ALBERT

Description: This data publication contains a seismic survey which was acquired in the Mont Terri Underground Rock Laboratory (URL) in January 2019. The aim of the SI-A experiment (Seismic Imaging Ahead of and around underground infrastructure) is to provide a seismic characterization at the meso scale and to investigate the feasibility of tomographic and reflection imaging in argillaceous environments. The survey covered the different facies types of Opalinus Clay: shaly facies, carbonate -rich sandy facies and sandy facies (Bossart et al. 2017). Three different seismic sources (impact, vibro, ELVIS) were used to acquire the seismic data. The impact and magnetostrictive vibro sources were particularly designed for seismic exploration in the underground (Giese et al. 2005, Richter et al. 2018). The ELVIS source was mainly designed for near-surface investigations on roads or in open terrain (Krawczyk et al. 2012). All data were recorded on 32 3-component geophones (GS-14-L3, 28 Hz) which were deployed in 2 m deep boreholes, fixed at the tip of rock anchors. The data publication covers raw and preprocessed data stored in SEG-Y format.

Description: From June to August 2021, we deployed a dense seismic nodal network across the Hengill geothermal area in southwest Iceland to image and characterize faults and high‐temperature zones at high resolution. The nodal network comprised 498 geophone nodes spread across the northern Nesjavellir and southern Hverahlíð geothermal fields and was complemented by an existing permanent and temporary backbone seismic network of a total of 44 short‐period and broadband stations. In addition, we recorded distributed acoustic sensing data along two fiber optic telecommunication cables near the Nesjavellir geothermal power plant with commercial interrogators. During the time of deployment, a vibroseis survey took place around the Nesjavellir power plant. Here, we describe the network and the recorded datasets. Furthermore, we show some initial results that indicate a high data quality and highlight the potential of the seismic records for various follow up studies, such as high‐resolution event location to delineate faults and body‐ and surface‐wave tomographies to image the subsurface velocity structure in great detail.

Description: BACKGROUND AND OBJECTIVES: Soil or rock types in a region are often interpreted qualitatively by visually comparing various geophysical properties such as seismic wave velocity and vulnerability, as well as gravity data. Better insight and less human-dependent interpretation of soil types can be obtained from a joint analysis of separated and independent geophysical parameters. This paper discusses the application of a neural network approach to derive rock properties and seismic vulnerability from horizontal-to-vertical seismic ratio and seismic wave velocity data recorded in Majalengka-West Java, Indonesia.METHODS: Seismic microtremors were recorded at 54 locations and additionally multichannel analyses of surface wave experiments were performed at 18 locations because the multichannel analyses of surface wave experiment needs more effort and space. From the two methods, the values of the average shear wave velocity for the upper 30 meters, peak amplitudes and the dominant frequency between the measurement points were obtained from the interpolation of those geophysical data. Neural network was then applied to adaptively cluster and map the geophysical parameters. Four learning model clusters were developed from the three input seismic parameters: shear wave velocity, peak amplitude, and dominant frequency. FINDINGS: Generally, the values of the horizontal to vertical spectral ratios in the west of the study area were low (less than 5) compared with those in the southeastern part. The dominant frequency values in the west were mostly low at around 0.1–3 Hertz, associated with thick sedimentary layer. The pattern of the shear wave velocity map correlates with that of the horizontal to vertical spectral ratio map as the amplification is related to the soil or rock rigidity represented by the shear wave velocity. The combination of the geophysical data showed new features which is not found on the geological map such as in the eastern part of the study area. CONCLUSION: The application of the neural network based clustering analysis to the geophysical data revealed four rock types which are difficult to observe visually. The four clusters classified based on the variation of the geophysical parameters show a good correlation to rock types obtained from previous geological surveys. The clustering classified safe and vulnerable regions although detailed investigation is still required for confirmation before further development. This study demonstrates that low-cost geophysical experiments combined with neural network-based clustering can provide additional information which is important for seismic hazard mitigation in densely populated areas.

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: Earthquakes and slow-slip events interact, however, detailed studies investigating their interplay are still limited. We generate the highest resolution microseismicity catalog to date for the northern Armutlu Peninsula in a ∼1-year period to perform a detailed seismicity distribution analysis and correlate the results with a local, geodetically observed slow-slip transient within the same period. Seismicity shows a transition of cluster-type behavior from swarm-like to burst-like, accompanied by an increasing relative proportion of clustered (non-Poissonian) relative to background (Poissonian) seismicity and gradually decreasing b-value as the geodetically observed slow-slip transient ends. The observed slow-slip transient decay correlates with gradually increasing effective-stress-drop values. The observed correlation between the b-value and geodetic transient highlights the influence of aseismic deformation on seismic deformation and the impact of slow-slip transients on local seismic hazard.

Description: The identification of the chemical status of groundwater is a prerequisite for the sustainable management and development of groundwater resources. A better assessment of the chemical status of the groundwater requires the knowledge and understanding of the natural background concentrations to establish threshold values of chemical pollutants in groundwater. The aim of this study is to estimate the natural background levels, the threshold value (TV) of nitrate-nitrogen (NO3--N) and to identify its source in the groundwater in the Bono, Ahafo and Bono East regions of Ghana. A total of 165 groundwater samples were taken from the crystalline and sedimentary aquifers in the study area. Iterative outlier removal technique and a Gaussian mixture model were then used to assess the natural background and threshold nitrate-nitrogen concentrations in the groundwater. Chloride data was then used to trace the NO3-–N source in the groundwater. The estimated NBL of –NO3-–N in the area ranges from [0.001–3.9] mg/L with an expected value of 1.25 mg/L and calculated TV of 6.95 mg/L. The data suggest that NO3-–N concentrations are homogeneous across all the lithologies underlying the study area. The results showed that agricultural, domestic, and denitrification contribute significantly to the loading of NO3-–N concentration in the groundwater. The estimated threshold range of NO3-–N provides the baseline nitrate-nitrogen concentration for future studies in the region. However, the results are inconclusive, and we recommend using isotopic detection (15N-NO3- and 18O- NO3-) in future studies through comprehensive and sustainable regional monitoring of the aquifer system in order to further limit the source of nitrate-nitrogen in the groundwater system.

Description: Ruptures of the largest earthquakes can last between a few seconds and several minutes. An early assessment of the final earthquake size is essential for early warning systems. However, it is still unclear when in the rupture history this final size can be predicted. Here we introduce a probabilistic view of rupture evolution - how likely is the event to become large - allowing for a clear and well-founded answer with implications for earthquake physics and early warning. We apply our approach to real time magnitude estimation based on either moment rate functions or broadband teleseismic P arrivals. In both cases, we find strong and principled evidence against early rupture predictability because differentiation between differently sized ruptures only occurs once half of the rupture has been observed. Even then, it is impossible to foresee future asperities. Our results hint toward a universal initiation behavior for small and large ruptures.

Description: An earthquake swarm affected the Bransfield Strait, Antarctica, a unique rift basin in transition from intra-arc rifting to ocean spreading. The swarm, counting ~85,000 volcano-tectonic earthquakes since August 2020, is located close to the Orca submarine volcano, previously considered inactive. Simultaneously, geodetic data reported up to ~11 cm northwestward displacement over King George Island. We use a broad variety of geophysical data and methods to reveal the complex migration of seismicity, accompanying the intrusion of 0.26–0.56 km3 of magma. Strike-slip earthquakes mark the intrusion at depth, while shallower normal faulting the ~20 km long lateral growth of a dike. Seismicity abruptly decreased after a Mw 6.0 earthquake, suggesting the magmatic dike lost pressure with the slipping of a large fault. A seafloor eruption is likely, but not confirmed by sea surface temperature anomalies. The unrest documents episodic magmatic intrusion in the Bransfield Strait, providing unique insights into active continental rifting.

Description: On 12 August 2021, a 〉220 s lasting complex earthquake with Mw 〉 8.2 hit the South Sandwich Trench. Due to its remote location and short interevent times, reported earthquake parameters varied significantly between different international agencies. We studied the complex rupture by combining different seismic source characterization techniques sensitive to different frequency ranges based on teleseismic broadband recordings from 0.001 to 2 Hz, including point and finite fault inversions and the back-projection of high-frequency signals. We also determined moment tensor solutions for 88 aftershocks. The rupture initiated simultaneously with a rupture equivalent to a Mw 7.6 thrust earthquake in the deep part of the seismogenic zone in the central subduction interface and a shallow megathrust rupture, which propagated unilaterally to the south with a very slow rupture velocity of 1.2 km/s and varying strike following the curvature of the trench. The slow rupture covered nearly two-thirds of the entire subduction zone length, and with Mw 8.2 released the bulk of the total moment of the whole earthquake. Tsunami modeling indicates the inferred shallow rupture can explain the tsunami records. The southern segment of the shallow rupture overlaps with another activation of the deeper part of the megathrust equivalent to Mw 7.6. The aftershock distribution confirms the extent and curvature of the rupture. Some mechanisms are consistent with the mainshocks, but many indicate also activation of secondary faults. Rupture velocities and radiated frequencies varied strongly between different stages of the rupture, which might explain the variability of published source parameters.