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Log data across the splayfault is key information

 Sean Toczko, Expedition 348 Project Manager explains, “In Expedition 348 we were setting the stage to collect data and sample a feature called the Megasplay fault that's connected to the plate interface and reaches all the way up to the sea floor. It's looked at as a proxy to what's going on below. In order to reach that depth we have to approach it in steps, setting casing pipes of gradually decreasing width, which prevents the drill hole from collapsing in on itself. When we drilled this section we had logging tools attached to the pipe that measured things like the gamma-rays inside the rock, the pressure inside the hole, sonic measurements of the bore-hole, etc. This year when we first ran casing down the existing hole it got stuck. We were able to recover and re-run it successfully. The goal is to get logging data across the splay-fault and use the data to get core-samples. That's the key information we want. We believe this is the major fault that has been slipping -for example the 1944 massive Tonankai earthquake and tsunami. We want to permanently monitor movement in this area, watching for things like fluids which move along the megasplay as a kind of highway from down deep, up to the surface of the sea floor, as well as watch what is considered a locked zone on the plate -where it's currently holding and most likely where it's going to suddenly release.

3 km bore-hole using Logging-while-drilling is major achievement

 Saffer points out “One of the main things we achieved in this expedition was to drill the entire bore-hole down to 3 km with a technology called logging-while-drilling(LWD). This allows us to make measurements in the bore hole, continuously, all the way down. We could measure things like the pressure in the bore hole, rock physical properties etc.. This allows us to extract information about the composition of the rocks and the conditions in the formation - things like the stresses and the pressures.”
“Another Expedition 348 achievement is the collection of all the non-core cuttings from the drilling which provide a really high fidelity continuous record, actually for the first time, into the heart of the upper plate of a subduction zone”. Saffer outlines the scientist's key questions: “What's there? What are its properties? What kind of rock is it? How compacted is it? How strong is it? What's its chemistry?” So we have two levels of records from both the LWD and the physical record of the samples from the deep holes.”

Need to combine research globally for long term monitoring in two or more subduction zones

 Voicing a world view based on his broad experience Saffer tells us, “we hope by combining research globally, we can find some common mechanisms and behaviour.” He goes on, “When we think about earthquake behaviour in different subduction zones, the older oceanic plate tends to have a colder subduction zone. We think one of the most promising models to explain why certain parts of these subduction faults host earthquakes and move more violently while other zones tend to creep along more peacefully is that it's controlled by temperature.”
 “The plate offshore southern Japan is warmer than some other areas, but whether they're more violent is going farther than we can right now - but that's one hypothesis.” Notes Saffer, “If we look globally at the very warm subduction zones with a thick layer of insulating sediment and very young subducting plates, those are places like Cascadia in the Pacific Northwest of the United States, southern Japan and parts of South America, where we typically have larger earthquakes. One of the reasons we're drilling in NanTroSEIZE is because it's younger. The earthquake processes seem to be shallower and closer to the surface here. We think we can actually reach what we call the seismogenic zone 5,000 meters below the sea floor. We would be just tapping into the very shallowest tip of that zone. So that's what's special about Nankai. My hope is that this project will really be a kind of model for how to study subduction zones globally.”
 “One of the real legacies of the NanTroSEIZE project”, Saffer points out, “are the long term observatories. The plan is to install them in the deep borehole as well as the shallow sites, so that we will have the opportunity to observe a major plate boundary in real time and monitor the conditions and the stress that are building in the earth through the seismic cycle leading up to, and during and after, the next earthquake. There is a possibility that this kind of new information could lead toward near-term prediction for damaging events in the future. So an important objective after drilling is long-term monitoring to characterize the fault zones. It needs to be done in at least a couple of subduction zones, not just Nankai, before we can start to see patterns.”