Global prevalence of double Benioff zones

Numerical modeling the process of deep slab dehydration and magmatism

This study uses a 2D high-resolution thermo-mechanical coupled model to investigate the dynamic processes of deep plate hydration, dehydration, and subsequent magmatic activity in ocean-continent subduction zones. We reveal the pathways and temporal evolution of water transport to the deep mantle during the subduction process. Plate dehydration plays a critical role in triggering partial melting of the deep mantle and related magmatic activity. Our study shows significant differences in the volumes of melt produced at different depths, with dehydration reactions in deeper regions being weaker compared to shallower ones. It takes a longer time to reach the suitable P-T conditions for hydrous melting in the deep mantle. The results highlight the geophysical significance of water transport in deep subduction zones and its role in magmatic processes, particularly in the formation of magma chambers beneath continental plates.

Anisotropic thermal conductivity of antigorite along slab subduction impacts seismicity of intermediate-depth earthquakes

Double seismic zones (DSZs) are a feature of some subducting slabs, where intermediate-depth earthquakes (~70-300 km) align along two separate planes. The upper seismic plane is generally attributed to dehydration embrittlement, whereas mechanisms forming the lower seismic plane are still debated. Thermal conductivity of slab minerals is expected to control the temperature evolution of subducting slabs, and therefore their seismicity. However, effects of the potential anisotropic thermal conductivity of layered serpentine minerals with crystal preferred orientation on slab's thermal evolution remain poorly understood. Here we measure the lattice thermal conductivity of antigorite, a hydrous serpentine mineral, along its crystallographic b- and c-axis at relevant high pressure-temperature conditions of subduction. We find that antigorite's thermal conductivity along the c-axis is ~3-4 folds smaller than the b-axis. Our numerical models further reveal that when the low-thermal-conductivity c-axis is aligned normal to the slab dip, antigorite's strongly anisotropic thermal conductivity enables heating at the top portion of the slab, facilitating dehydration embrittlement that causes the seismicity in the upper plane of DSZs. Potentially, the antigorite's thermal insulating effect also hinders the dissipation of frictional heat inside shear zones, promoting thermal runaway along serpentinized faults that could trigger intermediate-depth earthquakes.

Thermal ages of the Huatung Basin determined from seismic waveform modeling: insights into Southeast Asia's evolution

The Huatung Basin (HB), situated on the leading part of the Philippine Sea Plate, is directly involved in oblique subduction and mountain building in the Taiwan region. However, previous studies have reported a wide range of ages for the HB, from 30 to 130 Ma, making it difficult to properly constrain regional tectonics. We analyzed teleseismic waveforms recorded on Taiwan that traveled through the slab associated with the HB. By waveform matching, we have constrained the slab dimensions to approximately 400 km in length and 150 km in width, accompanied by an enhanced P-wave velocity of 6% within the slab core and an apparent dip angle of 55°. We used age-dependent subduction zone thermal models to estimate the thermal ages or the ages since the last thermal event of the HB. The best-fit thermal model indicates thermal ages ranging from 20 to 50 Ma, which is consistent with a suite of geophysical observations and the age inferred from geomagnetic anomaly data. However, our results differ considerably from the ages obtained through radiometric dating of rocks dredged from the seafloor. The discrepancy in age may be attributed to either thermal rejuvenation of the plate or dating of allochthonous samples dredged from the border of the basin.

Subduction age and stress state control on seismicity in the NW Pacific subducting plate

Intermediate depth (70-300 km) and deep (> 300 km) earthquakes have always been puzzling Earth scientists: their occurrence is a paradox, since the ductile behavior of rocks and the high confining pressure with increasing depths would theoretically preclude brittle failure and frictional sliding. The mechanisms proposed to explain deep earthquakes, mainly depending on the subducting plate age and stress state, are generally expressed by single parameters, unsuitable to comprehensively account for differences among distinct subduction zones or within the same slab. We analyze the Kurile and Izu-Bonin intraslab seismicity and detail the Gutenberg-Richter b-value along the subducted planes, interpreting its variation in terms of stress state, analogously to what usually done for shallow earthquakes. We demonstrate that, despite the slabs different properties (e.g., lithospheric age, stress state, dehydration rate), in both cases deep earthquakes are restricted to depths characterized by equal age from subduction initiation and are driven by stress regimes affected by the persistence of the metastable olivine wedge.

Metamorphism-facilitated faulting in deforming orthopyroxene: Implications for global intermediate-depth seismicity

The exothermic metamorphic reaction in orthopyroxene (Opx), a major component of oceanic lithospheric mantle, is shown to trigger brittle failure in laboratory deformation experiments under conditions where garnet exsolution takes place. The reaction product is an extremely fine-grained material, forming narrow reaction zones that are mechanically weak, thereby facilitating macroscopic faulting. Oceanic subduction zones are characterized by two separate bands of seismicity, known as the double seismic zone. The upper band of seismicity, located in the oceanic crust, is well explained by dehydration-induced mechanical instability. Our newly discovered metamorphism-induced mechanical instability provides an alternative physical mechanism for earthquakes in the lower band of seismicity (located in the oceanic lithospheric mantle), with no requirement of hydration/dehydration processes.

Effects of episodic slow slip on seismicity and stress near a subduction-zone megathrust

Slow slip phenomena deep in subduction zones reveal cyclic processes downdip of locked megathrusts. Here we analyze seismicity within a subducting oceanic slab, spanning ~50 major deep slow slip with tremor episodes over 17 years. Changes in rate, b-values, and stress orientations of in-slab seismicity are temporally associated with the episodes. Furthermore, although stress orientations in the slab below these slow slips may rotate slightly, in-slab orientations 20–50 km updip from there rotate farther, suggesting that previously-unrecognized transient slow slip occurs on the plate interface updip. We infer that fluid pressure propagates from slab to interface, promoting episodes of slow slip, which break mineral seals, allowing the pressure to propagate tens of km further updip along the interface where it promotes transient slow slips. The proposed methodology, based primarily on in-slab seismicity, may help monitor plate boundary conditions and slow slip phenomena, which can signal the beginning stages of megathrust earthquakes.

Large slow slip earthquakes and tremor occur in subduction zones near the locked megathrust. Combined analysis of changes in slab seismicity and stress field near the times of such slow slip events highlights the role of fluid in promoting slow slip.

Physical mechanisms of oceanic mantle earthquakes: Comparison of natural and experimental events

Because they provide information about the spatial distribution of brittle deformation, both seismologists and experimentalists use b-values to study earthquake populations. Here, we present the b-values for intermediate-depth intraslab earthquakes in the Pacific slab beneath the Tohoku and Hokkaido regions, northeastern Japan and find a difference in the lower-plane event b-values in the double seismic zone. Lower-plane events reveal significantly larger b-values beneath Tohoku (0.96) than Hokkaido (0.86), implying that the brittle deformation beneath Hokkaido is more localized and leads to higher ratio of relatively large lower-plane events than occur beneath Tohoku. We also estimated the b-values for experimental earthquakes, and found they increase with increasing antigorite content in serpentinized peridotite. These experimental earthquakes already led to the “dehydration driven stress transfer” (DDST) model, which suggests that a highly hydrated peridotite is not required when oceanic mantle events occur. A comparison of experimental and natural earthquake b-values implies that lower-plane peridotite is more hydrated beneath the Tohoku region, which could also explain the difference in oceanic-plate velocity structures near the trench identified in Ocean Bottom Seismometer studies off Tohoku and Hokkaido. These results suggest that lower-plane events occur in fresh peridotite near serpentinized faults.

Lithospheric folding by flexural slip in subduction zones as source for reverse fault intraslab earthquakes

Subduction requires the permanent generation of a bend fold in the subducting slab which mechanics is not well understood. Lithospheric bending of subducting slabs was traditionally considered to be accommodated by orthogonal flexure, generating extensional outer rise earthquakes responsible of the external arc elongation during folding. Here we explore the possibility of lithospheric flexure being accommodated through simple shear deformation parallel to the slab (folding by flexural slip) and evaluate this process as source of earthquakes. The seismicity predicted by flexural slip dominated slab bending explains a significant amount of intermediate earthquakes observed in subduction zones with different degrees of coupling. This mechanism predicts the generation of intraslab thrust earthquakes with fault planes subparallel to the slab top. Being the orientations of the fault planes the same for the interface thrust earthquakes and the flexural-slip intraslab earthquakes, the amount of seismic moment liberated by the interface could be significantly lower than considered before. This proposed seismic source should be taken into account in models and hazard studies of subduction zones. Determining the seismic generating processes in subduction zones and their characteristics is a fundamental issue for the correct assessment of the associated seismic and tsunami risk.

Phase-transformation-induced lubrication of earthquake sliding

Frictional failure is not possible at depth in Earth, hence earthquakes deeper than 30-50 km cannot initiate by overcoming dry friction. Moreover, the frequency distribution of earthquakes with depth is bimodal, suggesting another change of mechanism at about 350 km. Here I suggest that the change at 30-50 km is from overcoming dry friction to reduction of effective stress by dehydration embrittlement and that the change at 350 km is due to desiccation of slabs and initiation by phase-transformation-induced faulting. High-speed friction experiments at low pressure indicate that exceeding dry friction provokes shear heating that leads to endothermic reactions and pronounced weakening. Higher-pressure studies show nanocrystalline gouge accompanying dehydration and the highest pressure experiments initiate by exothermic polymorphic phase transformation. Here I discuss the characteristic nanostructures of experiments on high-speed friction and high-pressure faulting and show that all simulated earthquake systems yield very weak transformation-induced lubrication, most commonly nanometric gouge or melt. I also show that phase-transformation-induced faulting of olivine to spinel can propagate into material previously transformed to spinel, apparently by triggering melting analogous to high-speed friction studies at low pressure. These experiments taken as a whole suggest that earthquakes at all depths slide at low frictional resistance by a self-healing pulse mechanism with rapid strength recovery.This article is part of the themed issue 'Faulting, friction and weakening: from slow to fast motion'.

Seismic anisotropy evidence for dehydration embrittlement triggering intermediate-depth earthquakes

It has been proposed that dehydration embrittlement of hydrous materials can trigger intermediate-depth earthquakes and form a double seismic zone in a subducting slab. Seismic anisotropy may provide a possible insight into intermediate-depth intraslab seismicity, because anisotropic properties of minerals change with varying water distribution, temperature and pressure. Here we present a high-resolution model of P-wave radial anisotropy tomography of the Japan subduction zone down to ~400 km depth, which is obtained using a large number of arrival-time data of local earthquakes and teleseismic events. Our results reveal a close correlation between the pattern of intermediate-depth seismicity and anisotropic structures. The seismicity occurs in portions of the Pacific and Philippine Sea slabs where positive radial anisotropy (i.e., horizontal velocity being faster than vertical one) dominates due to dehydration, whereas the inferred anhydrous parts of the slabs are found to be aseismic where negative radial anisotropy (i.e., vertical velocity being faster than horizontal one) dominates. Our anisotropic results suggest that intermediate-depth earthquakes in Japan could be triggered by dehydration embrittlement of hydrous minerals in the subducting slabs.

Dehydration-driven stress transfer triggers intermediate-depth earthquakes

Intermediate-depth earthquakes (30–300 km) have been extensively documented within subducting oceanic slabs, but their mechanics remains enigmatic. Here we decipher the mechanism of these earthquakes by performing deformation experiments on dehydrating serpentinized peridotites (synthetic antigorite-olivine aggregates, minerals representative of subduction zones lithologies) at upper mantle conditions. At a pressure of 1.1 gigapascals, dehydration of deforming samples containing only 5 vol% of antigorite suffices to trigger acoustic emissions, a laboratory-scale analogue of earthquakes. At 3.5 gigapascals, acoustic emissions are recorded from samples with up to 50 vol% of antigorite. Experimentally produced faults, observed post-mortem, are sealed by fluid-bearing micro-pseudotachylytes. Microstructural observations demonstrate that antigorite dehydration triggered dynamic shear failure of the olivine load-bearing network. These laboratory analogues of intermediate-depth earthquakes demonstrate that little dehydration is required to trigger embrittlement. We propose an alternative model to dehydration-embrittlement in which dehydration-driven stress transfer, rather than fluid overpressure, causes embrittlement.

Intermediate-depth earthquakes (30-300 km) occur in subducting oceanic slabs, but their generation mechanism remains enigmatic. Here, the authors show through high-pressure and dehydration experiments of antigorite that dehydration-driven stress transfer triggers intermediate-depth earthquakes.

Slab temperature controls on the Tonga double seismic zone and slab mantle dehydration

Double seismic zones are two-layered distributions of intermediate-depth earthquakes that provide insight into the thermomechanical state of subducting slabs. We present new precise hypocenters of intermediate-depth earthquakes in the Tonga subduction zone obtained using data from local island-based, ocean-bottom, and global seismographs. The results show a downdip compressional upper plane and a downdip tensional lower plane with a separation of about 30 km. The double seismic zone in Tonga extends to a depth of about 300 km, deeper than in any other subduction system. This is due to the lower slab temperatures resulting from faster subduction, as indicated by a global trend toward deeper double seismic zones in colder slabs. In addition, a line of high seismicity in the upper plane is observed at a depth of 160 to 280 km, which shallows southward as the convergence rate decreases. Thermal modeling shows that the earthquakes in this "seismic belt" occur at various pressures but at a nearly constant temperature, highlighting the important role of temperature in triggering intermediate-depth earthquakes. This seismic belt may correspond to regions where the subducting mantle first reaches a temperature of ~500°C, implying that metamorphic dehydration of mantle minerals in the slab provides water to enhance faulting.

Seismic evidence of a two-layer lithospheric deformation in the Indian Ocean

Intra-plate deformation and associated earthquakes are enigmatic features on the Earth. The Wharton Basin in the Indian Ocean is one of the most active intra-plate deformation zones, confirmed by the occurrence of the 2012 great earthquakes (Mw≥8.2). These earthquakes seem to have ruptured the whole lithosphere, but how this deformation is distributed at depth remains unknown. Here we present seismic reflection images that show faults down to 45 km depth. The amplitude of these reflections in the mantle first decreases with depth down to 25 km and then remains constant down to 45 km. The number of faults imaged along the profile and the number of earthquakes as a function of depth show a similar pattern, suggesting that the lithospheric mantle deformation can be divided into two layers: a highly fractured fluid-filled serpentinized upper layer and a pristine brittle lithospheric mantle where great earthquakes initiate and large stress drops occur.

Seismic evidence of negligible water carried below 400-km depth in subducting lithosphere

Strong evidence exists that water is carried from the surface into the upper mantle by hydrous minerals in the uppermost 10-12 km of subducting lithosphere, and more water may be added as the lithosphere bends and goes downwards. Significant amounts of that water are released as the lithosphere heats up, triggering earthquakes and fluxing arc volcanism. In addition, there is experimental evidence for high solubility of water in olivine, the most abundant mineral in the upper mantle, for even higher solubility in olivine's high-pressure polymorphs, wadsleyite and ringwoodite, and for the existence of dense hydrous magnesium silicates that potentially could carry water well into the lower mantle (deeper than 1,000 km). Here we compare experimental and seismic evidence to test whether patterns of seismicity and the stabilities of these potentially relevant hydrous phases are consistent with a wet lithosphere. We show that there is nearly a one-to-one correlation between dehydration of minerals and seismicity at depths less than about 250 km, and conclude that the dehydration of minerals is the trigger of instability that leads to seismicity. At greater depths, however, we find no correlation between occurrences of earthquakes and depths where breakdown of hydrous phases is expected. Lastly, we note that there is compelling evidence for the existence of metastable olivine (which, if present, can explain the distribution of deep-focus earthquakes) west of and within the subducting Tonga slab and also in three other subduction zones, despite metastable olivine being incompatible with even extremely small amounts of water (of the order of 100 p.p.m. by weight). We conclude that subducting slabs are essentially dry at depths below 400 km and thus do not provide a pathway for significant amounts of water to enter the mantle transition zone or the lower mantle.