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, dunwang@eqh.dpri.kyoto-u.ac.jp.
Jim Mori,
DPRI, Kyoto University, Kyoto, Japan, mori@eqh.dpri.kyoto-u.ac.jp.
Takahiko Uchide,
DPRI, Kyoto University, Kyoto, Japan, uchide@eqh.dpri.kyoto-u.ac.jp.
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.