An integrated study of seismic hazard assessment in the area of aigion, gulf of corinth, greece

Previous studies have shown that the presence of tropospheric contributions in the interferograms represents one of the limits for the detection of vertical movements. The capability of correctly removing of tropospheric effects from the interferograms requires a very refined knowledge of the permanent scatterers of the area under study that can be obtained only through the analysis of a large population of interferograms. For that, using the CNES DIAPASON software, we derived 81 interferograms of the Gulf of Corinth from 38 ERS SAR raw images acquired between 1992 and 1999 on three different tracks. Thus, either the 1992 Ms=5.9 Galaxidi earthquake or the 1995 Ms=6.2 Aigion event have been covered in our database. We sampled very different time spans, ranging from 1 to 2710 days (7.42 years), to analyse the coherence of the region and the tropospheric effects at different time scales. A total number of 45 co-seismic interferograms were calculated in order to detect precisely the differences in the deformation field related to both the earthquakes occurred in the area. These differences could be obtained by atmospheric changes in the area during the images acquisitions. Some pre-seismic (5) and post-seismic (31) interferograms were computed to detect the presence of local and global tropospheric effects also in the epicentral area. Using all interferograms, we produced a coherence map of the area, which is expected to be less sensitive to local or temporal effects, as ground deformation or tropospheric contributions. That map collects all the pixels where the coherence is maintained over a long period of time, as towns or hard rock outcrops. It is used to mask the interferograms where the coherence is poor and the phase information lost. Almost all the southern side of the Gulf of Corinth is masked because of an important presence of vegetation and cultivated areas; only a few regions, in particular near the towns of Aigion and Xilokastro, remain coherent. Conversely, the northern side of the gulf has a fairly good coherence, especially the ~3-10km wide band located along the coast. In the field the area is rocky with little vegetation. Places with high coherence are Galaxidi area, the Ag. Pangalos peninsula south of Desfina and the Ag. Nikolaos peninsula north of the Alkionides islands. We developed a methodological approach to minimise the tropospheric contributions in the interferograms at both global and local scales. Firstly, all the interferograms are filtered to reduce the noise present in the data and then masked using the collective coherence map to leave only the most coherent pixels. Secondly, assuming a horizontally homogeneous troposphere model we searched for any correlation between the phase of the most coherent pixels and their elevation. On a large number of interferograms a simple linear correlation with the topography was observed. The resulting interferograms are then unwrapped in order to remove the phase ambiguity and a local-scale correction is performed. Comparing all the interferograms containing a given image, we are able to recognise all common anomalies. These local perturbations are then assigned to that image and subtracted from the interested interferograms. Finally, the corrected interferograms are stacked and averaged to reduce the high-frequency noise still present after the treatment and to put in evidence a smoothed deformation field related to the earthquakes occurred in the area. The deformation concerns a N-S 16km wide by E-W 28km long band, reaching the maximum value of ~243+/-5mm on Psaromita cape, a limestone mesozoic platform located on the northern side of the gulf, in front of the town of Aigion. Within the uncertainty of our displacement map (5mm), the comparison between each corrected interferogram and the average deformation map reveals that no post-seismic deformation is observed in the data in the period 1995-1999.

Contribution to these estimates derives from different approaches: - Translation of the footwall uplift rates to fault slip rates by using dislocation models by applying fault parameters based on WP8 fault mapping. A code based on standard (elastic half-space) dislocation theory (Ward and Valensise, 1989) was used because it provided the most conservative results always representing the maximum slip rates for the modelled fault. The long-term slip rate (over the past 2-300kyr) for each fault segment, obtained assuming a regional uplift rate of 0.2mm/yr was: -- East Eliki Fault 7-9mm/yr. -- West Eliki Fault 9-11mm/yr. -- Aigion Fault 9-11mm/yr. - Translation the vertical slip rate, measured on the trench walls, to slip rate on the fault plane, assuming an average fault dip of 50°, consistent for all the faults. The obtained mid-term slip rates (over the past 2-3000yr) for the two fault segments (where data is available) are: -- East Eliki Fault 1.7-6.0mm/yr. -- Aigion Fault 1.2-6.4mm/yr. - Translation of the vertical separation rates obtained from correlative deposits dated at about 30kyr across the Aigion fault (from the 200m deep well cored in Aigion harbour and an outcrop on the Aigion footwall) to slip on a 50°-dipping fault by simple trigonometric rules. This suggests a ca. 5.2-5.5mm/yr slip rate for the Aigion fault.

Description of the task (preliminary works carried out before and during the drilling of CORSSA boreholes): Performance of a 720 m-long High Resolution Seismic Profile across the Aigion harbour. Application of the result: - 2D expected geometry before drilling for geologic layers and faults. - Verification of the presence of a strong reflector at depth corresponding to the expected stiff conglomerate layer. - Assessment of the continuity of the stiff conglomerate layer (without lenses) at depth for the deep reference accelerometer host rocks. - Identification of strong reflectors that could be used in the expected geological cross section. - Establishment of a velocity law (Vp) for the first 200 metres.

Mapping of the Aigion and Elike faults was completed on a 1:50000 and 1:5000 scale and the results are included as layers in the GIS-database. We have also attempted to develop a segmentation model for the area based on geometric complexity, extent of co-seismic ruptures, and geomorphic and geologic considerations. Geometric and kinematic parameters for each fault as well as the maximum expected magnitude have been estimated on the basis of detailed field-work and empirical relations. Fault dip and rake were assumed according to field observations and geophysical information, whereas fault width was inferred on the basis of the instrumental seismicity distribution. These results are of relevance for any investigation in the active tectonics and seismo-genesis in the area and they could contribute as primary input to deterministic seismic hazard evaluations.

An important part of the work developed by INGV within WorkPackage 8 was the preparation of a GIS database useful for all the specific studies of the WorkPackage. This GIS database was developed on a Mapinfo platform (vers. 6.5 for PC). A html guided tour to the results contained in the database and mostly achieved by INGV was also prepared to make these results available through the internet but not with an interactive GIS interface. However, jpg images containing layers of the Database can be viewed in the html interface. Both the GIS database and the html-guided tour are enclosed in a CD-ROM. The geographic data concerning the study, forming the background information of the database were derived from the official Greek topographic maps scale 1:50000 and 1:5000 and can be viewed by layers. Digital elevation models were included too, both from SPOT imagery and topographic maps. Geologic, geomorphic and tectonic data where added to the database and updated continuously following the investigation development. The layering of all the data collected through the database provides a unique understanding of the amount of data, on one side, and on the consistency of the observations on the other. This database is thus a major result of the project but also a unique tool for investigating the tectonic, geologic and geomorphic setting or the area and to study the relationships among different elements. The structure of the database is ready to include also geographic data with attached information from other WP or from other ongoing projects in the Corinth Gulf.

In order to assess whether the western Eliki Fault has experienced recent tectonic reactivation, ground-penetrating radar surveys were made across a stepped sequence of morphological scarps along the main escarpment base. GPR investigations were focussed on the Nikolaiika Fan (NF), part of the Kartoula Fan (KF) complex, above the Eliki Plain. In the field, morphological scarps often coincide with agricultural terrace edges and so, in some cases may be of anthropogenic rather than tectonic origin. It is to ascertain whether these surficial scarps relate to concealed near-surface faults that recourse was made to ground penetrating radar studies. GPR surveys using PulseEKKO 50, 100 and 200MHz antennae were collected on three roads cutting across the fan surface, which run perpendicular to 0.5-1.0m high surface scarps identified in the field. On one road (L1) profiles were collected at 50, 100 and 200MHz. These profiles were collected across two scarps, which were visible in the field to the side of the road. On the other roads profiles were only collected at 50MHz. Data was post processed using Gradix 1.10 software. Around 250m of profiles were taken to image the faults beneath the Nikolaiika Fan. The results reveal near-surface tectonic disturbances on all profiles. In most cases, the subsurface disturbances can be correlated with breaks in slope in the surrounding fields. The faults are manifest in three ways: - Visible offsets in near-surface strata. - Concave features in the stratigraphy, representing zones of fracture and warping. - At depths of 2-5m, where some fault planes are steeply dipping, faults are imaged on GPR profiles as incoherent scattering from rupture planes. The GPR profiles confirm recent (Late Holocene) tectonic activity under the Nikolaiika Fan. Such recent activity in front of the footwall of the Western Eliki Fault may suggest that recent earthquake activity has been concentrated on fault splays from the main footwall fault, rather than along the footwall fault plane itself.

The CORNET network consists of 4 digital permanent seismological stations equipped with Lennartz 3D/5sec seismometers. The network consists of a Lennartz 5800 PCM system and operates on trigger mode. All remote stations are telemetric and all seismic signals are transmitted to a central in the Seismological Laboratory. The dynamic range is 120db and the sampling rate is 125 samples per second. Data are analysed in daily basis and results are presented at the website of the Department of Geophysics of the University of Athens. The CORNET network has been operating during the whole period of the Corseis project and has successfully monitored the earthquakes that occurred in the Gulf of Corinth and its surrounding area. During this period more than 650 earthquakes were recorded by the network. The located crustal seismicity has magnitudes less than 4.5R. The events located in the Gulf of Corinth indicate the most active areas.

Information about the CORSEIS project can be found at www.corinth-rift-lab.org with public and partners' restricted access. Moreover, in order to facilitate further research and data exploitation a dynamic web-site with advanced search facilities has been developed to insert, search and retrieve data-info concerning CORSEIS project. It is focused mainly in the accelerometric arrays (RASMON & CORSSA) that will continue to operate also after the end of this project. It is called CORSSA web-site and will be soon visible through its internet address www.corssa.info. The website is based on a commercial advanced web publishing Content Management platform developed with Java Server Pages and MySQL database. It has the capability of multi-author web-publishing of reports, news, articles, photos etc through a web-based WYSWYG editor. Each partner has its own login/password and can thus publish info in pre-specified areas with public or restricted access (available only for the other partners). Moreover, all acceleration data records along with their associated focal parameters estimates will be available in public through the already developed ACCEL database web API. This includes all data already collected through CORSEIS project, plus the data that will become available after the end of the project.

The areas of each fault footwall where the marine terraces are better preserved (profile B, F, H for the East Eliki, West Eliki and Aigion fault respectively) were selected. Direct dating was performed on some sediments collected on the footwall of the East Elike fault and more are underway. On the basis of these ages and because of geomorphic considerations, we correlated the prominent terrace sequences with major sea high stands. This provided a good approximation of long-term cumulative footwall uplift rates over the past 200-300kyr for each fault. Because these calculated uplift rates include a component of regional tectonic uplift, that, even if is not well constrained in the area of the Corinth Gulf, it is estimated to be of the order of 0.2mm/yr we corrected on this basis the calculated values and our best estimates of the footwall uplift for each fault are: - East Eliki fault 0.8mm/yr. - West Eliki fault 1.05mm/yr. - Aigion fault 0.85-1.0mm/yr. In general the net footwall uplift rates over the past 200-300kyr is found to be in the order of 1mm/yr for each fault.

The seismic ground response at CORSSA test-site has been studied theoretically, in order to identify different amplification characteristics of the 1D SH seismic waves propagating from the bedrock to the surface. The analyses performed with the software Cyber Quake, which uses the equivalent linear model to describe the non-linear soil behaviour during ground shaking. Two sets of analyses have been performed; the first one at the site of OTE building at the centre of the city, close to the Aigion fault and the second, at the experimental site of CORSSA. Preliminary studies have also been performed for liquefaction susceptibility and risk of the CORSSA soil formations. Potential application of the result: Definition of the typical "benchmark" 1D soil profile with full description of the dynamic and physical properties of the soil layers: Geo-technical description of the soils, Vs profile, soil densities, G/Go-g-D% curves for all soil layers, liquefaction susceptibility.

Paleo-seismological trenches have been opened and road cuts have been studied at different locations along the Elike and Aigion faults. Trenches from previous studies have been re-analysed too. Dendro-chronologically corrected radiocarbon ages (a list of which is contained in the Database) are used to constrain the timing of the recognised paleo-earthquakes. The obtained results are: - East Eliki fault: Most recent earthquake 1861, penultimate 980-1260, previous 100BC-100AD, slip per event 0.6-2.4m, vertical slip rate 1.3-4.6mm/yr. - West Eliki fault: At least two events during Holocene, slip per event >0.4m. - Aigion fault: Two, possibly three events between 1800 and 1160 AD, slip per event 0.4-0.7m, vertical slip rate 0.9-4.9mm/yr. The study of abrupt subsidence events, (possibly triggered by co-seismic subsidence) in the Eliki plain suggest evidence for one event in the 5th-6th century besides the 373BC historical event. These events would be attributed to the West Eliki fault.

One of the main purposes of this Work-package 7 Geochemistry was to provide information and to define the background geochemical and hydro-geological data also to select the best sites to install geochemical/hydrological monitoring stations and to record data of the most significant circulating fluids. All the gathered data (31 geochemical variables) have been geo-referenced in the frame of the GIS improved by the rest of partnership of the Corinth Rift Laboratory. In particular the geochemical data have been organized in the same GIS database used for the geo-structural and paleo-seismological data (Work-package 8). In October 2000 a first geochemical survey was carried out (Quattrocchi et al., 2001) in the western part of the Gulf of Corinth in order to define the main geochemical patterns of the shallow aquifers discharging along a NW-SE belt from Selianitika (NW) to Trapeza (SE), throughout the sectors of Aigion, Temeni, Diakofto and Eliki. The main purpose of the survey was both to find possible geochemical anomalous features in the circulating groundwater (thermal signature, gaseous emissions, enrichment in chemical elements and compounds indicating deepening of the hydrological circuits, etc) and to correlate them with the tectonic structures of this sector of the Corinth Rift, exploiting geochemical techniques, tested in other areas (Quattrocchi et al., 1999; Quattrocchi, 1999; Quattrocchi et al., 2000 c; Salvi et al. 2000, Pizzino et al., 2002; Quattrocchi et al., 2003). A total of 48 sites have been sampled (41 wells and 7 springs), while other 3 sites were collected in October 2001. On the other hand, chemical and isotopic analyses of the AG-10 well groundwater samples, collected at different depths during July 2002, in the frame of the 3F-Corinth, are still ongoing. On field, physico-chemical parameters (such as temperature, pH, Eh, electrical conductivity), alkalinity, H2S, NH4 and total dissolved CO2 content have been analysed. In laboratory, all samples have been selected for the analyses of 222Rn (Quattrocchi et al., 2001), major and some minor elements (Ca, Mg, Na, K, Cl, SO4, F, NO3, PO4). The d13C (TDC) analyses have been performed (values comprised between 5.26 to 15.12 per mill). Moreover, on 17 selected samples, analyses of minor and in trace elements (Li, B, Si, Fe, Mn, As, Br, Sr, Sb, Hg, U) have been performed. Only three sites were selected for dissolved gases analyses (CS 13, 41 and 50). The geochemical classification of the sampled groundwater highlighted: - Ca-HCO3 groundwater. They represent the bulk of our samples, and their chemistry indicates a very fast and shallow interaction with conglomerates and limestones cropping out in the area as a whole. These main patterns, together with the absence of fault pathfinder minor and in trace elements indicate a lack of deepening of hydrologic circuits along the regional tectonic structures and the absence of thermal signature. - Ca (Na-Mg)-HCO3 (Cl-SO4) groundwater. They represent a geochemical evolution of the calcium-bicarbonate groundwater, expressed as a chlorine, sulfate, sodium and magnesium enrichment, located in the Eliki area and for the artesian wells of the Nerazes and Trapeza areas (Na-HCO3 groundwater, see after); the only clear H2S signature (Eh negative value, -150mV) was found at the site CS 34 (Loutra Selianitika spa). - Mixing with seawater, interaction with marine sediments and/or with brackish sequence of silty sands, marls, for the samples Aigion spring and Rododafni School well, respectively, were invoked. It is worth noting that the Rododafni School is located only about one kilometre from the sea, just in correspondence of the Aigion fault and very close to the fractures opened in the ground when the M=6.2 Aigion earthquake occurred on June 15, 1995. Here the 222Rn survey in soil gases highlighted a clear positive anomaly just where the recent fracture field was discovered in 1995, at surface in the Agios Konstantinos garden, soon after the earthquake. - Na-HCO3 groundwater: this peculiar chemistry is pertaining only to the samples CS 13 and 40, located in the village of Nerazes and close to the town of Trapeza. They are the most pronounced artesian wells in the area, representing the expression at surface of a deeper aquifer with groundwater interacting with clayey strata during the uprising. Interaction of groundwater with alkali alumino-silicate (or silicate) minerals causes the pH increase, while the relatively high dissolved carbon dioxide is readily converted to bicarbonate with simultaneous introduction of alkali It is particularly sound to monitor continuously the relationships between seismic events and both hydrodynamic (being artesian) and geochemical variations. For these reasons it was selected to install a geochemical continuous monitoring station (GMS II, prototype).

The footwall of the Eliki and Aigion faults is indented by an impressive flight of flat to gently dipping platforms. Mapping of these surfaces was completed on a 1:50000 and 1:5000 scale, using detailed topographic maps, air photos, and field survey. The results are included as a layer in the GIS database. The errors in estimating elevations and positions of surfaces are evaluated as +/-10m. Field surveys helped in the recognition of marine sediments on some of these surfaces and thus we could interpret them as marine terraces, both depositional and erosional. In the footwall block of the Aigion Fault up to nine terraces can be distinguished and up to 10 marine terraces have been mapped in the footwall block of the Eliki Fault system. Some of the marine sediments collected on these surfaces have been dated (see. e.g. McNeill et al., 2002; Geoarcheology, in press), and more dating in underway. The present terrace elevations have been tentatively correlated with the eustathic sea-level curve (Lajoie et al., 1991) and the results are included as a layer in the GIS database.

In its first application to palaeoseismic studies, we show that that 210Pb dating, supported by 137Cs dating, is potentially useful for estimating the ages of very young (less than 150 years) colluvial deposits associated with recent earthquake activity. Colluvial deposits exposed in two fault trench sections were sampled for laboratory assay and analysed to confirm interpretations that they post-dated the 1861 surface rupture. Only one of the two sites, the Eastern Eliki Fault Trench 1A, was found to provide an appropriate sediment sequence for dating via 210Pb and 137Cs. Here, a soil horizon was found to have been buried by alluvial sedimentation in the late AD 1800s, consistent with reactivation during the 1861 earthquake. The precision of the dates obtained was found to depend largely on the date of occurrence of the earthquake event, and on the local sedimentary setting. The half-life of 210Pb (22.3 years) usually limits its dating range to 150 years (ca. 6-7 half lives), hence it is only useful in dating events which have occurred since ca. 1850. Extrapolation of the chronology to examine earlier events is not possible due to potential variations in sediment accumulation rate over time. Since errors on 210Pb dates generally increase as the excess 210Pb activity decays to low values (i.e. in older sediments), dating precision is likely to be better for more recent earthquake events, particularly those occurring after 1954 where the 210Pb ages can be corroborated by 137Cs dates. Where possible, 210Pb and 137Cs dating should be used in combination to confirm dates, and fully assess variations in sediment input and erosional processes. The results suggest that this novel geo-chronological technique can be applied more widely to date colluvial deposits typically encountered in fault-trench studies.

CRL seismological data are used to very precisely relocate earthquakes inside the CRL network. Multiplets, events that are located close to each other and have similar waveforms, are relocated in a relative sense with uncertainties of few tens of meters in all directions. This is possible because relative arrival times are picked using cross-correlation and cross-spectrum techniques with a precision of ~2msec. With these relocated hypocenters, it is possible to very precisely image active structures because the uncertainties are now of the order or smaller than the dimension of the earthquake source itself instead of ~10 times larger for standard locations. A joint analysis with focal mechanisms leads to a detailed study of the source of earthquakes of magnitude >3.5 for which aftershocks are present. Swarms of earthquakes that occur quite frequently in the area can also be studied with this technique. The March-July 2001 earthquake sequence that include the Ml=4.7, April 8 2001 earthquake, 10km south of the town of Aigion was studied with this technique. Over 10 different multiplets made of 10-20 events were relocated. It was found that the main-shock was a normal faulting event with right lateral component. The active fault plane has a strike of ~220° and a dip of ~40° to the north-west. Although this focal mechanism is consistent with the general state of stress in the area, the identified active plane does not correspond to any active structure observed at the surface.

The geodetic network deployed in the Gulf of Corinth is constituted by approximately 200 points: 50 1st order points, known with a 3mm horizontal accuracy for each survey, and about 150 2nd order points, known with a 1-2cm accuracy. The entire network covers a surface of about 100x80km, which corresponds, including sea surface, to 1 point every 5km. This dense network allows us to have a satisfactory sampling of the main active faults in the region. Since 1990, eleven field surveys on this network were already organised. Two of them (1993 and October 1995) interested the whole 1st order network points, whereas in the other campaigns only a part of them was measured. The last field survey in the Gulf of Corinth was organised in September 2001 in the framework of the CORSEIS project. The interest and aim of this survey was, first of all, to carry on with the monitoring of the deformation in the area, and, secondly, to detect the displacements occurred in the area in the last 11 years. During this survey, which spanned 12 days (from 18 to 29 September 2001), to ensure the maximum accuracy of the network and to have a certain redundance and repeatability, 33 1st order points of the 1991-1993 networks were observed three to four times. 19 among the 2nd order benchmarks were also observed for a single four-hours session. The GPS data of all the surveys carried out in the region since 1990 were processed with GAMIT/GLOBK software (King & Bock, 1998; Herring, 1998). The velocity field obtained from the GPS data analysis indicates an almost N-S opening direction and puts in evidence two main features: - Firstly, an important gradient of deformation localised offshore, on a very narrow band, in the central part of the Gulf of Corinth and quickly decreasing moving away through the interior. - Secondly, an increase of the opening rate ranging between 11mm/year in its central part and 16mm/year to the west. These rate values do not take into account the earthquakes contribution. Horizontal co-seismic displacements observed for the Ms=6.2 June 15, 1995 event reach 15cm in correspondence of the northern coast and quickly decrease moving away towards the North. The southern block appears undeformed, except the region of Aigion event. This means that the accumulation rate of deformation on the major faults of the southern part of the Gulf is slow, lower than 1 to 2mm/year, but still difficult to estimate precisely. Such rate is compatible with long recurrence periods for large earthquakes (Ms=7) on these major faults of the southern Gulf of Corinth coast. However, the area located immediately east of Patras, close to the Psathopyrgos fault, is discordant with the behaviour of the southern block (Peloponnisos) and it shows a deformation, which is concordant with the northern side of the Gulf.

Description of the task: - Study of palaeo-environments in the boreholes of the hanging wall of the Aigion fault and in the marine terraces outcrops of the hanging wall using ostracodes, foraminifers, nannoplancton, diatoms, malacofauna, pollens and dinokysts. - Geological cross section of the marine terraces. - Dating on shells and vegetal fragments in the cores of the boreholes and in the marine terraces. Application of the result: - Estimation of sedimentation rates and of the subsidence rates in the hanging wall formations; of the variability of the rates and interpretation in term of fault activity. - Estimation of the uplift rates in the footwall of the fault. - Reconstruction of the past environment in the boreholes since 60ka BP. - Correlation between boreholes deposits with regressive peaks during the Holocene transgression and between high stand sea level during the general low-level trend of the glacial period. - Correlation between marine deposits in the boreholes and outcrops of marine terraces. - Estimation of total offset between marine deposits in the boreholes and outcrops of marine terraces splitting into uplift rates in the footwall and subsidence rate in the hanging wall.

Shallow (<10m) coring campaigns in two locations attempted to establish a depositional record of fault activity along the western Eliki Fault based on detecting abrupt environmental changes in the adjacent coastal plain. One location was a former lake located in the upper part of the plain in immediate hanging wall of the fault. Here a series of boreholes revealed surficial silts and clays typical of a low-energy depositional setting. However, subsequent laboratory analysis of samples found that oxidation of the sediments had ensured there was no preservation of sensitive palaeo-ecological indicators (pollen, ostracods etc) of the depositional environment. Moreover, no material suitable for radiometric dating was obtained. This environment, probably a seasonal playa, was considered unlikely to preserve a useful sedimentological record of tectonic movements. A second and more favourable depositional environment was found in the lower part of the Eliki deltaic plain. According to historical accounts of the 373BC and 1861 earthquakes, surface faulting on the Eliki Fault is accompanied by the metre-scale tectonic subsidence and marine inundation of the coastal plain. An integrated programme of GPR surveys and shallow boreholes, accurately levelled in with respect to sea level, were carried out. Stratigraphic descriptions were made in the field and basic geo-technical tests were made on the intact core material (e.g. penetrometer measurements of bearing strength; shear vane measurements of shear strength). Subject to the limitations for core sample collection imposed by the archaeological permit under which the group was working, core material was obtained for laboratory analyses (particle size, trace element) and, where appropriate, radiometric dating (137Cs, 210Pb, 14C). GPR surveys in the coring sites proved to be unsuccessful in imaging the near-surface stratigraphy; in the heavily farmed fields of the plain the GPR profiles were found to be excessively degraded by offline reflectors (roots of olive trees, metal poles, irrigation channels, telegraph poles). The results from the sediment cores show a complex stratigraphy typical of late Holocene deltaic plains but do exhibit evidence of abrupt environmental changes. In places, there is stratigraphic evidence of a transition from basal lagoonal muds into a wetland peaty soil followed by an abrupt and erosional return to a lagoonal depositional environment. Although consistent with a sudden deepening of the coastal plain, the date of the transition is awaiting radiometric dating. Along a separate core transect, two abrupt environmental changes are recognised, one at ~0m and one at ~-2m. Sedimentological, geochemical and microfossil analysis indicates that at ~2 m below sea level a basal high-energy shoreline deposit abruptly changes into a low-energy lagoonal deposit. This change postdates a terrestrial shell age of 5th-6th centuries AD and may correspond to sudden subsidence event in Roman or Byzantine times, consistent with published coastal studies, archaeological evidence and palaeoseismic investigations.

Data from CRL array are used to build a reliable and complete catalogue of magnitude Ml>1.5 events in western part of the Corinth Gulf area. Events within the network have standard uncertainties of ~1km in x, y and depth. Data from the catalogue are public. They can be used for instance for statistical analysis of time-space fluctuations of seismicity, correlations with other geophysical observations, correlations with geological observations etc.

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