ALBERT

Description: Measurements of seismic anisotropy are commonly used to constrain deformation in the upper mantle. Observations of anisotropy at mid-mantle depths are, however, relatively sparse. In this study we probe the anisotropic structure of the mid-mantle (transition zone and uppermost lower mantle) beneath the Japan, Izu-Bonin, and South America subduction systems. We present source-side shear wave splitting measurements for direct teleseismic S phases from earthquakes deeper than 300 km that have been corrected for the effects of upper mantle anisotropy beneath the receiver. In each region, we observe consistent splitting with delay times as large as 1 s, indicating the presence of anisotropy at mid-mantle depths. Clear splitting of phases originating from depths as great as ∼600 km argues for a contribution from anisotropy in the uppermost lower mantle as well as the transition zone. Beneath Japan, fast splitting directions are perpendicular or oblique to the slab strike and do not appear to depend on the propagation direction of the waves. Beneath South America and Izu-Bonin, splitting directions vary from trench-parallel to trench-perpendicular and have an azimuthal dependence, indicating lateral heterogeneity. Our results provide evidence for the presence of laterally variable anisotropy and are indicative of variable deformation and dynamics at mid-mantle depths in the vicinity of subducting slabs.

Description: Understanding the dynamics of subduction is critical to our overall understanding of plate tectonics and the solid Earth system. Observations of seismic anisotropy can yield constraints on deformation patterns in the mantle surrounding subducting slabs, providing a tool for studying subduction dynamics. While many observations of seismic anisotropy have been made in subduction systems, our understanding of the mantle beneath subducting slabs remains tenuous due to the difficulty of constraining anisotropy in the sub‐slab region. Recently, the source‐side shear wave splitting technique has been refined and applied to several subduction systems worldwide, making accurate and direct measurements of sub‐slab anisotropy feasible and offering unprecedented spatial and depth coverage in the sub‐slab mantle. Here we present source‐side shear wave splitting measurements for the Central America, Alaska‐Aleutians, Sumatra, Ryukyu, and Izu‐Bonin‐Japan‐Kurile subduction systems. We find that measured fast splitting directions in these regions generally fall into two broad categories, aligning either with the strike of the trench or with the motion of the subducting slab relative to the overriding plate. Trench parallel fast splitting directions dominate beneath the Izu‐Bonin, Japan, and southern Kurile slabs and part of the Sumatra system, while fast directions that parallel the motion of the downgoing plate dominate in the Ryukyu, Central America, northern Kurile, western Sumatra, and Alaska‐Aleutian regions. We find that plate motion parallel fast splitting directions in the sub‐slab mantle are more common than previously thought. We observe a correlation between fast direction and age of the subducting lithosphere; older lithosphere (〉95 Ma) is associated with trench parallel splitting while younger lithosphere (〈95 Ma) is associated with plate motion parallel fast splitting directions. Finally, we observe source‐side splitting for deep earthquakes (transition zone depths) beneath Japan and Sumatra, suggesting the presence of anisotropy at midmantle depths beneath these regions.

Description: The lower mantle is dominated by two large structures with anomalously low shear wave velocities, known as Large Low‐Shear Velocity Provinces (LLSVPs). Several studies have documented evidence for strong seismic anisotropy at the base of the mantle near the edges of the African LLSVP. Recent work has identified a smaller structure with similar low‐shear wave velocities beneath Eurasia, dubbed the Perm Anomaly. Here we probe lowermost mantle anisotropy near the Perm Anomaly using the differential splitting of SKS and SKKS phases measured at stations in Europe. We find evidence for lowermost mantle anisotropy in the vicinity of the Perm Anomaly, with geographic trends hinting at lateral variations in anisotropy across the boundaries of the Perm Anomaly as well as across a previously unsampled portion of the African LLSVP border. Our observations suggest that deformation is concentrated at the boundaries of both the Perm Anomaly and the African LLSVP.

Description: Shear wave splitting of SK(K)S phases is often used to examine upper mantle anisotropy. In specific cases, however, splitting of these phases may reflect anisotropy in the lowermost mantle. Here we present SKS and SKKS splitting measurements for 233 event‐station pairs at 34 seismic stations that sample D″ beneath Africa. Of these, 36 pairs show significantly different splitting between the two phases, which likely reflects a contribution from lowermost mantle anisotropy. The vast majority of discrepant pairs sample the boundary of the African large low shear velocity province (LLSVP), which dominates the lower mantle structure beneath this region. In general, we observe little or no splitting of phases that have passed through the LLSVP itself and significant splitting for phases that have sampled the boundary of the LLSVP. We infer that the D″ region just outside the LLSVP boundary is strongly deformed, while its interior remains undeformed (or weakly deformed).

Description: Monitoring ground displacement produced by underground mining is essential to ensure the safety of infrastructure over mining areas. Differential synthetic aperture radar (DInSAR) can only obtain the one-dimensional (1D, i.e., along the line-of-sight (LOS) direction) displacement component. In this study, we present an improved algorithm for retrieving and predicting three-dimensional (3D) displacement fields induced by underground mining based on the LOS displacement derived from DInSAR and the probability integral method (PIM). Whole parameters included in the standard PIM model are involved in the improved algorithm. In addition, the interaction between multiple working panels is considered and incorporated into the model. Next, a stochastic optimization technique hybridizing the cultural algorithm and random particle swarm optimization (CA-rPSO) has been designed to retrieve model parameters, which can be used to retrieve and predict the 3D displacement field. Simulated experiments show that the RMSEs are 10 mm, 12 mm and 17 mm in the vertical, east-west and north-south directions, respectively, by comparing the simulated and retrieved 3D displacement. Furthermore, the capability of the proposed method is investigated and validated in the Xuehu mining area of China using three ALOS PALSAR acquisitions. Our results agree well with levelling measurements in the vertical direction with a RMSE of 38 mm. Although the retrieved horizontal displacement cannot be validated due to a lack of field surveys, these displacement fields coincide spatially with the evolution of mining excavation.

Description: Two arrays of broad-band seismic stations were deployed in the north central Andes between 8° and 21°S, the CAUGHT array over the normally subducting slab in northwestern Bolivia and southern Peru, and the PULSE array over the southern part of the Peruvian flat slab where the Nazca Ridge is subducting under South America. We apply finite frequency teleseismic P- and S-wave tomography to data from these arrays to investigate the subducting Nazca plate and the surrounding mantle in this region where the subduction angle changes from flat north of 14°S to normally dipping in the south. We present new constraints on the location and geometry of the Nazca slab under southern Peru and northwestern Bolivia from 95 to 660 km depth. Our tomographic images show that the Peruvian flat slab extends further inland than previously proposed along the projection of the Nazca Ridge. Once the slab re-steepens inboard of the flat slab region, the Nazca slab dips very steeply (∼70°) from about 150 km depth to 410 km depth. Below this the slab thickens and deforms in the mantle transition zone. We tentatively propose a ridge-parallel slab tear along the north edge of the Nazca Ridge between 130 and 350 km depth based on the offset between the slab anomaly north of the ridge and the location of the re-steepened Nazca slab inboard of the flat slab region, although additional work is needed to confirm the existence of this feature. The subslab mantle directly below the inboard projection of the Nazca Ridge is characterized by a prominent low-velocity anomaly. South of the Peruvian flat slab, fast anomalies are imaged in an area confined to the Eastern Cordillera and bounded to the east by well-resolved low-velocity anomalies. These low-velocity anomalies at depths greater than 100 km suggest that thick mantle lithosphere associated with underthrusting of cratonic crust from the east is not present. In northwestern Bolivia a vertically elongated fast anomaly under the Subandean Zone is interpreted as a block of delaminating lithosphere.

Description: Magnetic flux ropes are bundles of twisted magnetic field that harbor free magnetic energy and can be progenitors of coronal mass ejections (CMEs). However, identifying flux ropes on the Sun can be challenging. In this work, we show Hinode EUV Imaging Spectrometer observations of an active region that exhibits a sigmoidal configuration (indicative of a flux rope). We analyze the coronal plasma composition as the sigmoid (flux rope) forms before its eruption. Plasma with photospheric composition was observed in coronal loops close to the main polarity inversion line during episodes of significant flux cancellation, suggestive of the injection of photospheric plasma into these loops driven by photospheric flux cancellation. Concurrently, the increasingly sheared core field contained plasma with coronal composition. As flux cancellation decreased and a sigmoid/flux rope formed, the plasma evolved to an intermediate composition in between photospheric and typical active region coronal compositions. Finally, the flux rope contained predominantly photospheric plasma just before the CME. Hence, plasma composition observations strongly support models of flux rope formation by photospheric flux cancellation forcing magnetic reconnection first at the photospheric level then at the coronal level. We compare and contrast these findings with a flux rope that formed via reconnection directly in the corona, which exhibited coronal plasma composition and was determined to be located at a higher altitude. These results show that plasma composition provides key information on the specific configuration of a flux rope, which in turn affects its stability and the processes involved in CME onset.

Description: We discuss two types of ice shelf firn aquifers present in coastal Antarctica: melt-derived and brine-infiltration. Mapping of melt-derived firn aquifers and regions of melt-saturated firn using both passive and active L-band microwave data has identified perennial melt-derived aquifers in the southwestern Antarctic Peninsula, and several seasonal melt-saturation regions there and in widely separated sites around the ice sheet fringe. Extensive melt aquifers are identified on the Wilkins Ice Shelf, the northern George VI Ice Shelf, and the Müller Ice shelf, and are suspected on the former Jones and Wordie ice shelves. Field work has confirmed the aquifer presence in at least two sites on both the Wilkins and Müller shelves. We also present an improved assessment of the extent of brine aquifers based on their characteristics in modern airborne ice-penetrating radar profiles. Brine aquifers are relatively widespread and are found in shelf areas with porous firn at the waterline, as was previously recognized by Cook et al.(2018). Our study also assesses the relative risk of hydrofracture for ice shelves bearing melt and brine aquifers under various scenarios of firn density, aquifer depth, and brine density. We find that ponded surface melt poses the greatest threat, but that the increased density of brine may also be an issue if the brine layer is relatively shallow in the shelf. We also discuss the likely future expansion of melt aquifers on ice shelves under warming conditions, and consider the process of transitioning from a brine aquifer to a melt aquifer.

Description: The correlation between mantle flow and seismic anisotropy emphasizes its key role in understanding tectonic processes. While global geodynamic modelling of asthenospheric mantle flow results in a good correlation, the large spatial variation of seismic anisotropy beneath continents indicates a considerable complex contribution of the shallow lithosphere, which is not yet well understood. A detailed imaging of anisotropy with depth is therefore an important challenge for seismic investigation.Here, we present our current advances in improving the depth resolution based on receiver function and SKS-splitting techniques applied at the continental margins of the European Alps and Central Appalachians. Classical shear-wave splitting techniques are mostly used to infer a single anisotropic layer. This becomes misleading for continental margins with complex deformation. An azimuthal variation of splitting parameters is an indication for vertical layering of anisotropy and can be analyzed systematically providing insight into these complexities. However, this approach allows no direct constraint on the depth distribution. Recent developments involve the calculation of sensitivity kernels for Splitting Intensity observations, which allows us to consider the laterally broadened sensitivity for the anisotropic structure with depth. A requirement of this tomographic technique is a dense station spacing, which is satisfied by a growing number of seismic deployments.The increased lateral stress in deformational regimes at shallow levels results in a possible contamination of shear-wave splitting by crustal anisotropy. We suggest a stepwise approach in which receiver functions are examined for their harmonic variation with backazimuth to determine and correct for significant anisotropic crustal effects.

Description: Magnetotelluric (MT) signal is a natural electromagnetic (EM) response that can penetrate up to hundreds of kilometers deep in the earth's interior. When deployed at the seafloor, ocean bottom MT receivers provide us quality EM data that can decipher both regional and large-scale conductivity structures underneath the seafloor. Knowledge of such structures is critical in the studies of Earth's interior and marine geohazards. A key step in interpreting these deep sea EM data is accurate modelling of the data over various plausible marine conductivity models, where the existence of highly conductive sea water poses as a main challenge in improving the modelling accuracy. The accuracy degeneration issue caused by the seawater becomes more severe for three-dimensional (3D) studies in which fine discretizations of seawater layer are often required. To ensure modelling accuracy, we have developed high-order finite element (FE) modelling techniques in conjunction with unstructured triangular (2D) and tetrahedral (3D) meshes. Using the high-order FE methods, excessively large meshes are avoided since the seawater layer does not need to be finely discretized. Our modelling code has been tested and can provide accurate modelling results of marine MT data for a large range of frequencies. The modelling accuracy is demonstrated using both 2D and 3D examples. Using the newly developed code, we aim to study deep conductivity structures over various seafloors of interest.