Abstract
Abstract. The cause of intermediate-depth (>40 km) seismicity in subduction zones is not well understood. The viability of proposed mechanisms, which include dehydration embrittlement, shear instabilities and the presence of fluids in general, depends significantly on local conditions, including pressure, temperature and composition. The well-instrumented and well-studied subduction zone below Northern Japan (Tohoku and Hokkaido) provides an excellent testing ground to study the conditions under which intermediate-depth seismicity occurs. This study combines new finite element models that predict the dynamics and thermal structure of the Japan subduction system with a high-precision hypocenter data base. The upper plane of seismicity is principally contained in the crustal portion of the subducting slab and appears to thin and deepen within the crust at depths >80 km. The disappearance of seismicity overlaps in most of the region with the predicted phase change of blueschist to hydrous eclogite, which forms a major dehydration front in the crust. The correlation between the thermally predicted blueschist-out boundary and the disappearance of seismicity breaks down in the transition from the northern Japan to Kurile arc below western Hokkaido. Adjusted models that take into account the seismically imaged modified upper mantle structure in this region fail to adequately recover the correlation that is seen below Tohoku and eastern Hokkaido. We conclude that the thermal structure below Western Hokkaido is significantly affected by time-dependent, 3-D dynamics of the slab. This study generally supports the role of fluids in the generation of intermediate-depth seismicity.
Highlights
The subduction of the Pacific plate below central and northern Japan (Fig. 1) provides a classic cold end-member in the thermal structure of subduction zones, due to the high convergence rate and the old age of the oceanic lithosphere (e.g., Peacock and Wang, 1999)
We found differences of less than 1 ◦C in the temperature prediction for the oceanic crust, which is consistent with the performance of our models in a recent benchmark for subduction zones
Our new thermal models provide strong quantitative support for the hypothesis that intermediate-depth seismicity is caused by the presence of fluids that are generated by metamorphic dehydration reactions
Summary
The subduction of the Pacific plate below central and northern Japan (Fig. 1) provides a classic cold end-member in the thermal structure of subduction zones, due to the high convergence rate and the old age of the oceanic lithosphere (e.g., Peacock and Wang, 1999). Thermalpetrological modeling following van Keken et al (2011) strongly suggests that earthquakes in a particular cross section below northern Tohoku are limited by the blueschist-tohydrous-eclogite phase boundary (Fig. 2), which is the first main deep dehydration front in cold subduction zones (van Keken et al, 2011). It is important to note that the location of the blueschist-out boundary in Fig. 2 is based on independent thermal-petrological modeling and is not a fit to the observed seismicity. This independent correlation suggests the hypothesis that, at least in cold subduction zones, the upper
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