IODP

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doi:10.2204/iodp.pr.343343T.2012

Abstract

The main science goal of the Japan Trench Fast Drilling Project (JFAST) is to understand the physical mechanisms and dynamics of large slip earthquakes, which is fundamental to understanding the huge tsunami that caused extensive damage during the 2011 Tohoku earthquake. Specifically, the level of frictional stress during the earthquake rupture and the physical characteristics of the fault zone are investigated through drilling. The objectives of JFAST include locating the fault that ruptured during the Tohoku event using logging while drilling (LWD); characterizing the composition, architecture, and fundamental mechanisms of dynamic frictional slip and healing processes along the fault by taking core samples; and estimating the frictional heat and stress within and around the fault zone by placing a temperature measurement observatory across the fault.

During the main JFAST expedition (Integrated Ocean Drilling Program [IODP] Expedition 343 in April and May 2012), LWD was completed in a borehole drilled to 850.5 meters below seafloor (mbsf) (total depth [TD] = 7740 meters below sea level [mbsl]), and a coring hole was drilled to 844.5 mbsf (TD = 7734 mbsl) to acquire 21 cores that spanned the two main fault targets. Because of delays associated with severe weather and technical challenges of operating in great water depths, installation of the observatories for temperature and pressure measurements was not completed during the main expedition; however, temperature sensors were successfully deployed during a short technical extension of JFAST, IODP Expedition 343T (JFAST II), in July 2012.

Principal results of Expedition 343/343T include the following:

  1. The overall structure at the drill site consists of a prism of faulted and folded clayey to silty mudstones above, and in fault contact (at ~820 mbsf) with, a largely undeformed, relatively thin sequence of hemipelagic and pelagic sediments that were deposited on top of the incoming Pacific plate. The primary constituents of the mudstones that make up the prism are terrigenous silt and clay, vitric ash, and biogenic silica. The fault contact, interpreted as the plate boundary décollement, is defined by a subhorizontal, <5 m thick zone of highly sheared clay that displays penetrative scaly fabric and localized slip surfaces.
  2. Faults and bedding are variable in dip magnitude, but faults and bedding at all depths in the prism show a preferred northeast strike direction reflecting horizontal contraction and local extension (at shallower depths) approximately parallel to the plate convergence direction.

  3. Borehole breakouts are evident in image logs from the LWD hole and indicate several different in situ stress domains along the borehole. At shallow depths (from <200 to ~500 mbsf), the maximum horizontal compressive stress (SHmax) is variable and shows a complicated pattern. At deeper levels in the prism (537–820 mbsf), SHmax displays a single preferred orientation approximately 20° clockwise from the convergence direction.
  4. Fault slip during the 2011 event and other past earthquakes likely occurred on the plate boundary décollement. However, slip on other faults cutting the prism could have also occurred during the Tohoku event. One possible location of recent fault slip is identified at ~700 mbsf on the basis of a local H2, methane, and chlorinity anomaly. Several core samples from a fractured and brecciated zone at ~720 mbsf contain faults, the largest of which is a high-angle reverse fault that occurs at the same depth as a low-resistivity feature identified in image logs. The fault at ~720 mbsf and the décollement at ~820 mbsf are the two primary targets for the temperature measurement observatory.
  5. Successful recovery of ~1 m of highly sheared clay and neighboring sediments from the plate boundary décollement provide plenty of material for mechanical and physical properties testing, as well as for geochemical, mineralogical, and microstructural analyses. Three special interest structural whole-round samples taken from the sheared clay of the décollement, as well as two other structural whole rounds capturing secondary faults, will provide material for coordinated nondestructive and destructive investigations of structure, chemistry, and mineralogy of the faulted sediments.
  6. An observatory consisting of 55 temperature sensors and autonomous data loggers was successfully installed across the two fault targets. These instruments are monitoring the temperature distribution across the lower portion of the borehole to estimate the amount of frictional heat dissipated along the slip zone of the Tohoku earthquake in order to estimate the level of dynamic frictional strength during the earthquake.

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