Seismic Reflection Profiles Research Articles

Measurements of Geologic Characteristics and Geophysical Properties of Sediments From the New England Mud Patch

The characterization of physical, geological, and geophysical properties of sediments within the New England Mud Patch (NEMP) was undertaken to provide a physical basis for acoustic inversions associated with the SeaBed Characterization EXperiment 2017 (SBCEX17). Using a suite of 89 sediment cores (piston/trigger, gravity [acoustic], and vibracore), a comprehensive database of laboratory-based sediment analyses, geophysical core logs, and the results of seismic reflection profiling, we formulate a three-layer lithostratigraphic model of the area within and immediately adjacent to the SBCEX17 focus area, referred to as the seabed experiment area (SEA). The uppermost lithostratigraphic unit, Unit 1, is relatively homogenous clayey- to sandy silt, with consistent downcore textural, mineralogical, and physical property attributes. Unit 2 is a variable-thickness transitional layer between Unit 1 and Unit 3, whose properties reflect a decrease in proximal erosion and transition to a lower energy depositional environment. Unit 3 is clean quartz sand containing abundant shells and shell fragments that was regionally deposited during Holocene sea-level rise. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">210</sup> Pb and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> C radiocarbon geochronologies spanning the past 13 000 years are used to facilitate intercore comparison across the SEA. Analytical results and laboratory methods used in the derivation of those results are described in detail, serving as a reference for ongoing and future investigation of the SEA and entire NEMP. Although the derived lithostratigraphic model of the SEA is in good agreement with past evaluations of the regional sedimentology, comparisons of the lithostratigraphic and seismostratigraphic models highlight several significant incompatibilities that remain to be satisfactorily explained.

Densely Sampled Global Dynamic Topographic Observations and Their Significance

AbstractTopography and bathymetry are principally supported by some combination of crustal and sub‐crustal density variations. However, dynamic topography is generated by vertical deflection of the Earth's surface as a result of mantle convection. Isolating and quantifying observable dynamic topography yields valuable information about mantle processes. Here, we investigate global dynamic topography by calculating residual depth anomalies throughout the oceanic realm and residual topographic anomalies across the continents. We correct for sedimentary and crustal loading by exploiting a variety of seismologic datasets that include seismic reflection profiles, wide‐angle/refraction surveys, and receiver functions. In this way, an extensively revised and augmented global compilation of 10,874 oceanic residual depth measurements and 3,777 continental residual topographic measurements is constructed. In the oceanic realm, the methodology has been revised to improve accuracy and resolution. First, quartz/clay content of the sedimentary column is adjusted to remove minor skewness of residual depth anomalies as a function of plate age. Secondly, variation of bulk density as a function of crustal thickness is taken into account. Our global compilation is used to generate spherical harmonic representations of observable dynamic topography out to degree 40 (i.e., ∼1,000 km). Resultant spectra demonstrate that dynamic topographic power varies linearly with inverse wavenumber. The spectral slope directly reflects the way by which dynamic topography is generated by Stokes' flow. Our global results are consistent with independent and diverse geologic markers of uplift and subsidence together with Neogene‐Quaternary intraplate basalt magmatism.

Cost‐effective in‐mine seismic experiments to image platinum deposits and associated geological structures at Maseve platinum mine, South Africa

ABSTRACTThe detection of mineral deposits and their related geological structures is of great importance to the mining industry as structures (such as dykes and faults) can affect the safety, cost and efficiency of mining. With the goal of testing cost‐effective seismic methods for mineral exploration and mining, active and passive seismic experiments were conducted at Maseve platinum mine in the Bushveld Complex (South Africa) in 2020. The experiments involved surface‐passive (using 5 Hz wireless nodes; single vertical component) and in‐mine active reflection seismic surveys (using 4.5 Hz land streamer and 5 kg sledgehammer) to image geological structures and delineate economic platinum‐group elements bearing Merensky and Upper Group‐2 chromitite layers (known as reefs). This paper presents only the results from the in‐mine active seismic experiments. The in‐mine seismic surveys consisted of seven 2D reflection seismic profiles in the development tunnels, which were located ∼550 m below ground surface and a few tens of metres above known mineralizations: the Upper Group‐2 and Merensky Reef. The data were carefully processed to enhance the reflections and suppress noise generated by mine infrastructure (e.g., equipment and ventilation). We successfully imaged the Merensky Reef and Upper Group‐2 orebodies approximately 55 m and 124 m below the tunnel floor, respectively, and delineated faults and dykes that crosscut them. Furthermore, the seismic data reveal relatively strong amplitude and faulted reflections below the Upper Group‐2 that may represent deeper chromitite‐enriched orebodies. However, the economic value of these horizons can only be confirmed through drilling. The processed seismic data were combined with borehole data, synthetic modelling and geological models to constrain the interpretation. This study encourages the use of in‐mine seismics for future mineral exploration, mine development and planning.

Late Quaternary Evolution of a Submerged Karst Basin Influenced by Active Tectonics (Koločep Bay, Croatia)

Koločep bay is a 30 km elongated karst basin located between the Elafiti Islands and the mainland with a NW–SE general direction. The bay lies within the seismically active southern Dalmatia zone. Irregular grid sub-bottom profiles and two legacy reflection seismic profiles have been used to determine the overall morphology of the bay and to establish the seismic stratigraphy of its sedimentary infill. Three major seismic–stratigraphic units have been identified in the upper ~50 m of the ~120-meter-thick sedimentary sequence that lies above the karstified limestone bedrock. The karst polje basin was flooded due to sea-level rise before 12.1 kyr BP. The morphology of the bay implies complex influences of Late Glacial meltwater discharges, aeolian sand deposition, the existence of paleo–ponor/karst spring zones and buried Pleistocene river channels. The Pleistocene seismotectonic units are deformed in the NW and SE parts of the basin. The central part of the basin has no signatures of intensive tectonic activity during the Holocene. A major erosion event was identified that led to the formation of a basin within the older sedimentary infill. In the southern part of the basin, we have evidence of Holocene tectonic activity with the formation of erosional scarps on the seafloor of the bay.

Complex trans-ridge normal faults controlling large earthquakes

Studying faults capable of releasing moderate-to-strong earthquakes is fundamental for seismic hazard studies, especially in a territory that was subject to the strongest peninsular Italy earthquake (1857, Mw 7.1) and hosting the largest European oil field on-land. Fieldwork-based observations in the Campania-Lucania area highlight a SSW-dipping ~ 65 km-long normal-oblique-segmented fault, showing evidence of recent activity and possibly responsible for the 1857 earthquake. It crosses the Maddalena ridge, linking separate Quaternary basins. Two seismic reflection profiles cross the fault trace where it is buried beneath the Val d’Agri Quaternary deposits. Similarities between fault-controlled small basins in the highest portion of the massifs in the study area and the neighboring 1980 Irpinia area (1980 earthquake, Mw 6.9) are interpreted as evidence of trans-ridge fault activity. Kinematic analyses and the stress field inversion provide a N032-trending near-horizontal s3-axis, the same computed in literature for the Irpinia area, highlighting a deviation from the ~N045-axis which characterizes most of the Apennines. This study demonstrates how detailed fieldwork, supported by geophysics and innovative data analysis techniques, can unravel unknown faults while giving a novel interpretation of the trans-ridge faults' style in controlling strong earthquakes, moving away from classical interpretations, and providing a helpful approach in similar contexts worldwide.

The Next Decade of Seismic Oceanography: Possibilities, Challenges and Solutions

Seismic reflection profiling of thermohaline structure has the potential to transform our understanding of oceanic mixing and circulation. This profiling, which is known as seismic oceanography, yields acoustic images that extend from the sea surface to the sea bed and which span horizontal distances of hundreds of kilometers. Changes in temperature and salinity are detected in two, and sometimes three, dimensions at spatial resolutions of ~O(10) m. Due to its unique combination of extensive coverage and high spatial resolution, seismic oceanography is ideally placed to characterize the processes that sustain oceanic circulation by transferring energy between basin-scale currents and turbulent flow. To date, more than one hundred research papers have exploited seismic oceanographic data to gain insight into phenomena as varied as eddy formation, internal waves, and turbulent mixing. However, despite its promise, seismic oceanography suffers from three practical disadvantages that have slowed its development into a widely accepted tool. First, acquisition of high-quality data is expensive and logistically challenging. Second, it has proven difficult to obtain independent observational constraints that can be used to benchmark seismic oceanographic results. Third, computational workflows have not been standardized and made widely available. In addition to these practical challenges, the field has struggled to identify pressing scientific questions that it can systematically address. It thus remains a curiosity to many oceanographers. We suggest ways in which the practical challenges can be addressed through development of shared resources, and outline how these resources can be used to tackle important problems in physical oceanography. With this collaborative approach, seismic oceanography can become a key member of the next generation of methods for observing the ocean.

Sedimentary characteristics of deep-lacustrine fans: a case study on the lower Jurassic Sangonghe formation of the Turpan Basin, NW China

Ancient deep-lacustrine fans serve as prolific hydrocarbon reservoirs and provide records of climatic and tectonic activity. The deep-lacustrine fans of the Lower Jurassic Sangonghe Formation in Turpan Basin have become the most important exploration targets as stratigraphic traps. An integrated analysis of field profile investigation, core examination, well logging and seismic reflection profile interpretation, and grain size distribution patterns has enabled the identification of a series of deep-lacustrine fans. The deep-lacustrine fans of the Lower Jurassic Sangonghe Formation, deposited in the Turpan Basin consist of sandy debris flow, bottom current reworking and turbidite lithofacies. The deep-lacustrine fans are depicted by the following geological and geophysical features: (1) abundant soft-sediment deformation structures, and dish structures, normal graded beddings and inverse graded bedding structures developed in the strata, (2) the grain size distribution on the C–M diagram is parallel to the C = M baseline and cumulative probability curves display a bi-segment pattern with the suspension components to be the dominant parts, (3) gamma-ray logging curve are inverse-cycle features and seismic reflection profiles are characterized of poor continuity, high amplitudes, scattered and lenticular shapes. The deep-lacustrine fans are favorable target reservoirs, and have become the most important exploration targets in the Turpan Basin.

Basement structure of the Appalachian Basin in Pennsylvania

Basement structure beneath the Pennsylvanian portion of the Appalachian basin, where some of the deepest basement in the basin is found, is investigated by constructing a map of the Precambrian unconformity using 3939 new point estimates of basement depth determined from seismic reflection profiles and P-wave receiver functions, combined with geophysical and well log data used in other published basement maps. The map shows a gradual and uniform deepening of the basement from depths of ~1500 m in the northwest to ~6000 m in the southeast, consistent with a foreland basin plate flexure model. However, basement depths in two regions, when compared to depths predicted by a plate flexure model, show basin over-deepening. One to two km of over-deepening coincides with the Neoproterozoic-early Paleozoic Rome Trough in southwestern Pennsylvania, and the Northumberland Deep Basement Zone (NDBZ) in east-central Pennsylvania shows two to two and a half km of over-deepening. Basin over-deepening in the Rome Trough does not extend into northern Pennsylvania, pointing to a termination of the trough within central Pennsylvania. The origin of basin over-deepening in the NDBZ is less certain but overlaps spatially with the southern portion of the Neoproterozoic Scranton Rift. The preservation of one or more failed Neoproterozoic-early Paleozoic rifts within the basement structure of the Appalachian Basin suggests crystalline basement was not significantly deformed by the Paleozoic orogenic events associated with basin formation. It also supports models placing the locus of Laurentian rifting from Rodinia well to the east of the Appalachian Basin.

Hotspot volcanism along a leaky fracture zone contributes the formation of the 85°E Ridge at 11°N latitude, Bay of Bengal

The 85°E Ridge is a prominent linear structure in the northeastern Indian Ocean. Its nature and origin are still controversial due to the coverage of thick Bengal Fan sediments. Here we present a newly collected 400-km-long 2-D seismic reflection profile which crosses the 85°E Ridge near 11°N and shows the basement morphology and internal structures of the ridge. The ridge has a thickened crust characterized by two basement highs rising up to ~2–3 km above the surrounding oceanic crust and a Moho depression with amplitude of ~3–4 km. Typical volcanic structures formed by hotspot volcanism are identified in the ridge. Deep-rooted faults played important roles in the formation of the 85°E Ridge. They acted as the preferred conduit for the upwelling of hotspot melts, but their syn- and post-volcanic reactivations induced the collapse of volcanic sequences and the relative uplift of the East Ridge. These observations support that the 85°E Ridge near 11°N having formed by hotspot magmatism drained along a leaky fracture zone. Combined with previous results, a new single-hotspot formation model of the 85°E Ridge is proposed, in which the entire 85°E Ridge was formed by a weak and pulsating mantle plume acting on the northward drifting Indian Plate. Its along-strike variations in location, morphology, and structure in different segments are interpreted to reflect different elastic strength of the oceanic lithosphere and the existence of a pre-existing fracture zone which laterally redistributed the shallow melts.

Bidirectional subduction of the Bangong-Nujiang ocean revealed by deep-crustal seismic reflection profile

It is well known that the Tibetan plateau exposes a series of terranes that were drifted northward and accreted progressively onto the south margin of Eurasia resulting from ocean closure. However, the processes associated with the ocean closure remains poorly unconstrained. One such issue is whether the demise of the Bangong-Nujiang ocean between the Lhasa and Qiangtang terranes were caused by northward subduction only or both northward and southward subduction. To address this issue, we analyze high-resolution seismic reflection data along a 275-km north-south traverse across the Bangong-Nujiang suture zone. Our work shows that the Moho of the Lhasa terrane lies at 22.5 s (two-way travel time) and the Moho of the Qiangtang terrane lies at 20 s. In the Lhasa terrane, the reflection profile reveals an X-shaped structure that segmented south-dipping reflectors are truncated by continuous north-dipping reflectors. Based on the existing geological and geophysical observations and the cross-cutting relationships among seismic reflectors, we propose that the fossil relics of south-dipping reflectors with relatively weak amplitude in Lhasa terrane were created by the Mesozoic southward subduction of the Bangong-Nujiang ocean, and the north-dipping reflectors were triggered by the younger Cenozoic deformation during the India-Asia collision. In the Qiangtang terrane, our seismic reflection profile displays an antiformal structure with a set of north-dipping reflections in the lower crust, which we interpret as triggered by the northward subduction of the Bangong-Nujiang ocean. The structures as revealed by our seismic observations as a whole indicate that the closure of the Bangong-Nujiang ocean was created by bidirectional oceanic subduction.

Active construction of southernmost Tibet revealed by deep seismic imaging

Southernmost Tibet exhibits an anomalously twice the normal thickness of average continental crust. There is no available theory to explain and the driving mechanism remains uncertain. Here, we interpret a north-striking, 180 km-long deep seismic reflection profile traversing the southern Lhasa terrane (SLT) to the central Lhasa terrane (CLT). In addition to reflections showing subducting Indian crust, our results reveal lateral heterogeneity between the SLT and CLT, where north-dipping reflections beneath the CLT outline a tilted crystalline basement, while the non-reflective domain beneath the SLT represents homogeneous juvenile crust. Our integrated analysis leads to models calling upon episodic magmatism onto the southern margin of the basement to result in progressive construction of the SLT. We hypothesize that this crustal thickening via crustal-scale magma accretion contributed to surface uplift of the southern margin of the Tibetan plateau and leading to the development of the vast internal drainage system of Tibet.

The Amami Rift: Clarifying the roles of rifting and volcanism in the central Ryukyu Arc

Arc volcanism in the Ryukyu subduction system shows an along-arc variation characterized by the development of subaerial volcanoes in the northern arc and submarine ones in the southern arc. The central arc is a transitional area in arc volcanism, but its characteristics and tectonic controls remain largely unknown. This paper presents detailed new data on the tectonic and volcanic features of the Amami Calderas in the central Ryukyu Arc based on high-resolution bathymetry and a seismic reflection profile. Ship-based bathymetry reveals that ENE–WSW to E–W trending normal faults form a local rift structure which is ~60 km long, ~10 km wide and ~ 400 m deep. Small volcanic edifices and crater-like depressions are scattered in and around the rift. A number of bubble plumes were detected by shipboard multibeam sonar in the rift. AUV near-bottom mapping confirmed positive temperature anomalies near the bubble plumes, indicative of hydrothermal venting. Our observations demonstrate that the previously named Amami Calderas are not volcanic in origin, but are instead tectonic, and we therefore re-name these features the Amami Rift. The Amami Rift is the northernmost active rift along the central Ryukyu Arc and its unique location provides new insights into the interaction of rifting and arc volcanism. Yokoate-jima Island is a large volcanic edifice close to the Amami Rift whose surrounding seafloor shows no distinctive rift structures and few diffuse lineaments. In contrast, the Amami Rift consists of small scattered volcanic edifices and a prominent focused rift. This sharp volcano-tectonic contrast between Yokoate-jima Island and Amami Rift suggests that focused rifting makes arc volcanism scattered rather than focused in the area, with relatively small volcanic edifices. Another notable feature of the Amami Rift and surrounding area is that volcanic edifices, indicators of hydrothermal activity and bubble plumes are aligned not parallel to the rift, but in the cross-arc direction, indicating the presence of cross-arc volcanism. Cross-arc volcanism has been recently reported in the northern Ryukyu Arc, but this is the first time it has been observed in the central arc. Our observations combined with previous studies suggest a combination of cross-arc volcanism and crustal extension characterizes the spatial distribution of active magmatism in the central Ryukyu Arc.

Further evidence for the East Coast fault system and faults associated with the Summerville restraining bend and their possible relationship to the 1886 Charleston, South Carolina, earthquake, USA

The integration of aeromagnetic, LiDAR, and previously acquired seismic-reflection data and surficial geologic maps supports the existence of the East Coast fault system ( and faults associated with its 12⁰ Summerville restraining bend beneath the South Carolina Coastal Plain. Aeromagnetic data revealed a 10- to 15-km-wide zone of subtle, 22- to 35-km-long linear magnetic anomalies trending ~N10°E across the southern meizoseismal area of the 1886 Charleston earthquake that we postulate are associated with Cenozoic low-displacement brittle faults in the crystalline basement west of Charleston. We hypothesize that lineaments ML4 and ML5 represent the principal displacement zone along the southern end of the ECFS because they coincide with steeply dipping, west-side-up buried faults interpreted from previously acquired seismic-reflection profiles and a ~320-m dextral offset in the Brownsville Pleistocene beach ridge deposit. The alignment of the NNE-SSW-oriented Edisto dome, uplift along releveling line 9, gently upwarped longitudinal profiles along the Caw Caw and Horse Savanna swamps, local incision along the Ashley River, and exposures of the early Oligocene Ashley Formation near the incised part of the Ashley River support Quaternary uplift along the southern ECFS. The 12⁰ change in trend formed by lineaments ML4 and ML5 supports the existence of the Summerville restraining bend in the ECFS, east of which are numerous ENE-WSW- to NW-SE-oriented LiDAR lineaments that we postulate are surface expressions of faults that formed to compensate for the increased compression produced by dextral motion along the bend. Sinistral displacement along one of these proposed faults associated with the ~40-km-long, east-west-oriented Deer Park lineament may have produced the main shock of the 1886 Charleston earthquake.&#x0D; &#x0D;

Comprehensive Geophysical Measurement in Seismic Safety Evaluation: A Case Study of East Lake High-Tech Development Zone, China

Wuhan East Lake High-tech Development Zone is a National Innovation Demonstration Zone where several major projects are under construction. For the seismic safety evaluation of the zone, it is urgent to find out basic data such as stratigraphic structure, geometric characteristics of buried faults, and overburden thickness. In this study, seismic reflection and microtremor surveys are adopted in seismic safety evaluation in urban areas for the first time. Combined with borehole data, the overburden thickness and buried faults in the target area are detected and analyzed. The results are as follows: ① The seismic reflection detection is carried out for the possible hidden locations of the Xiangfan-Guangji Fault and Macheng-Tuanfeng Fault in the target area, and the obtained high-resolution seismic reflection profiles can be used to precisely divide the stratigraphic structure and preliminarily find out the geometric characteristics such as the strike, tendency, and the buried depth of the upper breakpoint of the buried faults. ② A microtremor survey is carried out in key regions of the target area, and the obtained velocity structure imaging can provide a perspective window into the urban underground space structure and accurately determine overburden thickness and the depth of strongly and moderately weathered bedrock. Comprehensive geophysical measurements can better realize the acquisition of basic data such as the stratigraphic structure of complex regions and characteristics of urban buried faults, which provides an important basis for seismic fortification. At the same time, the combined application of geophysical methods avoids the detection of the blind area and provides a guarantee for urban safety and sustainable development. Furthermore, the study has reference value for seismic safety evaluation in similar areas of the world. If comprehensive geophysical measurement can be popularized, its economic and scientific value will be immeasurable.

Geological interpretation of a seismic reflection profile in the eastern part of the Bohemian Cretaceous Basin

Geological interpretation of a seismic reflection profile in the eastern part of the Bohemian Cretaceous Basin

Revisit the Sedimentary Stratigraphic Evolution and Environmental Changes on the Outer Shelf of the East China Sea Since MIS 5

The East China Sea (ECS) is featured by the broad continental shelf, huge terrigenous sediment input and striking river-sea interaction during the Quaternary. Despite the numerous studies, mostly on the delta, coastal and inner shelf areas, the late Quaternary stratigraphy and sedimentary environment on the mid-outer ECS shelf were poorly documented. In particular, it is still controversial in issues such as the recognition of marine transgression deposits in the interglacial period and the occurrence and ages of deposits in the glacial period. This study presents new data of high-resolution seismic reflection profiles and collective evidences from lithology, microfossil assemblages and geochronology of borehole SFK-1 on the ECS outer shelf, aiming to revisit the sedimentary stratigraphic evolution and environmental changes since the Marine Isotopic Stage 5 (MIS 5). Sixty-one ages were obtained by radiocarbon and Optical Stimulation Luminescence (OSL), providing robust constraints on the chronostratigraphy. Three marine regression/transgression cycles were identified, which primarily constrained the sedimentary environmental evolution over the past 100 kyrs. Although the strong river-sea interaction accompanied with sea level fluctuations characterized the late Quaternary depositional environment on the shelf, this study clearly identified the stratigraphic boundaries between MIS 1 and MIS 5, and confirmed the tide-influenced fluvial deposition formed during the Last Glacial Maximum on the outer shelf. We infer that tidal forcing significantly superimposed its influence on sea level change and thus formed the most striking feature on the late Quaternary stratigraphy of the open ECS shelf. With the combination of a large number of absolute depositional ages with high-resolution core records, the major findings of this study may allow better understanding of the Quaternary sedimentary evolution and river-sea interaction on open shelves dominated by siliciclastic sedimentation.

Shallow Faulting and Folding in the Epicentral Area of the 1886 Charleston, South Carolina, Earthquake

ABSTRACT The moment magnitude (Mw) ∼7 earthquake that struck Charleston, South Carolina, on 31 August 1886 is the largest historical earthquake in the United States east of the Appalachian Mountains. The fault(s) that ruptured during this earthquake has never been conclusively identified, and conflicting fault models have been proposed. Here we interpret reprocessed seismic reflection profiles, reprocessed legacy aeromagnetic data, and newly collected ground penetrating radar (GPR) profiles to delineate faults deforming the Cretaceous and younger Atlantic Coastal Plain (ACP) strata in the epicentral area of the 1886 earthquake. The data show evidence for faults folding or vertically displacing ACP strata, including apparent displacements of near-surface strata (upper ∼20 m). Aeromagnetic data show several northeast (NE)-trending lineaments, two of which correlate with faults and folds with vertical displacements as great as 55 m on the seismic reflection and radar profiles. ACP strata show only minor thickness changes across these structures, indicating that much of the displacement postdates the shallowest well-imaged ACP strata of Eocene age. Faults imaged on the seismic reflection profiles appear on GPR profiles to displace the erosional surface at the top of the upper Eocene to Oligocene Cooper Group, including where railroad tracks were bent during the 1886 earthquake. Some faults coincide with changes in river trends, bifurcations of river channels, and unusual river meanders that could be related to recent fault motion. In contrast to our interpreted NE fault trends, earthquake locations and some focal mechanisms in the modern seismic zone have been interpreted as defining a nearly north-striking, west-dipping zone of aftershocks from the 1886 earthquake. The relationship between the modern seismicity and the faults we image is therefore enigmatic. However, multiple faults in the area clearly have been active since the Eocene and deform strata in the upper 20 m, providing potential targets for field-based geologic investigations.

Fault intersections control short period intraplate start-stop seismicity in the Korean Peninsula

After the devastating Tohoku-Oki earthquake (Mw 9.0, 2011) in Japan, the Korean Peninsula has experienced a higher number of large, plus Mw 5.0, earthquakes than recorded in the preceding half century of modern monitoring. In addition, seismicity has dramatically increased along with seismic waves arriving later than prior to the 2011 Tohoku-Oki earthquake, suggesting that the Korean crust has notably been perturbed. South Korea is densely populated, hence knowledge about active faults and earthquake mechanisms is of great relevance for public safety and risk mitigations. Quaternary faults, including the Chugaryeong crustal-scale fault, run through the Seoul metropolitan area and recent seismicity studies suggest that these faults are active. Based on two reflection seismic profiles, we provide compelling evidence that the depth clustered seismicity along the Chugaryeong fault is associated with the intersections of other fault systems. The two seismicity clusters, observed at two depth intervals of approximately 4.5–5 and 8–9 km, can be linked with two moderately-to-steeply-dipping bands of reflectivity interpreted to be splay faults and terminating at the Chugaryeong sub-vertical fault. We suggest that stress builds up at these fault intersections and is then released via strike-slip ruptures along the Chugaryeong fault. Time-clustered seismic events at the fault intersections support this hypothesis, indicating a start-stop mechanism is controlling the seismicity in the region at least based on nearly one decade of seismicity observations. The start-stop seismicity behaviour can possibly be used for forecasting earthquakes and their switching depth along the Chugaryeong fault.

Crustal configuration of the Naama and El Bayadh region of northwest Algeria: Inferences from gravity and magnetic analysis

Northwestern Algeria is located where the Saharan Atlas borders the Saharan Platform along the South Atlas Front. The origin of the South Atlas Front is controversial, thus a detailed gravity and magnetic analysis constrained by seismic reflection profiles and well data were conducted to determine the structural configuration of this region. A residual gravity anomaly map created using upward continuation and a reduced to the pole magnetic anomaly map indicated a series of northwest-trending maxima anomalies parallel to the Atlassic orogeny folds and faults and east-trending maxima within the Benoud foreland basin. These maxima mostly coincide with Paleozoic basement uplifts based on seismic reflection profiles. Depth analyses based on upward continuation, and two-dimensional forward modeling of the gravity and magnetic data indicated that the source of the maxima are mainly 5 km in depth with the magnetic sources being approximately 0.5 km deeper than the gravity sources. The gravity and magnetic models indicate that the source bodies are steep-sided and coincide with interpreted faults from the seismic reflection profiles. The maxima anomalies are mainly caused by basement uplifts instead of variations in density and/or magnetic susceptibility in the Paleozoic or older basement lithologies. The South Atlas Front and the Saharan Atlas on the central and eastern portions of Algeria is governed by a thin-skinned tectonics style involving the Mesozoic-Cenozoic sedimentary cover. While the western part was probably controlled by a mixed thin- and thick-skinned tectonic style with the reactivation of deeply rooted Triassic and Jurassic faults during Tertiary compressive events in the interpreted Paleozoic basement.Our gravity and magnetic models illustrate a crustal architecture model of the Paleozoic basement which is consistent with a north-dipping basement normal fault between less deformed Saharan Platform than the more deformed Atlassic domain. Such a crustal model aids in determining the nature of the Atlassic orogeny as geological and geophysical studies have been determining the variations of structural styles along its entire length.

Back-barrier evolution and along-strike variations in infilling of the Kosi Bay lake system, South Africa

This study examines both the inlet dynamics, recorded in the back-barrier stratigraphy of Kosi Bay, and the competing effects of waterbody segmentation on along-strike changes in the character and nature of an incised valley fill at Kosi Bay, South Africa. The Kosi Bay system comprises four interconnected lakes (Lake Makhawulani, Lake Mpungwini, Lake Nhlange and Lake Amanzimnyama) subject to varying degrees of marine influence. Formerly, the system's only connection to the ocean was through the Bhanga Nek palaeo-inlet. The location of this system behind a continuous coastal barrier predisposes it to excellent archives of environmental change related to changing sea-level and sediment supply. In this context, two cores were extracted from the system (KB2 and KB4), together with over 150 km of seismic reflection profiling, as well as AMS radiocarbon analyses and legacy data in order to present a new stratigraphic evolution model for this back-barrier system. A total of 5 seismic units were imaged (Units A-E). A major unconformity surface (SB), characterised by broad u-shaped incised valleys, incises the underlying unconsolidated sandy acoustic basement (Unit A). Unit B forms as thickly developed central basin deposits, partially infilling the incised valleys. Two forms of prograding spits are evident -Unit C1 progrades from the palaeo-highs of the valley interfluves into the nearest available accommodation space and Unit C2 progrades from the margins of the system into the basin. These are separated by an erosional surface (T-RS) formed by migrating tidal channels as sea-level rose and marine waters entered the system during the Holocene. Unit C2 spits are in turn associated with slump deposits formed on the steepest part of the margins (Unit D) that were dated to 2900 ± 165 cal. BP. The unusual depth of Surface T-RS is attributed to (1) limited sediment supply during sea-level rise, (2) the presence of easily erodible fine sediments in which this surface erodes and (3) a larger tidal prism when the Bhanga Nek inlet was still open and system segmentation by Units C2 and D had not yet occurred. Unit E caps the stratigraphy and is interpreted as lacustrine fines with a thin layer of acoustically transparent material (gyttja) on top. The deposition of Unit E signifies the closure of the system to the ocean and the overall shift from an estuarine/lagoon environment to a lacustrine one. Recalibrated AMS dates place the system closure to between 3160 and 2900 cal. BP, coeval with the formation of the prograding marginal spits and waterbody segmentation (Unit C2). Along-strike variation in the timing of the basin infilling, attributed to changes to the inlet functioning coupled to the segmentation of the waterbody, is characteristic of this area. At 1450 cal. BP, a new tidal inlet formed to the north of Lake Makhawulani, where impounded waters were diverted via a low point in the back-barrier coastal dune cordon. The formation of this inlet suggests a prolonged period of back-barrier flooding until a low point in the barrier was breached during the Holocene sea-level highstand of 1.5 m. Such inlet development and along-strike back flooding of incised valleys is an unusual attribute of coastal waterbodies of South Africa.