Çağatay, Namik M.

In their comment, Yamomoto and co-authors are primarily concerned with the existence and effect of large values of minimum and maximum phase residuals in our analysis and locations using the 2014 observations, as listed in Tables S7 and S8 in the supplementary material of our paper (Batsi et al, 2018). We retain these large residuals in the tables and analysis since they have vanishingly small effect on the NonLinLoc locations, since the used, equal differential time (EDT) location algorithm (Lomax, 2008; Lomax et al., 2009) is highly robust to outlier readings. In the case of our Marmara study, phases with residuals larger than 1-2sec have near zero weight in the locations and corrected phase data. However, we agree the larger residuals may have had adverse effect on the generation of station corrections, though this, in turn, would also be mitigated by the robust location procedure. As a result, we consider that the location discrepancies between Yamomoto et al (2017) and Batsi et al. (2018) are not due to effects of excessively large residuals on the station corrections or locations. Instead, we propose that, as in many seismicity studies, error and uncertainty in the absolute hypocenter locations is primarily related to error in the velocity model and insufficient geometrical coverage of the source zones by the available seismic stations. To support this proposition, and following the recommendation of Yamamoto et al., we recalculate station corrections for our 2014 data set and then relocate the 14 common events (Table A) that were located by both Yamamoto et al. (2017) and ourselves (see Table 9 in Batsi et al., 2018, with correct Yamomoto’s location for event 3: 40.8058N, 27.9504E, 13.411km). We first generate station corrections as described in Batsi et al. (2018) using all events from 2014 which comply with the Batsi et al. (2018) location criteria (number of stations ≥ 5; number of phases ≥ 6; (3) root mean square (rms) location error ≤ 0.5s; azimuthal gap ≤ 180°), except that we explicitly exclude from the analysis any P or S residuals > 3.0s when generating station corrections (Table B). We then relocate in the high‐resolution, 3D, P‐velocity model, as described in Batsi et al. (2018), the 14 common events using these station corrections. Figure 1 shows, for the 14 common events listed I Table A, the absolute NonLinLoc maximum likelihood and expectation hypocenters, and location probability density (pdf) clouds for our absolute relocations, along with the corresponding Yamamoto et al. (2017) double-difference relocations and Batsi et (2018) relative (NonDiffLoc) locations. For sake of clarity, calculation results are detailed in Figure 2 for each individual event (1 to 14). The full information on the earthquake location spatial uncertainty is shown by the pdf clouds, while the maximum-likelihood hypocenter is the best solution point and the expectation hypocenter shows a weighted mean or “center of mass” of the cloud. The pdf clouds show a large uncertainty in hypocenter depth, the formal standard error in depth ranges from 2-9km. There is also a large separation between the maximum likelihood and expectation hypocenters for some events. These results underline the large uncertainty in depth determination and corresponding instability in any one-point measure chosen as a hypocenter. However, despite these uncertainties and instabilities, the Yamamoto et al. (2017) hypocenters remain generally deeper than the maximum likelihood and expectation hypocenters for our relocations, positioned towards the deeper uncertainty limits of our locations (e.g. the lower portion of the pdf clouds), and the Yamamoto et al. (2017) epicenters fall near the Main Marmara fault (MMF) while our relocated epicenters define off axis seismicity, along secondary faults from the MMF system. Thus our relocated events, which explicitly exclude excessively large residuals, still show differences with the Yamamoto et al. (2017) events, but not as large as those we found in our original study. Based on our recalculated NonLinLoc absolute locations, we suspect that  Yamamoto et al (2017) results are systematically too deep and Batsi et al (2018) systematically too shallow, compared to what should be expected. These differences in epicenter and depth, along with the size and shape of the pdf clouds for our relocations, are most easily explained by differences in the 3D velocity models and by differences in available stations and the consequent network geometry . However, while the epicentral distances at most of the OBS stations are shorter than the focal depths, as noted by Yamomoto et al., the elongation of our pdf clouds in depth suggests that an increase in network aperture with more distant stations, along with an accurate 3D model, is required to better constrain depth. High-resolution earthquake epicenter and depth determinations below the Sea of Marmara is a difficult problem, yet of critical importance. To better understand why the two studies produce different results, and to obtain the best possible locations, the best action is to increase the number of constraints by merging the two OBS datasets, and examine, step by step, the effects of locations methods, network geometry and 3D velocity models from the two studies. Sharing the data (or phase picks and model) would provide an unique opportunity to give real, direct insight into these issues. We suspect that epicenters will shift as a function of used velocity model and station set, and that in all cases depth uncertainty is large, as is clearly represented in the NonLinLoc location, pdf clouds, while linearized location error estimates usually show lower uncertainty.

Understanding micro-seismicity is a critical question for earthquake hazard assessment. Since the devastating earthquakes of Izmit and Duzce in 1999, the seismicity along the submerged section of North Anatolian Fault within the Sea of Marmara (comprising the “Istanbul seismic gap”) has been extensively studied in order to infer its mechanical behaviour (creeping vs locked). So far, the seismicity has been interpreted only in terms of being tectonic-driven, although the Main Marmara Fault (MMF) is known to strike across multiple hydrocarbon gas sources. Here, we show that a large number of the aftershocks that followed the M 5.1 earthquake of July, 25th 2011 in the western Sea of Marmara, occurred within a zone of gas overpressuring in the 1.5–5 km depth range, from where pressurized gas is expected to migrate along the MMF, up to the surface sediment layers. Hence, gas-related processes should also be considered for a complete interpretation of the micro-seismicity (~M < 3) within the Istanbul offshore domain.

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 μmol, 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

A detailed study, based on ocean-bottom seismometers (OBSs) recordings from two recording periods (3.5 months in 2011 and 2 months in 2014) and on a high-resolution, 3D velocity model, is presented here, which provides an alternative view of the microseismicity along the submerged section of the North Anatolian fault (NAF) within the western Sea of Marmara (SoM). The nonlinear probabilistic software packages of NonLinLoc and NLDiffLoc were used for locating earthquakes. Only earthquakes that comply with the following location criteria (e.g., representing 20% of the total amount of events) were considered for analysis: (1) number of stations ≥ 5; (2) number of phases ≥ 6, including both P and S; (3) root mean square (rms) location error ≤ 0:5 s; and (4) azimuthal gap ≤ 180°. P and S travel times suggest that there are strong velocity anomalies along the Western High, with low Vp, low Vs, and ultra-high Vp=Vs in areas where mud volcanoes and gas-prone sediment layers are known to be present. The location results indicate that not all earthquakes occurred as strike-slip events at crustal depths (> 8 km) along the axis of the Main Marmara fault (MMF). In contrast, the following features were observed: (1) a significant number of earthquakes occurred off-axis (e.g., 24%), with predominantly normal focal mechanisms, at depths between 2 and 6 km, along tectonically active, structural trends oriented east–west or southwest–northeast, and (2) a great number of earthquakes was also found to occur within the upper sediment layers (at depths < 2 km), particularly in the areas where free gas is suspected to exist, based on high-resolution 3D seismics (e.g., 28%). Part of this ultra-shallow seismicity appears to occur in response to deep earthquakes of intermediate (ML ∼ 4–5) magnitude. Resolving the depth of the shallow seismicity requires adequate experimental design ensuring source–receiver distances of the same order as hypocentral depths. To reach this objective, deep-seafloor observatories with a sufficient number of geophone sensors near the fault trace are needed.

Episodic gas seepage occurs at the seafloor in the Gulf of Izmit (Sea of Marmara, NW Turkey) along the submerged segment of the North Anatolian Fault (NAF), which ruptured during the 1999 Mw7.4 Izmit earthquake, and caused tectonic loading of the fault segment in front of the Istanbul metropolitan area. In order to study gas seepage and seismic energy release along the NAF, a multiparametric benthic observatory (SN-4) was deployed in the gulf at the western end of the 1999 Izmit earthquake rupture, and operated for about 1 yr at 166 m water depth. The SN-4 payload included a three-component broad-band seismometer, as well as gas and oceanographic sensors. We analysed data collected continuously for 161 d in the first part of the experiment, from 2009 October to 2010 March. The main objective of our work was to verify whether tectonic deformation along the NAF could trigger methane seepage. For this reason, we considered only local seismicity, that is, within 100 km from the station. No significant (ML ≥ 3.6) local earthquakes occurred during this period; on the other hand, the seismometer recorded high-frequency SDEs (short duration events), which are not related to seismicity but to abrupt increases of dissolved methane concentration in the sea water that we called MPEs (methane peak events). Acquisition of current velocity, dissolved oxygen, turbidity, temperature and salinity, allowed us to analyse the local oceanographic setting during each event, and correlate SDEs to episodic gas discharges from the seabed. We noted that MPEs are the result of such gas releases, but are detected only under favourable oceanographic conditions. This stresses the importance of collecting long-term multiparametric time-series to address complex phenomena such as gas and seismic energy release at the seafloor. Results from the SN-4 experiment in the Sea of Marmara suggest that neither low-magnitude local seismicity, nor regional events affect intensity and frequency of gas flows from the seafloor.

In this paper we provide an overview of new knowledge on oxygen depletion (hypoxia) and related phenomena in aquatic systems resulting from the EU-FP7 project HYPOX (“In situ monitoring of oxygen depletion in hypoxic ecosystems of coastal and open seas, and landlocked water bodies”, www.hypox.net). In view of the anticipated oxygen loss in aquatic systems due to eutrophication and climate change, HYPOX was set up to improve capacities to monitor hypoxia as well as to understand its causes and consequences. Temporal dynamics and spatial patterns of hypoxia were analyzed in field studies in various aquatic environments, including the Baltic Sea, the Black Sea, Scottish and Scandinavian fjords, Ionian Sea lagoons and embayments, and Swiss lakes. Examples of episodic and rapid (hours) occurrences of hypoxia, as well as seasonal changes in bottom-water oxygenation in stratified systems, are discussed. Geologically driven hypoxia caused by gas seepage is demonstrated. Using novel technologies, temporal and spatial patterns of watercolumn oxygenation, from basin-scale seasonal patterns to meter-scale sub-micromolar oxygen distributions, were resolved. Existing multidecadal monitoring data were used to demonstrate the imprint of climate change and eutrophication on long-term oxygen distributions. Organic and inorganic proxies were used to extend investigations on past oxygen conditions to centennial and even longer timescales that cannot be resolved by monitoring. The effects of hypoxia on faunal communities and biogeochemical processes were also addressed in the project. An investigation of benthic fauna is presented as an example of hypoxia-devastated benthic communities that slowly recover upon a reduction in eutrophication in a system where naturally occurring hypoxia overlaps with anthropogenic hypoxia. Biogeochemical investigations reveal that oxygen intrusions have a strong effect on the microbially mediated redox cycling of elements. Observations and modeling studies of the sediments demonstrate the effect of seasonally changing oxygen conditions on benthic mineralization pathways and fluxes. Data quality and access are crucial in hypoxia research. Technical issues are therefore also addressed, including the availability of suitable sensor technology to resolve the gradual changes in bottom-water oxygen in marine systems that can be expected as a result of climate change. Using cabled observatories as examples, we show how the benefit of continuous oxygen monitoring can be maximized by adopting proper quality control. Finally, we discuss strategies for state-of-the-art data archiving and dissemination in compliance with global standards, and how ocean observations can contribute to global earth observation attempts.

We carried out a combined geophysical and gas-geochemical survey on an active fault strand along the North-Anatolian Fault (NAF) system in the Gulf of İzmit (eastern Sea of Marmara), providing for the first time in this area data on the distribution of methane (CH4) and other gases dissolved in the bottom seawater, as well as the CH4 isotopic composition. Based on high-resolution morphobathymetric data and chirp-sonar seismic reflection profiles we selected three areas with different tectonic features associated to the NAF system, where we performed visual and instrumental seafloor inspections, including in-situ measurements of dissolved CH4, and sampling of the bottom water. Starting from background values of 2-10 nM, methane concentration in the bottom seawater increases abruptly up to 20 nM over the main NAF trace. CH4 concentration peaks up to ~120 nM were detected above mounds related probably to gas and fluids expulsion. Methane is microbial (δ13CCH4: -67.3 and -76 ‰ vs. VPDB), and was found mainly associated with pre- Holocene deposits topped by a 10-20 m thick draping of marine mud. The correlation between tectonic structures and gas-seepages at the seafloor suggests that the NAF in the Gulf of İzmit could represent a key site for long-term combined monitoring of fluid exhalations and seismicity to assess their potential as earthquake precursors.

Methane-rich fluid vents have been widely observed and associated to active faults in the Sea of Marmara, along the submerged portion of the North Ana- tolian Fault (NAF). Episodic gas seepage also occurs in the Izmit Gulf, along the NAF segment that ruptured during the 1999 Izmit earthquake. This site is thus a unique area to test the hypothesis on the relation between strike-slip deforma- tion, seismic activity and gas expulsion within an active fault zone. A long-term multi-parametric experiment can be an effective way to study the irregular dy- namics of gas emission from seafloor and to understand its possible relation with seismic activity. A benthic seafloor observatory (SN-4) was deployed in the Izmit Gulf in 2009 using the R/V Urania as a demonstration mission in the framework of the EC ES- ONET (European Seas Observatory NETwork) project. Instrumental redundancy and specific cross-correlation of data from different sensors, proves to be funda- mental to distinguish actual seepage events from other signals related to ocean- ographic behaviour or even sensor biases. The observatory was equipped with a three component broad-band seismometer, a CTD with turbidity meter, two methane detectors, an oxygen sensor and a current-meter. All sensors installed on the observatory were managed by dedicated low-power electronics, which can manage a wide set of data streams with quite different sampling rates. A unique reference time, set by a central high-precision clock, is used to tag each datum. After six months of continuous monitoring, SN-4 was recovered in March 2010 in order to download the data and replace the batteries for a further six month mission period and finally recovered in October 2010. The data analysis clearly shows frequent degassing events, recorded as methane anomalies in seawater and as high-frequency short-duration signals recorded by the seismometer.. The time series of other oceanographic parameters (tempera- ture, oxygen concentration, turbidity and salinity) shows patterns that seem to be linked to both local gas seepage and to the circulation of water masses in the Gulf of Izmit. A comparative analysis of the various observables and their mutual correlation, can be a key tool to understand actual degassing events along the NAF. This analysis is first attempt in finding possible correlations be

Society’s needs for a network of in situ ocean observing systems cross many areas of earth and marine science. Here we review the science themes that benefit from data supplied from ocean observatories. Understanding from existing studies is fragmented to the extent that it lacks the coherent long-term monitoring needed to address questions at the scales essential to understand climate change and improve geo-hazard early warning. Data sets from the deep sea are particularly rare with long-term data available from only a few locations worldwide. These science areas have impacts on societal health and well-being and our awareness of ocean function in a shifting climate. Substantial efforts are underway to realise a network of open-ocean observatories around European Seas that will operate over multiple decades. Some systems are already collecting high-resolution data from surface, water column, seafloor, and sub-seafloor sensors linked to shore by satellite or cable connection in real or near-real time, along with samples and other data collected in a delayed mode. We expect that such observatories will contribute to answering major ocean science questions including: How can monitoring of factors such as seismic activity, pore fluid chemistry and pressure, and gas hydrate stability improve seismic, slope failure, and tsunami warning? What aspects of physical oceanography, biogeochemical cycling, and ecosystems will be most sensitive to climatic and anthropogenic change? What are natural versus anthropogenic changes? Most fundamentally, how are marine processes that occur at differing scales related? The development of ocean observatories provides a substantial opportunity for ocean science to evolve in Europe. Here we also describe some basic attributes of network design. Observatory networks provide the means to coordinate and integrate the collection of standardised data capable of bridging measurement scales across a dispersed area in European Seas adding needed certainty to estimates of future oceanic conditions. Observatory data can be analysed along with other data such as those from satellites, drifting floats, autonomous underwater vehicles, model analysis, and the known distribution and abundances of marine fauna in order to address some of the questions posed above. Standardised methods for information management are also becoming established to ensure better accessibility and traceability of these data sets and ultimately to increase their use for societal benefit. The connection of ocean observatory effort into larger frameworks including the Global Earth Observation System of Systems (GEOSS) and the Global Monitoring of Environment and Security (GMES) is integral to its success. It is in a greater integrated framework that the full potential of the component systems will be realised.