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Seismic precursors linked to super-critical fluids at oceanic transform faults
Large earthquakes on mid-ocean ridge transform faults are
commonly preceded by foreshocks[superscript 1â3] and changes in the
seismic properties of the fault zoneÂł. These seismic precursors
could be linked to fluid-related processes[superscript 2,3]. Hydrothermal
fluids within young, hot crust near the intersection of oceanic
transform faults are probably in a supercritical conditionâŽ. At
constant temperature, supercritical fluids become significantly
more compressible with decreasing pressure, with potential
impacts on fault behaviour. Here we use a theoretical model
to show that oceanic transform faults can switch from
dilatant and progressive deformation to rupture in response
to fluid-related processes. We assume that the fault core
material behaves according to a Cam-clay-typeâ” constitutive
law, which is commonly used to account for the behaviour of
clays. According to our model, we find that the fault is initially
stable, with stresses gradually increasing over a timescale
of years in response to tectonic loading. The fault evolves
into a metastable phase, lasting a few days, during which
the fault rocks dilate and pore pressures decrease, causing
the compressibility of the supercritical fluids to increase. This
in turn triggers fault-slip instability that creates foreshock
swarms. In the final phase, the fault fails in the mainshock
rupture. Our results imply that seismic precursors are caused
by changes in fluid pressure which result in variations in
fluid compressibility, in response to rock deformation just
before rupture
Geochemical Dynamics of the Natural-Gas Hydrate System in the Sea of Marmara, Offshore Turkey
Interseismic strain build-up on the submarine North Anatolian Fault offshore Istanbul
Using offshore geodetic observations, we show that a segment of the North Anatolian Fault in
the central Sea of Marmara is locked and therefore accumulating strain. The strain accumulation
along this fault segment was previously extrapolated from onshore observations or
inferred from the absence of seismicity, but both methods could not distinguish between fully
locked or fully creeping fault behavior. A network of acoustic transponders measured crustal
deformation with mm-precision on the seafloor for 2.5 years and did not detect any significant
fault displacement. Absence of deformation together with sparse seismicity monitored
by ocean bottom seismometers indicates complete fault locking to at least 3 km depth
and presumably into the crystalline basement. The slip-deficit of at least 4m since the last
known rupture in 1766 is equivalent to an earthquake of magnitude 7.1 to 7.4 in the Sea of
Marmara offshore metropolitan Istanbul
Multidisciplinary investigation on cold seeps with vigorous gas emissions in the Sea of Marmara (MarsiteCruise): Strategy for site detection and sampling and first scientific outcome
MarsiteCruise was undertaken in October/November 2014 in the Sea of Marmara to gain detailed insight into the fate of fluids migrating within the sedimentary column and partially released into the water column. The overall objective of the project was to achieve a more global understanding of cold-seep dynamics in the context of a major active strike-slip fault. Five remotely operated vehicle (ROV) dives were performed at selected areas along the North Anatolian Fault and inherited faults. To efficiently detect, select and sample the gas seeps, we applied an original procedure. It combines sequentially (1) the acquisition of ship-borne multibeam acoustic data from the water column prior to each dive to detect gas emission sites and to design the tracks of the ROV dives, (2) in situ and real-time Raman spectroscopy analysis of the gas stream, and (3) onboard determination of molecular and isotopic compositions of the collected gas bubbles. The in situ Raman spectroscopy was used as a decision-making tool to evaluate the need for continuing with the sampling of gases from the discovered seep, or to move to another one. Push cores were gathered to study buried carbonates and pore waters at the surficial sediment, while CTD-Rosette allowed collecting samples to measure dissolved-methane concentration within the water column followed by a comparison with measurements from samples collected with the submersible Nautile during the Marnaut cruise in 2007. Overall, the visited sites were characterized by a wide diversity of seeps. CO2- and oil-rich seeps were found at the westernmost part of the sea in the Tekirdag Basin, while amphipods, anemones and coral populated the sites visited at the easternmost part in the Cinarcik Basin. Methane-derived authigenic carbonates and bacterial mats were widespread on the seafloor at all sites with variable size and distributions. The measured methane concentrations in the water column were up to 377 ÎŒmol, and the dissolved pore-water profiles indicated the occurrence of sulfate depleting processes accompanied with carbonate precipitation. The pore-water profiles display evidence of biogeochemical transformations leading to the fast depletion of seawater sulfate within the first 25-cm depth of the sediment. These results show that the North Anatolian Fault and inherited faults are important migration paths for fluids for which a significant part is discharged into the water column, contributing to the increase of methane concentration at the bottom seawater and favoring the development of specific ecosystems
Ocean crust formation processes at very slow spreading centers: A model for the Mohns Ridge, near 72°N, based on magnetic, gravity, and seismic data
International audienceThe Mohns Ridge, in the Norwegian Greenland Sea, is one of the slowest spreading centers of the mid-ocean ridge system (8 mm/yr half rate). Sea Beam data acquired with R/V Jean Charcot near 72ĂžN show that its rift valley floor is characterizexl by en 6chelon volcanic ridges, oriented obliquely relatively to the average strike of the ridge axis. These ridges are regularly spaced along the axis, about every 40 km, and are separated by nontransform discontinuities. Sharp positive magnetic anomalies, centered over the topographic highs, suggest that they are eruptive centers, considered as the surficial expression of active spreading cells. Over the rift valley, Bouguer anomalies obtained by subtracting the predicted effects due to seafloor topography from the measured free-air gravity field are consistent with a low density body within the lower crust having its upper surface lying at about 2 km below the sea surface. This body, if it exists, probably corresponds to the zone of low viscosity that can be inferred from the model of Chen and Morgan (1990b), which predicts the existence of a decoupling region, between the upper crust and the asthenophere below. Its width varies rapidly along-strike, from less than about 5 km to more than 15 km. In plan view, it has a pinch and swell form, which defines a series of spreading cells, the center of one cell being where the Bouguer anomaly is widest. Short wavelength (less than 10 to 20 kin) along-strike variations, such as Bouguer anomaly lows centered on the topographic highs, reflect local effects associated with the presence of the eruptive centers. Seismic tomography data from a 20 x 10 km active oblique volcanic ridge near 72Ăž22'N tend to indicate that the links between the main, low-velocity body at depth, and the magma injections centers which lie within the rift valley inner floor are probably complex
Marmesonet Leg I. Cruise Report. November 4th - November 25th, 2009
The MARMESONET cruise is part of the Marmara Demonstration Mission Program supported by ESONET Network of Excellence (European Seafloor Observatory Network), within the 6th European Framework Programme. Main partners are: Ifremer, CNRS/CEREGE, Istanbul Technical University, TUBITAK, Institute of Marine Science and Technology of Dokuz EyluÌl Universitesi (Izmir), INGV (Rom) and ISMAR (Bologna). Marmesonet is also the follow-on of the Franco-Turk collaborative programme that resulted in numerous cruises in the Sea of Marmara since 2000. The objectives of the MARMESONET cruise were: 1) to study the relationship between fluids and seismicity along the Sea of Marmara fault system ; 2) to carryout site surveys prior to the implementation of permanent seafloor observatories in the Marmara Sea through ESONET. The cruise is divided in 2 parts: Leg I (from november 4th to november 25th, 2009), mainly dedicated to: i) the high resolution bathymetry at potential sites of interest for future permanent instrumentation using the Autonomous Unmanned Vehicle (AUV)Asterx of Ifremer/Insu ; ii) the systematic mapping of the gas emissions sites on the Marmara seafloor ; iii) the deployment of the Bubble Observatory Module (BOB) in the Ăinarçik basin. Leg II (from november 28th november to december 14th, 2009), for 3D, High Resolution Seismic imagery of the fluid conduits below the observatory site planned at the Western High. The present report only concerns Leg I. A total of 19 dives were completed during Leg I: 16 with the multibeam echosounder SIMRAD EM2000 (200 kHz), among which 12 were successful and 4 failed ;3 with the CHIRP sĂ©diment penetrator (1 test dive and 2 operational, both were unfortunately with early stop recording). Main results are: The absence of recent, visible deformation on the segment south of Istanbul. Wether or not this segment is locked or creeping remains an open question. The site south of Istanbul thus requires a massive effort to assess the deformation, particularly through submarine geodesy and piezometry. The plausible presence of a 4 km, right-lateral offset on the Western High, between N30 oriented structures related to cold seeps. Gas emission sites are systematically related to zones of High reflectivity mapped on the AUV imagery AUV imagery reveals the traces of intensive, human activity, which shows the necessity to ensure the security of the future cables by enforcing a clearance area Last but not least, the exact position of the future observatories is now established, at the Central High and at the Western High sites.La campagne constitue lâune des missions de dĂ©monstration soutenues par ESONET. Elle rĂ©sulte dâun partenariat entre lâIfremer, le CNRS, lâINSU, lâUniversitĂ© Technique dâIstanbul, lâInstitut des Sciences Marines dâIzmir, le CNR-ISMAR (Bologne) et lâINGV (Rome). La plupart des objectifs du premier leg de la campagne Marmesonet, du 4 au 25 novembre 2009, ont Ă©tĂ© atteints, grĂące Ă trois facteurs principaux: i) la mĂ©tĂ©o exceptionnellement favorable ; ii) la bienveillance des garde-cĂŽtes de la Marine Turque ; iii) le professionnalisme des Ă©quipes (Ă©quipage et sĂ©dentaires
A review of 20Â years (1999â2019) of TurkishâFrench collaboration in marine geoscience research in the Sea of Marmara
International audienceThis paper retraces the history and main achievements of the ongoing Turkish-French collaboration in marine geoscience research in the Sea of Marmara, which was initiated in the aftermath of the 1999, Izmit and Duzce earthquakes. The collaboration resulted in nine large oceanographic cruises along with six recovery operations involving diverse vessels, and in the participation in two major EU-funded programmes (ESONET-NoE and MARSITE) and to one bilateral project, e.g. the MAREGAMI Project, co-funded by TUBITAK and ANR (the Turkish and French national funding agencies for research, respectively). In this paper, we review the major scientific contributions on the tectonic evolution of the North-Anatolian Fault in the Marmara Region; on the relationships between faulting, seismicity, fluids and ecosystems; and on paleo-seismology and paleo-oceanography in the Sea of Marmara
Seismic imaging of the ocean internal structure: A new tool in physical oceanography?
Reflection seismics has been intensively used for the last four decades by marine geologists and geophysicists for imaging Earth structures below the seafloor. Because their subject of interest is below the sea bottom, solid Earth scientists do not usually consider the seismic signal propagating in the water column and most often do not even record it, in order to save data storage space.
Two physical oceanographers, Gonella and Michon [1988],first reported internal waves revealed by reflection seismics in the northeastern Atlantic. Only recently, though, has the scientific community realized the importance of this issue, after Holbrook et al. [2003] published reflection seismic sections of the water column off Newfoundland, Canada, showing reflectors related to the major oceanographic front between the Labrador Current and the North Atlantic Current
Estimation of methane bubbles volumetric flows from acoustic data of water column
Since few years, advances in technology and computer processing of devices initially designed for seafloor mapping allow to carry out acoustic surveys of the entire water column. These are revealing that gassy sediments and free gas emissions from the seafloor are much more frequent than expected which call for new experiments to characterize these plumes. Beside the primary scientific need to position the gas emission in the bathymetry of the area, quantification of bubbles volumetric flows is one of the important scientific issue to solve. The work proposed here explore the feasibility of assessing the volume of bubbles plumes and their temporal variations from acoustic data with a methodology similar to that used in acoustic fisheries, i.e. by inverse modeling of the acoustic backscattered data. Sounder used in these experiments is a split-beam fisheries sounder at 120 kHz installed on an autonomous module named BOB (Bubble OBservation Module). Experiments in sea water tank were done to have a validation of the inverse modeling on controlled bubbles flows, the method was then applied to in situ natural gas seeping from the seafloor obtained during a survey in Marmara Sea in 2009
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