Water Migration in the Subduction Mantle Wedge: A Two-Phase Flow Approach

Subduction zones are the main entry points of water into Earth's mantle and play an important role in the global water cycle. The progressive release of water by metamorphic dehydration induces important physical‐chemical processes, including subduction zone earthquakes. Yet, how water migrates in subduction zones is not well understood. We investigate this problem by explicitly modeling two‐phase flow processes, in which fluids migrate through a compacting and decompacting solid matrix. Our results show that water migration is strongly affected by subduction dynamics, which exhibits three characteristic stages in our models: (1) an early stage of subduction initiation; (2) an intermediate stage of gravity‐driven steepening of the slab; and (3) a late stage of quasi steady state subduction. Two main water pathways are found in the models: trenchward and arcward. They form in the first two stages and become steady in the third stage. Depending on the depth of water release from the subducting slab, water migration focuses in different pathways: a shallow release depth (e.g., 40 km) leads the water mainly through the trenchward pathway, a deep release depth (e.g., 120 km) promotes an arcward pathway and a long horizontal migration distance (~300 km) from the trench, and an intermediate release depth (e.g., 80 km) leads water to both pathways. We compare our models with seismic studies from southeast Japan (Saita et al., 2015, https://doi.org/10.1002/2015GL063084) and the west Hellenic subduction zone (Halpaap et al., 2018, https://doi.org/10.1002/2017JB015154) and provide geodynamical explanations for these seismic observations in natural subduction environments.publishedVersio

Localized crustal deformation along the central North Anatolian Fault Zone revealed by joint inversion of P-receiver functions and P-wave polarizations

The North Anatolian Fault Zone (NAFZ) is a major plate boundary that separates the Eurasian Plate to the north from the Anatolian Plate to the south and is associated with powerful damaging earthquakes. Despite numerous studies of the crust and upper mantle across the NAFZ, our understanding of the exact mechanisms and distribution of deformation with depth is still limited. Accurate models of the crustal velocity structure are key to assess seismic hazard associated with strike-slip deformation. Here, we address this need by employing a novel method that jointly inverts receiver function waveforms and P-wave polarizations to recover S-wave velocity structure from the surface to the upper mantle. The method is applied to a dense teleseismic data set collected across a segment of the central NAFZ in Turkey. The results provide important new constraints on the sedimentary thickness, depth to basement and Moho discontinuity beneath the region. Our estimates of uppermost sedimentary thickness range from 0 km in some areas (e.g. in the Central Pontides) to 6 km in the Çankırı Basin. Smaller basins are scattered along the NAFZ. A similar pattern is observed for the basement depth, with values exceeding 10 km beneath the Çankırı Basin, where the Moho is shallowest with a depth of ∼32 km. The Moho reaches a maximum depth of ∼42 km beneath the Central Pontides. Most other areas have an average Moho depth of 35–38 km. The results reveal clear structural–tectonic relationships in the crust: areas of fundamentally different sedimentary and crustal architecture are bounded by faults and suture zones. The NAFZ appears to accommodate small-scale basin and basement-highs, and acts as a thick-skinned (i.e. full crustal-scale) boundary between laterally displaced crustal blocks to the north and south. Seismicity clusters are centred on areas of low Vp/Vs ratios that may be representative of weak zones.publishedVersio

Constraints on the structure of the crust and lithosphere beneath the Azores Islands from teleseismic receiver functions

The Azores Archipelago is located near the Mid-Atlantic Ridge (MAR) and consists of nine islands, resting on both sides of the ridge. Various methods including seismic reflection, gravity and passive seismic imaging have previously been used to investigate the crustal thickness beneath the islands. They have yielded thickness estimates that range between roughly 10 and 30 km, but until now models of the more fine-scale crustal structure have been lacking. Pending questions include the thickness of the volcanic edifice beneath the islands and whether crustal intrusions or even underplating can be observed beneath any island. In this study, we use data from nine seismic stations located on the Azores Islands to investigate the crustal structure with teleseismic P-wave receiver functions. Our results indicate that the base of the volcanic edifice is located approximately 1 to 4 km depth beneath the different islands and that the crust–mantle boundary has an average depth of ∼17 km. There is strong evidence for magmatic underplating beneath the island of São Jorge, and indications that the underplating is also present beneath São Miguel and possibly Santa Maria. Additionally, the seismological lithosphere–asthenosphere boundary, defined as a seismic velocity drop in the uppermost mantle, seems to deepen with increasing distance from the MAR. It has a depth of ∼45 km beneath the islands close to the MAR, compared to depths >70 km beneath the more distal islands

The development of seismic anisotropy below South-Central Alaska: Evidence from local earthquake shear-wave splitting

The Transportable Array in south-central Alaska spans several subduction zone features: backarc, forearc and volcanic arc, making it an ideal tool to study subduction zone anisotropy. Shear wave splitting analysis of 157 local earthquakes of m b ≥ 3.0 that occurred between 2014 and 2019 yields 210 high-quality measurements at 23 stations. Splitting delay times (δt) are generally small (δt ≈ 0.3 s), increasing with distance from the trench. Arc-parallel fast directions, φ, are only seen in the forearc, but rotate to arc-perpendicular φ in the backarc. Observed φ values generally do not parallel teleseismic SKS splitting results, implying that the latter is sensitive primarily to subslab mantle flow, not mantle wedge dynamics. The forearc local-earthquake signal likely originates from anisotropic serpentinite in fractures atop the subducting Pacific Plate, with possible additional signal coming from fractures in the North American crust. Mantle wedge corner flow, potentially with additional arc-perpendicular anisotropy in the subducting slab, explains backarc anisotropy

Water Migration in the Subduction Mantle Wedge: A Two-Phase Flow Approach

Subduction zones are the main entry points of water into Earth's mantle and play an important role in the global water cycle. The progressive release of water by metamorphic dehydration induces important physical‐chemical processes, including subduction zone earthquakes. Yet, how water migrates in subduction zones is not well understood. We investigate this problem by explicitly modeling two‐phase flow processes, in which fluids migrate through a compacting and decompacting solid matrix. Our results show that water migration is strongly affected by subduction dynamics, which exhibits three characteristic stages in our models: (1) an early stage of subduction initiation; (2) an intermediate stage of gravity‐driven steepening of the slab; and (3) a late stage of quasi steady state subduction. Two main water pathways are found in the models: trenchward and arcward. They form in the first two stages and become steady in the third stage. Depending on the depth of water release from the subducting slab, water migration focuses in different pathways: a shallow release depth (e.g., 40 km) leads the water mainly through the trenchward pathway, a deep release depth (e.g., 120 km) promotes an arcward pathway and a long horizontal migration distance (~300 km) from the trench, and an intermediate release depth (e.g., 80 km) leads water to both pathways. We compare our models with seismic studies from southeast Japan (Saita et al., 2015, https://doi.org/10.1002/2015GL063084) and the west Hellenic subduction zone (Halpaap et al., 2018, https://doi.org/10.1002/2017JB015154) and provide geodynamical explanations for these seismic observations in natural subduction environments

Receiver functions obtained from the MEDUSA seismic array

Here we present a dataset of Receiver Functions from the South Line of the MEDUSA seismological array used in the studies of Pearce et al. (2012) and Millet et al. (2019). The geographical extend of the dataset is from (37.2N, 21.7E) to (39.0N, 24.6E). The data were aligned using multi-channel cross-correlation on the main P-wave arrival and obtained through frequency domain water-level type deconvolution. The data is three component (radial [R], transverse [T], and vertical [Z]) and the frequency band of the data is 0.03 to 0.3 Hz. It is written in a 4D matrix in netCDF format, with the first two dimensions corresponding to the source-receiver pairs grid, and the third and fourth to the time series data from the corresponding three component Receiver Function for all pairs. For source-receiver pairs where no recording is available the data is a string of zeroes. Metadata include information about the stations and events locations and timings, as well as recording parameters for the data

Multimode 3‐D Kirchhoff Migration of Receiver Functions at Continental Scale

Receiver function analysis is widely used to image sharp structures in the Earth, such as the Moho or transition zone discontinuities. Standard procedures either rely on the assumption that underlying discontinuities are horizontal (common conversion point stacking) or are computationally expensive and usually limited to 2‐D geometries (reverse time migration and generalized Radon transform). Here, we develop a teleseismic imaging method that uses fast 3‐D traveltime calculations with minimal assumption about the underlying structure. This allows us to achieve high computational efficiency without limiting ourselves to 1‐D or 2‐D geometries. In our method, we apply acoustic Kirchhoff migration to transmitted and reflected teleseismic waves (i.e., receiver functions). The approach expands on the work of Cheng et al. (2016, https://doi.org/10.1093/gji/ggw062) to account for free surface multiples. We use an Eikonal solver based on the fast marching method to compute traveltimes for all scattered phases. Three‐dimensional scattering patterns are computed to correct the amplitudes and polarities of the three component input signals. We consider three different stacking methods (linear, phase weighted, and second root) to enhance the structures that are most coherent across scattering modes and find that second‐root stack is the most effective. Results from synthetic tests show that our imaging principle can recover scattering structures accurately with minimal artifacts. Application to real data from the Multidisciplinary Experiments for Dynamic Understanding of Subduction under the Aegean Sea experiment in the Hellenic subduction zone yields images that are similar to those obtained by 2‐D generalized Radon transform migration at no additional computational cost, further supporting the robustness of our approach