Title: The influence of subduction zone deformation and geometry on the genesis of megathrust earthquakes and tsunamis
Abstract: The results presented in this cumulative ‘Habilitation’ document a nearly decade-long effort to image the interior regime of convergent margins. Subduction zones are formed by the underthrusting of a lithospheric plate (commonly of oceanic nature) underneath the overriding plate along the subduction thrust fault or ‘megathrust’. The subducting plate moves into the earth’s mantle at rates typically measured in centimeters per year. Stress accumulation in this high-friction setting may be released through seismic slip during an earthquake. The interplate environment hosts the seismogenic zone (typically at depths ranging from 5-40 km) where megathrust earthquakes are generated. These interplate earthquakes are the largest occurring on the planet. Due to the marine setting of subduction zones, tsunami waves are a common phenomenon in relation with megathrust events. As 60% of the global population lives within 50 km of the coast, damage and casualties related to subduction zone earthquakes or earthquaketriggered events rank highest among all geohazard-associated losses. The 2004 Sumatra earthquake and associated secondary processes served as a reminder that these hazards are major threats to society not only in regions from which they originate, but also on a global scale. Against this backdrop, the studies summarized here were initiated to improve our knowledge on the geological framework of the different types of subduction zones and on the control of the geometry of the subduction zone on seismic rupture and tsunami hazard. Data acquisition during seven research cruises in the Indian Ocean, Caribbean, and Pacific yielded a wealth of information that was analyzed in the course of these investigations. The results have been published in a series of inter-related scientific papers presented in the Appendix and referenced in bold font in the main text. The umbrella for these investigations is provided in the following chapters:
Chapter 1 sets the stage by introducing the motivation and aims of the studies. Based on the observation that some segments of subduction zones produce large megathrust events of moment magnitudes > 8.5, whereas other portions of the same margin experience only moderate size earthquakes, the implication is drawn that individual subduction zones must show a high variability in their structure and geometry to induce such diverse seismicity. Elucidating the structural diversity between different margin segments using exemplary field data from Indonesia’s Sunda Margin is therefore one of the goals of this work. Chapter 2 summarizes the current knowledge on convergent margin kinematics. The notion that the subduction channel, which hosts the material sandwiched between upper and lower plate, influences megathrust seismogenesis is introduced here in relation to the role of subducted basement relief serving as earthquake nucleation or termination patches. These concepts are expanded and discussed in Chapter 3, which presents the original research documented in the contributions comprising the Appendix and puts them in context. Chapter 3.1 reviews the framework concept on margin structure and geometry, which was developed based on the investigations published in the ten papers discussed in Chapter 3.1.1.1-3.1.4.2. The traditional perception of ‘steady-state’ accretion is disproved by the recognition of multiple kinematic boundaries in a single subduction complex, resulting in across-strike forearc segmentation. Comparison with erosive systems shows that many elements present in accretionary settings are also recognized in erosive margins. Furthermore, variations in lower to upper plate mass transfer result in along-strike margin segmentation. This chapter also inspects new analysis schemes developed during the course of the studies. Chapter 3.2 then lays the bridge between kinematics and dynamics and links three papers, which examine oceanic plateau and ridge subduction at locations in the Atlantic, Pacific, and Indian Ocean. The last section of Chapter 3 extracts the essence of the previous investigations and reviews the notion that subducting oceanic relief may act as ‘asperities’ or ‘barriers’ to seismic rupture and relates this to the observed absence of magnitude > 8 subduction earthquakes along the Java Trench.
A synopsis is provided in Chapter 4, which also inspects the results of a number of PhD-theses that emerged from the studies. The final Chapter 5 discusses unresolved issues and problems in subduction zone research and provides an outlook on future research strategies to tackle these open questions.
Publication Year: 2010
Publication Date: 2010-01-01
Language: en
Type: dissertation
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