Exploring North Sumatran subduction margin.
19 August 2016
Prof Kevin Pickering, is a shipboard scientist and Lead Sedimentologist on the Sumatra Seismogenic Zone - International Ocean Discovery Program (IODP) Expedition 362. IODP Expedition 362 aims to examine the role of seafloor sediments and their potential effect on seismogenesis and tsunamogenesis in this part of the North Sumatra seismogenic zone.
On December 26, 2004, a magnitude 9.2 earthquake struck North Sumatra and the Andaman-Nicobar Islands due to slip on a subduction zone plate boundary fault. The resulting tsunami inundated coastal communities around the Indian Ocean, killing over 250,000 people. This earthquake and the Japan Tohoku-oki earthquake in 2011 both ruptured to much shallower depths than expected, producing very large earthquakes and tsunami, and prompting a re-evaluation of earthquake slip potential and of the properties of shallow subduction faults. Subsequent large magnitude earthquakes have struck this margin since 2004, including unusually large earthquakes in the oceanic crust offshore North Sumatra in 2012. Therefore developing a better understanding of earthquake and tsunami behavior and potential is a priority for local communities, for the wider Indian Ocean, and for related subduction zones. The North Sumatran subduction margin has an unusual structure and morphology that is likely influenced by the properties of the sediments and rocks forming the margin. Although our understanding of this margin’s structure and development has increased enormously since 2004 due to geophysical data collection, as yet very little is known of the material properties of this subduction zone. This project will begin to investigate how input materials drive shallow earthquake generation and influence the shape of the continental margin. Our ultimate goal is to understand the hazard potential for this margin, and eventually others with similar material properties and margin morphology.
The materials comprising the Sumatra subduction zone and the sediments entering the system have never been sampled, other than with very shallow cores (maximum ~10 m). The sediments offshore North Sumatra are exceptionally thick due to the Bengal-Nicobar submarine fan sediment system that has developed over 10’s of millions of years from erosion of the Himalayas. This sediment thickness may be the cause of both the unusual observed structure and unexpected earthquake fault behavior. This ocean drilling expedition will for the first time drill scientific boreholes within the sediments entering this subduction zone, including the layer of sediment that eventually develops into the earthquake-generating fault. We know the sediments are of deep sea and terrestrial origin, including those eroded from the high Himalayas and transported 1000's km into the Bay of Bengal and eastern Indian Ocean. But we do not know how the sediments evolve as they become physically and chemically altered as the sediment section builds up to 4-5 km thickness before reaching the subduction zone. Increased burial depth and temperatures also affect fluids within the sediment pile, which are critical to earthquake fault behavior and to how the sediments accrete onto the continental margin. Sampling and measuring the properties of the materials in situ and then extrapolating their properties to greater burial depths using modeling and experimental techniques will together be important goals of this project.
To understand the sediments and fluids entering the North Sumatra subduction zone and to collect data for predicting their behavior and role in geohazards and margin evolution, this scientific ocean drilling (IODP) expedition will spend 56 days at sea coring, sampling, and characterizing the input system at two locations. The sites are located about 300 km offshore North Sumatra in 4000 m of water. At each site we will core from the seafloor to >1500 m below the seafloor. Each site will also include downhole measurements of in situ properties using tools within the boreholes. These measurements will for the first time give us the density, sediment type, temperature, deformation and pressures of these deep-sea sediments. Cores recovered from drilling will be used for laboratory tests to measure the strength of the sediments when buried and the how fluids behave within these sediments, both of which are important for understanding how faults develop and their earthquake potential. Lastly the international team of scientists will integrate results and develop models of how the sediments and fluids develop and then behave once they enter the subduction zone where the large earthquakes are generated.
This project is Expedition 362 of the International Ocean Discovery Program (IODP) and will utilize the research vessel JOIDES Resolution. The vessel is operated by the JOIDES Resolution Science Operator based at Texas A&M University (USA) on behalf of the US National Science Foundation. The expedition will include 33 scientists and 2 educators from 13 countries. IODP and its predecessors have led scientific ocean drilling worldwide since 1968. Currently, IODP has 26 international partners: USA, Australia, Austria, Belgium, Brazil, Canada, P.R. China, Denmark, Finland, France, Germany, India, Ireland, Israel, Italy, Japan, Korea, Netherlands, New Zealand, Norway, Poland, Portugal, Spain, Sweden, Switzerland, and the United Kingdom.