Seismic Reflection Images of Possible Mantle-Fluid Conduits and Basal Erosion in the 2011 Tohoku Earthquake Rupture Area

Abstract

Multi-channel seismic reflection and sub-bottom profiling data reveal landward-dipping normal faults as potential conduits for mantle-derived fluids in the coseismic slip area of the 2011 Tohoku earthquake (Mw9.0). Normal faults below the helium isotope anomaly sites appear to develop through the forearc crust (i.e., the seafloor sedimentary section and Cretaceous basement) and to evolve to lower dip angles as extension progresses deeper, potentially extending down to the mantle wedge, despite their intermittently continuous reflections. The faults are characterized by high-amplitude, reverse-polarity reflections within the Cretaceous basement. Moreover, deep extension of the faults connecting to a low-velocity region spreading from the Cretaceous basement into the mantle wedge across the forearc Moho suggests that the faults are overpressured by local filling with mantle-derived fluids. The locations of the normal faults are roughly consistent with aftershocks of the 2011 Tohoku earthquake, which show normal-faulting focal mechanisms. The 2011 Tohoku mainshock and subsequent aftershocks can lead the pre-existing normal faults to be reactive and more permeable so that locally trapped mantle fluids can migrate up to the seafloor through fault fracture zones. The reactivated normal faults may be an indicator of shallow coseismic slip to the trench. Locally elevated fluid pressures can decrease the effective normal stress for the fault plane, facilitating easier slip along the fault and local tsunami. The landward-dipping normal faults developing from the seafloor down into the Cretaceous basement are predominant in the middle slope region of the forearc. A possible shear zone with high-amplitude, reverse-polarity reflections above the plate interface, which is almost localized to the middle slope region, suggests more intense basal erosion of the overlying plate in that region.

Simple topographic parameter reveals the along-trench distribution of frictional properties on shallow plate boundary fault

“…Tsuji et al (2011) and JAMSTEC used different processing methods than Tsuru et al (2002), and uncertainty in depth conversions of seismic profiles may cause biases in estimating frictional strength, as we noted in the introduction. In particular, using the migrated-profile D6 of Park et al (2021), which is the nearest transect to that of Tsuji et al (2011), we obtained α and β values of 4.69° and 3.26°; we obtained μ b ' = 0.09 in D6, with the critical taper based on the parameters of Koge et al (2014). Now, our updated results for the Japan Trench highlight the usefulness of our α-μ b ' approximation.…”

Accretion versus erosion and sediment transfer balance near the subduction interface

“…The observation of widespread shallow normal faults cutting through the Neogene sedimentary cover of the margin had already been reported [von Huene and Lallemand, 1990] but not their deeper extent. In their interpretation, Park et al [2021] identified some of these at depths greater than 20 km. Their shallow dip suggests the reactivation of former splay faults into normal faults as similarly observed off Ecuador [Collot et al, 2008] or at the transition from Alaska and Aleutian subduction zones [Kahrizi et al, 2024].…”

“…It concerns both the penetration depth and the resolution of the data. Figure 1 is an example of the progress made, allowing the NE Japan margin to be imaged with unprecedented accuracy [Park et al, 2021]. The observation of widespread shallow normal faults cutting through the Neogene sedimentary cover of the margin had already been reported [von Huene and Lallemand, 1990] but not their deeper extent.…”

“…Given the progress made and the multitude of seismic lines acquired along the subduction zones since the 1980's and 1990's, we have undertaken to Park et al [2021]. Note the extent of normal faults (in dark blue) beyond the Moho, and the restriction of the thrust faults (in red) to the frontal part of the margin.…”

“…The detailed exploration of subduction interfaces, such as in Ecuador, Chile or Japan for example [e.g., Collot et al, 2008, Contreras-Reyes et al, 2010, Park et al, 2021, shows that the processes of growth or consumption of the margins evolve in space and time. It is now possible to detect evidence of subcrustal erosion down to depths of 20 km on a seismic profile (Figure 1) and to record uplift in the innermost parts potentially corresponding to the underplating of deeper sedimentary units like in Chile [Angiboust et al, 2021].…”