportional to magnitude, and the dashed line is the Sumatra trench. The inset shows peak beam power as a function of time. (d) same as panel (c), but for the M w 8.2 aftershock. The timedependent evolution is best evaluated by viewing Movie S2. 1

2 Figure S2 Long-period Seismic Energy Release Pattern. Maps showing the spatial patterns of long-period surface wave energy sources for the 11 April 2012 (M w 8.7) mainshock (epicenter indicated with a red outlined star) imaged using (a) R1 source time functions (STFs) only, (b) G1 STFs only, and (c) joint R1 and G1 STFs. Azimuthal weighting proportional to the R1 or G1 radiation patterns is used for each case to suppress instabilities near radiation nodes. 2

3 Figure S3 Time-varying Images of the Long-Period R1 and G1 Back-projection for the Mainshock. The back-projections of R1 and G1 source time functions (STFs) over the source region grid are shown for the indicated times, with the color image indicating locations of source radiation at long-period. The left column shows the images from just R1 STFs, the middle column shows the images from just G1 STFs, and the right column shows the images from joint R1+G1 imaging (as in Figure 3), where weighting proportional to radiation pattern coefficient has been applied. Linear stacking is used in each case. The secondary sources to the SW at 80s and to the west at 130 s have amplitudes about 20-30% of the peak near the hypocenter, consistent with seismic moment for M w subevents. 3

4 Figure S4 Long-period Seismic Energy Release Pattern for the M w 8.2 Aftershock. The maps show the spatial pattern of long-period surface wave energy release for the large aftershock on 11 April 2012 (M w 8.2) for which the red-outlined star indicates the epicenter, as imaged from (a) R1 source time functions (STFs) and (b) G1 STFs. The one-week aftershock distribution is shown by small circles with radii proportional to seismic magnitude. The color image indicates the peak stacked energy from the STFs back-projected relative to the epicentral location of the 10 January 2012 (M w 7.2) event (orange stars) which was used for empirical Green functions (EGFs). The star with black outline indicates the mainshock epicenter. The yellow circle in the images indicates a reference location of energy release that gives the move-out of R1 and G1 STF arrivals. The straight lines indicate the probable orientation of the fault that ruptured in the event. Panel (c) shows the azimuthally binned and stacked R1 (red) and G1 (blue) STFs plotted as functions of time relative to the EGF epicenter with a 100 s time shift from the aftershock origin time. These STFs represent seismic moment as a function of time observed in surface waves at different azimuths. The yellow curves correspond to the yellow circles in (a) and (b), showing the predicted arrival times of energy from that location, with the S-curve pattern caused by the relative propagation times toward different azimuths. The solid curve is for R1 waves using a velocity of 4.0 km/s and the dashed curve is for G1 waves using a velocity of 4.5 km/s. The STF signals aligned on such curves for each position in the map form the corresponding images in (a) and (b). 4

5 Figure S5a Observed and Predicted Broadband Waveforms for the 11 April 2012 M w 8.7 Mainshock. Broadband P wave ground displacement data filtered in the passband s are shown with black lines. The station name, angular distance, Δ, and azimuth from the source, φ, are shown. Similarly filtered synthetics for the preferred fault model in Figure 4 are shown with red traces. Some traces are truncated to avoid contamination from PP arrivals. All of these traces are vertical component P waves. 5

6 Figure S5b Observed and Predicted Broadband Waveforms for the 11 April 2012 M w 8.7 Mainshock. Broadband P wave ground displacement data filtered in the passband s are shown with black lines. The station name, angular distance, Δ, and azimuth from the source, φ, are shown. Similarly filtered synthetics for the preferred fault model in Figure 4 are shown with red traces. Some traces are truncated to avoid contamination from PP arrivals. All of these traces are vertical component P waves. 6

7 Figure S5c Observed and Predicted Broadband Waveforms for the 11 April 2012 M w 8.7 Mainshock. Broadband ground displacement data filtered in the passband s are shown with black (vertical component P waves) or blue (tangential component SH waves) lines. The station name, angular distance, Δ, and azimuth from the source, φ, are shown. Similarly filtered synthetics for the preferred fault model in Figure 4 are shown with red traces. Some traces are truncated to avoid contamination from PP or SS arrivals. 7

8 Figure S5d Observed and Predicted Broadband Waveforms for the 11 April 2012 M w 8.7 Mainshock. Broadband surface wave STF data stacked in 10 azimuthal bins are shown with blue lines (R1 STFs) and black lines (G1 STFs). The central azimuths from the source for each stacking bin, φ, are shown. Corresponding synthetics for the preferred fault model in Figure 4 are shown with red traces. 8

9 Figure S6 Finite-Fault Inversion For Choice of NNE-SSW Fault on Ninetyeast Ridge. The map shows fault segments activated during the 11 April 2012 M w 8.7 mainshock, specified for finite-fault slip model inversion of teleseismic broadband P and SH waves and G1 and R1 STFs. In this case, we choose the NNE-SSW orientation for segment f, with rupture trending along the Ninetyeast ridge. The red star indicates the epicentral location, and red dots indicate the placement of hypocenters (all at 30 km deep) on each fault segment. The time at which each segment begins to rupture is indicated by T0.The rectangles indicate the subfault strikes and dips (shallow edge is on the green side, deeper edge on the black side. We use 4 subfaults, two of which are subdivided, with onset times constrained by the short-period back-projections. The corresponding slip distributions on each subfault are shown below, with the subfault grids having 20 km spacing along strike and 10 km spacing along dip. The peak slip and M w of each subfault is indicated, as is the position of the hypocenter on each subfault from which the rupture expands. The rupture velocity is 2.0 km/s on all subfaults. This solution can be compared with that in Figure

10 Figure S7 Point source moment tensor representations for the 8.7 event. The top solution is the direct sum of all subfault moment tensors from the model in Figure 4, with no allowance for space-time variations. The second solution is the sum allowing for phase delay. The lower two are actual data inversions by the Global CMT and W-phase methods. The fault model solution produces only minor non-double couple component, although a complete calculation with the spatial effects will increase it, possibly to the level of the observed solutions. 10

11 Movie S1 QuickTime (.mov) animations of short-period P wave back-projections for the 11 April 2012 M w 8.7 event using stations in Europe and Japan. The left panel is for European recordings filtered in the Hz band. This corresponds to the information in Figure 2b,c. The middle panel is for the same European recordings filtered in the Hz band. The right panel is for the F-net broadband recordings in Japan filtered in the Hz band. The movie is also available at:

12 Movie S2 QuickTime (.mov) animations of short-period P wave back-projections for the 11 April 2012 M w 8.2 event using stations in Europe and Japan. The left panel is for European recordings filtered in the Hz band. This corresponds to the information in Figure 2d. The middle panel is for the same European recordings filtered in the Hz band. The right panel is for the F-net broadband recordings in Japan filtered in the Hz band. The movie is also available at: