Seismic imaging of slab metamorphism and genesis of intermediate-depth intraslab earthquakes

Abstract

We review studies of intermediate-depth seismicity and seismic imaging of the interior of subducting slabs in relation to slab metamorphism and their implications for the genesis of intermediate-depth earthquakes. Intermediate-depth events form a double seismic zone in the depth range of c. 40-180 km, which occur only at locations where hydrous minerals are present, and are particularly concentrated along dehydration reaction boundaries. Recent studies have revealed detailed spatial distributions of these events and a close relationship with slab metamorphism. Pressure-temperature paths of the crust for cold slabs encounter facies boundaries with large H 2 O production rates and positive total volume change, which are expected to cause highly active seismicity near the facies boundaries. A belt of upper-plane seismicity in the crust nearly parallel to 80-90 km depth contours of the slab surface has been detected in the cold Pacific slab beneath eastern Japan, and is probably caused by slab crust dehydration with a large H 2 O production rate. A seismic low-velocity layer in the slab crust persists down to the depth of this upper-plane seismic belt, which provides evidence for phase transformation of dehydration at this depth. Similar low-velocity subducting crust closely related with intraslab seismicity has been detected in several other subduction zones. Seismic tomography studies in NE Japan and northern Chile also revealed the presence of a P-wave low-velocity layer along the lower plane of a double seismic zone. However, in contrast to predictions based on the serpentinized mantle, S-wave velocity along this layer is not low. Seismic anisotropy and pore aspect ratio may play a role in generating this unique structure. Although further validation is required, observations of these distinct low P-wave velocities along the lower seismic plane suggest the presence of hydrated rocks or fluids within that layer. These observations support the hypothesis that dehydration-derived H 2 O causes intermediate-depth intraslab earthquakes. However, it is possible that dual mechanisms generate these earthquakes; the initiation of earthquake rupture may be caused by local excess pore pressure from H 2 O, and subsequent ruptures may propagate through thermal shear instability. In either case, slab-derived H 2 O plays an important role in generating intermediate-depth events.

Seismicity, Metamorphism, and Fluid Evolution Across the Northern Cascadia Fore Arc

“…We note that Wells et al (2017) have highlighted a statistically significant, alongstrike anticorrelation between tremor density and the presence of major crustal faults, suggesting that these deep faults are able to disrupt the plate boundary seal and, locally, tap fluids from the LVZ, thereby reducing tremor density. Hasegawa and Nakajima (2017) have also suggested that the spatial anticorrelation of LFEs and high levels of crustal seismicity observed at the Kii and Ise Gaps in southwest Japan, can be explained by a permeable fore arc that drains fluids at the megathrust, with these fluids migrating through crustal faults, reducing their shear strength, and promoting metamorphic reactions. The deep clusters of microseismicity below this structure in the vicinity of Victoria, displayed in Figure 6, may therefore manifest localized fluid delivery from the slab.…”

Revisiting Intraslab Earthquakes Beneath Kyushu, Japan: Effect of Ridge Subduction on Seismogenesis

“…Green arrows indicate the location of the upper‐plane seismic belt in the crust discussed in the text. Gray dashed lines denote the inferred location of the Kyushu‐Palau Ridge (KPR) taken from Hasegawa and Nakajima (). The location of Cluster A1 is indicated by black arrows in (a).…”

“…Hasegawa and Nakajima () pointed out that the upper‐plane seismic belt, which is defined as a belt‐like concentration of crustal seismicity distributed subparallel to the slab isodepth contours (Kita et al, ), is observed at depths of 50–70 km beneath central and southern Kyushu. Our classification of earthquakes shows that concentrated crustal seismicity is observed along the slab isodepth contour of ~60 km (green arrows in Figure a).…”

Source Parameter Variability of Intermediate‐Depth Earthquakes in Japanese Subduction Zones

“…Although dehydration appears to be an important factor in a wide range of slabs, the mechanism by which it actually enables earthquakes could differ depending on the location of seismicity and several hybrid mechanisms involving dehydration have been proposed in the literature. Embrittlement from blueschist‐facies dehydration may be more important in the upper plane of cold slabs like the Pacific (Okazaki & Hirth, ) whereas elsewhere, thermal shear instabilities can also be triggered by initial dehydration (Hasegawa & Nakajima, ). Ferrand et al () suggested from lab‐based evidence that dehydration reactions can enable intermediate‐depth earthquakes through stress transfer from dehydrating antigorite clusters to a surrounding olivine matrix rather than fluid overpressure, which also allows for strain localization followed by acceleration of said localization, a process termed “dehydration‐driven stress transfer” (DDST).…”