ABSTRACT

Dr Michael Stipp, GEOMAR, Kiel, Germany

Large earthquakes and tsunamis most frequently occur by subduction of oceanic plates below continental lithosphere at active continental margins. The International Ocean Discovery Program (IODP) investigates subduction processes and earthquake generation in two major research initiatives, the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) offshore SW-Japan and the Costa Rica Seismogenesis Project (CRISP). The two study areas are key examples for the two main types of active continental margins, which are either accretionary or erosive. Ocean drilling has been conducted at the continental forearc wedge and the incoming oceanic plates. Drill cores are dominated by clay-rich sediments in both study areas. Such clay-rich sediments are mechanically weak and deform aseismically as long as they have not been exposed to increasing metamorphic conditions. Despite the tectonic differences between the Nankai and the Middle America trench and forearc regions there is evidence for upper crustal faulting and tsunami generation in both areas. Rock mechanical data demonstrate that under certain circumstances even weak clay-rich sediments are amenable to faulting and surface rupture. This deformation behavior seems to be controlled by small compositional variabilities that result in major differences in mechanical behavior and sediment strength. Shear experiments also show that carbonate contents might be crucial for earthquake nucleation. At elevated temperature calcareous sediments tend to weaken and deform at low friction. This could explain the updip limit of the seismogenic zone at lower latitudes around the Pacific Ocean where carbonate subduction is important.