A hypothesis for the seismogenesis of a double seismic zone

Abstract

S U M M A R YThe seismogenesis of a double seismic zone, in particular the lower layer of a double seismic zone, has not been adequately explained in the literature. On the basis of seismic data and geothermal structures along three well-studied cross-sections in the Kuril-Kamchatka and Japan subduction zones, we investigate the temperature/pressure conditions associated with seismogenic structures of the double seismic zones. The corresponding TIP loci seem to suggest that earthquakes observed in the lower layer and in the lower part (below approximately 130+ 20 km) of the top layer of a double seismic zone were caused by metastable phase transition-a mechanism similar to that responsible for deep-focus earthquakes only at lower temperature/pressure conditions. Under this hypothesis, the wedge-shaped configuration of a double seismic zone is interpreted to represent the loci of the kinetic boundary of the phase transition. According to theoretical/experimental studies and the constraints imposed by our observations, a likely candidate for such a phase transition is the metastable Al-rich enstatite decomposing into the assemblage of Al-poor enstatite plus garnet. Earthquakes in the upper part of the top layer were most probably due to conventional mechanisms such as dehydration of subducted materials and/or facies change from basalt to eclogite. That the top layer involves more than one seismogenic mechanism is also implied by the distinct behaviour of seismicity in the vicinity of 130 f 20 km. Because the presence of deviatoric stress is critical to the reaction rate of a metastable phase transition, it is inferred that single seismic zones are also caused by the same mechanisms, except that the implicit layer of a supposed double seismic zone is missing, due to the insufficient amount of appropriate metastable minerals or to the lack of appropriate deviatoric stresses in the source region.

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;…”

Seismic anisotropy evidence for dehydration embrittlement triggering intermediate-depth earthquakes

“…Proposed hypotheses for the physical mechanism of intermediate-depth (50–250 km) earthquakes include transformational faulting 4 , thermal shear instability 5, 6 , and dehydration embrittlement 7, 8 . Currently, the hypothesis of dehydration embrittlement is considered to be the leading mechanism, which suggests that earthquakes in double seismic zones are linked to brittle failure associated with dehydration reactions of hydrous minerals in the slab crust and uppermost mantle 9–12 .…”

Double seismic zone and dehydration embrittlement of the subducting slab

“… Peacock [2001] emphasized the importance of the dehydration of slab mantle for the lower plane of the double seismic zone beneath northeast Japan. On the other hand, Nishiyama [1992], Kao and Liu [1995], and Kirby et al [1996] noted the possibility that the upper plane of the double seismic zone is caused by dehydration embrittlement of subducting oceanic crust. Peacock and Wang [1999] explained differences in the depths of seismicity between northeast and southwest Japan by differences in dehydration of the oceanic crust with different ages.…”