Boulder, Colo., USA - New geophysical data show that fault slip during the March 2005 magnitude 8.7 (Mw) earthquake off the west coast of northern Sumatra, Indonesia (also referred to as the Simeulue-Nias earthquake), was stopped by the topography on the downgoing plate.
Earthquakes in subduction zones, where one tectonic plate is forced beneath another, usually break only a part of the plate boundary fault. The pieces that break independently are known as segments. Topography on the top of the downgoing plate has often been suggested a cause of this segmentation, but there are few examples where this topography is as well-known as well as the details of earthquake rupture.
Data collected over the subduction zone offshore of Sumatra, Indonesia, has enabled the top of the downgoing plate to be mapped across a long-lived segment boundary at one end of the rupture zone. Seismic reflection data, similar to that used to find oil reserves, gives a detailed image of the shape of the downgoing plate. A 3-km high on the top of the plate over a 15-km by 30-km region matches where the 2005 earthquake rupture stopped. The topographic high appears to strengthen the plate boundary, and only very large earthquakes would break through this barrier.
This survey by Timothy Henstock and colleagues spans a complex segment boundary zone between the southern termination of the Mw 8.7 earthquake and the northern termination of a major 1797 earthquake that was partly filled by a Mw 7.7 event in 1935. They have identified an isolated 3 km basement high at the northern edge of this zone, close to the 2005 slip termination. They note that the high probably originated at the Wharton fossil ridge, and is almost aseismic in both local and global data sets, suggesting that while the region around it may be weakened by fracturing and fluids, the basement high locally strengthens the plate boundary, stopping rupture propagation.
FEATURED ARTICLE
Downgoing plate topography stopped rupture in the A.D. 2005 Sumatra earthquake
Timothy J. Henstock et al., National Oceanography Centre Southampton, University of Southampton, European Way, Southampton SO14 3ZH, UK. This article is OPEN ACCESS online at http://dx.doi.org/10.1130/G37258.1.
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Fault friction controls the generation of earthquakes. When slip occurs during earthquakes, the faults weaken dramatically. Understanding the mechanisms that cause fault weakening is fundamental to understanding earthquake propagation and slip. The proposed weakening mechanisms include thermally and non-thermally activated ones, but their dominance remains enigmatic because it is difficult to control temperature independently in high-speed sliding due to the coupling of frictional heating and experimental conditions (e.g. slip rates and stresses). This study introduces different thermally-conductive driving (host) blocks to sandwich incohesive fault rocks (gouges) in high-speed friction tests, and succeeds in varying temperature conditions by changing the speed of heat conduction out of the tested samples. The experiments show that high-velocity weakening of gouge becomes more pronounced as thermal conductivity of host blocks decreases. Temperature calculation and microstructural observations suggest big differences in the slip-zone temperature in the case of different host blocks. These results demonstrate that temperature rise driven by frictional heating is essential in causing dynamic weakening of fault gouge. The observed results are most likely to be explained by the flash-heating model that considers the role of the slip-zone temperature. Moreover, the presence of nanoparticles alone is not sufficient to cause dynamic weakening of faults.
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