ALBERT

Description: We study the crustal structure of Sri Lanka by analyzing data from a temporary seismic network deployed in 2016-2017 (Seneviratne et al., 2016) to shed light on the amalgamation process from the geophysical perspective. Rayleigh wave phase dispersion from ambient noise cross-correlation and receiver functions were jointly inverted using a transdimensional Bayesian approach (Bodin et al., 2012, Dreiling et al., 2019).

Description: The convergence between the Indian and Eurasian plates has produced the thick crust and uplifted the Tibetan plateau since about 50 Ma. However, the deformation of the mantle lithosphere is still a puzzle. The geometry of the subducting Indian mantle lithosphere beneath the plateau and the thickening or/and delamination of the Tibetan mantle lithosphere are the keys for understanding the continental collision process and the evolution of the plateau. However, knowledge has been restricted due to sparse data coverage in Tibet. In this study, S-wave receiver functions are calculated using tele-seismic waveforms recorded by two broadband arrays in central Tibet to image the lithospheric structure, mainly the depth variation of the lithosphere-asthenosphere boundary (LAB). Our results show that the depth of the Tibetan LAB decreases from ∼150 km in the west to ∼120 km in the east across the north-south trending Yadong-Gulu rift. Similarly, the LAB depth of the subducting Indian slab decreases from ∼270 km in the west to ∼200 km in the east, and the northernmost subducting Indian slab can be observed beneath the Bangong-Nujiang suture. These observations suggest that the thickness of the Tibetan lithosphere and the depth of the underlying Indian slab are segmented across the Yadong-Gulu rift in different degrees. The abrupt changes imply that the subducting Indian slab has been torn, which provided an upwelling channel for the asthenosphere contributing to the development of the rift.

Description: The tectonic activity and potential for linkage of adjacent active faults are crucial for seismic assessment. As the two largest faults that bound the Weihe Graben (central China), the Qinling Northern Piedmont Fault (QNF, ~200 km) and the Huashan Piedmont Fault (HPF, ~150 km) are mainly responsible for seismic risk in this densely-populated area, where the 1556 M 8.5 Huaxian earthquake occurred with 830,000 fatalities. However, their tectonic activity and the degree of interaction remain poorly constrained, hampering an adequate seismic risk assessment of the Weihe Graben. Here, we integrate 23 new 10Be-derived catchment-averaged denudation rates of ~0.06–0.32 mm/yr with topographic metrics to evaluate the seismic risk. The results demonstrate that the landscape of the Qinling and Huashan Mountains is in transient state in response to the tectonic perturbations of the QNF and the HPF, with tectonic knickpoints formed along main streams and tributaries, and widespread unstable drainage divides. These two faults have comparable tectonic activity, and are potentially capable of generating earthquakes with the maximum magnitude of Mw ~7.7–7.9. Moreover, they have likely started linking, posing a greater seismic risk than previously estimated.

Description: The Walvis Ridge (WR) is the most prominent hotspot track related to the opening in the South Atlantic Ocean. Several hypotheses have been developed to explain its origin and evolution. The presence of a massive magmatic structure at the landfall of the WR in Northwest Namibia raised speculation about the role of a hotspot during the opening of the South Atlantic ocean. To investigate its deeper velocity structure at the junction of the WR with the African continent was the focus of the amphibious seismological WALPASS experiment. In total 12 ocean-bottom seismometers and 28 broad-band land stations were installed between 2010 and 2012 to acquire seismological data. Here, we present the results of seismic ambient noise tomography to investigate to which extent the Tristan hotspot modified the crustal structure in the landward prolongation of the ridge and in the adjacent oceanic basins. For the tomography, vertical and hydrophone component cross correlations for 〉300 d for OBS stations and between 1 and 2 yr for land stations data were analysed. More than 49 000 velocity measurements (742 dispersion curves) were inverted for group velocity maps at 75 individual signal periods, which then had been inverted for a regional 3-D shear wave velocity model. The resulting 3-D model reveals structural features of the crust related to the continent–ocean transition and its disturbance caused by the initial formation of the WR ∼130 Ma. We found relatively thick continental crust below Northwest Namibia and below the near-shore part of the WR, a strong asymmetry offshore with typical, thin oceanic crust in the Namibe Basin (crossing over into the Angola Basin further offshore) to the North and a wide zone of transitional crust towards the Walvis Basin south of the WR.

Description: A sequence of three strong (M W 7.2–6.4) and several moderate (M W 4.4–5.7) earthquakes struck the Pamir Plateau and surrounding mountain ranges of Tajikistan, China, and Kyrgyzstan in 2015–2017. With a local seismic network in operation in the Xinjiang province since August 2015, an aftershock network on the Pamir Plateau of Tajikistan since February 2016, and additional permanent regional seismic stations, we were able to record the succession of the fore-, main-, and aftershock sequences at local distances with good azimuthal coverage. We located 11,784 seismic events and determined the moment tensor for 33 earthquakes. The seismicity delineates the major tectonic structures of the Pamir, i.e., the thrusts that absorb shortening along the plateau thrust front, and the strike-slip and normal faults that dissect the Plateau into a westward extruding and a northward advancing block. Fault ruptures were activated subsequently at increasing distances from the initial M W 7.2 Sarez. All mainshock areas but the initial one exhibited foreshock seismicity which was not modulated by the occurrence of the earlier earthquakes. The tabular ASCII data of the seismic event catalog consist of origin date, time, location, depth and magnitude of the events, along with the quality measures: number of P- and S-wave arrival time picks, location root-mean-square misfit and localization method. The tabular ASCII data of the moment tensor catalog consist of origin date, time, location, the six independent components of the moment tensor, the moment magnitude, and the orientation of the preferred fault plane parameterized as fault strike, dip and rake.

Description: The Pamir plateau protrudes ~300 km between the Tajik- and Tarim-basinlithosphere of Central Asia. We present a new local-seismicity catalog, a focal-mechanism catalog, and a P-wave velocity model of the of the collision system between the Pamir plateau and the Tarim basin. The data suggest a south-dipping Asian slab that overturns in its easternmost segment. The largest principal stress at depth acts normal on the slab and is orientated parallel to the plate convergence direction. In front (south) of the Asian slab, a volume of mantle with elevated velocities and lined by weak seismicity constitutes the postulated Indian mantle indenter. The data set consists of an earthquake catalog, an earthquake focal mechanism catalog and a subsurface P-wave velocity model of the central and eastern Pamir plateau and the adjacent north-western Tarim basin; between 36.8–40.0 °N and 72.2–78.0 °E. It was collected to identify the deep tectonic structures that determine the lithospheric architecture of the Pamir plateau. Earthquakes were recorded by two temporary seismic deployments. Earthquakes that occurred between 1st August 2008 and 6th June 2010 were primarily recorded by the TIPAGE network (Yuan et al., 2008); those, between 3rd August 2015 and 23rd June 2017 by the East Pamir and Sarez aftershock networks (Yuan et al., 2018a, b). The earthquake catalog contains 1,493 seismic events at depth 〉50 km. They were localized in the present 3-D velocity model. Some events were re-located with hypoDD. The focal mechanism catalog consists of double-couple fault-slip parameters for 38 events, 29 of which are newly determined using the HASH algorithm and 9 are moment tensors from Kufner et al. (2016). The P wave-velocity model has been determined using simulps from 2,264 seismic events with well-constrained P- and S-wave arrivals. It is parameterized as velocity gradients between nodes with a horizontal and vertical spacing of 40 and 15 km, respectively. Unresolved nodes were masked using a checkerboard resolution test. The full description of the methods is provided in the data description file.

Description: The Dead Sea Transform (DST) was formed in the Mid-Cenozoic, about 18 Myr ago, as a result of the breakaway of the Arabian plate from the African plate. Higher resolution information about the sub-Moho structure is still sparse in this region. Here we study seismic discontinuities in the mantle lithosphere in the region of the DST using a modified version of the P- and S-receiver function method. We use open data from permanent and temporary seismic stations. The results are displayed in a number of depth profiles through the study area. The Moho is observed on both sides of the transform at nearly 40 km depth by S-to-p and in P-to-s converted signals. The lithosphere-asthenosphere boundary (LAB) on the eastern side of the DST is observed near 180–200 km depth, which is according to our knowledge the first LAB observation at that depth in this region. This observation could lead to the conclusion that the thickness of the Arabian lithosphere east of the DST is likely cratonic. In addition, we observe in the entire area a negative velocity gradient (NVG) at 60–80 km depth, which was previously interpreted as LAB.

Description: The Bransfield Basin is a young (∼4 Ma) back-arc basin related to the remnant subduction of the Phoenix Plate that once existed along the entire Pacific margin of the Antarctic Peninsula. Based on a recently deployed amphibious seismic network, we use ambient noise tomography to obtain the S-wave velocity structure in the Central Bransfield Basin (CBB). Combining with the stress-field inverted from focal mechanisms, our images reveal that the CBB suffers a significant extension in the northwest-southeast direction. The extension is strongest in the northeastern CBB with associated mantle exhumation and weakens to the southwest with decoupled deformations between the upper crust and lithospheric mantle. Such an along-strike variation of extension can be explained by slab window formation and forearc rotation, which are associated with the Phoenix Plate detachment during the ridge–trench collisions at the southwest of the Hero Fracture Zone.

Description: This data publication includes digital data of the final 3D tomographic model from Chen et al. (2020: Lithospheric delamination beneath the southern Puna plateau resolved by local earthquake tomography). The 3D seismic velocity models are results of a local earthquake tomography which is performed to illuminate the crustal and uppermost mantle structure beneath the southern Puna plateau and to test the delamination hypothesis. The Southern Puna is distinctive from the rest of the Central Andean plateau in having a higher topographic elevation, a thinner lithosphere and in being flanked to the south by the Chilean flat slab region. Previous investigations involving geochemical, geological and geophysical observations, have invoked lithospheric delamination to explain the distinctive magmatic and structural history, elevation and lithospheric thickness of the region. In the present study, Vp and Vp/Vs ratios were obtained using travel time variations recorded by 75 temporary seismic stations between 2007 and 2009. The earthquakes catalog (Mulcahy et al., 2014) contains 1903 local earthquakes (25077 P- and 14059 S-picks). A minimum 1D model is derived with software VELEST (Kissling et al., 1995). The 3D tomographic inversion is performed with software SIMULPS (Thurber, 1983; Evans et al., 1994). Spread values are used to define well resolved model domains (6 for Vp and 5.5 for Vp/Vs), which are calculated from the model resolution matrix (Toomey & Foulger, 1989). The data are provided as one tar.gz archive. Individual ASCII files contain, at each depth from 0 to 200 km: - Vp model (model.vp.depth_???km), format: longitude, latitude, depth, Vp perturbation, absolute Vp - Vp/Vs model (model.vpvs.depth_???km), format: longitude, latitude, depth, Vp/Vs perturbation, absolute Vp/Vs - spread values for Vp (spread.vp.depth_???km), format: longitude, latitude, depth, spread value - spread values for Vp/Vs model (spread.vpvs.depth_???km), format: longitude, latitude, depth, spread value

Description: It is well established that slip on a frictionally‐weak low‐angle normal fault (LANF) can be more favorable than breaking a steep fault in strong crust. Very few studies, however, have considered the specific effect of crust and fault cohesion on LANF viability. We do so using Limit Analysis, a methodology for predicting the optimal orientation of faults with varying strength subjected to a specific set of boundary conditions. Accounting for crustal cohesion in our models reduces the lowest admissible LANF dip, and even allows slip on high‐friction LANFs if the contrast between crust and fault cohesion is large. Fault cohesion, however, increases the lowest admissible LANF dip, and introduces a locking depth above which LANF slip is not mechanically feasible. This is consistent with observations of steep splay faults rooting onto LANFs in a variety of settings. We further demonstrate that locking depth can help constrain LANF cohesion, friction, and fluid pressure on the Alto Tiberina (Italy) and western Corinth (Greece) LANFs. Specifically, assuming a measured fault friction of 0.2‐‐0.3, we find that the shallow locking depth of the Alto Tiberina fault requires either (1) moderate fluid overpressure (57% of lithostatic) with 8‐‐12 MPa of cohesion, or (2) strong overpressure 77% of lithostatic) with 13‐‐20 MPa of cohesion along the fault. By contrast, the larger locking depth characterizing the western Corinth LANF can reflect greater fault cohesion.