Subduction plate interface shear stress associated with rapid subduction at deep slow earthquake depths: example from the Sanbagawa belt, Southwest Japan

Abstract. Absolute maximum shear stress (“shear stress” in this study) along the subduction plate interface is important for understanding earthquake phenomena and is an important input parameter in subduction zone thermomechanical modelling. However, such shear stress is difficult to measure directly at depths more than a few kilometers and is generally estimated by simulation using a range of input parameters with large associated uncertainties. In addition, estimated values generally represent shear stress conditions over short observation timescales, which may not be directly applicable to long-timescale subduction zone modelling. Rocks originally located deep in subduction zones can record information about deformation processes, such as shear stress conditions, occurring in regions that cannot be directly accessed. The estimated shear stress is likely to be representative of shear stress experienced over geological timescales and be suitable to use in subduction zone modelling over time scales of millions to tens of millions of years. In this study, we estimated shear stress along a subduction plate interface by using samples from the Sanbagawa metamorphic belt of Southwest (SW) Japan, in which slivers of mantle wedge-derived serpentinite are widely distributed and in direct contact with metasedimentary rocks derived from the subducted oceanic plate. These areas can be related to the ancient subduction plate interface.

To obtain estimates of shear stress at the subduction interface, we focused on the microstructure of quartz-rich metamorphic rocks—quartz is the main component of the rocks we collected and its deformation stress is assumed to be representative of the region. Shear stress was calculated by applying deformation temperatures estimated by the crystallographic orientation of quartz (the quartz c-axis fabric opening-angle thermometer), and the apparent grain size of dynamically recrystallized quartz in a thin section to an appropriate piezometer. Combined with information on sample deformation depth, estimated from P–T path and deformation temperatures, it is suggested that there was nearly constant shear stress of 16–41 MPa in the depth range 17–27 km, assuming plane stress conditions even when uncertainties related to measurement direction of thin section and piezometer differences are included.

The Sanbagawa belt formed in a warm subduction zone. Deep slow earthquakes are commonly observed in modern-day warm subduction zones such as SW Japan, which has a similar thermal structure to the Sanbagawa belt. In addition, deep slow earthquakes are commonly observed to be concentrated in a domain under the shallow part of the mantle wedge. Samples showed the depth conditions near the mantle wedge, suggesting that these samples were formed in a region with features similar to the deep slow earthquakes domain. Estimated shear stress may not only be useful to long-timescale subduction zone modelling but also represent the initial conditions from which slow earthquakes in the same domain nucleated.