Geologically, a megathrust fault develops where an oceanic plate subducts under an oceanic or continental plate. Megathrust earthquakes are more powerful than any other type. Yet, some megathrust quakes only rupture limited areas, meaning a less destructive earthquake. The reasons are believed to involve structural factors in the subducting and overriding plates. The EU-funded project 'Controls on megathrust earthquake segmentation' (QUAKESEG) aimed to investigate subduction zone structures and factors affecting megathrust earthquake rupture. The team analysed data from three subduction zones, to document structural variation in the upper and lower plates and the resulting effects on earthquakes. After two years, QUAKESEG concluded in April 2014. Project members first completed a literature review on previous megathrust events. Next, they collated and analysed the necessary geophysical data, including seismic reflection and swath bathymetry. The analysis revealed variation in the plates' structures that may potentially limit earthquake rupture, and which could have been significant in the two recent powerful megathrust earthquakes in Sumatra (2004) and Chile (2010). The details — which among others involve composition and physical properties of subducting sediments — are outlined in conceptual models developed by the project. The results were presented at international conferences, and published as three journal papers. The new concepts have also been evaluated in terms of transferability to subduction zones that have not experienced large earthquakes in recent centuries. QUAKESEG has helped to reveal structural factors affecting the propagation and arrest of large seismic ruptures. The results are valuable for comparison with similar fault systems.
Megathrust faults, subduction zones, structural variation, megathrust earthquake