Report

Earthquake in a Maze: Compressional Rupture Branching During the 2012 Mw 8.6 Sumatra Earthquake

Science  10 Aug 2012:
Vol. 337, Issue 6095, pp. 724-726
DOI: 10.1126/science.1224030

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  1. Fig. 1

    Spatiotemporal distribution of HF radiation imaged by the (left) European and (right) Japanese networks. Colored circles and squares indicate the positions of primary and secondary peak HF radiation (from movies S1 and S2, respectively). Their size is scaled by beamforming amplitude, and their color indicates timing relative to hypocentral time (color scale in center). The secondary peaks of the MUSIC pseudo-spectrum are those at least 50% as large as the main peak in the same frame. The brown shaded circles in the right figure are the HF radiation peaks from the Mw 8.2 aftershock observed from Japan. The colored contours in the Sumatra subduction zone (left) represent the slip model of the 2004 Mw 9.1 Sumatra earthquake (28). The figure background is colored by the satellite gravity anomaly (left) in milligalileos (mgals) (color scale on bottom left) and the magnetic anomaly (right) in nanoteslas (color scale on bottom right). Black dots are the epicenters of the first day of aftershocks from the U.S. National Earthquake Information Center catalog. The big and small white stars indicate the hypocenter of the mainshock and Mw 8.2 aftershock. The moment tensors of the Mw 8.6 mainshock, Mw 8.2 aftershock, and double CMT solutions of the mainshock are shown as colored pink, yellow, red, and blue beach balls. The red line in the top left inset shows the boundary between the India (IN) and Sundaland (SU) plates (29). The patterned pink area is the diffuse deformation zone between the India and Australia plate. The red rectangular zone indicates the study area. The top right inset shows the interpreted fault planes (gray dashed lines) and rupture directions (colored arrows).

  2. Fig. 2

    Spatiotemporal details of the rupture process. (Left) Timing and position of the HF radiators relative to the hypocenter. The position is reported in alternation along the axes labeled X (red) and Y (blue) in Fig. 1, inset. Circles and squares are the results of Europe and Japan arrays, respectively. Solid and open symbols indicate principal and secondary HF radiators, respectively. (Inset) Shear strength (τ) versus normal stress (σ) diagram of a nonlinear strength envelope with small apparent friction coefficient μ (almost pressure-insensitive material) and large cohesion C, resulting in almost orthogonal failure planes (ϴ ~ 90°). (Right) Dynamic Coulomb stress changes induced near the tip of a right-lateral crack propagating at steady rupture speed, resolved onto orthogonal left-lateral faults in the compressional quadrant as a function of the ratio between rupture speed and shear-wave speed (Vr/Vs) (30). The symbols denote dynamic changes of normal stress (Δσxx, negative compressive, blue dashed line), shear stress (Δσxy, positive left-lateral, red dashed line), and Coulomb stress (Δσxy + μΔσxx, color solid curves, assuming various apparent friction coefficients μ indicated in the legend). Stresses are normalized based on the Mode II stress intensity factor (KII) and the distance to the crack tip (r). Rupture on the compressive side can be triggered (positive Coulomb stress change) only for low enough apparent friction and rupture speed.

  3. Fig. 3

    Bathymetry where the rupture crosses the NER. Colored background is global bathymetry from SRTM30+ overlain by multibeam bathymetry from cruise KNOX06RR and cruise DYNAMO, respectively. Black dots indicate aftershocks, and circles indicate HF source radiators. These indicate rupture through the NER during the last 15 s of the earthquake. The rupture plane is consistent with numerous fault scarps visible in the multibeam bathymetry.