High-resolution Seismic Reflection Data Research Articles

How was the Carboniferous Balkhash–West Junggar remnant ocean filled and closed? Insights from the Well Tacan-1 strata in the Tacheng Basin, NW China

The Balkhash–West Junggar remnant ocean is a conspicuous tectonic feature that had an important effect on the Paleozoic tectonic evolution of the Central Asian Orogenic Belt. How the Carboniferous Balkhash–West Junggar remnant ocean was filled and closed is poorly understood. In this study, we focus on the integrated tectonic–sedimentary evolution of the Tacheng Basin, which is located in the eastern part of the Balkhash–West Junggar remnant ocean, using high-resolution seismic reflection, well-line, geochronological and geochemical data to establish the filling of the basin by sediments and its evolution. The Carboniferous Balkhash–West Junggar remnant ocean basin was filled with Early Carboniferous deep marine mudstone and interbedded sandstone with minor andesite, basalt and tuff, and Late Carboniferous basalt, andesite, breccia and tuff and shallow marine mudstone and sandstone. The Carboniferous igneous assemblage mainly consists of basalt with high Nb content and (Nb/La)PM values and concomitant magnesian andesites with adakite geochemical characteristics of high Sr and low Y and Yb, suggesting a subduction-related tectonic setting. Zircons in the tuff that underlies the basalts and magnesian andesites were dated to ca. 315Ma by LA–ICP–MS U–Pb. The identified unconformity between the Carboniferous and Lower Permian from seismic reflection profile in Well Tacan-1 subdivided the Carboniferous–Early Permian strata into two tectonostratigraphic units. The Lower Permian sequences overlap the Carboniferous strata and the facies transition from the submarine Carboniferous to the terrigenous Lower Permian indicates that the Balkhash–West Junggar remnant ocean closed during the Early Permian. These new data suggest that the sedimentary filling of the basin was in response to subduction and accretionary processes and that oceanic subduction was an important mechanism that led to the shrinking, filling and closure of the Balkhash–West Junggar remnant ocean because of compression. Therefore, lateral crust growth through subduction–accretionary processes still played an important role in the Carboniferous construction of western Junggar.

On the prediction of settlement from high-resolution shear-wave reflection seismic data: The Trondheim harbour case study, mid Norway

This paper presents a novel approach to predict long-term settlement in the Trondheim harbour, mid Norway, from high-resolution shear-wave seismic reflection data, complemented with cone penetration test results (CPTU) and index soil properties from nearby boreholes in this reclaimed coastal area. The data enables sub-surface geo-characterisation in high-resolution, down to the basement at a depth of about 175m. Correlations between shear-wave velocity and Janbu's classical 1D consolidation parameters (m, M0, M1, σ′c and OCR) are used for settlement analysis. Both the magnitude and rate of time-settlement predictions compare well with the measured settlements from GPS levelling and PSInSAR data acquired since the late 1980s and the early 1990s in the area, respectively. The shear-wave reflection data further provide important input to understand the spatial patterns of land-subsidence in the Trondheim harbour. The highest settlement rates occur in areas where depth to bedrock is greatest and where thicker deposits with low stiffness are found. In summary, the data and workflow presented here are useful tools for predicting settlements based on correlations with shear-wave velocity measurements.

Origin of step-like and lobate seafloor features along the continental shelf off Rio de Janeiro State, Santos basin-Brazil

A combined analysis of seismic and morphological features identified in a set of high-resolution seismic reflection and bathymetric data, shows a systematic relationship between major modern seafloor morphological traces and the basinward migration of Late Pleistocene coastlines along the continental shelf of the Santos basin (Rio de Janeiro State, SE Brazil). Observed fairly continuous and sinuous mid-outer shelf escarpments are related to the sea-level variations and shelf exposure during the Last Glacial cycle. A bathymetric step at −110m is an erosional remnant of offlapping detached forced-regressive wedges that spread over 50km in the shelf-dip direction, probably developed during periods of falling sea level between MIS 3 and 2. A second major escarpment at −130m was interpreted as the shoreline during the LGM, at the time of most extensive subaerial exposure of the continental shelf. However, a distal escarpment at −150m is expressed as a straight contour feature along the two main shelf-edge embayments that characterize the shelf break. This escarpment is coupled with a basal seaward-inclined and highly eroded ramp, and was interpreted as the erosional action of bottom currents during the last transgression due to the displacement of the southward flowing Brazil Current towards the present-day outer shelf. Previously published articles have regarded the morphological features observed on the modern shelf as indicators of stillstands during the post-LGM transgression. We conclude that, on the contrary, most of these features are actually from earlier parts of the Late Pleistocene and were formed in a regressive scenario under oscillating and relative slow sea-level fall.

A revised Early Miocene age for the instigation of the Eirik Drift, offshore southern Greenland: Evidence from high-resolution seismic reflection data

The Eirik Drift lies on the continental slope south of Greenland, where it has been formed under the influence of Northern Component Water (NCW). NCW flow is an essential part of the global Thermohaline Circulation (THC), which is closely connected to the world's climate. Changes in pathways and intensity of NCW flow bear information about modifications of the North Atlantic THC in a changing climate. There is some disagreement about when deep-current controlled sedimentation at the Eirik Drift started. While the onset of drift building was previously dated as early Pliocene or late Miocene in age we suggest that the effect of large-scale current deposition had been initiated by at least 19–17 Ma based on the seismostratigraphic analysis of sedimentary structures identified in a set of high-resolution seismic reflection data. This assumption of an early Miocene onset of NCW flow is supported by regional evidence regarding the breaching of the Greenland–Scotland Ridge, which is documented in several erosional unconformities within the North Atlantic. After the onset of deep-current controlled sedimentation at the Eirik Drift, two major changes in the deep-current system are revealed during the Miocene: At the mid- to late Miocene boundary (12–10 Ma) and at 7.5 Ma.

Spatial distribution of gas hydrates from high-resolution seismic and core data, Woolsey Mound, Northern Gulf of Mexico

Integration of new jumbo piston cores with high-resolution seismic reflection data provides the first documentation on the subsurface distribution of gas hydrates at a deep-water cold seep. The study area, Woolsey Mound is a carbonate–hydrate mound of thermogenic origin, in the northern Gulf of Mexico where salt tectonics dominate the regional structure. Two typologies of unconventional seismo-acoustic data were used in this study: 1) surface-source deep-receiver (SSDR) data and 2) AUV-borne chirp sub-bottom profiler data. Correlation of the coring results with seismic interpretations supports the hypothesis that distinctive seismic brightening (high frequency scattering) present in the SSDR records may indicate the presence of solid hydrates. This suggests that such unconventional seismic survey may be prospective for mapping gas hydrates in complex deep-water settings. Coring results revealed the nature of the shallow gas hydrate system to be dominated by fine-grained sediments. Gas hydrates were found exclusively in fracture porosity in the vicinity of a major active fault. We present a model for Woolsey Mound where shallow gas hydrates are systematically distributed along segments of faults intersected by transit of thermogenic hydrocarbons. This dual nature of the faults being both gas hydrate reservoirs and gas migration routes suggests a very dynamic hydrate stability field for this site.

High-resolution reflection seismic investigations of quick-clay and associated formations at a landslide scar in southwest Sweden

We present high-resolution reflection seismic data from four lines (total 1.9 km) that cross a quick-clay landslide scar located close to the shore of the Göta River in southwest Sweden, and compare the results with geotechnical data from boreholes. The seismic data allow the imaging of bedrock topography and normally to weakly consolidated sediments to a subsurface depth of about 100 m. Different types of seismic sources, including sledgehammer, accelerated weight-drop and dynamite were utilized and compared with each other. Analysis of their power spectra suggests that weight-drop and dynamite have higher frequency content and energy than the sledgehammer, which makes these two sources suitable also for waveform tomography and surface-wave data analysis. The shallowest non-bedrock reflector is observed at about 10–20 m below the surface, it overlays the bedrock, and is interpreted to originate from the contact between clay formations above and a coarse-grained layer below. The coarse-grained layer appears to be spatially linked to the presence of quick-clays. It is a regional scale formation, laterally heterogeneous, which deepens to the west of the study area and correlates well with the available geotechnical data. Continuity of the coarse-grained layer becomes obscured by the landslide scar. There may be a link between the coarse-grained layer and landslides in the study area, although this possibility requires further hydrogeological and geotechnical investigations. Reflectors from the top of the bedrock suggest a depression zone with its deepest point below the landslide scar and a bowl-shaped structure in the northern portion of one of the seismic lines.

Uplift of the West Kunlun Range, northern Tibetan Plateau, dominated by brittle thickening of the upper crust

High-resolution seismic reflection data from the West Kunlun Range front show that crustal structures beneath the northwestern margin of the Tibetan Plateau are dominated by nappes of upper crustal rocks. Geological cross-section balancing suggests an average of 24.6–54 km (38%–52%) of upper crustal shortening. This horizontal shortening by brittle folding and faulting in the upper crust correlates positively with crustal thickening, an increase in Moho depth by 5–7 km, and the topography. Our work suggests that upper crustal shortening is a chief factor for topographic uplift and crustal thickening at the northwestern margin of the Tibetan Plateau. Such a mechanism is different from that proposed for the plateau south of the Karakax strike-slip fault, where crustal flow may have played a prominent role in plateau uplift.

Seismic structure and tectonics of the southern Arauco Basin, south-central Chile (~ 38°S)

The Arauco Basin is located on the continental shelf near the Chilean subduction zone (~38°S). Main deformational stages that affected this setting show a good correlation with Andean constructional phases. We studied the kinematic evolution of the Arauco Basin using high-resolution seismic reflection data across the southern Arauco forearc basin. Three structural domains are identified: (1) Inversion, (2) Extension and (3) Accretion. In addition, seven syn-kinematic sequences have been recognized overlying the Late Carboniferous to Triassic basement, and their relationships with four traditional tectonostratigraphic sequences of the Arauco Basin: (S1) Late Cretaceous syn-extension, (S2) Eocene syn-extension, (S3) and (S4) Eocene syn-inversion, (S5–S6) Miocene syn-inversion with middle extensional structures, and (S7) Pliocene-Quaternary post-inversion and syn-compression. Shortening began in the southern Arauco Basin coeval with a major readjustment of the plate convergence rate (~34Ma) that is represented by inversion structures and kinematic syn-inversion sequences. A marked erosional unconformity (34–23Ma) represents a subsequent event of erosion/uplift during the Oligocene. A contractional Miocene deformation phase reinforced the inversion tectonic and generated Miocene extensional structures. Since the Pliocene the rapid exhumation of the Nahuelbuta Range allows the emergence of the Arauco Basin. Within the basin, Pliocene and Quaternary sequences were affected by contractional structures including the inversion of the Miocene extensional faults. The contraction along the Arauco Basin can be related to the shift to accretionary conditions of the margin and the oblique collision of the Mocha Fracture Zone (~3.6Ma) at ~38°S. The mechanics of regional subsidence which affected the basin during the Miocene cannot be attributed to major scale extensional structures (>1.0km).

Very high-resolution seismic stratigraphy of Pleistocene eustatic minima markers as a tool to reconstruct the tectonic evolution of the northern Latium shelf (Tyrrhenian Sea, Italy)

This paper analyzes depth variations of seismically detected lowstand features (i.e., paleo–shelf break and lowstand submerged depositional terraces [LSDTs]), in order to define vertical movements along a continental margin. Narrowly spaced, high-resolution two-dimensional reflection seismic data were used, collected along a segment of the continental shelf of Latium (eastern Tyrrhenian Sea, Italy). Seismostratigraphic analysis allowed us to identify the six most-recent fourth-order depositional sequences formed in the past ∼500 k.y. They have a different degree of preservation in the outer and middle shelf, with almost no continuation in the inner shelf. Some wedge-shaped bodies interpreted as LSDTs can be identified in these units. The depth of paleo–shelf break and LSDTs of marine isotope stage 12 (ca. 435 ka) was measured, and gradually varies along the margin between ∼220 m and ∼131 m below current sea level. Differential subsidence rates have been estimated for the area, varying from 0 to −0.21 mm/a. The spatial variations agree with studies of uplifted coastal terraces on land, controlled by localized volcanic activity along the Latium section of the Tyrrhenian coast. The comparison of deformation onshore and offshore has proven to be a promising approach for understanding the evolution of uplifting continental margins.

Holocene subaqueous paleoseismology of Lake Tahoe

Gravity-flow deposits recovered in a suite of sediment cores in Lake Tahoe were examined to determine if the event deposits were triggered by strong shaking from earthquakes on active faults within and in close proximity to the Lake Tahoe Basin. The acoustic character and distribution of individual lacustrine deposits as well as potential source regions were constrained by high-resolution seismic Chirp reflection and multibeam bathymetric data. Between 14 and 17 Holocene event deposits have been identified in Lake Tahoe, and examination of their source areas suggests they originated from different initiation points along the steep margin, with some being synchronous around the basin, as opposed to flood-related deposits. Lithologic characteristics, magnetic susceptibility, carbon and nitrogen isotopic signatures, opal content, and 14 C dating indicate that these event deposits are reworked lacustrine material. Radiocarbon dates indicate that the emplacement of these event deposit sediments correlates well with the late Holocene paleoseismic earthquake record developed for the Tahoe Basin. When taken alone, the causality of these events may appear ambiguous, but when the evidence is examined comprehensively, it suggests that strong shaking may likely have been the primary trigger for many of the event deposits observed in the lake throughout the Holocene. For example, four event deposits are assigned to Tahoe Basin faults. The most recent earthquakes occurred on the Incline Village fault (between 630 and 120 cal. yr B.P.); the southern segment of the West Tahoe fault (between 4510 and 4070 cal. yr B.P.); on the central and northern segments of the West Tahoe fault (5600–5330 cal. yr B.P.); and on the West Tahoe fault (between 7890 and 7190 cal. yr B.P.). The oldest of the four associated Tahoe Basin events coincides with the beginning of an extended period when Lake Tahoe was likely not spilling or spilling intermittently, and this suggests that active faulting and footwall uplift cut off the outlet at this time, exaggerating drought conditions downstream. Likewise, the event between 5600 and 5330 cal. yr B.P. on the West Tahoe fault may have exaggerated a smaller drought reflected downstream in Pyramid Lake. This event may also be the most recent event (MRE) on the largest segment of the West Tahoe fault. If correct, the period since the last rupture is approximately twice the estimated average recurrence interval for the Rubicon segment of the West Tahoe fault. A more complete Holocene record of strong shaking greatly extends the paleoseismic record in the region and indicates a combined recurrence interval of between 750 and 800 yr for all faults in the region.

Seismic imaging of the Alpine Fault near Inchbonnie, New Zealand

[1] The transpressive Alpine Fault is the boundary between the Pacific and Australian plates across the South Island of New Zealand. Earthquakes on the Alpine Fault and related structures pose a serious risk to many urban centers, including the city of Christchurch. Although it is a major feature on satellite images, the Alpine Fault is a difficult target for surface studies along much of its length; it mostly traverses densely forested and mountainous terrain and where it occurs in the lowlands it is usually covered by recent sediments. To investigate the Alpine Fault at a rare accessible location (Inchbonnie), we have acquired high-resolution seismic reflection data along five 380–1200 m long lines. Images produced from these data reveal a glacially overdeepened valley containing a thick sequence of diverse glacigenic sediments that have been disrupted by three en echelon strands of the principal Alpine Fault and several secondary fault strands. Based on their seismic facies, the sedimentary sequence is interpreted to comprise basal lacustrine beds overlain successively by alluvial-colluvial deposits that possibly include the remnants of large landslides, deltaic-fan units, and braided river gravels. Whereas the principal Alpine Fault strands disrupt the entire post-glacial sedimentary section and likely offset basement at depths up to 400 m, most of the secondary faults either merge with the principal fault strands at shallow depths or are surficial features limited to the sedimentary section.

Paleozoic fault systems of the Tazhong Uplift, Tarim Basin, China

This report presents interpretations developed from a detailed study of new three-dimensional (3-D) high-resolution reflection seismic data in a portion of Tarim Basin. The tectonic history began with oceanic spreading during the Cambrian–Early Ordovician and continues beyond the Silurian–Devonian time of oceanic closure. Paleozoic faults of the Tazhong Uplift in the hinterland of Tarim Basin are capped by thick and undisturbed Meso-Cenozoic sedimentary sequences. Four Paleozoic fault systems have been recognized: (1) the Cambrian–Early Ordovician extensional faulting, (2) the Late Ordovician NWW trending thrust faulting, (3) the Silurian–Devonian NNE strike-slip faulting and (4) the Permian plutonism influenced by pre-existing fault planes. Zones of weakness created during Cambrian–Early Ordovician extensional faulting influenced subsequent tectonic movements. The Late Ordovician fault system divides the Tazhong Uplift into several deformation zones. Their mechanical characteristics vary across the study area, with stronger thrusting in the east. The Silurian–Devonian strike-slip fault system consists of three components: main faults, subordinate en echelon faults, and fault troughs. The main faults appear as steeply dipping, almost vertical, offsets on seismic map view, with associated flower structures on seismic profiles, together with other levels of faults. The Permian magmatic plugs have a spotty or bandy distribution, and are interpreted to have utilized former faults.

Morphologic and seismic features of the Gulf of Gökova, SW Anatolia: evidence of strike-slip faulting with compression in the Aegean extensional regime

Recently acquired (2005) multi-beam bathymetric and high-resolution seismic reflection data from the E–W-oriented Gulf of Gokova off SW Anatolia were evaluated in order to assess the uneven seafloor morphology and its evolution in terms of present-day active regional tectonics. Stratigraphically, the three identified seismic units, i.e., the basement, deltaic sediments deposited during Quaternary glacial periods, and modern gulf deposits, are consistent with those observed in previous studies. Structurally, the folds and faults with strike-slip and reverse components have been regionally mapped for the first time. Of these, NE–SW-oriented left-lateral strike-slip faults with compressional components forming the so-called Gokova Fault Zone intersect and displace two WNW–ESE-oriented submarine ridges and deep submarine plains. Thus, strike-slip faults are the youngest major structures in the gulf, and control present-day active tectonism. E–W-oriented folds on the inner and outer shelf, which are generally accompanied by reverse faults, delimit the margins of these submarine ridges, and deform the young basin deposits. These features also reveal the concomitant existence of a compressional tectonic regime. The compressional structures probably represent pressure ridges along left-lateral strike-slip fault segments. However, some E–W-oriented normal faults occur on the northwestern and partly also southern shelf, and along the borders of the adjacent deep submarine plains. They are intersected and displaced by the strike-slip faults. The lower seismicity along the normal faults relative to the NE–SW-oriented strike-slip faults suggests that the former are at present inactive or at least less active.

Delineating ophiolite-derived host rocks of massive sulfide Cu-Co-Zn deposits with 2D high-resolution seismic reflection data in Outokumpu, Finland

Seismic reflection data was applied to a study of the upper crustal structures in the Outokumpu mining and exploration area in eastern Finland. The Cu-Co-Zn sulfide ore deposits of the Outokumpu area are hosted by Palaeoproterozoic ophiolite-derived altered ultrabasic rocks (serpentinite, skarn rock, and quartz rock) and black schist within turbiditic mica schist. Mining in the Outokumpu area has produced a total of 36 Mt of ore from three historical and one active mine. Seismic data comprises 2D vibroseis data surveyed along a network of local roads. The seismic sections provide a comprehensive 3D view of the reflective structures. Acoustic rock properties from downhole logging and synthetic seismograms indicate that the strongly reflective packages shown in the seismic data can be identified as the host-rock environments of the deposits. Reflectors show excellent continuity along the structural grain of the ore belt, which allows correlating reflectors with surface geology, magnetic map, and drilling sections into a broad 3D model of the ore belt. Massive ores have acoustic properties that make them directly detectable with seismic reflection methods assuming the deposit size is sufficient for applied seismic wavelengths. The seismic data revealed numerous interesting high-amplitude reflectors within the interpreted host-rock suites potentially coinciding with sulfides.

Western limits of the Seattle fault zone and its interaction with the Olympic Peninsula, Washington

We present evidence that the Seattle fault zone of Washington State extends to the west edge of the Puget Lowland and is kinematically linked to active faults that border the Olympic Massif, including the Saddle Mountain deformation zone. Newly acquired high-resolution seismic reflection and marine magnetic data suggest that the Seattle fault zone extends west beyond the Seattle Basin to form a >100-km-long active fault zone. We provide evidence for a strain transfer zone, expressed as a broad set of faults and folds connecting the Seattle and Saddle Mountain deformation zones near Hood Canal. This connection provides an explanation for the apparent synchroneity of M7 earthquakes on the two fault systems ∼1100 yr ago. We redefine the boundary of the Tacoma Basin to include the previously termed Dewatto basin and show that the Tacoma fault, the southern part of which is a backthrust of the Seattle fault zone, links with a previously unidentified fault along the western margin of the Seattle uplift. We model this north-south fault, termed the Dewatto fault, along the western margin of the Seattle uplift as a low-angle thrust that initiated with exhumation of the Olympic Massif and today accommodates north-directed motion. The Tacoma and Dewatto faults likely control both the southern and western boundaries of the Seattle uplift. The inferred strain transfer zone linking the Seattle fault zone and Saddle Mountain deformation zone defines the northern margin of the Tacoma Basin, and the Saddle Mountain deformation zone forms the northwestern boundary of the Tacoma Basin. Our observations and model suggest that the western portions of the Seattle fault zone and Tacoma fault are complex, require temporal variations in principal strain directions, and cannot be modeled as a simple thrust and/or backthrust system.

On the application of Particle Swarm Optimization strategies on Scholte-wave inversion

We investigate different aspects concerning the application of swarm intelligence optimization to the inversion of Scholte-wave phase-slowness frequency (p—f) spectra with respect to shear wave velocity structure. Besides human influence due to the dependence on a priori information for starting models and interpretation of p—f spectra as well as noise, the model resolution of the inversion problem is strongly influenced by the multimodality of the misfit function. We thus tested the efficiency of global, stochastic optimization approaches with focus on swarm intelligence methods that can explore the multiple minima of the misfit function. A comparison among different PSO schemes by applying them to synthetic Scholte-wave spectra led to a hybrid of Particle Swarm Optimization and Downhill Simplex providing the best resolution of inverted shear wave velocity depth models. The results showed a very low spread of best fitting solutions of 7 per cent in shear wave velocity and an average of 9 per cent for noisy synthetic data and a very good fit to the true synthetic model used for computation of the input data. To classify this method we also compared the probability of finding a good fit in synthetic spectra inversion with that of Evolutionary Algorithm, Simulated Annealing, Neighbourhood Algorithm and Artificial Bee Colony algorithm. Again the hybrid optimization scheme showed its predominance. The usage of stochastic algorithms furthermore allowed a new way of misfit definition in terms of dispersion curve slowness residuals making the inversion scheme independent on Scholte-wave mode identification and allowing joint inversion of fundamental mode and higher mode information. Finally we used the hybrid optimization scheme and the misfit calculation for the inversion of 2-D shear wave velocity profiles from two locations in the North- and Baltic Sea. The models show acceptable resolution and a very good structural correlation to high resolution reflection seismic data.

Seismic results as a-priori knowledge for airborne TEM data inversion — A case study

Abstract We present a case study where a-priori knowledge from high resolution reflection seismic data is used to improve airborne time-domain electromagnetic data inversion. The a-priori knowledge constrains layer boundaries and lateral resistivity change during the inversion process. The object for the case study is the North Sea island Fohr that was affected by several glaciations and inherits a complex Quaternary and Tertiary geology. The freshwater lens and the local geology were investigated by several geophysical surveys. A SkyTEM survey with following laterally constrained inversion provides the resistivity distribution of the island and several 2-D high resolution seismic reflection surveys give structural information. Vertical seismic profiles provide reliable velocity information for time-to-depth-conversion. Seismic horizons and a combined interpretation yield to a-priori constraints for the SkyTEM inversion. We demonstrate how a-priori constraints enhance the inversion to a reliable direction. From the 3-D data cube of 1-D inversion models we extract an example and reinterpret the geophysical results. With the new inversion it is possible to image a till layer with subjacent sand in a larger range than before.

Frontal emplacement and mobility of sublacustrine landslides: Results from morphometric and seismostratigraphic analysis

The morphometric characteristics of 96 sublacustrine landslide complexes were measured on dense grids of high-resolution seismic reflection data from several lakes worldwide and were statistically analyzed. This analysis reveals that the morphology of the sublacustrine slopes exerts a strong control on the size of slope failures that occur on them, as the location of the headscarp and frontal ramp of these landslides are mainly determined by changes in the slope gradient. Our dataset also shows that the height drop of the failing slope section and the subsurface depth of the basal shear surface are the main parameters that determine whether a landslide will propagate in a frontally confined or frontally emergent manner. These parameters respectively represent the gravitational potential energy (driving force) of the sliding mass and the potential energy required (a resisting force) for it to emerge at its frontal ramp. These observations open perspectives for predicting the frontal emplacement style of future sublacustrine and submarine landslides and their associated natural hazards (e.g., tsunamis, dense flows). Although the investigated sublacustrine landslides have smaller dimensions than most submarine landslides, our data reveal mostly comparable inter-parameter correlations and relationships. However, frontally emergent landslides in lakes (and fjords) generally have a larger mobility and underwent a larger disintegration than what would have been expected by extrapolation of empirical relationships derived for ocean margin landslide datasets. This can be explained by the highly-unconsolidated material usually involved in the shallowly excavated slope failures in lakes and fjords.

Investigation of the weathering layer using seismic refraction and high-resolution seismic reflection methods, NE of Riyadh city

Five seismic refraction and five high-resolution seismic reflection (HRSR) profiles were carried out in northeastern part of Riyadh city to investigate depth of the weathering layer. Results obtained from seismic refraction survey reveal the depths of weathering layer at 12, 25, 17, 12, and 16 m, respectively. On the other hand, HRSR stack sections illustrate the depths of weathering layer at 14, 28, 20, 13, and 18 m, respectively. The weathering layer is composed of alluvial sediments and gravel, which is underlain by a sequence of limestone and dolomite layer. Seismic results from site no. 2 have been found to be in good agreement with lithological information reported from the adjacent water well. The HRSR data generally reveal better signal-to-noise ratio and enhanced resolution compared to the refraction data. Although, the HRSR data failed in achieving high-quality common midpoint (CMP) stacking profile at site no. 3, it provide an improved image of the subsurface features than the refraction data, recognizing it as a potential seismic technique.

The structure and formation of diapirs in the Yinggehai–Song Hong Basin, South China Sea

The occurrence of shale diapirs in the Yinggehai–Song Hong (YGH–SH) Basin is well documented, as is their association with big petroleum fields. In order to better understand how and why the diapirs form we performed a detailed geophysical analysis using a new regional compilation of high-resolution two- and three-dimensional seismic reflection data, as well as drilling data that cover the diapirs in YGH–SH Basin. As many as 18 diapirs were identified and are arranged in six N–S-striking vertical en échelon zones. On seismic reflection sections gas chimney structures, diapiric faults and palaeo-craters are genetically linked with the process of diapirism. Here we use geophysical and geological observations to propose a three-stage model for diapirism: initiation, emplacement, and collapse. During these three stages, different diapiric structure styles are formed, which we describe in detail. These include buried diapirs, piercing diapirs and collapsed diapirs. We link the diapirism to activity on the offshore continuation of the Red River Fault, as shown on our high-resolution seismic reflection data, which is also related to a high paleogeothermal gradient caused by crustal thinning. We also recognize the role of loading by the very large volume of sediment eroded from the edges of the Tibetan Plateau and delivered by the Red River to the basin.