High-resolution Seismic Reflection Research Articles

Diapiric Structures in the Tinto River Estuary (SW Spain) Caused by Artificial Load of an Industrial Stockpile

The mouth of the Tinto River is located on the southwest coast of the Iberian Peninsula in the northwest of the Gulf of Cadiz. The river flows into an estuarine system shared with the Odiel River, commonly known as the “Ría de Huelva”. In the 1960s, a wide area of ancient salt marshes was transformed by a stockpile of industrial wastes of phosphogypsum, reaching a height of 35 m above the level of the salt marsh at its highest point. Two surveys using high-resolution seismic reflection in conjunction with a parametric profiler were carried out in 2016 and 2018. The purpose of these geophysical studies was the realization of a 3D model of the sedimentary units constituting the most recent filling of the estuary. The records present abundant extrusion structures located on the margins of the waste stockpiles, which break the visible stratification of the surficial units of the estuary. In some sectors, these structures have reached the estuarine surface and have, therefore, a morphological expression on the estuarine floor. The origin of these structures is interpreted as a vertical escape of fluidized sediments from lower units caused by overpressure from stacking.

Late Pleistocene-Holocene sea level and climate changes in the Gulf of Saros: Evidence from seismostratigraphic record and sediment core data

A detailed record of sea level changes and climate oscillations in the Gulf of Saros during the late Pleistocene to the Holocene is provided by high-resolution seismic reflection profiles and analysis of a sediment core. The seismic stratigraphy reveals the formation of four main depositional units bounded by prominent reflection surfaces, reflecting high sea level variations. The Last Glacial Maximum period in the gulf is associated with the lowstand sea level, forming the deepest marine terrace at −148 m. The increasing sea level due to post-glacial warming was accompanied by the deposition of transgressive units characterized by coastal onlaps together with local channel-fills. This transgressive phase was modulated by three brief still-stands sea levels at 17 cal ka BP, 14.6 cal ka BP and 13.6 cal ka BP, producing the younger marine terraces in the gulf at −135 m, −112 m and −90 m, respectively. A comparison of their depths with the global sea level curve reveals elevation differences with decreasing subsidence rates from the older to younger ages, implying tectonic subsidence along the gulf floor.The close correlation of multi-proxy data from core SAG-22 in the gulf with the northern Aegean and Anatolia data strongly indicates that the timings of the past climate events generally agree with the global and regional climate patterns. The warm climate around the gulf during the late phase of Bølling/Allerød (13.3–12.6 cal ka BP) is represented by high marine biological productivity and improved ventilation in the deeper water due to subsequent transgression. Permafrost formation in the catchment area during the following cold and dry Younger Dryas period (12.6–11.7 cal ka BP) resulted in reduced soil erosion and sediment input to the gulf. Noticeable warming during the Early Holocene between 11.2 cal ka BP and 9.4 cal ka BP is recorded by the multi-proxy data of the core when the enhanced marine biological productivity occurred above the poorly oxygenated deep water column. The climatic deterioration to a cooler and drier phase during the Early to Middle Holocene transition (9.4–7.8 cal ka BP) resulted in intense physical weathering and erosion, causing the highest sedimentation in the gulf. The later period of the Holocene is associated with the formation of two discrete sapropels at 7.8 cal ka BP and 5.4 cal ka BP that were accompanied by warm and wet climates around the gulf.

Hazardous explosive eruptions of a recharging multi-cyclic island arc caldera

Caldera-forming eruptions of silicic volcanic systems are among the most devastating events on Earth. By contrast, post-collapse volcanic activity initiating new caldera cycles is generally considered less hazardous. Formed after Santorini’s latest caldera-forming eruption of ~1600 bce, the Kameni Volcano in the southern Aegean Sea enables the eruptive evolution of a recharging multi-cyclic caldera to be reconstructed. Kameni’s eruptive record has been documented by onshore products and historical descriptions of mainly effusive eruptions dating back to 197 bce. Here we combine high-resolution seismic reflection data with cored lithologies from International Ocean Discovery Program Expedition 398 at four sites to determine the submarine architecture and volcanic history of intra-caldera deposits from Kameni. Our shore-crossing analysis reveals the deposits of a submarine explosive eruption that produced up to 3.1 km3 of pumice and ash, which we relate to a historical eruption in 726 ce. The estimated volcanic explosivity index of magnitude 5 exceeds previously considered worst-case eruptive scenarios for Santorini. Our finding that the Santorini caldera is capable of producing large explosive eruptions at an early stage in the caldera cycle implies an elevated hazard potential for the eastern Mediterranean region, and potentially for other recharging silicic calderas.

Seismogenic fault of the 2021 Ms 6.0 Luxian induced earthquake in the Sichuan Basin, China constrained by high-resolution seismic reflection and dense seismic array

Accurate documentation of the location and geometry of seismogenic faults are critical for understanding strong seismicity and seismic hazard mitigation. A rising concern worldwide is induced seismicity caused by hydraulic fracturing, yet the geological characteristics and mechanisms of such seismogenic faults remain insufficiently understood. In particular, a first-order issue is whether the strong (M ≥ 5.0) induced earthquakes are caused directly by the slip of shallow faults near shale reservoirs or indirectly by deep basement faults. The September 16, 2021 Ms 6.0 (Mw 5.4) Luxian earthquake in the Sichuan Basin, China is one of the world's greatest induced earthquakes related to shale gas exploitation. Here we constrain the geometry of the seismogenic fault of this earthquake using three-dimensional seismic reflection data and a dense seismic array. Pre-existing faults in the sedimentary cover are clearly visible in seismic coherence slices. However, these shallow faults do not match the seismological parameters of the Luxian earthquake. High-resolution seismic reflection profiles combined with relocated earthquakes indicate that the seismogenic fault is located in the Precambrian basement. The results suggest that the mainshock is most likely caused by the poroelastic effects due to fluid injection. Hydraulic fracturing could have reactivated a large-scale basement fault and triggered the strong earthquakes under a relatively high geo-stress conditions in the study area. The basement fault slipped upward and ruptured the sedimentary cover, resulting in a significant difference between the focal and centroid depths of the mainshock. Our findings emphasize the importance of investigating regional tectonic and geological settings, local stress fields, and pre-existing faults for studying potential induced seismicity in shale gas fields.

Fault structure, seismicity, and magmatism of the Littoral Fault zone, northern South China sea: Insights from high-resolution seismic reflection data

Four destructive M7+ earthquakes and 18 M6+ earthquakes demonstrate that the Littoral Fault Zone (LFZ) is one of the most seismically active regions in the offshore area of South China. However, the location, geometry, and structural characteristics of the LFZ, which spans over 1000 km, have not been properly examined. In this study, we investigate the precise position of the LFZ offshore East Guangdong (EGD) and map the fault structure using recently acquired high-resolution multi-channel seismic (MCS) reflection data. The LFZ consists of an array of normal faults that collectively form a terraced sidewall fault zone linked to the continental rifting of the South China Sea (SCS). This fault zone is approximately 35–75 km wide and runs parallel to the shoreline. The primary faults of the LFZ EGD segment correlate to a high-gravity gradient zone and serve as the northern boundary of the Zhu-I Depression. During the syn-rift phase, the LFZ experienced significant intra-plate deformation, resulting in fault throws of up to 1000 m. This deformation was locally influenced by pre-existing structures, giving rise to a listric geometry at depth. In the post-rift period, the LFZ exhibited inherited activity along its rift-related faults. The presence of growth fault structures suggests that the LFZ remained active throughout the Cenozoic, with numerous active faults extending practically to the seafloor. The negative flower structure confirms the strike-slip character of the LFZ, which is influenced by the modern tectonic stress field. Intense multi-phase Cenozoic magmatism, including igneous intrusion, volcanism, and lava flows, has been detected in the offshore area of Nan'ao Island. The frequent earthquakes in the study area are primarily attributed to far-field stress release resulting from the subduction of the Philippine Sea Plate beneath the Eurasian Plate. Additionally, the intense magmatism weakens crustal structures, further facilitating stress release and the frequent occurrence of earthquakes.

Upper-plate conduits linked to plate boundary that hosts slow earthquakes

In shallow subduction zones, fluid behavior impacts various geodynamic processes capable of regulating slip behaviors and forming mud volcanoes. However, evidence of structures that control the fluid transfer within an overriding plate is limited and the physical properties at the source faults of slow earthquakes are poorly understood. Here we present high-resolution seismic velocity models and reflection images of the Hyuga-nada area, Japan, where the Kyushu-Palau ridge subducts. We image distinct kilometer-wide columns in the upper plate with reduced velocities that extend vertically from the seafloor down to 10–13 km depth. We interpret the low-velocity columns as damaged zones caused by seamount subduction and suggest that they serve as conduits, facilitating vertical fluid migration from the plate boundary. The lateral variation in upper-plate velocity and seismic reflectivity along the plate boundary correlates with the distribution of slow earthquakes, indicating that the upper-plate drainage system controls the complex pattern of seismic slip at subduction faults.

Estimation of Low-Velocity Landfill Thickness with Multi-Method Seismic Surveys

Conventional geophysical methods are suitable for estimating the thicknesses of subsoil layers. By combining several geophysical methods, the uncertainties can be assessed. Hence, the reliability of the results increases with a more accurate engineering solution. To estimate the base of an abandoned landfill, we collected data using classical approaches: high-resolution seismic reflection and refraction, with more modern methods including passive surface wave analysis and horizontal-to-vertical spectral ratio (HVSR) measurements. To evaluate the thickness of the landfill, three different datasets were acquired along each of the two seismic lines, and five different processing methods were applied for each of the two arrays. The results of all the classical methods indicate very consistent correlations and mostly converge to clear outcomes. However, since the shear wave velocity of the landfill is relatively low (<150 (m/s)), the uncertainty of the HVSR results is significant. All these methods are engineering-oriented, environmentally friendly, and relatively low-cost. They may be jointly interpreted to better assess uncertainties and therefore enable an efficient solution for environmental or engineering purposes.

Sedimentation since 140 ka in Te Tai-o-Aorere Tasman Bay, Aotearoa New Zealand

ABSTRACT Late Pleistocene and Holocene sedimentation records changes in glacio-eustatic cycles, oceanography, active tectonics, and sediment supply. Here, we use high-resolution seismic-reflection profiles in Te Tai-o-Aorere Tasman Bay to investigate sedimentary deposit distributions associated with sea-level cyclicity over ∼140 ka. We identify nine seismic stratigraphic units below the inner continental shelf, interpret their character based on stratal relationships and acoustic properties, and infer ages using global sea level. The sequence includes stacked deposits and unconformities associated with the shifting shoreline over glacial cycles. Prominent erosional features include a lowstand ravinement surface and two transgressive surfaces formed during sea-level rise following the last two glacial maxima of ∼20 ka and ∼140 ka. The youngest surface marks the base of post-glacial sediment accumulated since ∼12 ka in the study area. Post-glacial sediment includes five seismically mappable units, covering an area of ∼1400 km2 and deposit volume of ∼9 km3. Time-averaged sediment accumulation rates are estimated to range from a maximum of ∼1.6–2.8 m/kyr to ∼0.5–0.9 m/kyr. Sequence stratigraphy interpreted in Te Tai-o-Aorere Tasman Bay is representative of accumulation in a relatively wide-shelf, sheltered inner bay setting supplied by mountainous catchments, and provides stratigraphic context necessary for future stratigraphic and tectonics studies.

Out of sight, out of mind. Submarine springs in the Dead Sea — An underappreciated phenomenon

The drastic drop in Dead Sea lake levels during the last few decades has caused the formation of over 6000 dangerous sinkholes along its coasts. Concurrently, retreating shorelines have led to the exposure of an under-explored phenomenon – submarine spring systems. Once exposed on land, these features are labeled as the more familiar geological hazard – i.e. sinkholes, with no distinction between the two. While visually they may seem similar, the underlying formation mechanism is different, as may well be their hazard potential. This study utilizes high-resolution seismic reflection data collected in 1984 when lake levels were some 35 m higher, together with multibeam bathymetric data from 2014, visual observations and water chemistry data from a verified spring system in order to assess the underlying formation mechanisms of these features, their stability and morphology. Results show that the springs are relatively stable and long-lived systems. They “deserve” to be separated from the sinkholes and studied as a distinct phenomenon. The springs discharge into the lake a significant amount of freshwater from the adjacent aquifers and are therefore, a largely underestimated part of its hydrological budget and the connected fresh groundwater resources. The acceptance of submarine springs as a distinct geological phenomenon and their consideration as a major groundwater outlet into the lake will lead to more realistic groundwater resource models of the Judea and Samaria Eastern Mountain aquifer than exist today. This is extremely important given the increase aridification of the area and the increased demand for freshwater resources.

Formation of undulating seafloor bedforms during the Minoan eruption and their implications for eruption dynamics and slope stability at Santorini

The Minoan eruption of Santorini is one of the largest Holocene volcanic events and produced several cubic kilometers of pyroclastic flows emplaced on the submerged flanks of the volcano. Marine geophysical surveys reveal a multitude of undulating seafloor bedforms (USBs) around Santorini. While similar structures are known from other volcanoes worldwide, Santorini offers the unique opportunity to relate USB formation with volcanic processes during one of the best-studied volcanic eruptions worldwide. In this study, we combine high-resolution seismic reflection data with multibeam echosounder bathymetry to reveal the internal architecture of USBs around Santorini and to relate their morphological characteristics to formational processes. The USBs around Santorini were formed during the Minoan eruption and represent the seafloor expression of mass transport deposits. Three types of deposits differ in composition or origin. (1) Depositional USBs, which can only be found to the north of the island, where Minoan eruption ignimbrites reach their maximum thickness and the undulating topography is the result of thrusting within the deposit. (2) USBs related to slope failures of volcaniclastics from the entire Thera Pyroclastic Formation, which can be found east, south, and west of the island. (3) USBs associated with deep-seated deformation, which occurs on the southwestern flank along an area affected by rift tectonics and extends to a depth of more than 200 m below the seafloor. In cases (2) and (3), the USBs are formed upslope by block rotation and downslope by thrusting. Our study indicates that these processes may have contributed to the generation of the devastating Minoan tsunami. Since Santorini is located in one of the most tectonically active regions in the Mediterranean, capable of producing earthquakes with magnitude M7+, our study has important implications for hazard assessment. A strong earthquake located close to the island may have the potential to reactivate slope instabilities posing a previously undetected but potentially significant tsunami hazard.

High-resolution P- and S-wave reflection studies of an intraplate structure: The Azambuja fault, Portugal

The Azambuja fault is a NNE trending structure located 50 km north of Lisbon, the capital and most populous city of Portugal. The fault has been considered as a possible source for the historical, large earthquakes. Understanding this fault is a priority in seismic hazard evaluation of this region. The fault has a clear morphological signature. Miocene and Pliocene sediments are tilted eastward and cut by steeply dipping mesoscale fault segments, presenting reverse and normal offsets with a net downthrow to the east. Neotectonic studies indicate a Quaternary slip on the fault of 0.05–0.06 mm/year. However, no direct evidence of the Azambuja fault affecting the Pleistocene or Holocene sediments was found so far. Here, we present the findings from high-resolution seismic reflection studies using both P- and S-waves over the Holocene deposits. The detection of small-throw faulting in ductile sediments is a challenging task. We show that multiple signatures, like perturbations in the reflection hyperbolae visible in shot and CMP gathers, interruptions of reflectors in stacked sections, lateral seismic velocity variations obtained by horizon velocity analysis, all at coincident locations, strongly suggest that the activity of the Azambuja fault has affected the Holocene sediments in the study area. The lateral velocity variations are corroborated by wavepath eikonal traveltime tomography and velocity analysis supported by seismic modeling. By means of 2D viscoelastic modeling, we explain the absence of fault-related diffractions and negligible back-scattered energy from the fault. Using data from nearby boreholes, we find that the 15 ka old alluvium cover has indeed been disturbed by the presence of shallow fault strands. Considering the estimated vertical throws and the empirical relationships between fault length, co-seismic rupture and magnitude, a slip rate of 0.07 mm/y, slightly larger than previously thought, is expected for this fault.

Basement structures exert a crucial influence on the structural configuration of later rift development: Insights from the Shanan and Chengbei sag, Bohai Bay Basin

The Early Cretaceous and Paleogene rift structures of the Shanan and Chengbei sags were inherited from basement tectonic deformation of Indosinian and Yanshanian tectonism. However, the architecture of the basement structures and its influence on the structural configuration of later rift development are poorly understood, which impeded hydrocarbon exploration efforts in the study area. Based on 2D high-resolution seismic reflection data in the study area and onshore field data in the Luxi area, we recognize the Indosinian NW-striking low-angle fold-thrust belts and Yanshanian NE-striking high-angle thrusts in basement of the study area. The pre-rift NW- and NE-striking basement structures exert a crucial influence on the two-phase rift architecture and structural evolution of the study area: (1) Low-angle NW-striking Indosinian thrusts are dominantly reactivated as dip-slip major normal faults during Early Cretaceous, such as the Shanan fault and Chengbei faults. The high-angle Yanshanian NE-striking thrusts are primarily reactivated as left-lateral strike-slip faults during this stage, such as the Shaxi fault. (2) During the Paleogene, oblique extension took place across the pre-existing NW-striking reactivated normal faults. A set of half-grabens formed along the major NW-striking oblique-slip faults and large amount of ENE-striking secondary normal faults developed in the NW-striking sags. The pre-existing left-lateral NNE-striking faults shifted to become right-lateral and extensional faults. The Early Cretaceous rift was primarily controlled by NE-SW-directed extension, which was derived from the large-scale left-lateral strike-slip movement of the Tan-Lu Fault Zone. The Paleogene oblique rift was induced by NNW-SSE-directed extension related to the upwelling water-rich mantle wedge of the Pacific subduction zone.

Large earthquakes along slow converging plate margins: Calabrian Arc paleoseismicity based on the submarine turbidite record

The Calabrian Arc subduction-rollback system hosts seismogenic faults capable of generating earthquakes exceeding magnitude 7. Since earthquakes are the result of long-term geodynamic processes, documenting seismic activity during a sufficiently long time interval is of fundamental importance for hazard scenarios. Instrumental and historical data provide critical information on seismogenesis, but they cover time periods shorter than the recurrence times of large earthquakes, especially in areas with low deformation rates such as Calabria. If onshore paleoseismological studies are fundamental to compile earthquake catalogs, they are sometime affected by the relatively poor continuity of sedimentation in the subaerial environment.In this study we applied the paleoseismological approach to the submarine environment to reconstruct the record of high-energy sedimentary events triggered by seismic activity. We analyzed three gravity cores collected in disconnected sedimentary basins to reconstruct resedimentation processes during the Holocene, integrating inland information for a better assessment of tectonic activity and seismogenesis. Multiproxy analyses of the sedimentary record constrained by radiometric dating allowed reconstructing event stratigraphy and linking resedimented deposits to specific earthquakes.Onshore and offshore data allow to identify large-magnitude earthquakes in the central Calabrian Arc subduction system during the Holocene, with inferred epicenters located either along normal faults onshore and/or related to the slab dynamics. The turbidite record reveals 20 major events during the last 10 ka, with sources including crustal faults in Calabria (i.e. Lakes, Rossano and Cittanova faults). Analyses of sediment samples and high-resolution seismic reflection images allowed identification of different types of resedimented deposits during the last 30–50 ka. The basin-wide occurrence of three megaturbidites/homogenites suggests they are related to megatsunamis sourced by far field earthquakes along the Hellenic Arc. Megaturbidites with a more limited spatial extent are interpreted as subduction-type events in the Calabrian Arc, while thinner seismo-turbidites record the activity of crustal structures including faults onshore. Results suggest a recurrence time of 2–3 ka for major Calabrian Arc events that needs to be considered for a reliable hazard assessment in the Mediterranean region.

Shallow deformation on the Kirby Hills fault, Sacramento–San Joaquin Delta, California (USA), revealed from high-resolution seismic reflection data and coring in a fluvial system

Abstract The Sacramento–San Joaquin Delta (Delta) in California (USA) is an important part of the state’s freshwater system and is also a major source of agricultural and natural resources. However, the Delta is traversed by a series of faults that make up the easternmost part of the San Andreas fault system at this latitude and pose seismic hazard to this region. In this study, we use new high-resolution chirp subbottom data1 to map and characterize the shallow expression of the Kirby Hills fault, where it has been mapped to cross the Sacramento River at the western extent of the Delta. The fault is buried here, but we document a broad zone of deformation associated with the eastern strand of the fault that changes in character, along strike, across ~600 m of the river channel. Radiocarbon dates from sediment cores collected in the Sacramento River provide some minimum constraints on the age of deformation. We do not observe evidence of the western strand as previously mapped. We also discuss difficulties of conducting a paleoseismologic study in a fluvial environment.

Reading the sediment archive of the Eastern Campeche Bank (southern Gulf of Mexico): from the aftermath of the Chicxulub impact to Loop Current variability

This is the first high-resolution seismic study showing how the Chicxulub impact shaped the eastern slope of the Campeche Bank in the south-eastern Gulf of Mexico. The induced shock wave fractured Cretaceous strata causing the collapse of the upper slope and shelf over a length of ca. 200 km. Failed material was either transported downslope or remained in parts on the accommodation space created by the collapsed. In the Cenozoic, the East Campeche Plastered Drift developed within the created accommodation space, controlled by the inflowing surface current from the Caribbean, which forms the Loop Current. The internal reflection configuration of the drift shows that the closure of the Suwannee Strait in the Late Oligocene and the closure of the CAS in the Mid to Late Miocene controlled the variability of the southern Loop Current in time. Since the Loop Current transports heat and moisture from the western Atlantic warm water pool into the North Atlantic and further to NW Europe by the Gulf Stream, the drift represents an archive for controlling factors that influenced climate of the northern hemisphere. This first high-resolution seismic reflection study from the eastern Campeche Bank expands the understanding of destructive processes that a meteorite impact induces into the earth system. Furthermore, these data document that the East Campeche Plastered Drift bears the potential to understand the link between the climate variability of the northern hemisphere and oceanic processes in the equatorial western Atlantic.

Active tectonics in the Calabrian Arc: Insights from the Late Miocene to Recent structural evolution of the Squillace Basin (offshore eastern Calabria)

The Calabrian Arc represents one of the most active sectors of the upper plate of the Tyrrhenian-Ionian subduction system. This research aims to reconstruct the evolution of the Squillace Basin (Ionian offshore of the Calabrian Arc) from the Late Miocene to Recent times and recognise active shallow and deep structures using a multiscale approach. The latter is based on interpreting high-penetration and high-resolution seismic reflection profiles, calibrated with well-log data coupled with bathymetric data and the distribution of instrumental earthquakes. Data highlight three steps in the evolution of the Squillace Basin. A Late Miocene extensional event led to the formation of WNW-ESE oriented horst and half-graben structures. During the Pliocene, deformation was localised in the central and northern sectors of the basin and expressed by a WNW-ESE oriented strike-slip fault and NW-SE normal to trastensional faults, respectively. A transpressional event started in the Early Pleistocene, causing the positive inversion of deep (> 3 km) extensional faults and the formation of NW-SE to WNW-ESE oriented transpressional/reverse faults and related anticlines. The kinematics of these faults agree with the NW-SE oriented left-lateral Albi-Cosenza, Lamezia-Catanzaro and Petilia-Sosti crustal fault zones developed in north Calabria. The results of this work suggest that the transpressional structures in the northwestern sector of the basin likely represent the offshore prolongation of the Albi-Cosenza fault zone. NW-SE to WNW-ESE trending, shallow (<2 km) high-angle normal faults offset the younger deposits. Their depth and direction indicate that these faults are secondary structures formed in the extrados of the anticlines associated with the transpressional faults. The distribution of earthquakes shows events with M > 3 and depth <15 km located in the hanging wall of transpressional faults. The integrated data suggest that these structures are active and probably responsible for the major earthquakes that affected the Ionian offshore.

Detailed Imaging of a Seismogenic Fault That Potentially Induced the Two 2019 Weiyuan Moderate Earthquakes in the Sichuan Basin, China

Abstract Induced earthquakes are becoming a significant issue. Several destructive induced earthquakes (Ms ≥5.0) have occurred in shale gas fields located at the southwestern Sichuan basin, China, in the recent years. Specifically, two moderate earthquakes with magnitudes of Ms 5.4 (occurred on 8 September 2019) and 5.2 (occurred on 18 December 2019) exceeded the largest earthquake ever recorded in the Weiyuan. It remains unclear which seismogenic faults were responsible for the two moderate events due to insufficient knowledge of the subsurface structure and contradictory hypocenter depths obtained from different seismological methods. In this study, mainshock locations, focal mechanisms, and precise relocations of microseismicity were used to map the 3D geometry of the seismogenic fault. The result shows that the two moderate earthquakes occurred on the same fault. High-resolution seismic reflection profiles indicate several preexisting faults at various depths and a potential seismogenic fault. No large-scale faults are identified in the sedimentary cover above the Cambrian at the shallow centroid depth of these earthquakes. However, the seismic reflectors show phase discontinuous feature, indicating the existence of preexisting faults in the basement. A preexisting fault in the basement is consistent with a seismogenic fault that may have induced the two moderate earthquakes. Our study suggests that poroelastic stress propagation is likely responsible for the basement fault reactivation. The rupture on a stressed basement fault was triggered and produced two moderate earthquakes in a short period during hydraulic fracturing. The previous strategy of avoiding direct injection of fluids into preexisting faults to reduce seismic risk in operation region is not enough to avoid the possibility of the coupled poroelastic stress triggering deep fault rupture. We need to reevaluate the potential seismic risk of shale gas development in Weiyuan, the southern Sichuan basin shale gas field considering the preexisting faults in sedimentary and in basement and the effect of the fully coupled poroelastic stress field.

Effect of Dam Emplacement and Water Level Changes on Sublacustrine Geomorphology and Recent Sedimentation in Jackson Lake, Grand Teton National Park (Wyoming, United States)

Dam installation on a deep hydrologically open lake provides the experimental framework necessary to study the influence of outlet engineering and changing base levels on limnogeological processes. Here, high-resolution seismic reflection profiles, sediment cores, and historical water level elevation datasets were employed to assess the recent depositional history of Jackson Lake, a dammed glacial lake located adjacent to the Teton fault in western Wyoming (USA). Prograding clinoforms imaged in the shallow stratigraphy indicate a recent lake-wide episode of delta abandonment. Submerged ∼11–12 m below the lake surface, these Gilbert-type paleo-deltas represent extensive submerged coarse-grained deposits along the axial and lateral margins of Jackson Lake that resulted from shoreline transgression following dam construction in the early 20th century. Other paleo-lake margin environments, including delta plain, shoreline, and glacial (drumlins, moraines) landforms were likewise inundated following dam installation, and now form prominent features on the lake floor. In deepwater, a detailed chronology was established using137Cs,210Pb, and reservoir-corrected14C for a sediment core that spans ∼1654–2019 Common Era (CE). Dam emplacement (1908–1916 CE) correlates with a nearly five-fold acceleration in accumulation rates and a depositional shift towards carbonaceous sediments. Interbedded organic-rich black diatomaceous oozes and tan silts track changes in reservoir water level elevation, which oscillated in response to regional climate and downstream water needs between 1908 and 2019 CE. Chemostratigraphic patterns of carbon, phosphorus, and sulfur are consistent with a change in nutrient status and productivity, controlled initially by transgression-driven flooding of supralittoral soils and vegetation, and subsequently with water level changes. A thin gravity flow deposit punctuates the deepwater strata and provides a benchmark for turbidite characterization driven by hydroclimate change. Because the Teton fault is a major seismic hazard, end-member characterization of turbidites is a critical first step for accurate discrimination of mass transport deposits controlled by earthquakes in more ancient Jackson Lake strata. Results from this study illustrate the influence of dam installation on sublacustrine geomorphology and sedimentation, which has implications for lake management and ecosystem services. Further, this study demonstrates that Jackson Lake contains an expanded, untapped sedimentary archive recording environmental changes in the American West.

Quaternary Lowstand Prograding Wedges of the Salento Continental Shelf (Southern Adriatic Sea, Italy): Architectural Stacking Patterns and the Control of Glacio-Eustatic Sea Level Fluctuations and Foreland Tectonic Uplift

The performance of both the tectonic uplift and of the 4th-order glacial eustatic sea level fluctuations in controlling the stratigraphic architecture of Quaternary lowstand prograding wedges of the Salento continental shelf (Southern Adriatic sea, Italy) during a time interval spanning from the Middle Pleistocene to the Holocene has been pointed out through the interpretation of high-resolution seismic reflection profiles and their correlation to the curves of the isotopic stratigraphy. Three main transgressive surfaces of erosion (RS1, RS2 and RS3) punctuate the stratigraphic architecture of the Salento continental shelf, separating Quaternary lowstand prograding wedges between them. All along the Middle Pleistocene, increasing the tectonic uplift of the Puglia offshore, combining with 4th-order glacio-eustatic variations, have dealt with the pattern of a broad forced regression prograding wedge, favoring a platform progradation of approximately 15 km. The architectural stacking patterns of the overlying Late Pleistocene and Holocene prograding wedges are controlled by 4th-order glacio-eustatic sea level changes, allowing for the formation of incomplete depositional sequences. In this period, the eustatic signature overcomes the tectonic mark, implying a decline in the uplift of the Apulian foreland in the course of the final 250 ky.

Delineation of Coal Bed Geometry using High Resolution Seismic Reflection Data for Uninterrupted Directional Drilling Exploration

Delineation of Coal Bed Geometry using High Resolution Seismic Reflection Data for Uninterrupted Directional Drilling Exploration