CDEX Web Magazine　地球発見

CDEX

Understanding fault movements from frictional heat

　JFAST has two main objectives: collect geological samples in order to analyze the physical properties of the fault zone and measure the temperature near the fault in order to understand the changes in frictional heat produced by fault slip. Two boreholes will be drilled. Temperature sensors will be deployed in the first borehole to measure the temperature profile in the vicinity of the fault. In the second borehole geological samples from several geological strata near the fault will be collected and then temperature and pressure sensors will be installed.

　The frictional heat produced by fault slip during an earthquake can be calculated by measuring the change in temperature of the fault compared with surrounding areas. Clarifying the level of frictional stress on the fault is an important issue for explaining the processes of large slip in earthquake. The level of dynamic friction is necessary for understanding how earthquakes grow and what determines the final size of earthquakes. “Fault temperature rises rapidly from frictional heat when earthquakes occur,” explains Expedition Project Manager Nobuhisa Eguchi of the Center for Deep Earth Exploration (CDEX), which is providing general research support for the project. “This heat is absorbed by the surrounding strata over a period of several years, and the temperature returns to the original level. It is therefore vital to measure temperature as soon as possible.” The frictional properties of the rock can also be analyzed from collected samples. Laboratory experiments will be done to see what changes the rocks undergo during an actual earthquake. The conditions at the time when the faults slip during major earthquakes will be investigated from the altered rocks and other physical and chemical properties.

Schematic diagram of sub-seafloor structure at the planned drilling sites.
According to conventional models of subduction zone earthquakes, strain gradually builds up over time in the area of frictional resistance of the deep part of the plate boundary (yellow portion). A major earthquake occurs when this part ruptures and slips. In the 2011 Tohoku earthquake it is indicated that the rupture may have extended to the shallow portion (red portion) to close to the trench axis of the plate boundary, where there was thought to be low levels of stress accumuations between plates. The sea floor near the trench axis moved significantly both horizontally and vertically during the earthquake and displaced massive volumes of sea water, causing the huge tsunami.

　The CHIKYU brings together an international team of researchers who are determined to obtain new knowledge from this scientific drilling that can be put to use in the future. Says Eguchi: “The drilling that will be done in this expedition is very difficult. There are many challenges, but also much to be gained. If we can gain a scientific understanding of what happened on the fault that caused the Great East Japan Earthquake, it should help us to predict the scale of major earthquakes and tsunamis in other subduction zones with greater accuracy.”