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Controls on Megathrust Earthquake Segmentation

Final Report Summary - QUAKESEG (Controls on Megathrust Earthquake Segmentation)

Great subduction zone earthquakes rupture thousands of square-kilometres of a convergent continental margin with co-seismic slip up to some tens of meters. However, in some areas along and across the plate-boundary seismic rupture is arrested and the earthquake does not propagate any farther. Because the spatial extent of the rupture area controls the magnitude of the earthquake and the potentially resulting tsunami, it is crucial to better understand subduction-zone structures and in particular the parameters that control earthquake rupture in time and space. A prominent example to illustrate this is the difference between the 2004 Aceh-Andaman and the 2011 Tohoku-Oki (Japan) earthquakes on the one side, and the 2010 Maule (Central Chile) earthquake on the other side. During the 2004 Aceh-Andaman and the 2011 Tohoku-Oki earthquakes co-seismic rupture extended up-dip to the shallow-most part of the plate-boundary, in both cases enhancing the magnitudes of the disastrous tsunamis that followed the events. In contrast, during the 2010 Maule earthquake co-seismic rupture stopped much deeper in the subduction zone, and the resulting tsunami was significantly smaller. This project aimed to increase the understanding of factors that control the spatial extent of co-seismic rupture during large plate-boundary earthquakes. Marine reflection seismic and bathymetric data from a number of subduction zones (Sumatra, Central Chile, and Cascadia) was analyzed in order to document structural and morphological variations in the upper overriding and the lower subducting plate which impact on earthquake rupture. The results which are published in international peer-reviewed journals can help to support future global hazard assessment and mitigation efforts in regions that expect powerful subduction earthquakes.

Project objectives
The project objectives were defined as follows:
Objective 1: Synthesis and compilation of earthquake parameters from subduction zones worldwide in order to identify areas where co-seismic rupture ceased/stopped during past earthquakes.
Objective 2a: Analysis of bathymetric and marine reflection seismic data to document how structural variations in the upper overriding plate affect earthquake rupture in the different areas documented during objective (1).
Objective 2b: Analysis of bathymetric and marine reflection seismic data to document how structural variations and morphologic features on the lower subducting plate impact on earthquake rupture in the different areas documented during objective (1).
Objective 3: Application of results obtained in objective (2) to constrain possible future earthquake boundaries at continental margins where earthquake segmentation is not constrained (as identified during objective (1)) due to a lack of a detailed past earthquake records.

Work performed since the beginning of the project
During the first phase of the project a literature study helped to compile available information on spatial extend of earthquake rupture during past megathrust events. Once an overview on observed and proposed earthquake barriers was established, multi-resolution geophysical data, including seismic reflection and swath bathymetry, was assembled for the individual areas with the support of national and international project partners. The data was then analyzed with respect to structural and morphologic variations in the upper overriding and lower subducting plate that could potentially impact on (or limit) earthquake rupture. The obtained results were discussed with colleagues at the University of Southampton, UK and international collaborators as well as presented at international conferences. This has led to the preparation of two manuscripts which are already published in international peer-reviewed journals (Terra Nova, Geochemistry, Geophysics, Geosystems). During the last stage of the project the obtained results were evaluated with respect to their potential transferability to subduction zones that did not experience large plate-boundary earthquakes over the last 100-200 years (e.g. Cascadia, Southern Sumatra, and Northern Chile).

Main results achieved so far
Reflection seismic and bathymetric data from offshore Chile revealed how upper plate mass-wasting processes could have limited seismic rupture during the 1960 Great Chile and the 2010 Maule earthquakes. Based on the data I was able to show that the boundary between the two earthquakes was the site of a giant submarine slope failure with chaotic debris subducted to seismogenic zone depth today. Here, the plate-boundary is not continuous in space, whereas away from the slope failure, a continuous plate-boundary is seismically imaged. I developed a conceptual model that discusses how the subduction of of inhomogeneous slide deposits prevents the development of a spatially continuous plate-boundary, and thus the formation of a thin continuous slip zone necessary for earthquake rupture propagation. In this context it is the overriding plate that self-organizes seismicity and the distribution of earthquake slip in time and space. This work is published in Terra Nova, v25, p472–477, 2013.
During the giant 2004 Mw 9.2 Aceh-Andaman Earthquake shallow seismic rupture that extended up-dip close to the seafloor was primarily responsible for the disastrous Indian Ocean Tsunami that followed the event. Based on seismic reflection data I was able to show that the shallow rupture was enhanced by thick, clay rich sediments on the lower plate. At least in the southernmost part of the rupture area the formation of a strong bulk sediment section and a high fluidpressured predécollement, that likely enabled the 2004 rupture to reach the shallow plate-boundary, result from thermally controlled diagenetic processes in the upper oceanic basement and overlying sediments. Thickening of the sediment section to >2 km ~160 km seaward of the subduction zone increases temperatures at the sediment basement interface. At equivalent temperature / depth range mineral transformation and dehydration (e.g. smectite–illite) occur prior to subduction. The liberated fluids migrate into a layer of high porosity and permeability, which has been discovered in previous studies and is unique to the 2004 rupture area where they generate a distinct overpressured predécollement. Other effects of clay mineral transformation include processes of semi-lithification and induration of sediments. Coupled with compaction dewatering and amplified by the thick sediment section, these effects strengthen the bulk sediments. Farther to the south, where the 2005 Sumatra Earthquake did not include similar shallow rupture, sediment thickness on the oceanic plate is significantly smaller. Therefore, similar diagenetic processes occur at a later stage during subduction. The results are published in Geochemistry, Geophysics, Geosystems, v14, p3315-3323, 2013.

Expected final results and their potential impact and use
As this is the last reporting period of the project the final scientific results are mostly similar to the “Main Results” discussed above. The improved knowledge on what governed (and stopped) seismic rupture during the 2004 Aceh-Andaman and the 2010 Maule earthquakes can help to evaluate the possible extent of seismic rupture in subduction zones with a comparable geologic regime (sediment thickness, lithology, and thermal structure). However, it has to be noted that subduction-zones are among the geologically most complex regions on earth and a lot of future research and unfortunately also more destructive earthquakes are necessary to improve our knowledge on processes and mechanisms that govern co-seismic rupture.