Earthquake and tsunami generation at active continental margins - what do we learn from rock mechanical experiments?

Large earthquakes and tsunamis most frequently occur at active continental margins where oceanic plates are subducted beneath continental lithosphere. 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 complementary key examples as they represent the two main types, accretionary and erosive active continental margins, respectively. Ocean drilling has been conducted on the continental forearc and the incoming oceanic plates. Drill cores mostly consist of 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. Triaxial mechanical experiments demonstrate that under certain circumstances even weak clay-rich sediments are amenable to faulting and surface rupture. Deformation seems to be controlled by small compositional variabilities that result in major differences in mechanical behavior and sediment strength. Rotary shear experiments show that the carbonate content can be crucial for earthquake nucleation. At combined elevated temperature and pore fluid pressure calcareous sediments tend to weaken and deform at a low friction coefficient. This change in friction might control the updip limit of seismicity at lower latitudes around the Pacific Ocean where carbonate subduction is important.