Imaging an Active Plate Boundary Fault
6. The Seismogenic Zone
NanTroSEIZE - Imaging and Drilling the Megathrust and Seismogenic Zone
The Nankai Trough defines a trench along the seafloor extending approximately 770 kilometers from the Suruga Bay to where the Philippine Sea Plate slips (subducts) under southeast Japan. As a result of plate convergence and subduction, sediment on the downgoing Philippine Sea plate is continuously scraped off against and added to the overriding plate, forming a mass of highly deformed and faulted sediments called the accretionary prism. Plate convergence is this region has generated large-scale earthquakes and tsunamis every 120 years on average, including the historic earthquakes in 1944 and 1946 with magnitudes of 8.1 and 8.3, respectively. A primary goal of the NanTroSEIZE drilling project is gaining understanding of the deformation within the accretionary prism and its fault zones to better understand how earthquakes are generated and why some earthquakes cause disturbance at the seafloor that can produce tsunamis. However, drilling operations are very expensive, consequently every effort must be made to locate each drill hole accurately to sample the critical geologic features, such as the areas of the Megasplay fault and the Megathrust, along which fault slip results in devastating earthquakes, called the seismogenic zone (shown by red lines in the diagram below).
Rocks of the accretionary prism on either side of the seismogenic zone need to be sampled to understand the mechanics of faulting, and in one borehole, the top of the subducting Philippine Sea plate will be sampled by the drill. Sediments, shed from the nearby islands, sink through the ocean and are deposited on top of the accreationary prism. These sediments form layers that can provide information on the vertical motions of the underlying prism, potentially related to the history of seismic events.
Before drilling, we must first create the best possible image, in three-dimensions, extending 5 to 6 miles (8-9 kilometers) beneath the seafloor, which is the objective of this survey. The results of this survey will guide the selection of the drill sites, allow the samples to be related to the large-scale geologic structures, and will be critical to the placement of long-term monitoring instruments in the drillholes to measure any precusory events in advance of future earthquakes.
Diagram showing proposed locations of drillsites superimposed on older, two-dimensional seismic profile, showing a vertical slice through the earth's crust with the major rock bodies identified by the different colors. (Modified from. H. Tobin)