Multichannel Seismic Research Articles

The Protracted Evolution of a Plate Boundary: Eastern Cuba Block and Old Bahamas Channel

AbstractThe Eastern Cuban block has experienced a complex tectonic history characterized by plate interactions, resulting in a diverse array of geological features observable in the offshore sedimentary record. We investigate the tectonic evolution of offshore Eastern Cuba, specifically in the Old Bahamas Channel and its surrounding areas, by integrating multi‐channel seismic (MCS) reflection and published geological data. Our analysis employs stratigraphic frameworks and MCS data to assess deformation and key geological events in the region. We highlight the complex tectonic history of the Eastern Cuban block, marked by significant geodynamic events, such as rifting, the subduction of the oceanic Proto‐Caribbean plate, and syn‐orogenic and post‐orogenic phases. The seismic units observed in the majority of the study area reveal the early evolution of the Northern Proto‐Caribbean margin, subsequently impacted by the Cuban orogeny and the reactivation of the Cuban Transform Fault zone corresponding to a former plate boundary. We propose estimated ages for the seismic sequences, correlating them with available well data from neighboring regions. This study offers valuable insights into the tectonic history and geological evolution of offshore Eastern Cuba, contributing to a more comprehensive understanding of the region's geodynamic development.

Consequences of Continental Core Complexes on Rifting Patterns: Insights From Multichannel Seismic Data From the Northern Margin of the South China Sea

AbstractCore complex type structures are common in continental margins. Recent studies have proposed exhumation of deeper crustal and mantle levels during extension of the continental margin along the Northern South China Sea (NSCS) in a core complex mode. Here, we present high‐resolution seismic profiles across different sections of the NSCS that allow us to observe two types of core complex systems. The Kaiping Rift and eastern Baiyun Rift are characterized by a sub‐horizontal Moho and exhumation of ductile deeper crust beneath an extremely thinned brittle upper crust. The central Baiyun Rift in contrast is marked by homogeneous thinning of the entire crust that is floored by mantle unroofing with a concave‐down Moho geometry. While the former type, referred to as crustal core complexes, coincides with relatively intense syn‐rift magmatism, the latter, referred to as mantle core complexes, is marked by limited magmatism. Consistently, basin subsidence rates are lower over crustal core complexes than mantle core complexes. Compared to mantle core complexes at the magma‐poor Iberian margin, the widely distributed crustal core complexes in the NSCS indicate higher geotherms and more intense magmatism during extension of the continental lithosphere. We propose that the continental core complexes observed at the NSCS represent a typical outcome of extension of a gravitationally unstable lithosphere.

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.

Subsurface structures and nature of seafloor mounds in the northern South China Sea margin: Implications for Mesozoic hydrocarbon exploration

The Chaoshan Depression in the northern South China Sea (SCS) margin hosts thick (ca. 4–9 km) Mesozoic strata including two sets of hydrocarbon source rocks (the Late Triassic-Early Jurassic and Late Jurassic marine facies sequences). It was predicted to be petroliferous but unproven. Numerous submarine mounds are distributed on its seafloor, but whether they are mud volcanoes or magmatic volcanoes remains controversial due to poor knowledge of the subsurface structures. To reveal the nature of these mounds and the Mesozoic hydrocarbon potential in the Chaoshan Depression, multi-channel seismic (MCS) and ocean bottom seismometer (OBS) data along line CS-L1 across three seafloor mounds are processed and analyzed. The MCS profile reveals the existence of fuzzy reflection zones (FRZs) within the Mesozoic strata beneath these mounds. The overlying sedimentary sequences appear uplifted and deformed. The OBS velocity results exhibit that these FRZs feature significantly lower velocities than their host strata with velocity differences ranging from ∼0.25 km/s to 0.71 km/s, likely resulting from vigorous fluid charging rather than magma intrusion. Thus these FRZs are identified as mud-fluid diapirs. Between these diapirs and the overlying seafloor mounds, numerous high-angle faults exist, serving as vertical fluid feeder conduits. Additionally, several bright spots, acting as indicators of gas reservoirs, are observed near these faults. Moreover, bottom simulating reflectors with reverse polarity are imaged within the shallow strata around the seafloor mounds on an adjacent MCS profile, indicating rich gas trapping and/or gas hydrate accumulation. Therefore, we propose that these seafloor mounds are mud volcanoes with fluids coming from the underlying diapirs. As the diapirs extend downward into the Jurassic and Triassic strata, the hydrocarbon gases feeding the mud volcanoes and diapirs are reasoned to originate from the deep Mesozoic source rocks, implying significant Mesozoic hydrocarbon potential in the Chaoshan Depression of the SCS.

CASIE21-OBS: An Open-Access, OBS Controlled-Source Seismic Data Set for Investigating the Structure and Properties of the Cascadia Accretionary Wedge and the Downgoing Explorer-Juan de Fuca-Gorda Plate System

Abstract Geological processes at subduction zones and their associated geohazards (e.g., megathrust earthquakes, submarine landslides, tsunamis, and arc volcanism) are, to a large extent, controlled by the structure, physical properties and fluid content of the subducting plate, the accreted sediments, and the overriding plate. In these settings, modern seismic modeling and imaging techniques based on controlled-source, multicomponent ocean-bottom seismometer (OBS) data are some of the best tools available for determining the subseafloor elastic properties, which can be linked to the aforementioned properties. Here, we present CASIE21-OBS, a controlled-source marine wide-angle OBS data set recently collected across the Cascadia convergent margin as part of the larger CAscadia Seismic Imaging Experiment 2021 (CASIE21). The main component of CASIE21 is a long-offset multichannel seismic (MCS) survey of the Cascadia margin conducted in June–July 2021 onboard R/V M.G. Langseth (cruise MGL2104) aiming to characterize the incoming plate, the plate interface geometry and properties, and the overlying sediment stratigraphy and physical properties. CASIE21-OBS was conducted during R/V M.G. Langseth cruise MGL2103 (May 2021) and R/V Oceanus cruise OC2106A (June–July 2021). It consisted of 63 short-period four-component OBSs deployed at a total 120 stations along 10 across-trench profiles extending from ∼50 km seaward of the deformation front to the continental shelf, and from offshore northern Vancouver Island to offshore southern Oregon. The OBSs recorded the airgun signals of the CASIE21-MCS survey as well as natural seismicity occurring during the deployment period (24 May 2021 19:00 UTC–9 July 2021 09:00 UTC). The OBS data are archived and available at the Incorporated Research Institutions for Seismology Data Management Center under network code YR_2021 for continuous time series (miniSEED) and identifier 21-008 for assembled data set (SEG-Y).

3D and 2D topographic correction to estimated geothermal gradient from the base of gas hydrate stability zone in the Andaman Forearc Basin

Methane gas hydrate related bottom-simulating reflectors (BSRs) are imaged based on the in-line and cross-line multi-channel seismic (MCS) data from the Andaman Forearc Basin. The depth of the BSR depends on pressure and temperature and pore water salinity. With these assumptions, the BSR depth can be used to estimate the geothermal gradient (GTG) based on the availability of in-situ temperature measurements. This calculation is done assuming a 1D conductive model based on available in-situ temperature measurement at site NGHP-01-17 in the study area. However, in the presence of seafloor topography, the conductive temperature field in the subsurface is affected by lateral refraction of heat, which focuses heat in topographic lows and away from topographic highs. The 1D estimate of GTG in the Andaman Forearc Basin has been validated by drilling results from the NGHP-01 expedition. 2D analytic modeling to estimate the effects of topography is performed earlier along selected seismic profiles in the study area. The study extended to estimate the effect of topography in 3D using a numerical model. The corrected GTG data allow us to determine GTG values free of topographic effect. The difference between the estimated GTG and values corrected for the 3D topographic effect varies up to ∼5 °C/km. These conclude that the topographic correction is relatively small compared to other uncertainties in the 1D model and that apparent GTG determined with the 1D model captures the major features, although the correction is needed prior to interpreting subtle features of the derived GTG maps.

Regional zonation based on seismic vulnerability using local site effect analysis and potential damage to the city of Medan (North Sumatra, Indonesia) due to earthquake

BackgroundMicrozonation becomes important for big cities like Medan (North Sumatra, Indonesia) as population agglomeration increases in urban areas resulting in rapid and unplanned construction. Mitigation efforts must be carried out to minimize losses arising from earthquakes, such as loss of life and environmental damage, and it can be done by analyzing seismic susceptibility and local site effects.PurposeThis study analyzes the potential for environmental damage caused by seismic hazards in the city of Medan, North Sumatra, Indonesia. The purpose of this study is to provide an overview of the definition and methodology of seismic–geotechnical hazard zoning and the methodology used for making seismic hazard maps with the case study of Medan City.MethodsWhile determining the zoning, microzonation measurements and environmental data parameters are involved. To obtain comprehensive results, microzonation measurements were carried out using three methods, namely the Multichannel Seismic Analysis Surface Wave (MASW), Spatial Autocorrelation (SPAC), and Horizontal-To-Vertical Spectral Ratio (HVSR) with 200 measuring points spread throughout the city of Medan. Parameter weighting values such as population, site class, seismic susceptibility, dominant period, land cover, slope, and weather class were obtained using the analytic hierarchy process (AHP) method.ResultsWe presented the result of the survey, including the MASW, SPAC, and HVSR results. In general, medium soil and soft soil dominate almost all areas of Medan city. The estimated depth of engineering bedrock is at a depth of about 291.33 – 735.87 meters from the ground surface. The dominant period values ranged from 0.0907 s to 8.30723 s.ConclusionsWe classified Medan City (North Sumatra) into several zones, based on their seismic vulnerability using local site effect analysis and potential damage due to earthquakes. This study is necessary for future seismic hazard assessment.

Application of first arrival seismic tomography in a glaciated basin: implications for paleo-ice stream development

AbstractIce streams are sites of ice-sheet drainage and together with other processes, such as calving, have an impact on deglaciation rates and ice-sheet mass balance. Proglacial lake deposits provide records of ice-sheet deglaciation and have the potential to supplement other paleoclimate records. Oneida Lake, northeastern USA, contains a thick proglacial lake sequence that buries evidence of ice streaming and a paleo-calving margin that developed during retreat of the Laurentide Ice Sheet. Previous high-resolution digital elevation models identified the Oneida Ice Stream from glacial landforms northwest of the lake. In this study, we utilize seismic refractions from a multichannel seismic (MCS) reflection dataset to estimate the thickness of glacial deposits using seismic tomography. With this method we constrain the depth to top of Paleozoic strata, especially in areas where the reflection data yielded poor outcomes and validate our reflection data in regions of good coverage. We demonstrate that where long offset seismic data are available, the first-arrival tomography method is useful in studies of formerly glaciated basins. Our study identifies a ~108 m thick sedimentary section and potentially long paleoclimate record in Oneida Lake, and identifies a paleotopographic low that likely encouraged formation of the Oneida Ice Stream.

Revisiting the 1899 earthquake series using integrative geophysical analysis in Yakutat Bay, Alaska, USA

AbstractA series of large earthquakes in 1899 affected southeastern Alaska near Yakutat and Disenchantment Bays. The largest of the series, a MW 8.2 event on 10 September 1899, generated an ~12-m-high tsunami and as much as 14.4 m of coseismic uplift in Yakutat Bay, the largest coseismic uplift ever measured. Several complex fault systems in the area are associated with the Yakutat terrane collision with North America and the termination of the Fairweather strike-slip system, but because faults local to Yakutat Bay have been incompletely or poorly mapped, it is unclear which fault system(s) ruptured during the 10 September 1899 event. Using marine geophysical data collected in August 2012, we provide an improved tectonic framework for the Yakutat area, which advances our understanding of earthquake hazards. We combined 153 line km of 2012 high-resolution multichannel seismic (MCS) reflection data with compressed high-intensity radar pulse (Chirp) profiles, basin-scale MCS data, 2018 seafloor bathymetry, published geodetic models and thermochronology data, and previous measurements of coseismic uplift to better constrain fault geometry and subsurface structure in the Yakutat Bay area. We did not observe any active or concealed faults crossing Yakutat Bay in our high-resolution data, requiring faults to be located entirely onshore or nearshore. We interpreted onshore faults east of Yakutat Bay to be associated with the transpressional termination of the Fairweather fault system, forming a series of splay faults that exhibit a horsetail geometry. Thrust and reverse faults on the west side of the bay are related to Yakutat terrane underthrusting and collision with North America. Our results include an updated fault map, structural model of Yakutat Bay, and quantitative assessment of uncertainties for legacy geologic coseismic uplift measurements. Additionally, our results indicate the 10 September 1899 rupture was possibly related to stress loading from the earlier Yakutat terrane underthrusting event of 4 September 1899, with the majority of 10 September coseismic slip occurring on the Esker Creek system on the northwest side of Yakutat Bay. Limited (~2 m) coseismic or postseismic slip associated with the 1899 events occurred on faults located east of Yakutat Bay.

Field and Synthetic Waveform Tests on Using Large‐Offset Seismic Streamer Data to Derive Shallow Seabed Shear‐Wave Velocity and Geotechnical Properties

AbstractCharacterizing properties of marine subsurface sediment helps with siting for offshore infrastructure. Shear‐wave velocity (Vs) provides information on the geotechnical properties of the seabed. We present our initial efforts to obtain a detailed two‐dimensional model of Vs for a large‐offset multi‐channel seismic (MCS) transect collected in shallow waters across the Taiwan Strait using surface waves excited by a large volume airgun. We derived the dispersion curves of the Scholte waves along the 37.5‐km‐long transect using the phase‐shift method and then conducted multimodal inversion to obtain a Vs model down to a depth of 150 m. To estimate the dynamic Poisson's ratio across the transect, we combined the Vs model with a compressional wave velocity model derived from the traditional MCS semblance velocity analysis. Lastly, we approximated the seismic attenuation of the profile. Our results show a large lateral variation in shear‐wave velocity. In the north, a low‐velocity zone with shear‐wave velocities of about 150 m/s was identified, while in the south, the shear‐wave velocity was found to be 300 m/s. With synthetic data, several sensitivity tests were performed to derive optimal parameters for offshore large‐offset streamer data. We particularly focused on the depth of the streamer and source and the water depth in combination with different seabed properties. Our results show that we can robustly derive the shear‐wave velocity, along with the Poisson's ratio, using large‐offset streamer data elsewhere based on the criteria we have tested using field and synthetic data sets.

Downward continuation of marine seismic reflection data: an undervalued tool to improve velocity models

SUMMARY The purpose of marine seismic experiments is to provide information of the structure and physical properties of the subsurface. The P-wave velocity distribution is the most commonly modelled property, usually by inversion of arrival times or waveform attributes. In wide-angle seismic reflection/refraction (WAS) experiments, arrival times of seismic phases identified in data recorded by ocean bottom seismometers (OBS) are used to image relatively deep structures. Most WAS experiments have relatively low redundancy and produce robust velocity models of limited resolution. The shallow subsurface is also commonly studied with multichannel seismic (MCS) data recorded by towed streamers, a technique that is highly complementary to WAS. In this case, the recording of refractions as first arrivals is limited primarily by the streamer length and by features like water depth and the velocity structure and, in general, most refractions are masked by reflections and noise. However, MCS data of variable quality are available in many regions where no other data exist and previous work has shown that these data can also be used to retrieve velocity models through traveltime and full waveform inversion provided that first arrival information is properly extracted from the record sections. The most widely used tool to extract refractions as first arrivals from MCS recordings is the so-called downward continuation technique, which is designed for redatuming streamer field data to the seafloor. In this new virtual configuration, the early refractions transform to seismic phases that are becoming visible as first arrivals from nearly zero offset, facilitating their identification and use in traveltime tomography. However, there is limited literature, let alone available codes, to be used with the available MCS data sets. This work presents a user-friendly open source HPC software for redatuming 2-D streamer field data to the sea bottom for any seafloor relief. The main ingredient is the acoustic wave equation used backwards in time, allowing first the redatuming of the receivers and then the redatuming of the sources. Assessment tools are provided to evaluate the information available after redatuming for specific data acquisition configurations. Also, we present a step-by-step analysis that defines the most important features that influence the quality of the virtual, redatumed recordings.

The Rift and Continent‐Ocean Transition Structure Under the Tagus Abyssal Plain West of the Iberia

AbstractThe West Iberia margin is the focus of intense research since the 1980s, with some of the most exemplary geophysical cross‐sections and drilling expeditions. Those data sets have been used to create conceptual models of rifting used as a template to interpret margins worldwide. We present two collocated ∼350 km long lines of multi‐channel seismic (MCS) streamer data and wide‐angle seismic (WAS) data collected across the Tagus Abyssal Plain (TAP). We use travel‐times of first arrivals identified at WAS and reflected seismic phases identified at both WAS and MCS records to jointly invert for the P wave velocity (Vp) distribution and the geometry of a sediment unconformity, the top of the basement, and the Moho boundary. The Vp model shows that the TAP basement is more complex than previously inferred, presenting abrupt boundaries between five domains. Domain I under the foot of the slope and Domain III under the abyssal plain display Vp values and gradients of thin continental crust. In between, Domain II displays a steep Vp gradient and high Vp values at shallow depth that support that basement is made of exhumed partly serpentinized mantle. Domain IV and Domain V, further oceanward, have oceanic crust Vp structure. The new results support an unanticipated complex rift history during the initial separation of Iberia and America. We propose a geodynamic scenario characterized by two phases of extension separated by a jump of the locus of extension, caused by the northward propagation of the oceanic spreading center during the J‐anomaly formation, which terminated continental rifting.

Temporal Variability of Thermohaline Fine-Structure Associated With the Subtropical Front Off the Southeast Coast of New Zealand in High-Frequency Short-Streamer Multi-Channel Seismic Data

Seismic oceanography generally makes use of multi-channel seismic reflection data sourced by air gun arrays and long hydrophone streamers to image oceanographic water masses and processes—often piggybacking on surveys that target deeper geological features below the seafloor. However, due to the acquisition methods employed, shallow (upper 200 m or so) regions of the ocean can be poorly imaged with this technique, and resolution is often lower than desirable for imaging fine-structure within the water column. In 2012, we collected a set of higher-resolution seismic lines off the southeast coast of New Zealand, with a generator-injector airgun source and hydrophone streamer configuration designed to improve images of shallower water masses and their boundaries. The seismic lines were acquired with coincident expendable bathythermograph deployments which provides direct ties between physical oceanographic data and seismic data, allowing for definitive identification of the Subtropical Front and associated water masses in the subsurface. Repeat acquisition along the same transect shows significant temporal variability on the scale of hours, illustrating the highly dynamic nature of this important ocean boundary. Comparisons to conventional low-frequency seismic data in the same location show the value of high-resolution acquisition methods in imaging the near-surface of the ocean and allowing subsurface features to be linked to their expressions at the surface.

A Complex Structure in the northwestern of Sumatra Fore-arc

The Sumatran offshore region is known as an active seismogenic zone. Consequently, there were many records of great events that caused a big catastrophe, such as the 2004 Sumatra-Andaman earthquake. In order to investigate the geometry and nature of the earthquake ruptures and structures, we utilize the bathymetry and Multi-Channel Seismic (MCS) profiles from the SO-198 research project in 2008 (R/V Sonne) and KH-10-5 research project in 2010 (R/V Hakuho-Maru). We find that the area along the Sunda Trench to the fore-arc high in the northwestern Sumatra are characterized by parallel anticlinal ridges with distinctive vergence regimes. These vergence regimes classified as seaward, mix and landward vergence system. Our data interpretation recognized structures such as Main thrust, Lower thrust, Middle thrust, Upper thrust, piggy-back basin and West Andaman Fault within this region. Our conclusion suggest that the anticlinal seaward and landward vergence are considered as the product of recent deformation rupture. The co-seismic slip from the past earthquake ruptures tends that it was occurred from the deeper part in the entire northwestern of Sumatra fore-arc.

Multichannel Seismic Reflection Data Processing: Multiple Attenuation of Line 250 at the Coast of Southern California Using Hyperbolic Radon Transform

Multichannel seismic reflection data processing of line 250 at the Coast of Southern California has been done by using Echos®. The purpose of this research is to separate multiple reflections from the primary reflection. Multiple reflections yield dramatic effect especially on marine seismic survey. Multiple reflections in marine seismic data is the noise caused by waves that trapped in a layer of sea water or rock layers. One of geophysics exploration problem is removing multiple reflections from primary reflection. Multiple reflections often destructively interfere with the primary reflection making interpretation difficult, so it’s needed to get seismic section free from multiple reflections. There are some methods to attenuate the multiple reflections, one of them is Hyperbolic Radon transform. Hyperbolic Radon transform method works by transforming the data from time-offset domain into the Radon domain. Primary reflection and multiple reflections can be separated into Radon domain. In the end of this research, seismic section before and after multiple attenuation using hyperbolic Radon transform were compared. Based on the result of processing, it can be concluded that hyperbolic Radon transform is effective to attenuate multiple reflections at 0,24-l,25s TWT. But, it can eliminate some of data and makes seismic section seen unclear.

Igneous Activity and Structural Development of the Mianhua Terrace, Offshore North Taiwan

Using bathymetric and multichannel seismic (MCS) data, we explored the volcanic influence on the bathymetric and stratigraphic features of the Mianhua Terrace. The Mianhua Terrace occupies the marine counterpart of the Northern Taiwan Volcanic Zone (NTVZ) along the collapsed Taiwan orogenic wedge and is dominated by post-collisional magmatism and extensional structures. The bathymetric data showed several semicircular-shaped features near the shelf break. The MCS profiles showed that the Pleistocene unconformity buried beneath the Mianhua Terrace is partly difficult to observe due to seafloor multiples, suggesting that the seafloor is dominated by physically hard lithology, probably volcanic lavas. We interpreted the high-amplitude reflectors and their projected seafloor relief as intrusive sills and associated extrusive edifice. Similarly, we interpreted high-amplitude reflectors in the vicinity of normal faults as intrusive sills emplaced and facilitated by fault structures. A volcanic or hydrothermal mound was also recognized. We propose that the Mianhua Terrace is a breached ramp in a transfer zone between the tips of two successive normal faults along the shelf break. Once the fault tips reactivate and extend toward each other, the Mianhua Terrace may continue to collapse, leading to catastrophic volcanic or associated hydrothermal events.

Gas Emissions in a Transtensile Regime Along the Western Slope of the Mid-Okinawa Trough

Gas emissions from the seabed are favored by tectonically active settings and their distribution is often linked to the nearby faults. Here we use the multi-beam echo-sounder (MBES) and the multi-channel seismic (MCS) data and a sediment core to show multiple gas emissions near the fault complex out of the shelf of the Mid-Okinawa Trough. The features indicating the gas emissions include 1) a set of the conical positive reliefs at the seabed, 2) the bundle-shaped clusters of the high-backscattering intensities in the water column, and 3) the sub-circular medium-to high-backscattering patches at the level of the seabed. These features together show that the free gases can escape from the marine sediments then rise in the water column at present, while some other gases trapped in the sub-seafloor sediment might contribute to the precipitation of the authigenic carbonates in the past. The spatial relationship between the gas emissions and the faults suggests that the faulting driven by the back-arc extension should provide the permeable migration pathways for the gas emissions to operate, and thus determines where most of them could potentially occur. The area surrounding the restraining bend concentrates part of the gas emissions rather than along the fault lines, due to the lateral compression and the structural complexity. This is demonstrated by the results of the numerical model of finite element method (FEM), which shows two gas emissions are within the compressed zone of the modeled restraining step-over. This study provides new evidence of the role of the tectonic stresses in determining the sites of degassing of marine sediments.

Subducting oceanic basement roughness impacts on upper-plate tectonic structure and a backstop splay fault zone activated in the southern Kodiak aftershock region of the Mw 9.2, 1964 megathrust rupture, Alaska

AbstractIn 1964, the Alaska margin ruptured in a giant Mw 9.2 megathrust earthquake, the second largest during worldwide instrumental recording. The coseismic slip and aftershock region offshore Kodiak Island was surveyed in 1977–1981 to understand the region’s tectonics. We re-processed multichannel seismic (MCS) field data using current standard Kirchhoff depth migration and/or MCS traveltime tomography. Additional surveys in 1994 added P-wave velocity structure from wide-angle seismic lines and multibeam bathymetry. Published regional gravity, backscatter, and earthquake compilations also became available at this time.Beneath the trench, rough oceanic crust is covered by ∼3–5-km-thick sediment. Sediment on the subducting plate modulates the plate interface relief. The imbricate thrust faults of the accreted prism have a complex P-wave velocity structure. Landward, an accelerated increase in P-wave velocities is marked by a backstop splay fault zone (BSFZ) that marks a transition from the prism to the higher rigidity rock beneath the middle and upper slope. Structures associated with this feature may indicate fluid flow. Farther upslope, another fault extends >100 km along strike across the middle slope. Erosion from subducting seamounts leaves embayments in the frontal prism.Plate interface roughness varies along the subduction zone. Beneath the lower and middle slope, 2.5D plate interface images show modest relief, whereas the oceanic basement image is rougher. The 1964 earthquake slip maximum coincides with the leading and/or landward flank of a subducting seamount and the BSFZ. The BSFZ is a potentially active structure and should be considered in tsunami hazard assessments.

A reliable velocity estimation in a complex deep-water environment using downward continued long offset multi-channel seismic (MCS) data

The estimation of a reliable velocity–depth model from towed streamer marine seismic data recorded in deep water, especially with a complex seafloor environment, is challenging. The determination of interval velocities from the normal move-out (NMO) of the reflected seismic signals for shallow reflectors (<1 km below the seafloor) is compromised by the combination of a long wave path in the water column and the complex ray paths due to topography, leading to small move-out differences between reflectors. Furthermore, low sediment velocities and deep water produce refraction arrivals only at limited far offsets that contain information about deeper structures. Here, we present an innovative method where towed streamer seismic data are downward continued to the seafloor leading to the collapse of the seafloor reflection and the emergence of refraction events as first arrivals close to zero offset, which are used to determine a high-resolution near surface velocity–depth model using an efficient tomographic method. These velocities are then used to perform pre-stack depth migration. We found that the velocity–depth model derived from tomography of downward continued towed streamer data provides a far superior pre-stack depth migrated image than those produced from velocity–depth models derived from conventional velocity estimation techniques.

Structural controls on widespread methane seeps in the back-arc basin of the Mid-Okinawa Trough

Methane trapped in marine sediments surrounding continental margins can be driven by tectonic stress to escape into the oceans. What geological processes control their spatial occurrences in an extension-dominated tectonic setting is not fully understood. Here we use the multibeam echo-sounder (MBES) and the multichannel seismic (MCS) data to reveal the distribution of the gas seeps in the back-arc basin of the Mid-Okinawa Trough in the years 2013–2016. They are manifested by (a) the gas plumes in the water column, occurring either in isolation or as clusters; (b) the irregularly shaped areas of the medium- to the high-backscatter intensity at the level of the seabed and (c) some seismic gas chimneys below these areas. The spatial combination of these signature points toward the widespread upward migration of the gas-rich pore fluids. Two types of gas seeps (A and B) sustained by methane supply have been proposed. The results of the isotopic analysis (δ13CCH4 and δDCH4) show that methane having a mixture of thermogenic and microbial sources migrated upwards along the faults to feed type A gas seeps, while in type B ones the thermogenic methane preferentially migrated through the fracture clusters embodied as the seismic gas chimneys. The tensile faults formed during the ongoing rifting episode of the incipient back-arc basin and provided the permeable migration conduits for the gases trapped in the shallow subsurface. We propose that this faulting driven by back-arc extension determines, to a large extent, the potential locations of the temporal gas seeps in the Mid-Okinawa Trough. Some of the other free gases accumulated in the less faulted sediments and the spatial distribution of their seepage is controlled by the topographic high. Tensile faulting can disturb the overpressure build-up within the shallow sediments by releasing it prior to the seal failure in the extension-dominated regime.