Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12
DOI 10.1186/s40645-017-0126-9

REVIEW

Progress in Earth andPlanetary Science

Open Access

Seismic imaging of slab metamorphismand genesis of intermediate-depth intraslab
earthquakes
Akira Hasegawa1*

and Junichi Nakajima2

Abstract
We review studies of intermediate-depth seismicity and seismic imaging of the interior of subducting slabs in relation toslab 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 H2O 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 H2O 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 validationis 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 H2O
causes intermediate-depth intraslab earthquakes. However, it is possible that dual mechanisms generate theseearthquakes; the initiation of earthquake rupture may be caused by local excess pore pressure from H2O, andsubsequent ruptures may propagate through thermal shear instability. In either case, slab-derived H2O plays
an important role in generating intermediate-depth events.
Keywords: Internal slab structure, Slab metamorphism, Dehydration-related embrittlement, Intermediate-depth
earthquake, Intraslab earthquake, Earthquake generation mechanism, Subduction zone

Introduction
Subduction of oceanic lithosphere causes high seismic
activity in subduction zones. The shallow portion of theplate interface between a subducting oceanic plate andan overriding continental plate is locked by friction. To
release the stress that accumulates from this interplate* Correspondence: akira.hasegawa.d8@tohoku.ac.jp
1
Research Center for Prediction of Earthquakes and Volcanic Eruptions,
Graduate School of Science, Tohoku University, Aramaki, Aoba-ku, Sendai
980-8578, Japan
Full list of author information is available at the end of the article

locking, the locked portion of the plate interface
occasionally slips dynamically and causes interplate
earthquakes. Plate convergence at subduction zones also
causes stress accumulation within both the overriding
continental lithosphere and the subducting oceanic
lithosphere. To release this accumulated stress, earthquakes take place both in the overriding and subducting
plates. Earthquakes that occur in a subducting plate arecalled intraslab earthquakes because the subducting
oceanic lithosphere is referred to as the slab. Intraslab
earthquakes can occur as deeply as the bottom of the© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

upper mantle. With the exception of shallow events, allearthquakes occur within the subducting slab. Therefore,
the maximum depth of earthquakes on Earth is c.
660 km, or potentially slightly deeper. The global distribution of the focal depths of seismicity is bimodal(Fig. 1). The first, and largest, peak of earthquake frequency with depth occurs at very shallow depths between 0 and 30 km. The frequency decreases almost
exponentially to a depth of c. 300 km, where a minimum
emerges between c. 300 and 450 km in depth. The frequency then increases to a depth of c. 550 km, where it
reaches a local maximum of 550–600 km (Frohlich,
2006; Houston, 2007). Generally, intraslab earthquakes
are divided into two groups based on focal depth. Events
shallower than 300 km are defined as intermediatedepth earthquakes, and those deeper than 300 km areconsidered deep-focus earthquakes.
At the depths of intraslab earthquakes, lithostatic pressure, and therefore frictional strength, becomes too greatto allow dynamic slips, i.e., earthquakes, along fault planes.
In order to generate earthquakes at such great depths,
shear stress must exceed that very large frictional strength.
However, it is difficult to envision the application of suchlarge shear stress on a fault where intermediate-depth anddeep-focus events take place. Nevertheless, intermediate0

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mantle transition
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Fig. 1 Focal depth distribution of global seismicity for the period of
1960–2007. The shallower peak is truncated to show the deeper peakin detailPage 2 of 31

depth and deep-focus earthquakes do occur in the subducting slab, and therefore the frictional strength mustnot be as high as expected. Therefore, some special mechanism must operate to lower the strength of friction andgenerate intermediate-depth and deep-focus intraslab
earthquakes (e.g., Frohlich, 1994, 2006).
Although several plausible mechanisms have been proposed, the exact mechanism that generates intermediatedepth and deep-focus earthquakes remains unresolved.
Some early studies explored the possibility that implosive phase change of mantle materials could cause intraslab earthquakes (e.g., Leith and Sharpe, 1936). However,subsequent studies have revealed that both intermediatedepth and deep-focus earthquakes generally do not havethe large isotropic components that would be expected if
this mechanism occurred (e.g., Hara et al. 1996; Kawakatsu,
1991, 1996; Kuge and Kawakatsu, 1993; Okal, 1996;
Russakoff et al. 1997; Estabrook, 1999). Therefore, someother mechanism must exist to generate intraslab events.
To date, leading hypotheses proposed to explain the occurrence of earthquakes at such great depths include thermal
shear instability of plastic deformation (e.g., Ogawa, 1987;
Hobbs and Ord, 1988; Karato et al. 2001; Kelemen and
Hirth, 2007), transformational faulting and anticrack formation (e.g., Kirby, 1987; Green and Burnley, 1989; Burnley etal. 1991; Green and Houston, 1995; Kirby et al. 1996), anddehydration embrittlement (e.g., Griggs and Handin, 1960;
Raleigh and Paterson 1965; Raleigh, 1967; Meade and Jeanloz, 1991; Nishiyama, 1992; Green and Houston, 1995;
Kirby, 1995; Kirby et al. 1996; Seno and Yamanaka, 1996).
The thermal shear instability model assumes that
intraslab earthquakes occur as follows. If plastic deformation progresses at a faster rate than the rate at which
the generated heat dissipates, a positive feedback is produced that further promotes plastic deformation. Consequently, plastic deformation proceeds rapidly untilmaterials in that portion of the slab start to melt, which
results in sudden slips, i.e., earthquakes. The transformational faulting hypothesis assumes the following mechanism. As the slab subducts into the mantle transition
zone, the phase transformation from olivine to spinel
proceeds in the slab. Initially, small lens-shaped spinel
crystals (spinel lenses) are formed in this phase transformation. Under the deviatoric stress field within theslab, spinel lenses grow analogously to the cracks thatdevelop at shallow depths. These spinel lenses alignalong a plane that obliquely intersects with the axis of
maximum compressive stress, and eventually, the crystals coalesce in that plane, which results in abrupt slips
and therefore earthquakes. The dehydration embrittlement hypothesis posits that hydrous minerals that were
present in the lithosphere before subduction decompose
as the slab subducts and that the released H2O lowers
the effective normal stress, thereby enabling brittle

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

fractures that cause earthquakes. However, all three ofthese hypotheses are based on extrapolation from the results of laboratory experiments or theoretical calculations to simulate the great depths and scale of natural
intraslab earthquakes; therefore, the exact mechanism
has not yet been conclusively determined.
The present manuscript is focused on the genesis ofintermediate-depth intraslab earthquakes. The transformational faulting model applies only for events that
occur at the great depths of the mantle transition zone;
therefore, it is not applicable to intermediate-depth
events. Dehydration-related embrittlement is thought tobe a promising hypothesis as a mechanism of lowering
frictional strength sufficiently to generate intermediatedepth intraslab events (Nishiyama, 1992; Kirby, 1995;
Seno and Yamanaka, 1996). According to this hypothesis, temperature and pressure increase within the slab
with subduction, and as a result, hydrous minerals thathave been fixed in the slab are decomposed. H2O is liberated by this process then lowers the effective normal
stress. This effect would allow brittle faulting or dynamic
slip along a fault even at depths with high lithostaticpressure. The traditional dehydration embrittlement hypothesis refers only to the embrittlement of rock that
occurs during dehydration reactions. In a broader sense,
however, dehydration-related embrittlement may also include the embrittlement caused by fluid released
through dehydration reactions at any distance from thefault and at any time (e.g., Barcheck et al. 2012). We use
the term in the latter, broader sense. The thermal shear
instability model is another plausible mechanism forcausing not only deep-focus earthquakes but alsointermediate-depth intraslab seismicity. In some recent
studies, the results of laboratory experiments and numerical simulations have supported this hypothesis (e.g.,
Kelemen and Hirth, 2007; Chernak and Hirth, 2011;
Proctor and Hirth 2015).
In the following sections, we review recent researchfindings about intermediate-depth intraslab seismicity
and the internal structure of subducting slabs in relation
to slab metamorphism and discuss the generation mechanisms of intermediate-depth intraslab earthquakes.

Review
The double seismic zone and dehydration ofmetamorphosed crust and serpentinized mantle

Seismic studies of precise intermediate-depth seismicity
and the detailed seismic structures of subducting slabs
have provided growing evidence that supports thedehydration-related embrittlement hypothesis for earthquakes nucleation.
A non-negligible amount of H2O is contained within
subducting lithosphere in the form of hydrous minerals.
Hydration of oceanic lithosphere can occur in a number

Page 3 of 31

of tectonic settings. (1) When new lithosphere is createdat a mid-ocean ridge, hydrous minerals are formed in
the shallow portion of the crust by hydrothermal circulation and reactions with seawater (e.g., Dixon et al. 2002;
Staudigel et al. 1995). (2) As oceanic lithosphere approaches a trench, it begins to sink and deforms underthe bending force, which produces the outer rise. Thisbending force causes normal faulting in the trench–outer rise, and seawater enters along faults, in which hydrous minerals are then formed (Fig. 2) (e.g., Kirby,
1995; Peacock, 2001; Ranero et al. 2003; Faccenda et al.
2009; Iyer et al. 2012). (3) Seawater enters along transform faults near mid-ocean ridges because of fault
movement, and hydrated minerals form along thesefaults as well (e.g., Bonatti and Honnorez, 1976; Gregget al. 2007). (4) In addition, Seno and Yamanaka (1996)
described a scenario whereby H2O is fixed to the oceanic
plate in the form of hydrous minerals as the plate passes
directly over mantle plumes or superplumes during its
horizontal movement along the earth’s surface. Magmaof the plumes or superplumes intrudes into or accretes
to the overlying oceanic lithosphere, and H2O containedin the magma is added and eventually fixed in theoceanic plate.
H2O is thus incorporated into oceanic lithosphere andfixed in the form of hydrous minerals through several
different processes. It is therefore estimated that oceanic
lithosphere contains a considerable amount of H2Oprior to its subduction (e.g., Rüpke et al. 2004; Faccenda,
2014). When lithosphere reaches a trench, it then sinks
beneath the continental plate, and its temperature andpressure rise with increasing depth. Hydrous minerals
that were fixed in the oceanic lithosphere then decompose and release H2O. It is proposed in the dehydrationrelated embrittlement hypothesis that this liberated H2O
causes intraslab earthquakes, especially intermediatedepth events.
If hydrous minerals are present along normal faults inthe trench–outer rise region prior to plate subduction,
those hydrous minerals decompose and expel H2O along
the faults as the plate subducts to depth. It has been
proposed that intraslab earthquakes occur along thesepreexisting planes of weakness that originated as normal
faults prior to subduction (e.g., Kirby, 1995). Jiao et al.
(2000) tested this hypothesis by examining fault planesolutions of intraslab events in Tonga. The results showthat for outer rise events, features of the geometry of thefault system prior to subduction persist down to c.
450 km depth in the subducted slab, which suggests that
intraslab events down to this depth are caused by the reactivation of preexisting faults containing hydrous minerals that formed prior to subduction. Similar results wereobtained for intermediate-depth intraslab events in the
Middle America and Chile subduction zones (Ranero etHasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

Page 4 of 31

arc volcano

trenchouter rize

crust
mantlewedge

diapirhydrated faults

es

uak

thq
ear

anic
oce

plate

Fig. 2 Schematic diagram showing normal faults in the trench–outer rise, intermediate-depth intraslab earthquakes, and arc magmatism. Estimatedtransportation paths of H2O from the subducting slab to the arc crust are indicated by blue arrows (Hasegawa and Nakajima, 2004)

al. 2005). These observations support the dehydrationrelated embrittlement hypothesis for the genesis ofintermediate-depth intraslab events, especially those that
occur within the slab crust. However, subsequent studies
by Warren et al. (2007, 2008) have shown that this issue
may be more complex. By examining the fault planeorientations of intraslab events in Tonga and Middle
America, they identified many subhorizontal faults, whichsuggest the formation of new faults. Therefore, the outerrise faults are not always reactivated. Further investigations are required to confirm the hypothesized reactivation of preexisting faults.
If intermediate-depth intraslab events are caused bydehydration-derived H2O, these events would not occur
everywhere throughout a slab; they would occur only inparts of the slab where hydrous minerals or H2O exist.
In particular, it is hypothesized that many of these eventsoccur at the phase boundaries (facies boundaries) of dehydration reactions across which H2O content changes.
As described in detail below, recent studies have demonstrated that intermediate-depth intraslab events do infact show such distribution. To verify whether fluidrelated embrittlement is the cause of intermediate-depth
events, it is necessary to determine where hydrous minerals are present within the slab and where they decompose and expel H2O. Based on experimentally andtheoretically derived phase diagrams of the rocks thatcompose the oceanic crust and mantle (Fig. 3), Hacker

et al. (2003a, b) estimated the existence range of hydrous
minerals and the locations of the facies boundaries ofdehydration reactions within the slabs beneath the subduction zones of Cascadia, Costa Rica and NE and SW
Japan. As predicted, comparing the phase diagrams withhypocenter distributions of intermediate-depth intraslab
earthquakes in these subduction zones shows that theearthquakes appear to occur only within the existence
range of hydrous minerals and not outside this range. Examples from NE Japan and Costa Rica are shown in Fig. 4.
Moreover, Yamasaki and Seno (2003) found thatintermediate-depth intraslab events tend to concentrate
along the dehydration reaction boundaries within slabs.
Based on experimentally derived phase diagrams, they
estimated the locations of dehydration decomposition inmetamorphosed crust and serpentinized mantle within
the slabs of subduction zones beneath NE and SW
Japan, Taiwan, northern Chile, Cape Mendocino, and theeastern Aleutians, and compared these estimated locations with the hypocenter distributions of intermediatedepth earthquakes. Results of this analysis for the Pacificslabs beneath NE Japan and the eastern Aleutians areshown in Fig. 5. In NE Japan, intermediate-depth earthquakes occur in a double-planed deep seismic zone 70–
150 km in depth, with subparallel upper and lower
planes 30–40 km apart from each other (Tsumura, 1973;
Umino and Hasegawa, 1975: Hasegawa et al. 1978a, b).
Figure 5a shows that the estimated locations of theHasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

Upper
surface

Slab
Moho

5

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Temperature [°C]

Temperature [°C]Fig. 3 Metamorphic facies and maximum H2O contents. a Mid-ocean ridge basalt (MORB) and b harzburgite. H2O contents in wt% are shown by
the color scale. Estimated pressure–temperature paths for the slab crust under Tohoku, Costa Rica, Nankai, and Cascadia are indicated as shaded
areas in a PA, prehnite–actinolite, eA epidote amphibolite, eB epidote blueschist, and jeB jadeite–epidote blueschist. (After Hacker et al. 2003b)

0

Depth (km)

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0Depth (km)

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Distance (km)Fig. 4 Across-arc vertical cross-sections of intraslab seismicity and existence range of hydrous minerals in the slab. a Tohoku and b Costa Rica
subduction zones. Maximum H2O contents are shown by the color scale. Dots show earthquakes. (After Hacker et al. 2003)

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

Page 6 of 31

0

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NE Japan
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E Aleutians
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dehydration loci ofmetamorphosed crust
dehydration loci ofserpentinized mantle

Fig. 5 Across-arc vertical cross-sections of intraslab earthquakes and dehydration loci in the slabs. a Tohoku and b eastern Aleutian subduction
zones. Dehydration loci of metamorphosed crust and serpentinized mantle are indicated by the green and red lines, respectively. Red broken lines
and red dotted lines are those based on the antigorite reaction boundaries of Ulmer and Trommsdorff (1995) and Wunder and Schreyer (1997), respectively.
Blue dots are earthquakes. Thermal structure is shown as isothermal contours. (After Yamasaki and Seno, 2003)

dehydration boundary correspond approximately to theupper and lower planes of this double seismic zone.
Under the eastern Aleutians, the Wadati–Benioff zone
also comprises the double seismic zone, with a slightly
smaller separation between the upper and lower planes
than that of the NE Japan slab. Figure 5b again showsthat the estimated dehydration reaction boundaries
roughly correspond to the upper and lower seismic
planes. Similar results were also obtained for theremaining four subduction zones included in the study.
This correspondence further explains why the lowerplane events of the double seismic zone are generated in
the middle of the lithospheric mantle, as previously reported by Peacock (2001).
Since the discovery of an unambiguous double seismiczone in Tohoku (Hasegawa et al. 1978a, b), numerous
subduction zones across the world have been similarly
investigated, and double seismic zones have been detected with certainty in several subduction zones (e.g.,
Frohlich, 2006), such as in slabs subducted below
Hokkaido (Suzuki et al. 1983), Kuril–Kamchatka (Kaoand Chen, 1994), Alaska (Abers, 1992, 1996), Cascadia(Cassidy and Waldhauser, 2003), Mendocino (Smith etal. 1993), northern Chile (Comte and Suarez, 1994;
Comte et al. 1999), New Zealand (Reyners et al. 1997),
Tonga (Kawakatsu, 1985, 1986), and northern Taiwan(Kao and Rau, 1999). However, double seismic zones are
not apparent in all subduction zones worldwide. Furthermore, even when a double subduction zone ispresent, its depth may range from c. 40 to 180 km, andthe separation distance between the upper and lower
planes may range between c. 10 and 40 km.
Hypocenters of earthquakes must be accurately determined, especially their focal depths, to evaluate whether
a double seismic zone is present in a given subduction
zone. Dense seismic observation network data are required for accurate determination of earthquake hypocenters, but few subduction zones are installed withsuch dense networks. Without relying on data from such
local seismic networks, Brudzinski et al. (2007) investigated a number of subduction zones worldwide to evaluate whether double seismic zones were present. They
used global seismic network data and selected eventswith well-constrained focal depths, such as those determined based on depth phases. Using hypocenter datafrom the EHB catalog (Engdahl et al. 1998; Engdahl andHasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

Villaseñor, 2002), they verified the presence of the
double seismic zone by investigating whether two peaks
appear in the frequency distribution of intermediatedepth intraslab earthquakes with respect to the distancefrom the slab upper surface. The two peaks in the frequency distribution are interpreted to correspond to theupper and lower planes of the double seismic zone. It
thus became clear that (1) the two peaks were apparent
for all 30 of the investigated subduction zones, and (2)the distance between the two peaks, i.e., the separation
distance between the upper and lower planes, varies bysubduction zone, and is proportional to the age of the
subducting plate (Fig. 6). These findings are consistent
with the predictions of the dehydration-related embrittlement hypothesis. In fact, the lowerplane events
correspond to the expected positions of dehydration decomposition in the serpentinized mantle in the slab, asshown in Fig. 6.
Detailed analyses of intermediate-depth intraslab
earthquakes in NE Japan have further revealed a characteristic structure of the Wadati–Benioff zone. Igarashi etal. (2001) reported that normal fault type events also
occur at the top of the upper plane of the double seismiczone in the depth range of c. 70–100 km. In terms of
focal mechanisms, the Wadati–Benioff zone beneath NE
Japan is triple-planed; from top to bottom, these threeplanes are characterized by normal fault, down-dip
compressional and down-dip extensional events. Wang

Page 7 of 31(2002) argued that plate unbending combined withdehydration-related embrittlement may account for themorphology and stresses of this triple-planed seismic
zone. Metamorphic reactions densify materials andtherefore induce tensile stress in the uppermost transformed crust. This tensile stress, as well as high pore
fluid pressure, in the uppermost slab crust thus generates the normal fault type seismicity that occurs there. A
recent study by Nakajima et al. (2013) revealed that evenin a small seismic cluster, normal fault type events occur
just 1 km above the down-dip compressional events inthe lower crust of the subducting Pacific Plate beneath
Niigata, NE Japan. These observations provide evidence
for the role of densification-related stresses caused bymetamorphic reactions and fluid-related embrittlementin earthquake nucleation within the slab crust.
Seismic wave velocity structure and upper-plane seismicity
in the slab crust
A belt of intraslab seismicity and phase transition in theslab crust beneath eastern JapanAs described in the previous section, the locations ofupper-plane events in the Pacific slab crust beneath NE
Japan roughly correspond to the dehydration reaction
boundaries of the metamorphosed crust (Fig. 5a). Recent
studies have revealed a more detailed spatial distributionof this upper-plane seismicity, as well as a clear relationship between that distribution and the seismic wave

Plate Age (Myr)
10

20

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2 00

0

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tig

ori

DSZ(double seismic zone) Separation (km)

te

De

hy

dra

tio

10

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lor

ite

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n

hy

dra

tio

n

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Fig. 6 Separation of the double seismic zone plotted against the age of the subducting slab. Distances between the upper and lower seismic
planes are shown by three different symbols: triangles show typical down-dip compression overlying down-dip extension, circles indicate a different
pattern, and crosses denote unknown. Vertical lines attached to the symbols are one-standard deviation uncertainties from Gaussian fits. Predicted
separations of the double seismic zones based on the lower seismic planes resulting from the dehydration of antigorite and chlorite based on Hacker etal. (2003) are shown as blue and green shaded areas, respectively. (After Brudzinski et al. 2007)

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

velocity structure of the slab crust, which is likely associated with dehydration reactions. This relationship may
be well explained if dehydration-related embrittlement is
the cause of intermediate-depth intraslab events.
The hypocenter distribution of events that occurred in
the crust of the Pacific slab under eastern Japan is shownin Fig. 7a (Kita et al. 2010). As shown with arrows in thisfigure, events in the slab crust in Tohoku and eastern
Hokkaido form a significant belt-like seismic zone nearlyparallel to the 70–90 km depth contours of the uppersurface of the slab. The frequency distributions of eventsin the slab crust as a function of depth along lines (b)through (l) in Fig. 7a are shown in Fig. 7b–l, respectively.
The distribution for each of these lines shows a clear
peak between the depths of 70 and 90 km, as indicatedin the figure by arrows. Figure 8a shows an across-arc
vertical cross-section of intraslab seismicity undercentral Tohoku. The belt-like concentrated earthquake
140°E

142°

Page 8 of 31

activity shown in the plan view of Fig. 7a is visible at c.
70–90 km in depth in the vertical cross-sectional view in
Fig. 8a. Kita et al. (2006) named this seismically active
band-shaped region of the slab crust the “upper-plane
seismic belt.” If intermediate-depth intraslab events arecaused by dehydration-related embrittlement, thisupper-plane seismic belt should be located in the existence range of hydrous minerals in the slab crust, particularly around the facies boundaries of the dehydration
reaction.
Beneath Kanto, the upper-plane seismic belt is notparallel to the 70–90 -km depth contours of the uppersurface of the slab; it becomes deeper locally and is oblique to the 70–90 -km depth contours, as shown in
Fig. 7a. This difference is caused by the shielding effectof the Philippine Sea slab that directly overlies the
Pacific slab (Hasegawa et al. 2007, 2009, 2016). Highquality data obtained in Japan with a nationwide seismic

144°

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44°N

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Fig. 7 A belt of upper-plane seismicity in the slab crust. a Hypocenter distribution of earthquakes in the crust of the Pacific Plate subducting
under eastern Japan. Events 0–10 km below the upper plate surface are shown by red dots. Bold arrows denote estimated locations of the upper-plane
seismic belt. The contact zone between the Philippine Sea and Pacific Plate beneath Kanto and that between the subducted crust and the Pacific Platebeneath western Hokkaido are indicated by blue thick broken curves. Solid curves, red triangles, and gray stars denote iso-depth contours of the upper platesurface, active volcanoes, and small repeating earthquakes, respectively. b–l Frequency distributions of upper-plane events along the lines b through l in
Fig. 7a plotted against depth. Frequency is plotted for intervals of 10 km. The black arrows show the peak that corresponds to the upper-plane seismic
belt. (After Kita et al. 2010)

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

(a)WPage 9 of 31

VolcanoesE60elan lt)r-p
pe ic be
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140E

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Depth (km)

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jadeite lawsonite
blueschist3hoeclogite
Lawsonite amphibole
eclogite

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Mo

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Tohoku0(b)40Depth (km)

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S-wave velocity (km/s)
Fig. 8 Seismicity and facies boundaries in the slab crust and low S-wave velocity layer. a Across-arc vertical cross-section of intraslab earthquakes
and facies boundaries in the crust of the Pacific slab under central Tohoku (modified from Kita et al.(2006)). Facies boundaries are indicated bybroken black lines and a long and short dashed red line. Open circles are earthquakes. Solid and thin dashed lines are the upper plate interface andthe Moho within the slab, respectively. b Across-arc vertical cross-section of S-wave velocity in central Tohoku (Tsuji et al. 2008). S-wave velocity is
indicated by the color scale at the bottom. Solid line is the upper plate interface, and A denotes the upper-plane seismic belt

network have enabled precise estimation of the positions
and shapes of both the Pacific and the Philippine Sea
Plates subducting under the Japanese archipelago, as
well as the contact of the two plates over a wide area
under Kanto (Nakajima and Hasegawa, 2007, 2010;
Nakajima et al. 2009b; Hirose et al. 2008; Kita et al.
2010a; Uchida et al. 2009, 2010). As shown in Fig. 9a, b,
the Pacific slab under Kanto is directly overlain by, andin contact with, the Philippine Sea slab. The extent of
the slab–slab contact area, which is enclosed by two
thick, broken green lines in Fig. 7a, corresponds

approximately to the extent of the Kanto Plain. Figure 7ashows that the down-dip end of the contact area is parallel to the upper-plane seismic belt, adjacent to its shallow side, and oblique to the 70–90 -km depth contoursof the upper surface of the slab. This clear spatial correspondence can be understood if we consider the following scenario. (1) The Pacific slab subducting under
Kanto is capped by the overlying Philippine Sea slab. (2)This cap prevents temperature rise in the Pacific slab
due to heating from the hot mantle wedge above. (3)Therefore, the phase transition associated with theHasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12(a)Page 10 of 31

(c)

44ºN
43º
42º
41º
140ºE 142º 144º

(b)

(d)

Fig. 9 The contact of the Pacific slab with the overlying Philippine Sea slab or forearc crust. a Vertical cross-section of S-wave velocity along a line
under Kanto in the inset map and b schematic figure showing the contact between the Philippine Sea and Pacific slabs (Nakajima et al. 2009b). c
Across-arc vertical cross-section of S-wave velocity under western Hokkaido along a line in the inset map and d schematic figure showing thecontact between the subducted forearc crust and the Pacific slab (Kita et al. 2010a). S-wave velocity is indicated by the color scale at the bottom

dehydration reaction is significantly delayed in the Pacific slab crust. (4) Consequently, the upper-plane seismic belt in the Pacific slab deepens locally and is obliqueto the 70–90 km depth contours of the upper surface ofthe slab, as shown in Fig. 7a.
A similar local deepening of the upper-plane seismic
belt was detected below SW Hokkaido, associated withthe junction of the NE Japan and Kuril arcs (Kita et al.
2010). As indicated by the arrows in Fig. 7a, e, theupper-plane seismic belt becomes deeper locally in thearea beneath this arc–arc junction and reaches the vicinity of the 110 -km depth contour of the slab’s upper surface. Beneath Hokkaido, on the southwestern edge ofthe Kuril arc, the Pacific Plate is subducting obliquely tothe trench axis. This oblique subduction causes the forearc sliver of the Kuril arc to collide with the forearc of
the NE Japan arc (e.g., Kimura, 1986). A seismic tomography study by Kita et al. (2010) clearly imaged the crust
portion of the colliding forearc, which has subducted
into the mantle wedge and reached the upper surface of
the Pacific slab immediately below (Fig. 9c, d). The estimated contact area between the subducted crust thus
imaged, and the Pacific slab is enclosed by a thick,broken green line in Fig. 7a. This figure shows thatthe contact area corresponds closely with the local

deepening of the upper-plane seismic belt. Therefore,
similarly to the case in Kanto, the local deepening of
the upper-plane seismic belt in SW Hokkaido can beunderstood if we consider the following. (1) The Pacific slab under SW Hokkaido is capped by subductedcrust that directly overlies it (2) his capping prevents
the Pacific slab from being heated by the mantle wedge
immediately above it (3) herefore, the phase transition anddehydration is delayed in the slab crust (4) his delay causes
local deepening of the upper-plane seismic belt in thisregion, as well (Kita et al. 2010).
If the phase transition of the dehydration reaction occurs in the slab crust, seismic wave velocity is expected to
become faster across the phase boundary. Therefore, it
can be verified based on the distribution of seismic wave
velocities obtained through seismic tomography whether
the phase transition of the dehydration reaction causes
the formation of the upper-plane seismic belt. Figure 8b
shows an across-arc vertical cross-section of S-wave velocity in central Tohoku obtained by Tsuji et al. (2008)
using the double-difference (DD) tomography method of
Zhang and Thurber (2003). A low-velocity layer that corresponds to the slab crust extends down to the depth of
the upper-plane seismic belt (A in the figure), and no further. Figure 10a shows the distribution of S-wave velocity

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12(a)Page 11 of 31(b)Fig. 10 Seismicity and S-wave velocity in the crust of the Pacific slab. a S-wave velocity distribution in the crust of the Pacific slab under NE Japanon a curved plane 5 km below the upper plate surface. b Hypocenters of earthquakes within the crust of the Pacific slab. Events 0–10 km below
the upper plate surface are shown as blue dots, and the upper-plane seismic belt is shown in pink. Broken black curves denote iso-depth contoursof the upper plate surface. The slab–slab contact zone is enclosed by two green broken curves. Red triangles are active volcanoes. (After Nakajima et al. 2009b)in the Pacific slab crust estimated through DD tomography (Nakajima et al. 2009). As also shown in Fig. 8b, thelow-velocity layer under Tohoku extends to a depth of approximately 80 km, the depth of the upper-plane seismic
belt, and no further. In contrast, the low-velocity layer
under Kanto becomes locally deeper in the area that corresponds to contact with the overlying Philippine Sea slab;
this area is enclosed by two thick, broken green lines inFig. 10a. Figure 10a shows that the areal extent of the lowvelocity layer is reasonably consistent with the slab–slab
contact area. Comparison of Fig. 10a with the hypocenter
distribution in the slab crust (Fig. 10b) shows that the location of the locally deepened upper-plane seismic belt is
in excellent agreement with the down-dip end of the lowvelocity layer in the slab crust. Consequently, the obtained
distribution of S-wave velocity in the slab crust further
supports the prediction that the phase transition is locallydelayed in this region.
Mode-converted seismic waves at the upper plateinterface from intermediate-depth intraslab events also
provide evidence that the seismic low-velocity layer corresponds to the subducting slab crust (e.g., Matsuzawa
et al. 1986, 1990; Zhao et al. 1997; Abers, 2005). P-to-S
mode-converted waves at the upper surface of the plate
propagate through the slab crust for longer distances

than do direct P- and S-waves; therefore, their arrivaltime data are convenient for estimating the seismic velocity structure within the slab crust. A recent analysis of
P-to-S converted waves under central Tohoku by Shiina
et al. (2013) reported that the seismic low-velocity layer
persists down to the depth of the upper-plane seismic
belt in the subducted oceanic crust. Estimated P-wave
velocity at the depth of the upper-plane seismic belt inthe low-velocity crust is c. 10% lower than theoretically
predicted values for metamorphosed mid-ocean ridge
basalt (MORB). Shiina et al. (2013) proposed the coexistence of fluids and hydrous minerals to explain this substantial decrease in P-wave velocity, which provides
additional seismic evidence that earthquakes in the slab
crust are facilitated by fluid-related embrittlement.
Thus far, we have discussed the phase transition basedon the seismic observations that the belt-like concentration of seismic activity is detectable in the slab crust andthat the seismic wave low-velocity layer extends down to
the depth of this upper-plane seismic belt. Here, we also
compare the locations of the upper-plane seismic belt
and the down-dip end of the seismic low-velocity layer
with thermal–petrological estimates of the positions of
the dehydration reaction boundaries in the slab crust, although there is high uncertainty in their positions

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

because of error in the estimation of temperature distribution in the subducting slab. The phase boundaries ofjadeite lawsonite blueschist (hereafter JLB) → lawsonite
amphibole eclogite (hereafter LAE) and of LAE → eclogite in the slab crust beneath central Tohoku are indicated by the broken black lines B and C, respectively, inFig. 8a. These boundaries are estimated based on the
MORB phase diagram of Hacker et al. (2003b) (Fig. 3a)and the temperature distribution of the slab by Peacockand Wang (1999) (the P–T trajectory of the oceanic
crust of Tohoku shown in Fig. 3a). The upper-plane seismic belt indicated as A in the figure is located near oron the shallower side of the phase boundary of JLB witha H2O content of 5.4% → LAE with a H2O content of
3.0%, as indicated by B in the figure. These findings areroughly consistent with the predictions based on thedehydration-related embrittlement hypothesis. Furthermore, earthquakes occur only in the lower crust on the
deeper side of the phase boundary of JLB → LAE, andno longer occur after exceeding the phase boundary of
LAE → eclogite, as indicated by C in the figure. This observation is also consistent with the predictions drawn
from the dehydration-related embrittlement hypothesis.
For another example, the estimated dehydration reaction
boundary based on the MORB phase diagram by Omori
et al. (2009) and the temperature distribution of the slab
given by van Keken et al. (2002) is indicated by a thick,
one-dot-chain red line in Fig. 8a. In this case, the phaseboundary also roughly coincides with the position of theupper-plane seismic belt as predicted based on thedehydration-related embrittlement hypothesis.
The production rate of dehydration-derived H2O appears to clearly explain the formation of the belt-like
concentration of upper-plane seismicity at depths of c.
80–120 km in the oceanic crust of the Pacific slab subducting under eastern Japan. Abers et al. (2013) calculated the accumulated amount of dehydration-derived
H2O, the H2O production rate, and the overall system
volume change from the dehydration reaction, as a function of depth for two representative slabs: the Pacificslab beneath eastern Hokkaido as a cold slab and thePhilippine Sea slab beneath Kii as a warm slab
(Fig. 11b–d). Their calculations were based on estimatedP–T (pressure–temperature) trajectories for the subducting slab crust on the P–T diagram of maximum
H2O contents for a hydrated MORB metabasalt, asshown in Fig. 11a. The frequency of earthquakes within
the slab crust as a function of depth beneath eastern
Hokkaido (Kita et al. 2010) is shown in Fig. 11e for comparison. These figures show that the H2O production
rate, the overall volume change from dehydration, andthe frequency of earthquakes in the slab crust below
eastern Hokkaido (blue lines in Fig. 11c–e) all havepeaks at a depth of c. 80–90 km. Although Fig. 11c, d isPage 12 of 31

based on calculations for the top of the slab, the strong
correlations of both the H2O production rate and thetotal volume change with seismicity in the slab cruststrongly support the interpretation that embrittlement
caused by increased pore pressure from dehydrationderived H2O is the reason for the upper-plane seismic belt.
At present, however, there are large uncertainties in
estimating the temperature distribution within subducting slab crust; therefore, further verification of the thermal–petrological estimates of the position of thedehydration reaction boundary is necessary. Nevertheless, the following seismic observations strongly support
the dehydration-related embrittlement hypothesis: (1)that there is a belt of upper-plane seismicity nearly parallel to the 80–90 km depth contours of the upper slab
surface, (2) that the seismic low-velocity layer corresponding to the slab crust persists down to the upperplane seismic belt and, no further, (3) that both theupper-plane seismic belt and the down-dip end of theseismic low-velocity layer deepen locally under Kanto
and SW Hokkaido, with these local deepenings coinciding spatially with the areas of contact with the Philippine
Sea slab and the overlying subducted forearc crust, and(4) that both the H2O production rate and the net volume change caused by dehydration in the slab crust
have peak depths at c. 80–90 km, which is approximately consistent with the location of the upper-plane
seismic belt.
The upper-plane seismic belt and seismic low-velocity layer
in slab crust in other subduction zones

Does a similar upper-plane seismic belt form in the slab
crust in other subduction zones? The formation of
upper-plane seismic belts should be common to all subduction zones if they are caused by fluids expelled by
the dehydration of slab crust materials.
Abers et al. (2013) systematically investigated the relationship between trends in earthquake locations andthermal–petrological conditions within the crust of subducting slabs by comparing several well-characterized
subduction zones from different parts of the world. They
found that in young (and therefore warm) subduction
zones, earthquakes preferentially lie within the slab
mantle or very close to the slab Moho. In contrast, inold (and therefore cold) subduction zones, earthquakes
occur throughout the oceanic slab crust as well as theslab mantle. They proposed that separation into thesetwo classes is caused by differences in the P–T paths
that the slab crust follows as it descends into the mantle.
The subducting slab P–T trajectories of young andwarm subduction zones and those of old and cold subduction zones encounter different facies boundaries,
which produce differences in the way that H2O is released via dehydration. Estimated temperatures along

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

Depth, km

(a)

(b)

(c)

(d)
(e)

Page 13 of 31

Fig. 11 P–T path, H2O production, system volume change, andfrequency of earthquakes in the crust. a Pressure–temperature (P–T)
paths of the subducting oceanic crust for six subduction zone
segments plotted on a diagram of maximum H2O contents (Aberset al. 2013). P–T paths for the upper slab surface and slab Moho areshown as colored solid and dot dashed lines, respectively. Shadingshows maximum H2O contents in wt% for a hydrated metabasalt of
MORB bulk composition (Hacker, 2008). Major dehydration reactions
are indicated by A (lawsonite breakdown), B (zoisite breakdown), andC (amphibole breakdown). b–d Evolution of pore fluid pressure andH2O content for cold and warm slabs (Abers et al. 2013). Hokkaido isindicated by blue lines as a representative cold slab and Kii by redlines as a representative warm slab. b Accumulated dehydration-derived
H2O content in wt% as dehydration reactions progress. c Rate of H2O
production, with Kii scaled by a factor of 10 relative to Hokkaido. d Total
system (solid + H2O) volume. Shaded in light blue and yellow denote
depth intervals of major dehydration for Hokkaido and Kii, respectively. A,
B, and C denote specific reactions shown in Fig. 11a. e Frequency ofupper-plane earthquakes beneath central Hokkaido (Fig. 7d) as a functionof depth (Kita et al. 2010)

the upper surface of the slab and the slab Moho for eachof the six subduction zone segments are plotted on apressure–temperature diagram of H2O content for hydrated MORB in Fig. 11a. The figure shows that the P–
T paths for the cold and warm slabs are characterizedby considerably different conditions. At high pressures,the main dehydration reaction is the breakdown of lawsonite (A in Fig. 11a), and the crusts of cold slabs, suchas those beneath Hokkaido, Tohoku, and central Alaska,
experience this dehydration reaction to eclogite. At
lower pressures, the breakdown of zoisite and amphibole(B and C in Fig. 11a) takes place and the crusts of warm
slabs, such as those beneath Cascadia, Tokai, and Kii,
undergo this dehydration reaction.
The above two dehydration reaction boundaries produce different H2O production rates, as shown in
Fig. 11b, c. These figures show a gradual release of H2O
in the depth range of c. 20–60 km for the warm slab beneath Kii, but a considerably abrupt release in the depth
range of c. 80–95 km for the cold slab beneath
Hokkaido. The production rate of H2O in cold slabs isan order of magnitude higher than that for warm slabs(Fig. 11c). Moreover, because of the positive Clapeyron
slope, the overall volume change from the dehydration
reaction is positive for the cold slab (Fig. 11d), whichcauses effective increase in pore fluid pressure and thus
favors overpressuring. The large H2O production rate
and the increase in total volume through the breakdownof blueschist are likely to promote earthquake occurrence in cold slabs. In contrast, the slow H2O production rate and decrease in total volume through the
breakdown of zoisite and amphibole probably inhibit, orat least do not promote, earthquake occurrence in thecrusts of warm slabs. Abers et al. (2013) showed that theboundary between these two cases can be expressed by

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

the thermal parameter (Kirby, 1995; (plate age) × (vertical descent rate of plate)) of c. 500–1000 km.
The results given by Abers et al. (2013) predict thatthe formation of the upper-plane seismic belt in the slab
crust does not occur only in the subducting Pacific slab
beneath eastern Japan but occurs also commonly in cold
subduction zones with thermal parameters larger than c.
500–1000 km. In contrast, in the crust of warm slabs
with thermal parameters less than c. 500–1000 km,
earthquakes occur preferentially within the slab mantle,
and therefore, the upper-plane seismic belt does notform. Dense local seismic network data and intense seismic activity in the slab crust are required to ascertain
the presence of an upper-plane seismic belt. Few suchsubduction zones exist, and consequently, clear evidenceof the presence of upper-plane seismic belts in othersubduction zones has not yet been reported.

Page 14 of 31

The nationwide dense seismic network of Japan also
covers the SW Japan subduction zone where the Philippine
Sea slab is subducting beneath it. The age of the subducting Philippine Sea Plate at the trench axis varies in thealong-arc direction. It is as young as 15–25 Ma between
western Shikoku and Tokai, whereas it is older in Kanto(>40 Ma) and Kyushu (> 50 Ma) (Seno and Maruyama,
1984). Therefore, it may be possible to ascertain the presence or absence of the upper-plane seismic belt. Figure 12
shows the hypocenter distribution of events in the crust of
the Philippine Sea slab. This figure shows that events inthe slab crust under the region from western Shikoku toTokai are distributed to a maximum depth of c. 40, except
for some outliers. In contrast, the down-dip end of theseismicity in the slab crust is deeper under regions where
older lithosphere of the Philippine Sea Plate is subducting;it is as deep as 80–90 km in Kanto and is c. 100 km deep

Fig. 12 Hypocenter distribution of earthquakes in the crust of the Philippine Sea plate under western Japan. Events 0–10 km below the upper
plate surface are shown as red dots. Bold arrows denote estimated locations of the upper-plane seismic belt. The contact zone between the Philippine
Sea and Pacific plates beneath Kanto is indicated by thick broken curves. Solid curves, red triangles, and orange curves denote iso-depth contours of theupper plate surface, active volcanoes, and source areas of large interplate earthquakes, respectively. Thick, broken gray line shows the Kyushu–Palau Ridge
and its northeastern prolongation on top of the Philippine Sea slab. The northward bend near the northern end appears because of the projection of
the slab’s top surface with an increasing subduction angle relative to Earth’s surface. (After Hasegawa et al. 2010)

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

in Kyushu. These findings are consistent with the expectation that the colder Philippine Sea slab under Kanto andKyushu is able to generate earthquakes at greater depths.
Another prominent feature of Fig. 12 is a belt-like concentration of seismicity, as indicated by the arrow, thatis nearly parallel to the c. 60 km depth contour of theupper surface of the slab under southern Kyushu(strictly speaking, under the region to the south of the
NW extension of the Kyushu–Palau ridge (thick brokenline)). This feature is similar to the upper-plane seismic
belt detected in the crust of the Pacific slab under eastern Japan. Although a similar belt-like concentration ofseismic activity seems to be detectable in the depth
range of 40–50 km beneath Kanto and may reflect thesame activity, it is not discussed further here because it
is unclear. In contrast, such a significant belt-like concentrated earthquake zone is not apparent in the muchlarger area from western Shikoku to Tokai. This alongarc variation in crustal seismicity is also inferred to be
the result of the difference in age (or temperature) of thePhilippine Sea slab (Hasegawa et al. 2009). The thermal
parameters calculated by Syracuse et al. (2010) indicatethat the slab beneath southern Kyushu at 31.22°N,
130.57°E has a value of 1560 km, which means that it belongs to the cold slab group. In contrast, the slab beneath Nankai at 35.3°N, 137.3°E has a value of 450 km,
which means it belongs to the warm slab group. Consequently, the belt-like concentrated seismicity in the crustof the older Philippine Sea slab beneath southern
Kyushu, which occurs at depths approximately 20 km
shallower than the belt of the Pacific slab beneath
Hokkaido and Tohoku, is probably caused by the same
dehydration reactions of slab crust materials. The absenceof such a clear belt-like concentrated seismicity in the crustof the younger portion of the Philippine Sea slab beneaththe area from Tokai to western Shikoku (or northern
Kyushu) is attributed to the warmer conditions there.
As shown in Fig. 12, the estimated upper-plane seismic
belt in southern Kyushu abruptly disappears north of the
NW prolongation of the Kyushu–Palau ridge (thick,broken gray line in Fig. 12). Figure 13 illustrates an alongarc vertical cross-section of earthquakes that occurred
within the rectangle shown in Fig. 12. This figure also
shows that the upper-plane seismic belt, which is visible atdepths of c. 50–70 km, and abruptly disappears to thenorth of the NW prolongation of the Kyushu–Palau ridge,
as shown by the thick, broken red line. The sudden disappearance of the upper-plane seismic belt strongly suggests that the formation of the belt is controlled by slab
age or temperature, as shown in Fig. 11, because the slab
age abruptly changes between the north and south of the
Kyushu–Palau ridge.
Studies of several subduction zones have revealed thepresence of seismic low-velocity zones that correspondPage 15 of 31

to the subducting crust, although the upper-plane seismic belt has not yet been detected at other sites (e.g.,
Fukao et al. 1983; Hori et al. 1985; Hori, 1990; Helffrich,
1996; Reyners et al. 1999; Abers, 2000, 2005; Bostok et
al. 2002; Kodaira et al. 2000, 2004; Eberhart-Phillips etal. 2005; Hirose et al. 2008a, b; Syracuse et al. 2008; Rondenay et al. 2008, 2010; Abers et al. 2009, 2013;
Eberhart-Phillips and Bannister, 2015). These seismic
low-velocity layers in subducting crust have been interpreted as evidence for the presence of hydrated rocks or
fluids liberated by dehydration reactions. Typical examples of these layers are shown in Figs. 14, 15, and 16.
Figure 14 shows across-arc vertical cross-sections of
the P-wave velocity in the slab below Nicaragua andCosta Rica estimated based on inversions of P- and Swave arrival times of local earthquakes (Syracuse et al.
2008). Nicaragua and Costa Rica lie on the Caribbean
Plate, under which the young Cocos Plate is subducting
northeastward at a rate of c. 88 mm/year. Inversion results show the presence of P-wave low-velocity zones inthis area that probably correspond to the crust of the subducted Cocos Plate. This observation is particularly clear
in Fig. 14c, which shows a P-wave low-velocity zone that
extends as far as 70 km in depth. The down-dip end may
be produced by the phase transformation of materials thatcompose the slab crust. Moreover, P-wave low-velocity
zones were also imaged at the locations of the slab mantle
c. 20–30 km below Wadati–Benioff zone seismicity,
depths of c. 140 km beneath Nicaragua, and c. 60 km beneath Costa Rica. Syracuse et al. (2008) interpreted theselow-velocity zones beneath Nicaragua as 10–20% serpentinized mantle of the subducted Cocos Plate, which may
introduce relatively high amounts of H2O into the mantle
wedge beneath Nicaragua.
Figure 15 shows across-arc vertical cross-sections of Pwave velocity and Vp/Vs (the ratio of P-wave velocity to Swave velocity) along the northern Hikurangi subduction
zone of New Zealand. These data were obtained by inverting arrival-time data of local earthquakes (Eberhart-Phillips
and Bannister, 2015). Onshore–offshore marine-seismic
data were also incorporated in these inversions, which improved constraints on the shallow portion of the target area.
In this area, the Pacific Plate is subducting obliquely underthe Australian Plate, on which the North Island of New
Zealand lies, at a rate of 42–47 mm/year. The obtained
seismic velocity images shown in Fig. 15 clearly indicate thepresence of a P-wave low-velocity zone and a high Vp/Vs
zone to depths of 80–100 km associated with the oceanic
crust of the subducted Pacific slab. This zone of low Vp
and high Vp/Vs is probably caused by hydrated rocks or
fluids liberated by dehydration reactions.
Figure 16a shows an across-arc seismic section beneath central Alaska obtained through seismic receiverfunction analyses (Rondenay et al. 2008, 2010). In thisHasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

SSW

Depth (km)NNEPage 16 of 31

Distance (km)Fig. 13 Along-arc vertical cross-section of earthquakes beneath Kyushu. Earthquakes that occurred in the rectangular region shown in Fig. 12 are shownas circles. The distance of each event from the east-southeastern edge of the rectangular region is shown as the color scale. The estimated northeastern
prolongation of the Kyushu–Palau ridge from the top of the slab is shown as the thick, broken red line. Red triangles at the top denote volcanoes

region, the Pacific Plate has an age of 35–45 Ma at the
trench axis and converges with the North American
Plate at a rate of c. 55 mm/year, which produces a thermal structure of the slab that falls in the middle of therange globally (Kirby et al. 1996; Rondenay et al. 2008).
Along the profile shown in Fig. 16a, the Yakutat terrane,
which has a thickness from 15 to 30 km and can be described as an oceanic plateau, is subducting. This subduction was clearly imaged as a dipping low-velocity
layer to a depth of c. 130 km. Intraslab earthquakes
occur near the top of the subducting slab crust at depths
shallower than c. 80 km and then gradually deepen toward the oceanic Moho with increasing depth. Both theintraslab seismicity and the low-velocity slab crust terminate at c. 120–130 km in depth, which suggests thatthe eclogite-forming phase transition operates in thisdepth range (Abers et al. 2013).
Figure 16b shows an across-arc seismic migration
image below central Cascadia obtained via seismic
receiver-function analyses (Abers et al. 2009). In this region, the Juan de Fuca Plate, with an age of 6–9 Ma at
the trench axis, is subducting obliquely at a rate of 35–
45 mm/year. This slow subduction rate in the verticaldirection and the young age of the plate make it one of
the hottest slabs in the world (Wada and Wang, 2009;
Syracuse et al. 2010). The subducting slab crust was
clearly imaged as a dipping low-velocity layer with athickness of 7–9 km to a depth of c. 40 km. Almost all
intraslab earthquakes in this region are located at or justbelow the slab Moho, and very little seismicity is
produced within the subducting crust. The shallow

termination depths of the low-velocity slab crust and thevery low level of seismicity in the slab crust are consistent with the high-temperature conditions of this slab
(Abers et al. 2013). Based on estimations of the thermal
structure of the slab and on the phase diagram of crustmaterials, Rondenay et al. (2008) compared two subduction zones, central Alaska and central Cascadia, andclaimed that the termination depths of the low-velocity
subducting crust correspond to eclogitization of themetamorphosed crust through dehydration of hydrous
minerals, which depends largely on the thermal structure of the subducting slab crust. Overall, seismicity inthe slab crust appears to be closely related to the dehydration reactions, although the upper-plane seismic belt
is not apparent even in the case of the cold slab described here, which can most likely be attributed to thevery small number of earthquake data.
All of these observations seem to support thedehydration-related embrittlement hypothesis as themechanism behind the genesis of intermediate-depth
events within the slab crust.
Seismic wave velocity structure and lower-plane seismicity
in the slab mantle: seismic low-velocity zones andserpentinized slab mantle

Until recently, seismic imaging of the subducting slab
was performed not to image detailed, inhomogeneous
structures inside the slab, but to determine the shape ofthe subducting slab as a seismic high-velocity zone separate from the surrounding mantle with normal seismic
wave velocity. Very high spatial resolution is required toHasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

Depth (km)Page 17 of 31

(d)

(b)

(e)

(c)

(f)

Depth (km)Depth (km)(a)

Distance (km)Distance (km)14ºN

Vp (km/s)

12º

10º

8º
88ºW

86º

84º

82º

Fig. 14 P-wave velocity structure in Nicaragua and Costa Rica (Syracuse et al. 2008). P-wave velocity is shown on across-arc vertical cross-sections
along lines a through f in the inset map by the color scale at the bottom. Regions surrounded by white contours contain 50 or more seismic
rays. Earthquakes within 40 km to the northwest of each cross-section line are shown in white and those within 40 km to the southeast are inblack. Red inverted triangles show the trench axis

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

Page 18 of 31

Fig. 15 P-wave velocity and Vp/Vs ratio structures in northern Hikurangi, New Zealand (Eberhart-Phillips and Bannister, 2015). P-wave velocity and
Vp/Vs values are shown on across-arc vertical cross-sections by the color scales at the bottom in a and c and b and d, respectively. a–b Raukumara
Peninsula and c–d Hawke Bay. White and yellow lines denote the limit of resolution for the spread function = 3.5. Earthquakes and volcanoes areshown as crosses and triangles, respectively. Magenta and green lines show the upper surface of the slab

detect the inhomogeneous structure inside the slab, andin the past, this resolution could not be achieved. A
high-density seismic network is required for such highresolution seismic imaging, and recently, this resolution
has become possible through dense observation network
data. The nationwide dense seismic network has been
deployed in Japan, a typical subduction zone with a very
high seismic activity, and this development has enabled
us to image even the internal structure of the subductedslab below Japan. The seismic low-velocity layer thatpersists to the depth of the upper-plane seismic belt inthe slab crust, as described in the “The double seismiczone and dehydration of metamorphosed crust and 149
serpentinized mantle” section, is one example of such internal structure. Recent seismic tomography studies
based on nationwide dense network data have also revealed the detailed structure of seismic velocity insideeven the slab mantle.
If hydrous minerals or the H2O released through dehydration reactions are present in some regions of theslab mantle, those regions are expected to have low seismic wave velocities. Precisely, imaging of heterogeneous
structures inside the slab mantle with seismic tomography

can be challenging, however. The slab mantle exists atgreater depths than the slab crust, and consequently, seismic rays arriving at seismic stations above earthquake hypocenters do not sufficiently crisscross the slab mantle,
which results in poor resolution in seismic velocity imaging. Therefore, research into the heterogeneous seismic
velocity structure of the slab mantle remains very limited.
If many densely distributed events occur in a particular area, that area can be imaged very precisely via
seismic tomography because a large number of ray paths
will crisscross the area. The occurrence of numerous aftershocks associated with a large intraslab earthquake
provides such a rare opportunity. Based on the tomography method of Zhao et al. (1992, 1993), Mishra andZhao (2004) determined the detailed seismic velocity
structure in and around the hypocenter of the 2003 M
7.1 Miyagi–Oki earthquake, which occurred at a depthof c. 70 km in the Pacific slab near the oceanic Moho
beneath NE Japan. In their tomographic inversions, theyused many data from aftershocks in addition to thebackground earthquake data, which enabled precise imaging of the source area within the slab. P-wave velocity
structures revealed through this method are shown inHasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

Page 19 of 31

(a)
64ºN

Depth

60º

156ºW

152º

148º

Depth

(b)

48ºN

44º
124ºW

120º

Fig. 16 S-wave velocity structures in central Alaska and central Cascadia. Across-arc seismic sections are shown along lines in the inset maps
under a central Alaska and b central Cascadia (Abers et al. 2013, originally from Rondenay et al. 2008, 2010 and Abers et al. 2009). Colored images
show migrated teleseismic P coda as dVs/Vs (S-wave velocity perturbation). dVs/Vs values are shown by the color scale at the bottom; red and blue
correspond to slow and fast velocities, respectively. Black circles denote earthquakes. Dotted and solid lines indicate the upper slab surface and theslab Moho, respectively. Inverted black triangles show seismic stations used, and the red triangle in b shows a volcano

Fig. 17 in N–S and E–W vertical cross-sections that passthrough the mainshock hypocenter. This figure showsan anomalously low-velocity zone across the crust andmantle in the Pacific slab and that the source area of theM 7.1 intermediate-depth intraslab event is located inthis zone characterized by a velocity anomaly lower thanits surroundings in the slab. Mishra and Zhao (2004)

concluded that this low-velocity anomaly and the occurrence of earthquakes within it could be attributed to theprocess of fluid-related embrittlement caused by the dehydration of hydrous minerals in the slab.
Nakajima et al. (2011) examined the seismic wave velocity structure of the source area of the 2011 M 7.1
Miyagi–Oki earthquake that took place at a depth of c.

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

Page 20 of 31

Depth (km)

0
50

100

150

(a)

(c)

(b)

(d)

Depth (km)

0

50

100

150

Fig. 17 P-wave velocity structure in and around the source area of the 2003 M 7.3 Miyagi–Oki intraslab earthquake. P-wave velocity perturbations
are plotted based on the color scale at the bottom for a, c N–S and b, d E–W vertical cross-sections along lines shown in the inset map. White dots
in a and b are aftershocks of the M 7.3 event, and those in c and d are background seismicity. Hypocenter of the M 7.3 event is indicated by a star.
Thick curves are the Moho of the upper plate and the top surface of the subducting Pacific slab, in order from the top. (After Mishra and Zhao, 2004)

65 km in the slab mantle about 1 month after the2011 Mw 9.0 Tohoku–Oki earthquake. As shown in
Fig. 18, the seismic wave velocity structure obtained byusing numerous aftershock data in conjunction with thebackground seismicity data revealed the existence of alow-velocity zone around the fault plane of this M 7.1
intermediate-depth intraslab event. The lateral extent of
the low-velocity zone is comparable to the extent of thefault plane of the M 7.1 event. The angle between themainshock fault plane and the upper slab surface is c.
60°, which is consistent with the dip of the oceanwarddipping normal faults in the outer rise region. Based onthese observations, Nakajima et al. (2011) suggested that
the M 7.1 intraslab earthquake was generated by reactivation of a buried hydrated fault that had formed in theplate prior to its subduction.
Zhang et al. (2004) were the first to determine the detailed internal structure of the slab mantle to the depth ofthe lower seismic plane of the double seismic zone. Their

tomographic imaging of the slab beneath central Tohokuwith DD tomography revealed the existence of a P-wave
low-velocity layer along the lower seismic plane, as shownin Fig. 19. The similar P-wave low-velocity layer along thelower seismic plane of the slab mantle was identified insubsequent seismic tomography studies by Shelly et al.
(2006) and Nakajima et al. (2009c) under southern Tohoku
and eastern Hokkaido, respectively. Figure 20 shows
across-arc vertical cross-sections of P- and S-wave velocities under eastern Hokkaido as determined by Nakajima etal. (2009c). A clear P-wave low-velocity layer is visible atthe lower plane of the double seismic zone. However, both
Figs. 19 and 20 indicate that S-wave velocity is not necessarily low for that layer. Therefore, the Vp/Vs ratio for thislayer is not necessarily as high as is expected for the serpentinized mantle, but may be fairly low, as shown in Fig. 19c.
Reynard et al. (2010) investigated the cause of the
low Vp and Vp/Vs values imaged via seismic tomography for this layer of the mid-slab mantle below

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

140ºE

140.5º141ºPage 21 of 31

141.5º142º142.5º
39ºN

38.5º

38º

Fig. 18 Seismic velocity structure in and around the source area of the 2011 M 7.1 Miyagi–Oki intraslab earthquake. a Across-arc vertical crosssections of S-wave velocity under central Tohoku along three lines: A–A', B–B', and C–C' in the inset map e. White stars in line A–A' and line C–C'
denote the hypocenter of the 2011 M 7.1 event and the 2003 M 7.1 event, respectively. Solid and dashed black curves denote the upper surfaceof the Pacific slab and the Moho within it, respectively. White rectangle in a shows the area enlarged in b and c. b Across-arc vertical cross-sections of Swave velocity and c Vp/Vs within the area outlined by the white rectangle in a. Black lines in A–A' and in B–B' show the fault plane of the 2011 M 7.1
event. d Schematic figure showing fault slips of the 2011 M 9.0 Tohoku–Oki earthquake (purple arrows) and the M 7.1 intraslab event (white arrows). e
Map showing the aftershock distribution of the 2011 and 2003 intraslab events and the locations of the three lines, A–A', B–B', and C–C'.
(After Nakajima et al. 2011)

eastern Hokkaido (Fig. 20). They claimed that thisanomalous layer is the result of the anisotropy of anhydrous peridotite, which suggests that the same mineralogical composition occurs along the lower-plane
seismicity as in the surrounding slab mantle. Based on
this interpretation, they further argued that the shear
heating instability of plastic deformation (Griggs andHandin, 1960; Ogawa, 1987; Hobbs and Ord, 1988)
would therefore be the mechanism behind the lowerplane earthquakes. Conventionally, the thermal shear
instability model was thought to be unable to explain
the observation that the lower plane of the double seismic zone formed along the isotherm of approximately
600–700 °C in the slab mantle (see Figs. 5 and 6; Yamasaki
and Seno, 2003;. Brudzinski et al. 2007). However, a recent
investigation by Kelemen and Hirth (2007) based on numerical modeling of a fine-grained viscous shear zone in acoarse-grained elastic half-space revealed that the shear
heating instability of peridotite occurs effectively within anarrow temperature range of 650–800 °C. If this is
the case, the shear heating instability model of plastic
deformation can also explain the observations thatthe lower-plane events occur along the isotherm of
approximately 600–700 °C.
However, if we inspect the seismic tomography images(Fig. 20) of Nakajima et al. (2009c) in detail, regions with
P-wave velocity below 7 km/s are distributed entirely
along the lower plane of the double seismic zone. It isdifficult to explain the distribution of this low P-wave
velocity anomaly along the lower seismic plane, but notin the surrounding mantle, based on the anisotropy ofanhydrous peridotite alone. The low Vp values over theentire lower seismic plane imaged with seismic tomography suggest that materials with lower Vp were more

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12(a) VpVp (km/s)

10
9.5
9
8.5
8
7.5
7
6.5
6
5.5
5

Depth (km)

20
40
60
80
100
120
140
160

41ºN

40º

39º

38º
140ºE

20 40 60
Vs (km/s)(b) Vs

142º

143º

Vp/Vs

20
5

40
60

4.5

80
4

100
120

3.5

140

40
60
80
100
120
140

3

160
-80 -60 -40 -20 0
X (km)(c) Vp/Vs

5.5

20

Depth (km)

141º

0

Depth (km)-80 -60 -40 -20

Page 22 of 31

20 40 60160
-80 -60 -40 -20 0 20 40 60
X (km)

1.95
1.9
1.85
1.8
1.75
1.7
1.65
1.6
1.55
1.5

Fig. 19 Seismic velocity structure along the lower seismic plane under central Tohoku. Shown are across-arc vertical cross-sections of a P-wave
velocity (Vp), b S-wave velocity (Vs), and c Vp/Vs ratio under central Tohoku. P-wave velocity, S-wave velocity, and Vp/Vs ratio along the line in
the inset map are shown by the color scale at the right of each figure. Black dots are earthquakes. (After Zhang et al. 2004)

abundant there than in the surrounding slab mantle, although the anisotropy effect of those materials may contribute in part to the lower Vp and Vp/Vs values
detected via seismic tomography. The low Vp and Vp/Vs
layer observed along the lower seismic plane is still not
fully explained, and further examination and verification
are required to determine the cause. Moreover, seismicity in the slab mantle is limited not only in the upperand lower planes of the double seismic zone but also between these planes, although the amount of seismic activity is lower (Kita et al. 2010b). Such interplane events
are also shown in Fig. 20. In particular, the aftershock
activity associated with the 1993 M 7.5 Kushiro–Oki
earthquake at a depth of c. 100 km was distributed along
a distinct P-wave low-velocity layer that extended horizontally from the lower seismic plane to the vicinity of
the upper seismic plane (black arrow in Fig. 20c). This
low P-wave velocity is also difficult to explain throughthe mechanism of peridotite anisotropy alone. The presence of interplane events (Kita et al. 2010b) indicatesthat intermediate-depth intraslab events occur even at
temperatures much lower than 650–800 °C, whichfurther contradicts predictions based on the estimate of
Kelemen and Hirth (2007).
Similar P-wave low-velocity layers along the lower
plane of the double seismic zone were detected beneath
northern Chile using DD tomography (Dorbath et al.
2008), although these results were less clear than thosefor beneath Tohoku and Hokkaido (Zhang et al. 2004;
Shelly et al. 2006; Nakajima et al. 2009c). Across-arc vertical cross-sections of the estimated P- and S-wave velocities are shown in Fig. 21. This figure shows thepresence of P-wave low-velocity zones along the lower
seismic plane in all three cross-sections. As in the Pacificslab below eastern Japan, S-wave velocities are not low,
but normal along the lower plane, which suggests thatthe low P-wave velocity and normal S-wave velocity
along the lower seismic plane is a common featureacross subduction zones. Dorbath et al. (2008) inferredthat these P-wave low-velocity zones form along theslab’s flexural neutral plane where fluids released
through dehydration of the surrounding hydrated minerals accumulate. Therefore, the P-wave velocities are
low and trigger lower-plane seismicity.

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

Page 23 of 31

AB(a) Vp(c) VpAB(b) Vs(d) VsFig. 20 Seismic velocity structure along the lower seismic plane under eastern Hokkaido. Shown are across-arc vertical cross-sections of a, c P-wave
velocity and b, d S-wave velocity under eastern Hokkaido. P- and S-wave velocities along two lines in the inset map are shown by the color scales at
the right. Aftershock activity of the 1993 M 7.5 Kushiro–Oki intraslab earthquake is shown by an arrow in c. Solid lines denote the upper surface of the
subducting slab. Solid lines and red triangles at the top show the land area and active volcanoes, respectively. (After Nakajima et al. 2009c)

For lower-plane events to be triggered by the aqueous
fluids liberated from hydrous minerals via dehydration
reactions, at least part of the slab mantle near the lower
plane must be hydrated. However, the lower seismic
plane is usually located at depths of several tens of kilometers, to as deep as c. 40 km, below the upper slab surface. In contrast, hydrothermal circulation and alteration
at mid-ocean ridges are limited in many cases to shallow
crustal levels. Peacock (2001) suggested that normal
faulting in the trench–outer rise regions promotes infiltration of seawater several tens of kilometers into theoceanic plate, which facilitates the serpentinization of affected mantle materials. However, this hypothesis of the
deep hydration of the oceanic mantle before the platesubduction has received criticism, and it is still controversial whether seawater infiltration extends to suchgreat depths through bending-related normal faulting.
Seismic reflection/refraction surveys conducted at
trench–outer rise regions have revealed the presence ofsuch normal faults that cut across the crust and penetrate into the mantle of the incoming oceanic plate (e.g.,
Ranero et al. 2003; Ranero and Sallares, 2004). Seismicreflection/refraction profiling experiments have provided
further evidence of alteration of the crust and mantle ofthe incoming plate: anomalously low seismic wave velocities in the crust and mantle (Ivandic et al. 2008; Grevemeyer et al. 2007; Contreras-Reyes et al. 2007, 2011;
Fujie et al. 2013). Seismic surveys have thus detectedtraces of the altered crust and mantle of the incoming
oceanic plate, but these surveys are restricted to theuppermost mantle levels, with a maximum depth of c.
20 km. However, this limitation is mainly related to poor
resolution at such great depths in the mid-mantle ofoceanic plates associated with seismic surveys. In fact, arecent seismic reflection survey along a 233 km-longprofile beneath the Wharton Basin in the Indian Ocean
clearly imaged a fault that extends down to 40 km depth
below the plate upper surface (Qin and Singh, 2015).
The negative polarity of the reflection indicates thepresence of a low P-wave velocity zone along the fault,
likely formed by serpentinization of mantle materials
caused by infiltration of seawater. Although it is unclear
whether the fault was formed by the bending stresses of
the trench–outer rise region, this observation suggests
the possibility of deep hydration of the oceanic mantle
prior to plate subduction. Moreover, an estimation
by Garth and Rietbrock (2014) based on waveformmodeling of P-wave coda from intermediate-depthHasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

Page 24 of 31

73°

72°

71°

70°

69°

68°W
17°S

18°
P1
P2
P3

19°

20°

P1

P2

P3

Depth (km)010 km/s
9

40

8

80
120

(a)Vp

(c)Vp

6

(e)Vp

160
0
Depth (km)

Vp

7

5
5.5km/s
5

40

4.5

80

Vs
4

120
160

3.5

(b)Vs

(d)Vs

(f)Vs
3

200 160

120

80

Distance (km)40

0

200 160

120

80

Distance (km)40

0

200 160

120

80

40

0

Distance (km)Fig. 21 Seismic velocity structure along the lower seismic plane under northern Chile. Shown are across-arc vertical cross-sections of a–c P-wave
velocity and d–f S-wave velocity beneath northern Chile. P- and S-wave velocities along three lines in the inset map are indicated by the color
scales to the right. Earthquakes within a 20 km-wide section are plotted as dots. White lines denote the limit of resolution where the derivative
weight sum is more than 10. (After Dorbath et al. 2008)

events, whereby a 40 km-thick layer is inferred to be
17–31% serpentinized, indicates bulk hydration of
the oceanic mantle.
A possible alternative mechanism for hydration of the
deep mantle of an incoming oceanic plate is the hypothesis
proposed by Seno and Yamanaka (1996). These workers
discussed the possibility of hydration of the middle-to-deep
portion of the oceanic plate through H2O released fromsolidified magma that was involved in plumes. Most of themagma solidifies and releases H2O at the plume head
immediately below the oceanic plate; this H2O thenaccretes to the plate as the plate cools and thickens. In
addition, magma injected into the oceanic plate releases
H2O as it solidifies. H2O thus released reacts with the surrounding mantle materials, which results in the hydrationof the mid-mantle. There are arguments against this hypothesis, as well; for example, Omori et al. (2009) argued
that extensive hydration of the lithospheric mantle would
not result from such a mechanism because the amount of
H2O released decreases at greater depths due to depthbased variation in the solubility of H2O in the melt, andbecause magma reservoirs are present at shallow depths in
the crust rather than such great depths in the mantle.

A dynamic thermomechanical–petrological modelingstudy of plate subduction by Faccenda et al. (2012) has
presented another possibility for the penetration of H2O
into the deep slab mantle. The results of their simulation
show that during slab subduction, fluids liberated
through dehydration of hydrous minerals follow a bimodal
migration pattern. Fluids released from the slab crust andthe sub-Moho mantle flow upward, whereas those released from deeper levels of the mantle are driven into thecold core of the slab because of the pressure gradients
generated by plate unbending stresses. Fluids thus driven
downward into the cold core of the slab accumulate progressively and percolate updip along a layer nearly parallelto the slab surface (Fig. 22). The predicted location of thislayer of accumulated fluids coincides with the lower planeof the double seismic zone. This lower layer moves deeper
with increasing plate age, and the separation distance
between this lower layer and the middle of the slab crustis roughly consistent with the separation of double seismic
zones observed globally by Brudzinski et al. (2007), as
shown in Fig. 23. Both free fluids and serpentinizedperidotite are present in this lower layer in the middle of the slab mantle because the mantle rocks there

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

Page 25 of 310

Depth (km)

50

100

150

200

250

300
km 800Air/Sea
water

850

900

950

1,000

Sediments Basaltic/gabbroic Dry mantle
oceanic crust

1,050

1,100

Serpentinizedmantle1,150

Wet mantle

1,200

Free-fluids

Fig. 22 Simulated hydration and dehydration of the subducting slab. Shown is an example of simulation results of a dynamic thermomechanical–
petrological model that shows the hydration and dehydration of the slab caused by its subduction and bending/unbending stresses. A snapshot of
the hydration and dehydration at 13 Ma after subduction is shown on an across-arc vertical cross-section of the case in which an oceanic plate withan age of 52 Ma is subducting. White lines are isotherms. (After Faccenda et al. 2012)

are saturated with bound H2O to the assumed upper
limit of serpentinization. If this assumption is accurate and the slab dip is not sufficiently shallow, extremely deep hydration of the slab mantle prior to
subduction is unnecessary to explain the lower-plane
seismicity. Moreover, if the free fluids are intensively
distributed along the lower seismic plane as demonstrated here and if the pores that hold these fluids havesmall aspect ratios, the lower seismic plane should havelow Vp and low Vp/Vs values (e.g., Nakajima et al. 2001;
Takei, 2002). As suggested by Dorbath et al. (2008), thispossibility may partially explain the low Vp and low Vp/Vs
along the lower plane, as is commonly observed in
Tohoku, Hokkaido, and northern Chile (Figs. 19, 20,
and 21).
The possibility of deep hydration of the slab mantle
prior to subduction was also demonstrated by a recent 2D reactive-flow model simulation of hydration processes
at the trench–outer rise region (Iyer et al. 2012). Iyer et al.
(2012) developed a reaction-transport model that enables
better quantification of the amount and depth at which
hydration occurs in an incoming oceanic plate due to normal faulting caused by the bending force. The hydration
patterns determined with this model show the formationof a highly serpentinized band in the slab mantle centeredat the 270 °C isotherm. The depth of the 270 °C isotherm

increases with increasing plate age and is roughly consistent with the separation distance of globally observed
double seismic zones (Fig. 6; Brudzinski et al. 2007). If thismechanism actually operates in the mantle, it could
explain the formation of double seismic zones. Iyer et al.
(2012) suggested that the double hydrated zone at thedepths of the slab mantle, through slab unbending (Fig. 22;
Faccenda et al. 2012) and maximum serpentinization at c.
270 °C, may together cause the variations in the observed
separation of the double seismic zone.
Recently, Korenaga (2017) demonstrated another possible mechanism for deep hydration of the slab mantle
prior to subduction. He proposed that mantle hydration
first occurs through thermal cracking associated withthe cooling of oceanic plates and is later enhanced bybending-related faults in the outer rise region. The
amount of fluids required to generate the lower plane ofthe double seismic zones is not well quantified, but
Korenaga estimated the lower bound of the amount ofwater contained in the slab mantle (0.03–0.07 wt%) andargued that this amount is sufficient to explain thelower-plane seismicity of double seismic zones.
Overall, observations of distinct low P-wave velocities
along the lower plane of the double seismic zones under
Tohoku, Hokkaido, and northern Chile, as shown in
Figs. 19, 20, and 21, strongly suggest the presence ofHasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

Page 26 of 31

Plate Age (Myr)
10

20

50

1 00

200

DSZ(double seismic zone) Separation (km)

0

10

20

30

40

Fig. 23 Separation of the double seismic zone plotted against plate age. The separation of the double seismic zone obtained from a dynamic
thermomechanical–petrological model is plotted against the age of the subducting slab. The distances from the middle of the oceanic crust to
the level where fluids concentrate are shown by red and green dots. Vertical lines show error bars. Red and green dots are based on models ofantigorite dehydration by Wunder and Schreyer (1997) and by Schmidt and Poli (1998), respectively. Blue dots are globally observed data reported
by Brudzinski et al. (2007). (After Faccenda et al. 2012)hydrous minerals or fluids in the middle portion of theslab mantle, although further investigation is necessary.
We expect this research to progress in the future.
Genesis of intermediate-depth intraslab earthquakes

As described in the previous sections, spatial distributionof intraslab seismicity and the internal seismic structuresof subducting slabs provide evidence that strongly supports the dehydration-related embrittlement hypothesis
for nucleating intermediate-depth intraslab events. However, much of this evidence is circumstantial. The
dehydration-related embrittlement hypothesis posits that
H2O expelled from hydrous minerals within the subducting slab generates earthquakes, but the details of theprocess that actually lead from a state of local excess
pore fluid pressure to earthquake rupture are still subject to debate (e.g., Raleigh and Paterson, 1965; Rutterand Brodie, 1988; Ko et al. 1997; Dobson et al. 2002;
Jung and Green, 2004; Jung et al. 2004, 2006, 2009;
Kelemen and Hirth, 2007; Chernak and Hirth, 2010,2011; Brantut and Sulem, 2012; Sawai et al. 2013; Proctor and Hirth, 2015; Auzende et al. 2015).
Based on triaxial compression experiments on serpentinite samples at temperatures up to 700 °C and confiningpressures up to 0.5 GPa in undrained conditions, Raleighand Paterson (1965) reported a significant reduction instrength of the samples, and brittle fracturing occurredat temperatures higher than 375 °C, whereas plastic

deformation occurred below 340 °C. They attributed
the embrittlement and weakening over 375 °C to reduction in the effective normal stress caused by the releaseof the pore pressure of H2O via serpentine dehydration.
This mechanism of dehydration embrittlement would
certainly apply at pressures below c. 0.5 GPa (corresponding to c. 15 km depth). However, it is not evident whether the same mechanism can be applied tointermediate-depth intraslab events that occur atmuch higher pressures. The effective stress law forsliding friction works at shallow depths, as seen in
the experimental results of Raleigh and Paterson(1965), but this law does not work efficiently below
the brittle–ductile transition depth (c. 15 km depth)(e.g., Hirth and Beeler, 2015). Another point, which
is favorable to the dehydration-related embrittlement,in the experiments of Raleigh and Paterson (1965) is
the positive Clapeyron slope of serpentine dehydration. The positive overall volume change associated with the dehydration in this case leads to anincrease in pore pressure, which should operate topromote brittle fracture. However, at pressureshigher than c. 2.2 GPa (which corresponds to c.
60 km depth), the Clapeyron slope of serpentine
dehydration is negative (see Fig. 3b), and thus, theoverall volume change that accompanies the dehydration becomes negative, which may suppress theoccurrence of brittle instability.

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

In contrast, some laboratory experiments have shownthat hydraulic embrittlement occurs even at this highpressure range. For example, Dobson et al. (2002) detected strong acoustic emission signals during dehydration in serpentinite compression experiments that useda multianvil press at pressures up to 8.5 GPa and temperatures up to 900 °C. Recovered samples show characteristics of the brittle deformation associated with highpore fluid pressures, which suggests that dehydration
embrittlement occurs even at pressures over 2.2 GPa.
Based on triaxial compression tests of antigorite at pressures up to 6 GPa and temperatures up to 820 °C, Jung
et al. (2004) found that antigorite dehydration results in
faults delineated by ultrafine-grained solid reaction
products independent of the sign of the Clapeyron slope.
Jung et al. (2009) conducted in situ measurements ofacoustic emissions under compression tests of serpentinite and showed that acoustic waves are generated by dehydration embrittlement even at pressures higher than
2.2 GPa. These experimental results suggest that dehydration embrittlement is a viable mechanism for generating earthquakes independent of depth.
On the contrary, the results of some recent laboratory
experiments do not support the dehydration embrittlement hypothesis. For example, Chernak and Hirth(2011) performed deformation experiments on antigorite
designed to maximize the possibility of dehydration
instability. They showed that unstable fault behavior did
not develop in the specimens, but rather that slow slip
events took place. Compression experiments of antigoriterich serpentinite by Gasc et al. (2011), who used a newly
developed experimental setup that allows simultaneouscollection of in situ X-ray diffraction data and recordingof the full waveforms of acoustic emissions with a multianvil device, showed that the serpentinite samples did not
produce detectable acoustic waves over the course of theirdehydration. Proctor and Hirth (2015) conducted deformation tests with a new experimental method to vary thepore fluid pressure. Their results showed that dehydrating serpentinite causes deformation through semibrittle flow with grain-scale ductile deformation,
which suggests that earthquakes do not nucleate due
to unstable frictional sliding.
Triaxial compression tests on antigorite conducted recently by Okazaki and Hirth (2016) using a Griggs-type
deformation apparatus showed that antigorite samples
are associated with slow slips during the dehydration reaction without producing acoustic waves. Their similar
deformation tests on lawsonite, however, showed that
unstable fault slip certainly takes place and that acoustic
emissions are continuously generated in the dehydrationof lawsonite, which suggests that lawsonite dehydration
may be the cause of intermediate-depth events in the subducting slab crust. These results support the hypothesis

Page 27 of 31

that embrittlement through blueschist-facies dehydration
reactions in the slab crust may lead to the formation of
the belt of upper-plane seismicity observed in the crust of
the cold slabs, such as that detected under eastern Japan.
As described above, the detailed process that leads toearthquake rupture is still under debate, and is left to
future works. However, because intermediate-depth
earthquakes have been found to occur at the dehydration boundaries within the subducting slab as describedin the previous sections, a process analogous to hydraulicfracturing may be occurring due to local excess pore fluidpressure formed through dehydration and expulsion of
H2O from hydrous minerals. This phenomenon may causeintermediate-depth earthquakes within the slab. Alternatively, intermediate-depth events may be caused by dual
mechanisms, both fluid-related embrittlement and thermal shear instability, i.e., the local excess pore fluid pressure may initiate earthquake rupture via hydraulicfracturing, which may then trigger subsequent earthquake
ruptures through thermal shear instability. Nevertheless, it is estimated that H2O released by the dehydration of slab materials plays an important role ingenerating intermediate-depth intraslab earthquakes.

Conclusions
Intermediate-depth intraslab earthquakes occur within
the existence range of hydrous minerals in the slab crustand slab mantle. In particular, intermediate-depth events
tend to concentrate on the dehydration reaction boundaries in the slab, which forms the double seismic zone in
the depth range of c. 40–180 km. Recent studies haverevealed detailed spatial distribution of intermediatedepth seismicity that shows a close relationship with slab
metamorphism. The pressure–temperature path of thecrust for cold subducting slabs encounters facies boundaries with large rates of H2O production and positive
total volume change, which are likely to promote the occurrence of earthquakes near the facies boundaries in
the crust due to increase in pore fluid pressure. Conversely, the path for warm slabs encounters boundaries
with slow H2O production rates and negative volumetricchange, which may not effectively promote earthquake
occurrence. Observations show that intermediate-depth
earthquakes in warm slabs occur preferentially within
the slab mantle or very close to the slab Moho, whereas
those in cold slabs occur throughout the slab crust as
well as in the slab mantle. Precise hypocenter locationsof intermediate-depth events in NE Japan further demonstrate the presence of a belt of upper-plane seismicity
in the crust of the cold Pacific slab that is nearly parallelto the 80–90 km depth contours of the upper slab surface. This concentration of seismic activity in the slab
crust is probably caused by dehydration reactions withlarge H2O production rates and positive net volume

Hasegawa and Nakajima Progress in Earth and Planetary Science (2017) 4:12

change. A seismic low-velocity layer detected in the slab
crust persists down to the depth of this upper-plane seismic belt, but no further, which provides evidence thatthe phase transformation of the dehydration reaction occurs there. Both the belt of the upper-plane seismicity
and the down-dip end of the seismic low-velocity layer
deepen locally under Kanto and SW Hokkaido, andthese deepenings coincide spatially with the areas ofcontact with the overlying Philippine Sea slab and subducted forearc crust, respectively. This local deepening
is probably caused by delay of the phase transformationof the slab crust because of the thermal shielding effectof the overlying cold bodies. Although the belt of theupper-plane seismicity was also detected in the cold
Philippine Sea slab beneath southern Kyushu, it has notbeen detected in cold slabs of other subduction zones of
the world. This absence of evidence may simply reflect
the shortage of high-quality data for intermediate-depth
events precisely located in those subduction zones.
However, similar low-velocity subducting crust with aclose relationship with intraslab seismicity has been detected in several other subduction zones. Anomalously,
low P- and S-wave velocity zones have been detected in
the source area of M7-class large, intraslab events that
occurred in the slab crust and the uppermost portion of
the slab mantle in NE Japan, which suggests the presence of hydrous minerals or fluids. The situation forlower-plane events is somewhat more complicated. Seismic tomography studies in both eastern Japan andnorthern Chile again revealed the presence of a P-wave
low-velocity layer along the lower plane of the double
seismic zone. This P-wave low-velocity layer is not associated with low S-wave velocity, however, which is inconsistent with predictions based on the serpentinizedmantle materials. Seismic anisotropy and pore aspect ratio may also play important roles in generating thisunique structure. Although further validation is necessary for confirmation, the widespread distribution of adefinite low-velocity P-wave layer along the entire lower
seismic plane suggests the presence of hydrous minerals
or fluids at that level, as well as the fluid-facilitated generation of lower-plane seismicity. All of these observations support the hypothesis that slab-derived H2O
causes intermediate-depth intraslab earthquakes. However, the possibility has not been ruled out that H2O
contributes only to earthquake nucleation and that seismic ruptures propagate through another mechanism. In
other words, intermediate-depth events may be causedby both fluid-related embrittlement and thermal shear
instability, i.e., local excess pore pressure may initiate
earthquake rupture, and thus trigger subsequent earthquake ruptures through thermal shear instability. We
anticipate significant progress in this area of research in
the future to further verify this hypothesis for the

Page 28 of 31

genesis of intermediate-depth earthquakes, such as improving the accuracy of the temperature model, increasing the spatial resolution of seismic tomography imagingof the internal structures of subducting slabs, and deepening our understanding of the detailed processes thatlead to earthquake rupture from a state of local excess
pore fluid pressure based on rock deformation experiments or numerical simulations.
Acknowledgements
The authors thank T. Okada, N. Uchida, S. Kita, T. Matsuzawa, S. Kirby, S. Maruyama,
and S. Omori for valuable discussions. The authors also wish to thank the editor
Fenglin Niu and two anonymous reviewers for their constructive comments,
which greatly improved the manuscript.
Funding
Not applicable.
Authors’ contributions
AH proposed the topic, wrote the manuscript, and prepared the figures. JN
collaborated with the corresponding author in the construction of themanuscript and in the preparation of the figures. Both authors read andapproved the final manuscript.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in publishedmaps and institutional affiliations.
Author details
1
Research Center for Prediction of Earthquakes and Volcanic Eruptions,
Graduate School of Science, Tohoku University, Aramaki, Aoba-ku, Sendai
980-8578, Japan. 2Department of Earth and Planetary Sciences, School of
Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo
152-8551, Japan.
Received: 27 October 2016 Accepted: 22 March 2017

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