A pockmark field in the Patras Gulf (Greece) and its activation during the 14/7/93 seismic event

During a recent oceanographical-geophysical survey carried out in the southeastern part of the Gulf of Patras in Western Greece for the construction of an outfall, an active pockmark field was found. The pockmark field was formed in soft layered Holocene silts. The pockmarks are associated with acoustic anomalies attributed to gas-charged sediments. The pockmarks vary in size and shape from 25 to 250 m in diameter and from 0.5 to 15 m in depth and are among the largest and deepest observed in the world.

On July 14th, 1993, during the survey, a major earthquake of magnitude 5.4 on the Richter scale occurred in the area. During the 24 hour period prior to the earthquake the bottom water temperature anomalously increased on three occasions, whilst for a few days after the earthquake it was noted that the majority of the pockmarks were venting gas bublles.

It is considered that the three abrupt sea-water temperature increases were probably the result of upward migrating high-temperature gas bubbles in the water column. It is further suggested that the earthquake was the triggering mechanism and that the gas expulsion was caused by the reduction in the pore volume in the sediments resulting from changes in the stress regime prior to the earthquake. Therefore, it can be suggested that in seismic areas adjacent to pockmark fields, earthquake prediction may be achieved by monitoring the water temperature and/or the rate of gas venting in the pockmark field.

Our analysis indicates that the pockmark field in the Patras Gulf has formed slowly during the Holocene by continuous gas venting, which is periodically being interrupted by short-duration events of enhanced gas seepage triggered by earthquakes.     

Tectono-sedimentary processes along an active marine/lacustrine half-graben margin: Alkyonides Gulf, E. Gulf of Corinth, Greece

ABSTRACT The Alkyonides half‐graben is separated from the Gerania Range to the south by active faults whose offshore traces are mapped in detail. The East Alkyonides and Psatha Faults have well‐defined, Holocene‐active tip zones and cannot be extrapolated from the onshore Skinos Fault into a single continuous surface trace. During the late Quaternary, catchments draining the step‐faulted range front have supplied sediment to alluvial fans along a subsiding marine ramp margin in the hangingwall of the Skinos Fault, to shelf ledge fans on the uplifting footwall to the East Alkyonides Fault and to the Alepochori submarine fan in the hangingwall of the latter. During late Pleistocene lowstand times (c. 70–12 ka), sediment was deposited in Lake Corinth as fan deltas on the subsiding Skinos shelf ramp which acted as a sediment trap for the adjacent 360 m deep submarine basin plain. At the same time, the uplifting eastern shelf ledge was exposed, eroded and bypassed in favour of deposition on the Alepochori submarine fan. During Holocene times, the Skinos bajada was first the site of stability and soil formation, and then of substantial deposition before modern marine erosion cut a prominent cliffline. The uplifting eastern shelf ledge has developed substantial Holocene fan lobe depositional sequences as sediment‐laden underflows have traversed it via outlet channels. We estimate mean Holocene displacement rates towards the tip of the Psatha Fault in the range 0.7–0.8 mm year^?1. Raised Holocene coastal notches indicate that this may be further partitioned into about 0.2 mm year^?1 of footwall uplift and hence 0.5–0.6 mm year^?1 of hangingwall subsidence. Holocene displacement rates towards the tip of the active East Alkyonides Fault are in the range 0.2–0.3 mm year^?1. Any uplift of the West Alkyonides Fault footwall is not keeping pace with subsidence of the Skinos Fault hangingwall, as revealed by lowstand shelf fan deltas which show internal clinoforms indicative of aggradational deposition in response to relative base‐level rise due to active hangingwall subsidence along the Skinos Fault. Total subsidence here during the last 58 kyr lowstand interval of Lake Corinth was some 20 m, indicating a reduced net displacement rate compared to estimates of late Holocene (< 2000 bp ) activity from onshore palaeoseismology. This discrepancy may be due to the competition between uplift on the West Alkyonides Fault and subsidence on the onshore Skinos Fault, or may reflect unsteady rates of Skinos Fault displacement over tens of thousands of years.     

Use of Factor Analysis to Study the Distribution of Metalliferous Bauxitic Tailings in the Seabed of the Gulf of Corinth,Greece

In 1994, a detailed marine environmental survey was carried out in surface sediments of the northern flank (Antikyra Bay) and the basin floor of the Gulf of Corinth. Metalliferous tailings (red-mud slurry) of a bauxite processing plant are discharged through a pipeline at a water depth of 100 m, in the Antikyra Bay, covering an area of 16 km². The bauxitic tailings are detached from the main deposit at the outfalls, flow as turbidity currents downslope, and are redeposited on the basin floor of the Gulf of Corinth, where they cover an area of about 277 km². One hundred sediment samples, that were collected from red-mud deposits and the surrounding natural sediments, were analyzed for Ag, Al, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Ni, Pb, Si, Ti, V, and Zn concentrations. Statistical analysis of the heavy metals concentrations using factor analysis allowed (i) an examination of the interrelations among metals and (ii) distinguishing possible sample groups on the basis of metal concentrations in order to study the mechanisms of transport of the red mud and the degree of mixing with natural sediments. Factor 1 (Al, Cr, Ti, Fe, Co, Ni, Pb, Ag, Hg, V, Cd, and Cu) and the positive pole of Factor 2 (Cu, Ag, Cd, and Hg) are red-mud factors, reflecting different metal behaviors, which are related to processes that take place during the transport and redeposition of the red mud. The negative poles of Factors 2 and 3 and the positive pole of Factor 4 are related to natural sediment supply processes. Q-mode factor analysis identifies three distinct sediment groups located in different areas, on the base of the degree of mixing of red mud with natural sediments.     

Monitoring of a methane-seeping pockmark by cabled benthic observatory (Patras Gulf, Greece)

A new seafloor observatory, the gas monitoring module (GMM), has been developed for continuous and long-term measurements of methane and hydrogen sulphide concentrations in seawater, integrated with temperature (T), pressure (P) and conductivity data at the seafloor. GMM was deployed in April 2004 within an active gas-bearing pockmark in the Gulf of Patras (Greece), at a water depth of 42 m. Through a submarine cable linked to an onshore station, it was possible to remotely check, via direct phone connection, GMM functioning and to receive data in near-real time. Recordings were carried out in two consecutive campaigns over the periods April–July 2004, and September 2004–January 2005, amounting to a combined dataset of ca. 6.5 months. This represents the first long-term monitoring ever done on gas leakage from pockmarks by means of CH₄+H₂S+T+P sensors. The results show frequent T and P drops associated with gas peaks, more than 60 events in 6.5 months, likely due to intermittent, pulsation-like seepage. Decreases in temperature in the order of 0.1–1°C (up to 1.7°C) below an ambient T of ca. 17°C (annual average) were associated with short-lived pulses (10–60 min) of increased CH₄+H₂S concentrations. This seepage “pulsation” can either be an active process driven by pressure build-up in the pockmark sediments, or a passive fluid release due to hydrostatic pressure drops induced by bottom currents cascading into the pockmark depression. Redundancy and comparison of data from different sensors were fundamental to interpret subtle proxy signals of temperature and pressure which would not be understood using only one sensor.     

Fan delta development and processes offshore a seasonal river in a seismically active region, NW Gulf of Corinth

High-resolution geophysical surveys (seismic, side-scan sonar) offshore of the Eratini River, a seasonally flowing river in the NW Gulf of Corinth, Greece, revealed a small fan delta with a variety of bottom features (blocky deposits, chutes and sediment instabilities). Considering the relatively small size of this river, however, these features could not be explained as being produced solely by river flow processes. Based on morphological features, the fan delta can be subdivided into a high- and a low-energy area. Sedimentation processes in the fan delta are associated with flood-derived sediment input, hyperpycnal flows which erode the fan surface, mud settling from suspension plumes, shelf sedimentation and sediment failures. The observed blocky deposits are considered to be the result of earthquake-induced mass flows in 1965 and 1995, whereas the chutes would be produced both by erosive mass flows and by hyperpycnal currents. The bulk block sediment volume has probably resulted from the 1965 earthquake. The 1965 evacuation zone and the related chutes were buried by the prograding fan delta. The main causative factor triggering the observed sediment instabilities is considered to be liquefaction, which is caused by (1) frequent earthquake-induced cyclic loading and (2) low sediment shear strengths created by rapid deposition during floods, in both cases associated with high pore-water pressures.     

Sediment distribution and transport processes on the outer continental shelf of the Hebridean Sea

The major topographical features of the study area within the Hebridean Sea consist of two submarine banks lying at the edge of the outer continental shelf, an offshore island slope surrounding the Island of Barra-Head, a basin crossing the area in a SSE—NNW direction, and a deep trench.The present investigation suggests that the area was covered with glacial drifts and morainic accumulations during the Pleistocene and that during a subsequent low sea level the drifts were reworked around the topographically high areas. Much of the clay material has been transported to and deposited in regions off the continental slope.Recent conditions are represented by an erosional and depositional environment. The erosional environment is represented by the summit and leeward side of the two banks and the offshore island slope. In these areas tidal-induced currents and south-westerly swells continue to sweep the remaining sand, resulting in the formation of lag gravel deposits. The depositional environment in the basin results in homogeneous sand cover.     

Water masses in Thermaicos Gulf,north-western Aegean Sea

Physical oceanographic data were collected during September 1975 at stations in the shallow water (max. depth 45 m) embayments of Salonica Bay and Thermaicos Gulf, and comparisons are made with data collected during other seasons. The distribution of temperature and salinity in the water column indicated the following features: fresh surface water predominantly related to river discharge; an intermediate water mass, apparently formed by mixing between the fresh and open sea waters; a thermocline, and deep bottom water open sea characteristics. The summer distribution of density in the surface and bottom waters, and the seasonal variation in the pattern of surface salinity, suggested an anti-clockwise water circulation throughout the embayments.     

Tidally induced secondary circulations and their associated sedimentation processes

Fine grained (mud) deposits or coarse grained (sand) deposits, which occur in tidally dominated coastal environments (bays, tidal inlets and estuaries), are associated with topographical (small-scale) tidally induced eddies; these are considered to be caused by the phenomenon of flow separation, downstream from headlands and at abrupt coastal discontinuities. Extensive sand deposits, which have been described as bed-load convergence zones can, similarly, be associated with residual (large-scale) circulation eddies.     

Offshore geological hazards in the Hellenic arc

Abstract

The Hellenic Arc is located within one of world's most seismically active areas and has experienced extreme tectonism through Tertiary and Quaternary times. This activity controls the rates of uplift and subsidence and determines the sediments supply and depot centers. This paper discusses the various geological hazards detected in selected parts of the Hellenic Arc and examines the causative factors. The areas surveyed were the North Aegean Trough, the Kythera Ridge in the outer island arc, the Corfu/Kefalinia/Zante shelf/slope, and the Thermaicos, Corinth, Patras, Amvrakia, and Malliacos gulfs, as well as the Trichonis Lake. The potential geological hazards observed are (1) active faulting, (2) sediment instabilities, (3) gas‐charged sediments, (4) salt doming, (5) erosion, transport, and deposition of sediments, and (6) seismicity, volcanism, and tsunamis. The major types of sediment instabilities that have been documented on seabed include (1) surficial sediment creep in slopes ranging from 1 to 2° resulting infolding and faulting of the surficial sediments, (2) translational and rotational slides in slopes ranging from 2 to 40°, (3) debris flow, and (4) turbidity currents. Factors that contribute to slope instability in the Hellenic Arc are (1) sloping bottom, (2) thick accumulations of Plio‐Quatemary sediments, (3) present day high rates of sedimentation, (4) closely spaced active faults, (5) earthquakes, and (6) active diapirism. The contribution of long period waves to slope failure in these areas appears to be of minor importance, since the slope failures occur in depths of more than 150 m. Gas‐charged sediments and pockmarks have been observed in areas associated with deltaic, fjord‐like, and open sea environments. This gas is presumably formed by the decomposition of biogenic material. Numerous disasters that took place during historical times and greatly affected the coastal zone were caused by the above‐mentioned hazards. Damage to offshore installations are limited to cable failure.