Double seismic zone and dehydration embrittlement of the subducting slab

Abstract

[1] Dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate-depth earthquakes. We explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast Japan, southwest Japan, northeast Taiwan, northern Chile, Cape Mendocino, and eastern Aleutians. We calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions. We then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams. The depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age. The dehydration loci of serpentine produce a double-layered structure. Because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab, we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust, and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab. We find that the lower plane seismicity is located at the lower dehydration loci of serpentine, which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement.INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 7209 Seismology: Earthquake dynamics and mechanics; 7220 Seismology: Oceanic crust; KEYWORDS: double seismic zone, dehydration embrittlement, serpentine, oceanic crust, subducting plate Citation: Yamasaki, T., and T. Seno, Double seismic zone and dehydration embrittlement of the subducting slab,

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

“…A number of processes have been suggested for causing local weakening. These include shear instabilities (Ogawa, 1987;Kelemen and Hirth, 2007), thermal runaways (John et al, 2010), and local weakening and embrittlement of the crust due to dehydration (e.g., Raleigh and Paterson, 1965;Kirby et al, 1996;Seno and Yamanaka, 1996;Hacker et al, 2003;Yamasaki and Seno, 2003). Possible consequences of dehydration include stress changes to the change in volume of the oceanic crust , pore pressure changes (Wong et al, 1997) or just the release and transport of water (e.g., Jung et al, 2004;John and Schenk, 2006).…”

Very low frequency earthquakes excited by the 2004 off the Kii peninsula earthquakes: A dynamic deformation process in the large accretionary prism

“…Reverse faults in the accretionary prism take the role of a drain for fluids generated by dehydrating the subducting oceanic plate. If the intraslab earthquakes are caused by the embrittlement due to the dehydration process (Yamasaki and Seno, 2003), the 2004 off the Kii peninsula earthquakes might have affected the dehydration process and the fluid flow. On the other hand, the strong shaking by these Mw 7.2 and Mw 7.5 earthquakes might have increased instability of the fold-and-thrust structures in the accretionary wedge and caused a chain-like occurrence of slip events, which are the VLF earthquakes.…”

Geographical distribution of 3He/4He ratios and seismic tomography in Japan

“…Based on the magma aging calculation, it is possible to maintain medium-Ra in the subducting Philippine Sea plate at present. Aqueous fluids supplied from the oceanic crust at the depth between 30-50 km by metamorphic dehydration reactions (Hacker et al, 2003;Yamasaki and Seno, 2003) may be the principle source of medium-Ra in Shikoku. In the case of NE Japan, expelled fluids produce secondary minerals which may have been dragged down to deep in the mantle.…”