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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.

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.

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 fr

In this study we present a 3D P-wave upper-mantle tomography model of the SW Iberian margin and Alboran Sea based on teleseismic arrival times recorded by Iberian and Moroccan land stations and by a seafloor network deployed for 1 year in the Gulf of Cadiz area during the EC-NEAREST project. The 3D model was computed down to 600 km depth. The tomographic images exhibit significant velocity contrasts, as large as 3%, confirming the complex evolution of this plate boundary region. Prominent high-velocity anomalies are found beneath Betics-Alboran Sea, off-shore SW Portugal, and N Portugal, at sublithospheric depths. The transition zones between high and low velocity anomalies in SW and S Iberia are associated to the contact of oceanic and continental lithosphere. The fast structure below the Alboran Sea-Granada area depicts a L-shaped body steeply dipping from the uppermost mantle to the transition zone where it becomes less curved. This anomaly is consistent with the results of previous tomographic investigations and recent geophysical data such as stress distribution, GPS measurements of plate motion, and anisotropy patterns. In the Atlantic domain, under the Horseshoe Abyssal Plain, the main feature is a high-velocity zone found at uppermost mantle depths. This feature appears laterally separated from the positive anomaly recovered in the Alboran domain by the interposition of low-velocity zones which characterize the lithosphere beneath the SW Iberian peninsula margin, suggesting that there is no continuity between the high velocity anomalies of the two domains west and east of the Gibraltar Strait.

The gulf of Cadiz is a region considered as a complex seismic area, where strong earthquakes occur and where the plate boundary between the African and Eurasian plates is not exactly known. In this paper, we use high resolution seismic data recorded by a network of OBS stations deployed for one year in the Gulf of Cadiz as well as eight permanent Portugal land seismic stations. The OBS network was deployed within an experiment of the NEAREST project. Nearly 600 seismic events are extracted from the recorded data set and their analysis revealed that most of them occur at 20 to 80 km depths, with clusters of seismicity that occur mainly at the Gorringe Bank, within the SW segment of the Horseshoe fault and the Marques de Pombal Plateau and the S. Vicente Fault. A new NW-SE trend of seismicity has been revealed with depths that extend from 35 to 80 km. This seismicity trend is close and nearly parallel to the SWIM (South West Iberian Margin) faults lineament. We further present in this paper, the first regional-scale high resolution P- and S-velocity distributions across the Gulf of Cadiz region. These velocity models are obtained using three-dimensional seismic tomography to invert the OBS data-set. The results show that the patterns of anomalies in the Gulf of Cadiz are in general, oriented in NE-SW and NW-SE directions. They also show the presence of a low velocity zone (LVZ) to the SE of our study area. At shallow depth, this LVZ is interpreted as due to a large accumulation of sediments within the accretionary wedge, while at a greater crustal depth, it may reflect a continental crustal composition rather than an oceanic crust. Moreover, seismic velocity profiles show that under this region of the Cadiz Gulf, the Moho averages a 30-km depth. The Gorringe Bank and the Marquise de Pombal plateau are found to be deeply rooted and represent expressions of mantle uplifting. The association of these deep anomalies with active seismicity that occurs at their levels, indi

We present results from a seismicity study in the Gulf of Cadiz, one of the major potential source regions of Tsunamis close to the European coast. Between the summers 2007 and 2008 24 broadband ocean-bottom seismometers and the GEOSTAR deep sea observatory was deployed in the area within the framework of EU funded project NEAREST (Integrated observation from NEAR shore sourcES of Tsunamis: towards an early warning system; GOCE, contract n. 037110). In this contribution we focus on 36 of the larger events located by the Institute of Meteorology (IM) of Portugal inside the OBS network (magnitude up to ML 4.7). We re-located the events using only the OBS recordings and compare the results to the previous land-based locations. In contrast to the preliminary bulletin of IM, most of these events do not occur in the crust, but as deep as 40 to 60 km within the oceanic lithosphere. They form two clusters of events in the vicinity of the Horseshoe Fault Zone, one of the major faults identified in the bathymetry charts. Furthermore, focal mechanisms were analysed by P wave polarities, showing the potential to study source kinematics of low-magnitude earthquakes by means of OBS networks.

Bisher durchgeführte geophysikalische und geologische Untersuchungen im Golf von Cadiz geben uns eine Vorstellung von den größten aktiven Störungen, an denen zerstörerische Erdbeben stattfinden und damit verheerende Tsunamis ausgelöst werden können, vergleichbar den Ereignissen am 1. November 1755 in Lissabon und entlang der Küsten SW Europas und NW Afrikas. Jedoch sind die Störungskinematik und die seismische Aktivität bisher unzureichend bekannt, da die seismischen Stationen an Land keine genaue Lokalisierung und Bestimmung der Herdparameter für kleinere Ereignisse erlauben.Unter anderem deshalb wurde das durch die EU geförderte Projekt NEAREST (Integrated observation from NEAR shore sourcES of Tsunamis: towards an early warning system) ins Leben gerufen (GOCE, contract n. 037110). Eines der Hauptziele des Projektes ist die Charakterisierung der möglichen Tsunami-Quellen im Golf von Cadiz durch herdnahe kontinuierliche Beobachtungen der natür-lichen Seismizität. Dazu wurden für einen Zeitraum von elf Monaten das GEOSTAR Multiparameter-Tiefseeobservatorium (verantwortlich: INGV Rom) und 24 Breitband-Ozeanbodenseismometer (OBS) des deutschen DEPAS Gerätepools ausgesetzt. Die Stationen befanden sich in Wassertiefen zwischen 2000 und 5100 m. Zusammen mit dem dichten seismo-logischen Netz an Land erlaubt das OBS-Netzwerk die Lokalisierung und Charak-terisierung auch sehr kleiner Erdbeben (ML = 1) in bisher unerreichter Genauig-keit. Automatische Trigger und Spektro-gramme wurden zur Detektion bisher unbekannter Erdbeben verwendet.Eine der zentralen Fragen der Untersuchungen ist die maximale Herdtiefe der Beben unter dem Golf von Cadiz. Im Zeitraum September 2007 bis August 2008 wurden etwa 300 Ereignisse innerhalb des OBS-Netzwerkes durch die Portugiesischen Landstationen lokalisiert. Die Magnituden reichen von 1 bis 4.7 (ML, Quelle: Institute of Meteorology Lisbon, Portugal). Mit dem OBS-Netzwerk konnten eine Vielzahl von Ereignissen detektiert werden, die bisher nich

The paper presents an overview of recent seafloor long-term single-frame multiparameter platform developed in the framework of the European Commission and Italian projects starting from the GEOSTAR prototype. The main features of the different systems are described as well as the sea missions that led to their validation. The ORION seafloor observatory network recently developed, based on the GEOSTAR-type platforms and engaged in a deep-sea mission at 3300 m w.d. in the Mediterranean Sea, is also described.

The paper presents an overview of recent seafloor long-term single-frame multiparameter platform developed in the framework of the European Commission and Italian projects starting from the GEOSTAR prototype. The main features of the different systems are described as well as the sea missions that led to their validation. The ORION seafloor observatory network recently developed, based on the GEOSTAR-type platforms and engaged in a deep-sea mission at 3300 m w.d. in the Mediterranean Sea, is also described