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Abstract

Drilling the input materials of the north Sumatran subduction zone, part of the 5000 km long Sunda subduction zone system and the origin of the Mw ~9.2 earthquake and tsunami that devastated coastal communities around the Indian Ocean in 2004, was designed to groundtruth the material properties causing unexpectedly shallow seismogenic slip and a distinctive forearc prism structure. The intriguing seismogenic behavior and forearc structure are not well explained by existing models or by relationships observed at margins where seismogenic slip typically occurs farther landward. The input materials of the north Sumatran subduction zone are a distinctively thick (as thick as 4-5 km) succession of primarily Bengal-Nicobar Fan-related sediments. The correspondence between the 2004 rupture location and the overlying prism plateau, as well as evidence for a strengthened input section, suggest the input materials are key to driving the distinctive slip behavior and long-term forearc structure. During Expedition 362, two sites on the Indian oceanic plate ~250 km southwest of the subduction zone, Sites U1480 and U1481, were drilled, cored, and logged to a maximum depth of 1500 meters below seafloor. The succession of sediment/rocks that will develop into the plate boundary detachment and will drive growth of the forearc were sampled, and their progressive mechanical, frictional, and hydrogeological property evolution will be analyzed through postcruise experimental and modeling studies. Large penetration depths with good core recovery and successful wireline logging in the challenging submarine fan materials will enable evaluation of the role of thick sedimentary subduction zone input sections in driving shallow slip and amplifying earthquake and tsunami magnitudes, at the Sunda subduction zone and globally at other subduction zones where submarine faninfluenced sections are being subducted.

Sedimentology, stratigraphy and architecture of the Nicobar Fan (Bengal–Nicobar Fan System), Indian Ocean: Results from International Ocean Discovery Program Expedition 362

“…1). They were drilled, cored and logged to a maximum depth of 1500 m below seafloor (mbsf), and reached the 60 to 70 Ma igneous oceanic crust of the Indian plate (Dugan et al, 2017;McNeill et al, 2017a,b). The two sites are ca 35 km apart making them at the same proximity to the source of the fan (Fig.…”

“…Figs 2 and 3), some of which are divided into lithological subunits based on the vertical organization of the dominant lithologies (Dugan et al, 2017). Lithological units and subunits I, IIA to IIC and IIIA contain siliciclastic sediments of the Nicobar Fan to a depth of 1250.35 mbsf (Site U1480).…”

“…Subunits LU-IIIB to LU-V are interpreted as pre-fan deposits while LU-VI is identified as the basaltic crust of the Indian Plate (Dugan et al, 2017;McNeill et al, 2017b). The basal part of the Nicobar Fan (LU-IIIA), unlike LU-IIC with thin sandstone beds, only contains siliciclastic mudstones and is here interpreted as recording the very earliest SGFs (muddy turbidity currents) to reach this part of the Indian Ocean.…”

“…International Ocean Discovery Program (IODP) Expedition 362 (August to October 2016) sampled and logged a complete section of the Nicobar Fan seaward of the subduction zone of North Sumatra down to the oceanic crust (Dugan et al, 2017;McNeill et al, 2017a,b). Designed to investigate the causes of shallow seismogenesis, tsunamigenesis and forearc development of the Sunda subduction zone system where the 2004 M w 9.2 earthquake struck, IODP expedition 362 also offers a unique opportunity to precisely determine the lithologies, properties and sedimentation rates of the material emplaced in the Nicobar Fan since its formation.…”

Processes Governing Giant Subduction Earthquakes: IODP Drilling to Sample and Instrument Subduction Zone Megathrusts

“…Ocean drilling sites in the Indian Ocean prior to the 2004 Sumatra-Andaman earthquake were mainly located on the Bengal-Nicobar Fan and the Ninety East Ridge, where drilling was designed to reveal the links between Himalayan tectonics and the climate of Asian monsoon (Deep Sea Drilling Project Leg 22, Ocean Drilling Program [ODP] Legs 116 and 121, and IODP Expedition 353 and 354). IODP Expedition 362 targeted two drilling sites on the incoming plate in order to reveal what factors controlled the 2004 great earthquake and tsunami in relation to the development of the Bengal-Nicobar Fan and the huge accretionary prism characteristic of the hanging wall of the rupture area of the 2004 earthquake (Dugan et al, 2017).…”