Intermediate depth earthquakes (70–300 km depth) occur at pressures and temperatures (P/T) that should produce ductile deformation rather than brittle failure (e.g., Green and Houston, 1995). There has long been significant debate about the role of dehydration reactions in the formation of intermediate depth seismicity. The P/T conditions at which intermediate depth earthquakes occur have repeatedly been shown to be consistent with dehydration phase boundaries for both crustal and mantle lithospheric hydrous compositions (e.g., Kirby, 1995; Kirby et al., 1996; Peacock, 2001; Hacker et al., 2003; Yamasaki and Seno, 2003; Abers et al., 2013). The most commonly cited link between dehydration and intermediate depth seismicity is dehydration embrittlement. As originally conceived, the term “dehydration embrittlement” means the hydraulic fracturing of rock caused by the production of free fluids and resultant overpressure in pore spaces that in turn could overcome the ambient pressure (e.g., Raleigh and Paterson, 1965; Meade and Jeanloz, 1991). However, more recent experimentation has shown that (a) above some pressure, volume change associated with dehydration reactions is likely negative, complicating the overpressure argument (e.g., Dobson et al., 2002; Jung et al., 2004; Gasc et al., 2011), and (b) some experiments on antigorite and other hydrous phases have produced no acoustic emissions (AEs) upon dehydration and/or show evidence solely of stable sliding or distributed deformation, bringing the concept of “dehydration embrittlement” into question (e.g., Chernak and Hirth, 2010; Chernak and Hirth, 2011; Gasc et al., 2011; Proctor and Hirth, 2015; Gasc et al., 2017). One possible contribution from dehydration reactions unrelated to pore pressure is the formation of nanocrystalline reaction products whose small grain size might contribute to vein growth, channel flow, and/or localized weakening (e.g., Incel et al., 2017; Plümper et al., 2017). Other theories have focused more directly on localized weakening due to slip, i.e., thermal run-away/shear heating mechanisms that also incorporate reduced grain size along discrete planes of weakness (e.g., Kelemen and Hirth, 2007; John et al., 2009; Prieto et al., 2013; Poli et al., 2016; Proctor and Hirth, 2016; Ohuchi et al., 2017). Thermal run-away mechanisms may be self-initiating or may serve as a propagation mechanism for events triggered by another mechanism (e.g., Zhan, 2017). A recent theory that matches many of these observations is the “dehydration destabilization stress transfer” mechanism (DDST; Ferrand et al., 2017; Ferrand, 2019). In this model, hydrated peridotite produces AEs at specific conditions that depend on the amount of hydrated material present within the peridotite matrix and the pressure at which the dehydration reaction occurs. Rather than increasing pore pressure to hydraulically fracture the surrounding rock matrix, the dehydration of antigorite (or other hydrous phases) results in an extreme weakening and volume decrease of the dehydrating phase, which in turn transfers ambient stresses onto the enclosing dry peridotite rock matrix. The amount of hydrous phase in the rock has to be sufficient that its dehydration-related weakening will sufficiently load the surrounding matrix to the point of failure, but not so great that the surrounding matrix does not possess sufficient strength to accumulate strain. According to the calculations of Ferrand et al. (2017) at 3.5 GPa, relatively little antigorite (∼5%) is needed to induce seismicity, consistent with small amounts of hydrated material that might be present deep within the mantle lithosphere of subducted slabs.

There has also been increased interest in the relationship between observations of heterogeneities within the oceanic plate as it enters the trench, and seismicity patterns in the subducting slab at intermediate depths (e.g., Shillington et al., 2015). Recent studies looking at the seismic velocity structure of oceanic plates as they approach the trench show that outer-rise faulting consistently results in extensive hydration of the oceanic plate (see Grevemeyer et al., 2018 and references therein). In studies with sufficiently deep resolution, hydration is observed to extend ∼10–12 km below the oceanic Moho (e.g., Ranero et al., 2003). Fromm et al. (2006) used aftershock locations from a M_w 6.7 outer-rise earthquake to show faulting to depths of ∼ 25 km. In rare cases (e.g., Bloch et al., 2018; Cai et al., 2018), evidence exists for even deeper hydration. Recent work by Boneh et al. (2019) shows a correlation between the observed fault offsets on outer-rise faults from bathymetry data and the rate of intermediate depth seismicity observed down-dip, which suggests the extent of hydration in the oceanic plate due to outer rise faulting may control the amount of seismicity that is possible at intermediate depths.

While links between hydration and seismicity are consistent with dehydration embrittlement or related mechanisms, they do not address whether it is the dehydration of the oceanic crust or mantle that is responsible for the generation of intermediate depth earthquakes. Our hypothesis is that regions with thinner oceanic crust produce intermediate depth seismicity due to mantle dehydration reactions, and that slabs with very thick oceanic crust do not produce intermediate depth events because the previously hydrated overthickened crust has already experienced its most important dehydration reactions by this point, and because the mantle below was never hydrated to begin with and therefore cannot produce the dehydration reactions needed to generate seismicity.

Our hypothesis is based on the clearest and simplest observation of this pattern: along the Nazca Ridge in Peru (∼18 km thick oceanic crust) (Hampel et al., 2004; Kumar et al., 2016). We present our hypothesis in the context of this location, and investigate other possible factors such as plate age, evidence for outer rise faulting, oceanic crustal structure, and any available direct evidence for oceanic mantle hydration. We also investigate the plausibility of our hypothesis in terms of the metamorphic reactions and pressure-temperature conditions required for this to be true. We then assess the hypothesis at a number of other locations where overthickened oceanic crust is being subducted, including the Juan Fernandez Ridge in Chile which produces an overabundance of seismicity rather than an absence thereof, the Carnegie Ridge subducting beneath Ecuador, and the Cocos Ridge subducting beneath Costa Rica. We show that while these other situations are often more complicated and less clear, they are each consistent with our hypothesis given existing data. Finally, we suggest a number of regions where additional research could be used for further testing of this hypothesis, and could help to constrain the relationship between water penetration depth, crustal thickness, and intermediate depth seismicity.

The Nazca Ridge comprises a 1200 km long, ∼200 km wide seamount chain that extends southwest from the southern Peruvian margin. The crustal structures of the Nazca Ridge and adjacent regions of “typical” Nazca plate are constrained by analyses of a suite of trench perpendicular and trench parallel active source surveys and gravity studies (e.g., Hampel et al., 2004; Krabbenhoeft et al., 2004; Grevemeyer et al., 2018; Contreras-Reyes et al., 2019). These show a ∼100 km wide (trench parallel), 17 km thick crustal root beneath the Nazca Ridge, thinning to ∼6.4 km on either side (Figure 1). These crustal thickness results are consistent with earlier surface wave dispersion estimates of ∼18 km (Woods and Okal, 1994). The Nazca Ridge’s conjugate on the Pacific Plate, the Tuamotu Ridge, has been used to determine the initial timing of collision of this ridge with the South American continent at ∼15–12 Ma, and to project its current location onto the subducted slab (i.e., Pilger, 1981; Woods and Okal, 1994; Gutscher et al., 1999b; Hampel, 2002; Rosenbaum et al., 2005). The difference in the strike of the Nazca Ridge and the relative convergence direction between the Nazca and South American plates (see Figure 1) has resulted in a progressive southward migration of the ridge relative to the overriding continental margin since that time (i.e., Gutscher et al., 1999b; Rosenbaum et al., 2005). This, together with the inferred subduction of the conjugate to the Marquesas Plateau, named the Inca Plateau, are often cited as contributing to the formation of the unusually long (along strike) extent of the Peruvian flat slab (i.e., Gutscher et al., 1999b; Antonijevic et al., 2015). The current location of the Nazca Ridge beneath the South American continent has been assessed using adjacent oceanic magnetic anomalies that allow for an alignment of the projection of the conjugate Tuamotu Plateau both along the Nazca Ridge and to regions of the slab already subducted (Hampel, 2002). Note that this location depends only on observations of magnetic anomalies and does not make assumptions about current or past relative plate motions.

Over the past decade, three broadband seismic deployments (PULSE: Wagner et al., 2010; CAUGHT: Beck et al., 2010; PeruSE, 2013) have collected data in central and southern Peru, providing dense seismic coverage across the projected location of the Nazca Ridge. Tomographic and scattered wave imaging based on these data indicate that, as with many regions with subducted overthickened oceanic crust, the projected location of the subducted Nazca Ridge coincides closely with the shallowest portion of the southern Peruvian flat slab (Antonijevic et al., 2015, 2016; Bishop et al., 2017; Wagner and Okal, 2019).

The hypothesis that thicker oceanic crust could lead to an absence of intermediate depth seismicity was first proposed by Kumar et al. (2016) in their study of intermediate depth seismicity using the PULSE, CAUGHT, and PeruSE networks in Peru. The earthquake locations from recent studies using local data (Kumar et al., 2016; Lim et al., 2018) are roughly consistent with catalog data available from the International Seismological Center for events occurring 1990–2017 (International Seismological Centre, 2019 Reviewed Event Catalog, retrieved October 1, 2019). Notably, there is a distinct gap in seismicity located in the middle of the PULSE array where event locations using these data should be most complete (Hampel, 2002; Kumar et al., 2016). This gap corresponds closely to the projected location of the subducted Nazca Ridge based on the projected location of the Tuamotu Plateau (Figure 1). The Tuamotu Plateau, however, is much broader than the unsubducted portions of the Nazca Ridge, highlighting the uncertainties about the geometry of the Nazca Ridge location and crustal thickness at depth. However, we note that the region of the observed seismic gap is well within the contours of the projected Tuamotu Plateau, consistent with our hypothesis, and consistent with previous observations of the correlation between the modern projection of the Tuamotu Plateau and the observed gap in seismicity (e.g., Hampel, 2002). We note, however, that at depths shallower than 60 km (i.e., closer to the trench), there is no gap in seismicity along the projected Nazca Ridge track. The right-hand panel of Figure 1 (A,A′) shows the locations of intermediate depth earthquakes within 75 km of the roughly ridge-perpendicular cross section extending from (−11.3°S; −76.9°W) to (−16.8°S; −71.1°W). Also shown is the oceanic Moho topography and bathymetry from Contreras-Reyes et al. (2019), showing the wavelength of the overthickened crust associated with the Nazca Ridge, which is similar to the size of the gap in seismicity. The red line plotted beneath the seismicity profile indicates the top of the subducted slab as determined by Antonijevic et al. (2015) and Wagner and Okal (2019). Unfortunately, while receiver function studies such as Bishop et al. (2017) are able to identify seismic conversions from the oceanic Moho in some places, the errors in absolute depth for both the receiver functions and the earthquake locations make a determination of whether these events are occurring in the slab crust or mantle impossible.

While it is not currently possible to know the detailed structure of the already subducted Nazca plate collocated with these observed seismicity patterns, it is instructive to look directly offshore at the structure of the Nazca Ridge and adjacent areas that are about to be subducted to infer some constraints on possible structures that could be influencing intermediate depth seismicity. The age of the Nazca plate at the Peruvian margin along and adjacent to the Nazca Ridge varies only slightly from 43 to 46 Ma, except south of the Nazca Fracture Zone (south of our study area) where ages increase to ∼52 Ma (Mueller et al., 2008). This is significant because the age of the plate strongly affects the temperature, and therefore the potential locations of dehydration reactions. In this case, not only is the overall range in ages small, intermediate depth seismicity is located on either side of the gap at the Nazca Ridge, while plate age increases monotonically from north to south. It is therefore unlikely that plate age or slab temperature are responsible for the observed seismicity patterns.

Another possible explanation for this pattern is that the thick crust of the subducting Nazca Ridge does not form as many outer-rise faults as the adjacent thinner oceanic crust, and is therefore less hydrated overall. However, side swath bathymetry surveys show evidence of trench parallel normal faults across both the Nazca Ridge and the adjacent normal oceanic plate with similar vertical offsets on the order of ∼200 m across our study area irrespective of crustal thickness (Hussong et al., 1988; Hagan and Moberly, 1994; Hampel et al., 2004; Kukowski et al., 2008). A more direct way to determine the hydration state of the oceanic plate is through tomographic modeling of seismic velocities. Wide-angle seismic studies north of the Nazca Ridge indicate average uppermost mantle velocities of ∼7.8 km/s, consistent with moderate hydration (Krabbenhoeft et al., 2004). However, along the ridge, somewhat higher uppermost mantle velocities are observed (∼8.0 km/s) consistent with an absence of mantle hydration (Contreras-Reyes et al., 2019), though the resolution of these studies at upper mantle depths is limited. Finally, Hu and Liu (2016) propose a slab tear as a possible cause for the observed absence in seismicity along the projected Nazca Ridge track based on geodynamic modeling and patterns of volcanism. However, given the clear evidence from seismic imaging for a continuous slab in this region from receiver functions (i.e., Bishop et al., 2017, 2018), teleseismic tomography (Scire et al., 2015), and surface wave tomography (Antonijevic et al., 2015, 2016), this explanation for the observed seismic gap is inconsistent with existing detailed imaging of the region.

Figure 2A shows a simplified schematic version of the cross section shown in Figure 1 (A,A′) that we use to model our hypothesis. We assume the top of the oceanic crust is at the same depth along the transect, placing it at 70 km depth based on the depths of observed seismicity and the thickness of the overriding continental crust (Bishop et al., 2017). The base of the “normal” oceanic crust to either side of the ridge is assumed to be 77 km depth (assuming a 7 km thick crust) and the base of the Nazca Ridge crust is at 88 km depth (based on the 18 km crustal thickness imaged offshore). Here we show the penetration depth of water into the Nazca plate to also be 18 km beneath the surface of the oceanic crust (i.e., at 88 km depth), consistent with a number of previous studies investigating the depth of mantle hydration (e.g., Ranero et al., 2003; Contreras-Reyes et al., 2011; Lefeldt et al., 2012). We note that in order for our model to hold, the penetration depth of water simply needs to be greater than normal oceanic crustal thickness, and less than or equal to the ridge crustal thickness. For clarity, we have projected seismicity from cross-section A,A′ (Figure 1) vertically to depths between the normal oceanic Moho and the maximum penetration depth of water into the slab. This allows us to demonstrate along-strike variability in intermediate depth seismicity, and is reasonable given that (a) the slab is unlikely to be perfectly horizontal along the cross section and (b) depth errors in hypocentral locations typically significantly exceed errors in latitude and longitude.

According to our hypothesis, the blue region in Figure 2A (hydrated mantle lithosphere) generates seismicity at these depths because it is undergoing dehydration reactions and the brown region (oceanic crust) is not dehydrating, and is therefore not generating seismicity. Because there is no hydrated mantle below the thick crust of the Nazca Ridge, there is a seismic gap. The discriminating region, then, lies between the 77 km depth of normal oceanic Moho (black dashed line in Figure 2C) and the assumed 88 km depth that marks the lowermost penetration depth of water into outer rise faults (magenta dashed line in Figure 2C). If our hypothesis is correct, the temperature of this region should correspond to a P/T condition at which hydrated peridotite would dehydrate but hydrated MORB would not, based on the assumption that temperature would be the same at these depths, regardless of whether the material is MORB or hydrated oceanic mantle lithosphere.

Figures 2B,C show the amount of structurally bound H₂O that can be incorporated into altered crust and depleted mantle, respectively. These diagrams are based on phase equilibria that were calculated for a composite crustal composition from DSDP sites 417 and 418 (Staudigel et al., 1989, 1996) and for harzburgitic mantle (Ringwood, 1982). Sufficient H₂O was added to each bulk composition to saturate mineral assemblages at every P/T condition with fluid, and the amount of water contained in the coexisting mineral assemblage is contoured in Figure 2 (see also Hacker, 2008, for water contents of a larger range of possible rock composition). Calculations utilized Perple_X (Connolly, 2005) and a 2004 update to the Holland and Powell (1998) thermodynamic dataset for mineral end-members. We used the suite of mineral solution models employed by Baxter and Caddick (2013), with the exception of updated models for antigorite (Padrón-Navarta et al., 2013) and amphibole and omphacite (Diener and Powell, 2012). Silicate melt was not considered. Calculated mineral assemblages are labeled in more detail in the Supplementary Information, where calculated densities of both rocks are also shown.

The dashed lines in Figures 2B,C correspond to the dashed lines of the same colors in Figure 2A – indicating the pressures at the base of the normal oceanic crust (∼2.4 GPa) and at the maximum penetration depth of water into the oceanic plate (∼2.8 GPa) as determined by converting the associated depths to pressures based on the Preliminary Reference Earth Model (PREM; Dziewonski and Anderson, 1981). We identify three temperature ranges within this pressure range at which hydrated peridotite releases a significant amount of water: ∼500°C upon the loss of brucite (box 1, Figure 2C), ∼640°C upon the loss of antigorite (box 2, Figure 2C) and ∼800°C upon chlorite breakdown (box 3, Figure 2C). We note that ∼640°C is similar to the temperature modeled for a flat slab at similar depth by van Hunen et al. (2002). If we then plot these same temperature boxes on the MORB phase diagram, we note that indeed, boxes 2 and 3 lie in regions at which MORB has already undergone extensive dehydration and is not dehydrating further, consistent with our hypothesis. Colder subduction that intersects box 1 would, however, be associated with crust that retains water in both lawsonite and amphibole.

According to our hypothesis, the oceanic crust along and adjacent to the subducted Nazca Ridge will be undergoing little dehydration by the time it reaches the P/T conditions of our seismic gap, implying that it has already fully dehydrated or that its H₂O content is sequestered in stable phases at these conditions. Subducting oceanic crust undergoes a sequence of dehydration reactions, but one of the most commonly attributed to the formation of intermediate depth seismicity is the breakdown of lawsonite (e.g., Kim et al., 2015; Okazaki and Hirth, 2016; Incel et al., 2017). In the Nazca Ridge case, this suggests that the oceanic crust must have exceeded the maximum temperatures of lawsonite (and amphibole) stability by the time the top of the slab reaches 70 km depth and the bottom of the hydrated layer of the slab reaches 88 km depth. Our hypothesis requires that off-ridge regions of thinner crust are still undergoing peridotite dehydration reactions at these same P/T conditions, which is true in Figures 2B,C for box 2 (dehydration of antigorite in the slab mantle) and box 3 (dehydration of chlorite in the slab mantle), but not box 1 (dehydration of brucite in the slab mantle that would be accompanied by dehydration of crust in the on-ridge section). Choosing whether peridotite dehydration at 640 or 800°C is most appropriate is more equivocal, given that the temperatures within regions of flat slab subduction are highly dependent on the three-dimensional geometry of the slab and the duration of time the overriding plate has been insulated from asthenospheric temperatures by the slab. Detailed time-dependent thermal modeling is required to better assess the implications of these predicted temperatures. However, either option demonstrates that our hypothesis is viable from a simplified petrologic perspective.

The nature of the relationship (if any) between oceanic crustal structures and intermediate/deep seismicity has long been discussed. For example, Kirby et al. (1996) consider the relationship between the “mechanically flawed and heterogeneous lithosphere” and intermediate depth seismicity. In particular, they note the effects of fractures, both from outer-rise faults and fracture zones on the hydration of the oceanic lithosphere and associated potential for increased intermediate depth seismicity. We note that similar patterns are visible in all of our study areas along the projected locations of the Nazca Fracture Zone, Challenger Fracture Zone, Grijalva Fracture Zone, and Panama Fracture Zone for the study areas near the Nazca, Juan Fernandez, Carnegie, and Cocos ridges, respectively. However, to our knowledge, we are the first to suggest that the presence of overthickened oceanic crust can result in an absence of intermediate depth seismicity if crustal thickness exceeds the maximum penetration depth of water into the subducted plate. Kirby et al. (1996) do mention the role of ridges in the formation of seismicity, and while they note the apparent absence or dearth of seismicity associated with the Nazca, Cocos, Carnegie and D’Entrecasteaux ridges, they focus instead on the overabundance of seismicity present along the Juan Fernandez Ridge.

We would argue that the Juan Fernandez Ridge is fundamentally different from the type of ridge we would expect to produce a gap in intermediate depth seismicity, both because of its limited crustal root and because of its extensive ridge-parallel faulting. Interestingly, these structures may be related to the location and timing of ridge emplacement – the Juan Fernandez Ridge is the only one considered here that did not form at or near a spreading center. This may affect the architecture of the crust in important ways. The Juan Fernandez Ridge crustal thickness is primarily due to increases in upper crustal layers, but a relatively constant thickness lower crust (e.g., Kopp et al., 2004). In contrast, the Nazca, Carnegie, and Cocos ridges all attained their increased Moho depths through thickening of the lower crust, with little to no increase in the thicknesses of layers in the upper crust (e.g., Sallares et al., 2003; Krabbenhoeft et al., 2004; Sallares et al., 2005; Contreras-Reyes et al., 2019). An intriguing analog to the Juan Fernandez Ridge might be the Louisville Ridge on the Pacific plate that also formed at an intraplate hotspot (the Louisville hotspot, Watts et al., 1988). Here, as with the Juan Fernandez Ridge, most of the additional crustal volume is added intrusively into the upper crust with only modest variations in lower crustal thickness (Contreras-Reyes et al., 2010; Robinson et al., 2018). However, unlike the Juan Fernandez Ridge, there is no evidence of ridge-parallel fracturing or hydration of the mantle beneath the ridge itself (Contreras-Reyes et al., 2010). While the Louisville Ridge has been associated with a reduction in seismicity at shallow to intermediate depths (Giardini and Woodhouse, 1984; Habermann et al., 1986; Scholtz and Small, 1997), the seismicity pattern is less clear in part due to the oblique subduction of the ridge, and uncertainty about its projected location. More work is needed to understand how crustal thickening mechanisms, Moho depth, and seismicity patterns are related in this region.

At this stage, we know of no example that contradicts our hypothesis that if the oceanic crust is thicker than the maximum penetration depth of water into the slab, intermediate depth seismicity will be reduced or absent. The precise mechanism by which dehydration results in intermediate depth seismicity, whether it be by traditional “dehydration embrittlement” via an increase in pore pressure, by the formation of nanocrystalline/fine grained dehydration products or fault plane weakening, by DDST, or some combination of the above, may depend on the precise P/T conditions of the subducting slab, and is certainly a field of ongoing scientific research. Our hypothesis does, however, strongly suggest that dehydration plays a dominant role in the generation of intermediate depth seismicity, even if the precise mechanism remains uncertain.

For future work, more information is needed on the crust and mantle structures of subducting bathymetric features. Better constraints on the depth of hydration at trench outer-rise settings, both in regions with and without overthickened crust will dramatically improve our understanding of the role of slab mantle hydration on the genesis of intermediate depth seismicity. If our hypothesis is true, then intermediate depth seismicity adjacent to seismic gaps like the one in Peru can be used as a virtual thermometer to provide direct evidence of the temperature at a specific range of depths within a subducting plate at significant distance from the trench. These types of constraints could prove very valuable as ground truth for geodynamic models of subduction zone thermal structures.

Publicly available datasets were analyzed in this study. This data can be found here: https://doi.org/10.31905/D808B830, http://www.isc.ac.uk/iscbulletin/search/catalogue/.

LW is responsible for the original content and initial manuscript of this work. MC provided the Perplex modeling and contributions to the metamorphic petrology portions of this manuscript. LW, ML, and SB led the collection of the PULSE dataset. SB and LW were leaders in the collection of the CAUGHT dataset. SB, AK, and ML contributed to discussions and revisions of the manuscript. All authors read and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.