JFAST – Japan Trench Fast Drilling Project
The Japan Trench Fast Drilling Project (JFAST) is a seismological and geological investigation to understand the very large slip that occurred on the shallow portion of the fault during the 2011 Tohoku earthquake. This huge amount of slip (30 to 50 meters) was largely responsible for the devastating tsunami that caused much damage and took over 19,000 lives on the northeast coast of Honshu, Japan.
Using the Deep Sea Scientific Drilling Vessel Chikyu, boreholes will be drilled into the main fault zone of the earthquake. There are two main objectives for the drilling expedition. First, obtain a sample of the fault zone so that the physical properties can be analyzed. Second, is to make temperature measurements in order to estimate the frictional stress on the fault during the earthquake
Figure 1. Sketch of JFAST borehole site and subduction fault zone of the Tohoku earthquake
Fault Zone Sampling
Obtaining a sample of the fault that moved tens of meters during the earthquake will provide significant information about the physical properties of fault. No one has ever seen a piece of a fault that has had such huge slip in a recent earthquake.
Detailed analyses of textures and small-scale structures of core samples of the fault zone will be used to infer the role of fluids and pressurization during rupture. We will look for evidence of melting and other processes that contribute to dynamic strength reduction. Trace elements will be used to estimate the thermal history of the recent and past events.
Another important component will be laboratory experiments on fault zone samples from the core. High-speed friction and petrophysical experiments on fault material can be used to characterize the frictional behavior of the fault.
Figure 2. Fault zone sample taken from the Nankai Subduction zone during IODP Expedition 322
A key to understanding the dynamics of large ruptures, is knowing the level of friction on the fault. One of the most direct ways to estimate the fault friction during the earthquake, is to measure the residual heat in the fault zone.
The Tohoku earthquake represents the rare opportunity to measure the friction for the largest slip (30 to 50 m) that has ever been observed for an earthquake. Because the temperature signal dissipates with time, we need to make the measurements rapidly after the earthquake. The JFAST project will try to measure the temperature across the fault quickly (within 18 months) after the earthquake. At the time of the earthquake, the temperatures were likely several hundred degrees, but will drop to 1 degree or less in about a year.
Figure 3. Estimated temperatures across the fault for various levels of friction (m). These calculations assume the temperature is measured 18 months after the earthquake for a slip of 30 meters.
The temperature measurements will be made with a string of about 50 thermistors placed across the fault zone. Since the drilling disturbs the temperatures in the borehole, the measurements cannot be made at the time of the drilling. The thermistors will be installed in the borehole during the JFAST expedition and will continuously record the temperature for about 4 years. Data will be retrieved with the use of a Remotely Operated Vehicle (ROV). The first retrieval of data is scheduled for about 6 months after the installation.
Past temperature measurements have only been done for the 1999 Chi-Chi,Taiwan and 2008 Wenchuan, China earthquakes. The results from both of these earthquakes have shown much lower values of friction that expected. Such low values of dynamic friction need to be verified
Figure 4. Configuration of
temperature sensors to be installed in the JFAST Hole A.
The location for the JFAST drilling is about 220 km east of Sendai, in the region where there was very large slip on the fault during the earthquake. The planned boreholes are expected to reach the fault at a depth of about 850 meters. The water depth is located in about 7000 meters.
Figure 5. Location of JFAST drilling site.
Past thinking was that fault zone in the region of the accretionary wedge is weak, so that stress does not accumulate and large slip does not occur there during great subduction earthquakes. The 2011 Tohoku earthquake has shown this is not always the case. The huge amount of slip on this shallow portion of the fault, along with the large water depth, were the main source of the very large tsunami that caused so much damage and loss of lives along the coast of northeast Honshu.
Importance for Understanding the Tsunami
Understanding the stress conditions for this shallow portion of the megathrust may be the most important seismological issue for this earthquake. This also has obvious consequences for evaluating future tsunami hazards at other subduction zones around the world, such as the Nankai Trough and Cascadia.
The JFAST project is complementary to the IODP drilling project in the Nankai Trough (NanTroSEIZE). JFAST is looking at the stress and physical properties of the fault zone soon after a large earthquake, while NanTroSEIZE is studying the fault zone before the occurrence of a large earthquake.
A significant borehole has never been drilled in such deep water. Drilling 850 meters beneath the sea floor in 7000 meters of water means there is a drill string of nearly 8000 meters (more than twice the height of Mt. Fuji). Engineers have been carefully studying the technical aspects for this difficult drilling. Previously, a project in the Marianas trench drilled in 7034 meters of water, but the borehole was only 15 meters.