Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

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 wellinstrumented and well-studied subduction zone below Northern Japan (Tohoku and Hokkaido) provides an excellent testing ground to study the conditions under which intermediatedepth 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 timedependent, 3-D dynamics of the slab. This study generally supports the role of fluids in the generation of intermediatedepth seismicity.

Fluid Migration in a Subducting Viscoelastic Slab

“…It means that the fluid phase does not affect the matrix phase. This geometry roughly mimics the expected geometry of the metamorphic dehydration reaction that generally dips into the slab (e.g., van Keken et al, 2012). The spatial extent of the fluid source and fluid production rate are similar to that in previous work (Wilson et al, 2014).…”

“…We find that the distribution of compaction pressure differs significantly depending on the assumed parameters. Many previous studies have discussed the relationship between seismicity and fluid mainly based on the predicted location of fluid source (e.g., Hacker et al, 2003;Peacock & Wang, 1999;van Keken et al, 2012). Considering that compaction pressure is defined as the pressure difference in between fluid and matrix phases it suggests that the temporal change in pore fluid pressure is nonuniform even when the same amount of fluid production rate is given along the source.…”

Genesis of Intermediate‐Depth and Deep Intraslab Earthquakes beneath Japan Constrained by Seismic Tomography, Seismicity, and Thermal Modeling

“…Various mechanisms have been proposed to explain the triggering of these earthquakes, which include geometric unbending of the elastic core of the slab (Engdahl & Scholz, 1977;Kao & Ruey-Juin, 1999;Kawakatsu, 1985;McGuire & Wiens, 1995), thermal shear instability due to a positive feedback between deformation-induced heating and deformation (Hobbs & Ord, 1988;John et al, 2009;Karato et al, 2001;Ohuchi et al, 2017), dehydration embrittlement caused by breakdown of hydrous minerals and subsequently in situ positive volume change (Dobson et al, 2002;Jung et al, 2004;Kirby et al, 1996;Okazaki & Hirth, 2016;Peacock, 2001;Yamasaki & Seno, 2003), fluid-related embrittlement related to the presence of fluid in general (Van Keken et al, 2012;Wei et al, 2017), and phase transformational faulting along the boundaries of metastable mineral wedges (Green & Houston, 1995;Kao & Liu, 1995;Kirby et al, 1991). Various mechanisms have been proposed to explain the triggering of these earthquakes, which include geometric unbending of the elastic core of the slab (Engdahl & Scholz, 1977;Kao & Ruey-Juin, 1999;Kawakatsu, 1985;McGuire & Wiens, 1995), thermal shear instability due to a positive feedback between deformation-induced heating and deformation (Hobbs & Ord, 1988;John et al, 2009;Karato et al, 2001;Ohuchi et al, 2017), dehydration embrittlement caused by breakdown of hydrous minerals and subsequently in situ positive volume change (Dobson et al, 2002;Jung et al, 2004;Kirby et al, 1996;Okazaki & Hirth, 2016;Peacock, 2001;…”

Slab temperature controls on the Tonga double seismic zone and slab mantle dehydration

“…A variety of mechanisms have been proposed to explain these earthquakes, including dehydration embrittlement ( 11 ), plastic shear instability ( 12 ), transformational faulting ( 6 ), and fluid-related embrittlement ( 13 ). The upper zone hypocenters coincide well with locations where free fluids can be produced by dehydration reactions.…”