Earthquake faulting in subduction zones: insights from fault rocks in accretionary prisms

Abstract

Subduction earthquakes on plate-boundary megathrusts accommodate most of the global seismic moment release, frequently resulting in devastating damage by ground shaking and tsunamis. As many earthquakes occur in deep-sea regions, the dynamics of earthquake faulting in subduction zones is poorly understood. However, the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) and fault rock studies in accretionary prisms exhumed from source depths of subduction earthquakes have greatly improved our understanding of earthquake faulting in subduction zones. Here, we review key advances that have been made over the last decade in the studies of fault rocks and in laboratory experiments using fault zone materials, with a particular focus on the Nankai Trough subduction zone and its on-land analog, the Shimanto accretionary complex in Japan. New insights into earthquake faulting in subduction zones are summarized in terms of the following: (1) the occurrence of seismic slip along velocity-strengthening materials both at shallow and deep depths; (2) dynamic weakening of faults by melt lubrication and fluidization, and possible factors controlling coseismic deformation mechanisms; (3) fluid-rock interactions and mineralogical and geochemical changes during earthquakes; and (4) geological and experimental aspects of slow earthquakes.

Multiple major faults at the Japan Trench: Chemostratigraphy of the plate boundary at IODP Exp. 343: JFAST

“…We also note that because the JFAST core did not penetrate to basement, significant faults could have been missed below the bottom of the recovered JFAST section. A thicker décollement (at least 10 m) would be more in keeping with observations in other subduction zone settings such as Barbados , although estimates from Nankai suggest a décollement more on the order of tens to hundreds of meters (Moore et al, 1990;Rowe et al, 2013;Ujiie and Kimura, 2014). Our results imply that structures with insignificant appearance in the core, such as thin shear surfaces with little notable damage surrounding them, may be responsible for significant displacement.…”

Structure and lithology of the Japan Trench subduction plate boundary fault

“…If it is a tectonic contact, the maximum width of the plate boundary fault is around 15 m (Figure 3). This estimate is notably less than the ∼50 m thick fractured and brecciated zone that defines the plate boundary thrust at the Nankai margin [Ujiie and Kimura, 2014], which is more typical for shallow subduction thrusts [Rowe et al, 2013].…”

Vitrinite reflectance and Raman spectra of carbonaceous material as indicators of frictional heating on faults: Constraints from friction experiments

“…The fault sample for the friction experiments was the same as that used by Ujiie and Tsutsumi (2010), which is obtained from microbrecciated hemipelagic mudstone immediately above the 10-mm-thick fault gouge in the shallow portion (271 m below sea floor) of the megasplay fault at site C0004 in the Nankai accretionary prism, southwest Japan (Fig. 1).…”