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
Temperature
Measurements
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.
Site
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.
Technical Challenges
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.
Links
Expedition 343
Official website