Rupture Process of the April 11, 2012 Sumatra (Mw 8.6)  Earthquake

Imaged with Back-Projection of Hi-net data.


Dun Wang and Jim Mori

(GRL, in press)

A great (Mw8.6) strike-slip earthquake occurred at shallow depth off the west coast of northern Sumatra on April 12, 2012. The earthquake was located on the Indo-Australian plate, west of the 2004 Sumatra-Andaman earthquake (Mw9.1).


Methods and Data

We applied a back-projection analysis to study the rupture process. The back-projection was done using the same method described in Wang and Mori (2011).


P waveforms from Hi-net stations were used in a back projection analysis of the rupture for this earthquake. We used unfiltered Hi-net data (vertical components) for the back projection. The stations in Japan at distances of about 41 to 61 degrees from the earthquake epicenter. The azimuth is about 45 degrees.



The results show a complex pattern of four conjugate faults over about 180 sec (see Figure 1). There is a striking correspondence between the lengths and orientations of our rupture pattern with the distribution of aftershocks. Each of the first three stages of the rupture corresponds to a clear lineation in the aftershocks, with lengths of 200 to 400 km (Figure 2). Rupture speeds for several of the fault segments were very high, about 5 km/s, and exceed the local S-wave velocity (Figure 3). This is the first example of an oceanic earthquake with supershear rupture speed





Figure 1. Animation of the rupture propagation.





Figure 2. Rupture propagation and radiated energies. (a) Cumulative short-period radiated energy of the 2012 Mw 8.6 Sumatra earthquake for Stage 1, 2, 3 and 4. (b) Locations of aftershocks (yellow open circle) that occurred two days following the mainshock. The red and black stars indicate the USGS determined epicenters for the 2012 Off Northern Sumatra (Mw 8.6) and 2004 Sumatra-Andaman (Mw 9.1), respectively. (c) Locations, timings and amplitudes for the stack with the maximum correlation at each time step (2 s). The dashed gray line is the Sumatra trench. The black triangle indicates the reference point for calculating the rupture speed for Stages 3 and 4. Left small inset shows a model for the four Stages.



Figure 3. Rupture speed and energy release. (a) Normalized value of the maximum amplitudes (sums of squared amplitude stacks) in each time window as a function of time. (b) Distance as a function of time for the locations of the maximum stack amplitude in each time window, as measured from the epicenter for the first 100 s and from the location of the reference point shown in Figure 2c after 100 s. The colored lines show the slopes of rupture speeds for reference.



Wang, D. J. Mori, and T. Uchide (2012), Supershear rupture on multiple faults for the Mw 8.6 Off Northern Sumatra, Indonesia earthquake of April 11, 2012, Geophys. Res. Lett., 39, L21307, doi:10.1029/2012GL053622.


Wang, D. and J. Mori, Rupture Process of the 2011 off the Pacific Coast of Tohoku Earthquake (Mw 9.0) as Imaged with Back-Projection of Teleseismic P-waves, Earth, Planets and Space, 63, 603-608,  doi:10.5047/eps.2011.05.029, 2011.