Western Gulf Of Corinth Research Articles

The 2020–2021 seismic sequence in the Western Gulf of Corinth: Insights on the triggering mechanisms through high resolution seismological and geodetic data analysis

The 2020–2021 seismic sequence at the Western Gulf of Corinth, Central Greece, has been thoroughly analyzed using seismological and geodetic data processing. We present a high-resolution dataset of over 4000 relocated earthquakes between June 2020 and February 2021, delineating the activated structures. We examine the evolution of various clusters that were triggered during a three-stage sequence which began on December 23, 2020, with an Mw = 4.6 event near Marathias, migrated eastwards, following an Mw = 5.0 event on January 12, 2021 near Trizonia Island, and culminated on February 17 with an Mw = 5.3 offshore event north of Psathopyrgos. Focal mechanisms for the 20 stronger events, determined by moment tensor inversion, and 36 weaker ones, obtained through first motion polarities measurements, revealed dominant normal faulting. The fault plane of the February 17, 2021 major event is modeled through the inversion of geodetic data. Results suggest a very shallow geodetic centroid at 1.5 km depth, consistent with the seismic centroid (3.5 km), but different from the relocated hypocenter (7.5 km). Spatiotemporal analysis reveals seismic migration, following a diffusion law with D values in the range 0.06–0.25 m2s−1, consistent with triggering due to pore-pressure diffusion by fluids intrusion. This enables failure on non-optimally oriented faults, which explains the significant proportion of the observed strike-slip and oblique-normal faulting. Coulomb stress transfer due to the major events shows only weak stress-loading on the fault of the January 12, 2021 earthquake and a higher level of positive stress transfer to the south-dipping than to the north-dipping nodal plane of the 17 February event. The latter could have facilitated the nucleation of the rupture at 7.5 km and its further propagation to shallower depths along a south-dipping plane, where the bulk of its seismic and geodetic moment was released.

Deep Vs structure under the city of PATRAS, Greece – Application of the SASW method utilising earthquake recordings at pairs of accelerograph stations

Earthquake recordings (vertical component of motion) by the dense urban accelerographs stations of the UPAN network (Patras, Greece) were utilized for estimation of the deep (∼10 km) Vs structure under the city, utilizing the dispersion of surface waves. Multiple pairs of accelerographs of the network were selected (with interstation distances varying from 0.7 km to 6.3 km) that recorded seismic events with epicenters surrounding the city and located on the extrapolation of the straight line defined by the particular pair of accelerographs. The recordings were processed by applying the SASW method and estimating the inverted 1D Vs-depth profiles at the midpoints of the interstation distance. A total of 28 1D Vs-depth profiles were estimated in this way at different locations of the urban area; a deep (∼10 km) 3D model was then derived using a 2D interpolation technique. The resolution of the derived 3D Vs-depth model for shallow depths (∼80 m) was enhanced by embedding a shallow model derived from MASW and ReMi measurements. The new Vs-depth model for the city of Patras (PATRAS-VS) was compared to a number of published Vs models derived for the urban area of Patras and for the broader vicinity of the city. The comparisons indicated an impressive agreement with published models pertaining to the Patras areas, and a general good agreement with models developed for the areas of Rio-Antirrio Straight and the western Corinth Gulf. The new 3D Vs-depth model for the city of Patras developed in the present study is expected to be a useful tool for: (i) retrieving the physical properties of shallow subsurface with adequate resolution, (ii) site characterization and classification as well as for seismic hazard analyses for the Patras area, (iii) mapping the depth to the “seismic” and “geologic” bedrock of the urban area and associated site periods, and (iv) performing seismic response analyses for deep soil profiles.

The mid-6th century AD enigmatic mega earthquake and tsunami in central Greece: a seismotectonic, archeological, and historical reexamination

In AD 552 the area around the Gulf of Corinth, Central Greece, was reportedly hit by large destructive earthquakes. At the same time a tsunami supposedly hit Maliac Bay. The case calls for a realistic interpretation since earthquake scenarios failed to explain rationally the narration of the contemporary Byzantine historian Procopius, the only literary source. Scholars have some doubts about Procopius’ reliability since he frequently copied classical authors. We found impressive similarities between the texts of Procopius and those by classic authors regarding the 426 BC central Greece tsunamigenic earthquake. Very likely the AD 552 Maliac Bay tsunami is a fake event in mimic of the 426 BC one. Sixth-century seismic disasters in southern Greece, reported away from the Gulf of Corinth have been attributed as a deus ex machina to the AD 552 earthquake(s) due to the lack of supporting literary sources. After critical examination of the Procopius’ narration and of relevant geological, seismotectonic, and archeological evidence, we concluded with a scenario comprising two strong (magnitude ~6.5) earthquakes but without tsunami generation. The first earthquake perhaps happened in Boeotia, NE Corinth Gulf, as palaeoseismological evidence also indicates. The second earthquake, in Patras-Naupactus area, western Corinth Gulf, is supported by convincing archeological findings. However, seismic destruction horizons in southern Greece are interpreted by distinct sixth-century earthquakes independent from the Procopius’ ones.

Probabilistic and Scenario-Based Seismic Hazard Assessment on the Western Gulf of Corinth (Central Greece)

The Gulf of Corinth (Central Greece) is one of the most rapidly extending rifts worldwide, with its western part being the most seismically active, hosting numerous strong (M ≥ 6.0) earthquakes that have caused significant damage. The main objective of this study was the evaluation of seismic hazard through a probabilistic and stochastic methodology. The implementation of three seismotectonic models in the form of area source zones via a logic tree framework revealed the expected level of peak ground acceleration and velocity for return periods of 475 and 950 years. Moreover, PGA values were obtained through the stochastic simulation of strong ground motion by adopting worst-case seismic scenarios of potential earthquake occurrences for known active faults in the area. Site-specific analysis of the most populated urban areas (Patras, Aigion, Nafpaktos) was performed by constructing uniform hazard spectra in terms of spectral acceleration. The relative contribution of each selected fault segment to the seismic hazard characterizing each site was evaluated through response spectra obtained for the adopted scenarios. Almost all parts of the study area were found to exceed the reference value proposed by the current Greek National Building Code; however, the three urban areas are covered by the Eurocode 8 regulations.

Multisegment ruptures andVp/Vsvariations during the 2020–2021 seismic crisis in western Corinth Gulf, Greece

SUMMARYOn 2020 December 23, a seismic crisis initiated in the western Corinth Gulf offshore Marathias, lasted several months, and generated thousands of small magnitude earthquakes. The Gulf of Corinth is well known for earthquake swarm occurrence and short-lived burst-like earthquake sequences, mostly triggered by crustal fluids. Here, we perform a detailed seismic analysis aiming to identifying earthquake clusters within the seismic crisis and define their spatial and temporal characteristics. Thanks to the dense seismic station coverage in the area, operated by the Hellenic Unified Seismological Network and Corinth Rift Laboratory, we relocate shallow seismicity and compile a high-resolution earthquake catalogue containing ∼1400 earthquakes spanning the first two months of the seismic crisis. We identify 19 earthquake clusters by applying spatio-temporal criteria and define the geometry (strike and dip) using principal component analysis for 11 of them. Our results are consistent with moment tensor solutions computed for the largest earthquake in each cluster. A striking feature of the seismic activity is the west-towards-east migration with a notable increase in Vp/Vs values for each cluster and a slight increase of the dip angle for the identified fault segments. Furthermore, we find that each cluster contains several burst-like, short interevent time, repeating earthquakes, which could be related to aseismic slip or fluid migration. Overall, we show that the 2020–2021 seismic crisis consists of earthquake clusters that bifurcate between swarm-like and main shock–aftershock-like sequences and ruptured both north- and south-dipping high-angle fault segments. The 2020–2021 seismic activity is located between 5 to 8 km, shallower than the low angle north-dipping (∼10°) seismic zone which hosts long-lived repeating sequences at ∼9–10 km depth. This study supports a hypothesis that the low-angle north-dipping seismicity defines the brittle-ductile transition in the western Corinth Gulf, with seismic bursts occurring at shallower depths in the crust.

Spatiotemporal Properties of Seismicity and Variations of Shear-Wave Splitting Parameters in the Western Gulf of Corinth (Greece)

The Western Gulf of Corinth (WGoC) exhibits significant seismicity patterns, combining intense microseismic background activity with both seismic swarms and short-lived aftershock sequences. Herein, we present a catalogue of ~9000 events, derived by manual analysis and double-difference relocation, for the seismicity of the WGoC during 2013–2014. The high spatial resolution of the hypocentral distribution permitted the delineation of the activated structures and their relation to major mapped faults on the surface. The spatiotemporal analysis of seismicity revealed a 32-km-long earthquake migration pattern, related to pore-pressure diffusion, triggering moderate mainshock-aftershock sequences, as fluids propagated eastwards in the course of ~15 months. The anisotropic properties of the upper crust were examined through automatic shear-wave splitting (SWS) analysis, with over 2000 SWS measurements at local stations. An average fast shear-wave polarization direction of N98.8°E ± 2.8° was determined, consistent with the direction of the maximum horizontal regional stress. Temporal variations of normalized time-delays between fast and slow shear-waves imply alterations in the level of stress or microcrack fluid saturation during the long-lasting pore-pressure diffusion episode, particularly before major events. The present study provides novel insights regarding seismicity patterns, active fault structures, anisotropic properties of the upper crust and triggering mechanisms of seismicity in the WGoC.

The Western Gulf of Corinth (Greece) 2020–2021 Seismic Crisis and Cascading Events: First Results from the Corinth Rift Laboratory Network

AbstractWe investigate a seismic crisis that occurred in the western Gulf of Corinth (Greece) between December 2020 and February 2021. This area is the main focus of the Corinth Rift Laboratory (CRL) network, and has been closely monitored with local seismological and geodetic networks for 20 yr. The 2020–2021 seismic crisis evolved in three stages: It started with an Mw 4.6 event near the northern shore of the Gulf, opposite of Aigion, then migrated eastward toward Trizonia Island after an Mw 5.0 event, and eventually culminated with an Mw 5.3 event, ∼3 km northeast of the Psathopyrgos fault. Aftershocks gradually migrated westward, triggering another cluster near the junction with the Rion–Patras fault. Moment tensor inversion revealed mainly normal faulting; however, some strike-slip mechanisms also exist, composing a complex tectonic regime in this region dominated by east–west normal faults. We employ seismic and geodetic observations to constrain the geometry and kinematics of the structures that hosted the major events. We discuss possible triggering mechanisms of the second and third stages of the sequence, including fluids migration and aseismic creep, and propose potential implications of the Mw 5.3 mainshock for the seismic hazard of the region.

Earthquake Diffusion Variations in the Western Gulf of Corinth (Greece)

Earthquake Diffusion Variations in the Western Gulf of Corinth (Greece)

Tropospheric Correction of Sentinel-1 Synthetic Aperture Radar Interferograms Using a High-Resolution Weather Model Validated by GNSS Measurements

Synthetic Aperture Radar Interferometry (InSAR) is a space geodetic technique used for mapping deformations of the Earth’s surface. It has been developed and used increasingly during the last thirty years to measure displacements produced by earthquakes, volcanic activity and other crustal deformations. A limiting factor to this technique is the effect of the troposphere, as spatial and temporal variations in temperature, pressure, and relative humidity introduce significant phase delays in the microwave imagery, thus “masking” surface displacements due to tectonic or other geophysical processes. The use of Numerical Weather Prediction (NWP) models as a tropospheric correction method in InSAR can tackle several of the problems faced with other correction techniques (such as timing, spatial coverage and data availability issues). High-resolution tropospheric modelling is particularly useful in the case of single interferograms, where the removal of the atmospheric phase screen (and especially the highly variable turbulent component) can reveal large-amplitude deformation signals (as in the case of an earthquake). In the western Gulf of Corinth, prominent topography makes the removal of both the stratified and turbulent atmospheric phase screens a challenging task. Here, we investigate the extent to which a high-resolution WRF 1-km re-analysis can produce detailed tropospheric delay maps of the required accuracy by coupling its output (in terms of Zenith Total Delay or ZTD) with the vertical delay component in GNSS measurements. The model is operated with varying physical parameterization in order to identify the best configuration, and validated with GNSS zenithal tropospheric delays, providing a benchmark of real atmospheric conditions. We correct sixteen Sentinel-1A interferograms with differential delay maps at the line-of-sight (LOS) produced by WRF re-analysis. In most cases, corrections lead to a decrease in the phase gradient, with average root-mean-square (RMS) and standard deviation (SD) reductions in the wrapped phase of 6.0% and 19.3%, respectively. Results suggest a high potential of the model to reproduce both the long-wavelength stratified atmospheric signal and the short-wave turbulent atmospheric component which are evident in the interferograms.

Use of GNSS Tropospheric Delay Measurements for the Parameterization and Validation of WRF High-Resolution Re-Analysis over the Western Gulf of Corinth, Greece: The PaTrop Experiment

In the last thirty years, Synthetic Aperture Radar interferometry (InSAR) and the Global Navigation Satellite System (GNSS) have become fundamental space geodetic techniques for mapping surface deformations due to tectonic movements. One major limiting factor to those techniques is the effect of the troposphere, as surface velocities are of the order of a few mm yr−1, and high accuracy (to mm level) is required. The troposphere introduces a path delay in the microwave signal, which, in the case of GNSS Precise Point Positioning (PPP), can nowadays be partly removed with the use of specialized mapping functions. Moreover, tropospheric stratification and short wavelength spatial turbulences produce an additive noise to the low amplitude ground deformations calculated by the (multitemporal) InSAR methodology. InSAR atmospheric phase delay corrections are much more challenging, as opposed to GNSS PPP, due to the single pass geometry and the gridded nature of the acquired data. Thus, the precise knowledge of the tropospheric parameters along the propagation medium is extremely useful for the estimation and correction of the atmospheric phase delay. In this context, the PaTrop experiment aims to maximize the potential of using a high-resolution Limited-Area Model for the calculation and removal of the tropospheric noise from InSAR data, by following a synergistic approach and integrating all the latest advances in the fields of remote sensing meteorology (GNSS and InSAR) and Numerical Weather Forecasting (WRF). In the first phase of the experiment, presented in the current paper, we investigate the extent to which a high-resolution 1 km WRF weather re-analysis can produce detailed tropospheric delay maps of the required accuracy, by coupling its output (in terms of Zenith Total Delay or ZTD) with the vertical delay component in GNSS measurements. The model is initially operated with varying parameterization, with GNSS measurements providing a benchmark of real atmospheric conditions. Subsequently, the final WRF daily re-analysis run covers an extended period of one year, based on the optimum model parameterization scheme demonstrated by the parametric analysis. The two datasets (predicted and observed) are compared and statistically evaluated, in order to investigate the extent to which meteorological parameters that affect ZTD can be simulated accurately by the model under different weather conditions. Results demonstrate a strong correlation between predicted and observed ZTDs at the 19 GNSS stations throughout the year (R ranges from 0.91 to 0.93), with an average mean bias (MB) of –19.2 mm, indicating that the model tends to slightly underestimate the tropospheric ZTD as compared to the GNSS derived values. With respect to the seasonal component, model performance is better during the autumn period (October–December), followed by spring (April–June). Setting the acceptable bias range at ±23 mm (equal to the amplitude of one Sentinel-1 C-band phase cycle when projected to the zenithal distance), it is demonstrated that the model produces satisfactory results, with a percentage of ZTD values within the bias margin ranging from 57% in summer to 63% in autumn.

Using Kinematic GNSS Data to Assess the Accuracy and Precision of the TanDEM-X DEM Resampled at 1-m Resolution Over the Western Corinth Gulf, Greece

We assess the accuracy and the precision of the TanDEM-X digital elevation model (DEM) of the western Gulf of Corinth, Greece. We use a dense set of accurate ground coordinates obtained by kinematic Global Navigation Satellite Systems (GNSS) observations. Between 2001 and 2019, 148 surveys were made, at a 1 s sampling rate, along highways, roads, and tracks, with a total traveled distance of ∼25 000 km. The data are processed with the online Canadian Spatial Reference System precise point positioning software. From the output files, we select 885 252 coordinates from epochs with theoretical uncertainty below 0.1 m in horizontal and 0.2 m in vertical. Using specific calibration surveys, we estimate the mean vertical accuracy of the GNSS coordinates at 0.2 m. Resampling the DEM by a factor of 10 allows one to compare it with the GNSS in pixels of metric size, smaller than the width of the roads, even the small trails. The best fit is obtained by shifting the DEM by 0.47 ± 0.03 m upward, 0.10 ± 0.1 m westward, and 0.36 ± 0.1 m southward. Those values are 20 times below the nominal resolution of the DEM. Once the shift is corrected, the root mean square deviation between TanDEM-X DEM and GNSS elevations is 1.125 m. In forest and urban areas, the shift between the DEM and the GNSS increases by ∼0.5 m. The metric accuracy of the TanDEM-X DEM paves the way for new applications for long-term deformation monitoring of this area.

Pytheas: An open-source software solution for local shear-wave splitting studies

Shear-wave splitting analysis of local earthquakes is a challenge tackled by a long history of published software. However, there is currently no complete solution that integrates manual and (partially or fully) automatic methods. The Pytheas software aims to achieve this by offering a collection of popular analysis techniques accompanied by an intuitive Graphical User Interface. The software offers its own implementation of the following methods: (a) Visual inspection of polarigrams and hodograms (manual), (b) Rotation-Correlation (semi-automatic), (c) Eigenvalue (semi-automatic) and (d) Minimum Energy (semi-automatic). Moreover, it automates the last three operations by using Cluster Analysis. The software also utilizes databases to effectively manage results storing. It is compatible with standard file formats for event and station metadata (QuakeML and StationXML), as well as waveform data (such as SAC and miniSEED). Pytheas also introduces a novel algorithm for automatically assigning quality grades to measurements. To showcase the utility of the software, we analyzed a set of 263 events recorded in the Western Gulf of Corinth (Greece). The workflow included the establishment of a control group through manual analysis, the fully automatic processing of the catalogue and the verification of the results through visual inspection of a sample. Automatic and manual measurements show good agreement, validating the potential of Cluster Analysis. The presented software aims to provide a friendly experience to the end-user in all stages of its usage (installation, operation and database management) and trivialize the time-consuming part of a local shear-wave splitting study, leaving more time for interpretation and analysis of results.

Impact of a pre‐existing transverse drainage system on active rift stratigraphy: An example from the Corinth Rift, Greece

AbstractModels to explain alluvial system development in rift settings commonly depict fans that are sourced directly from catchments formed in newly uplifted footwalls, which leads to the development of steep‐sided talus‐cone fans in the actively subsiding basin depocentre. The impact of basin evolution on antecedent drainage networks orientated close to perpendicular to a rift axis, and flowing over the developing hangingwall dip slope, remains relatively poorly understood. The aim of this study is to better understand the responses to rift margin uplift and subsequent intrabasinal fault development in determining sedimentation patterns in alluvial deposits of a major antecedent drainage system. Field‐acquired data from a coarse‐grained alluvial syn‐rift succession in the western Gulf of Corinth, Greece (sedimentological logging and mapping) has allowed analysis of the spatial distribution of facies associations, stratigraphic architectural elements and patterns of palaeoflow. During the earliest rifting phase, newly uplifted footwalls redirected a previously established fluvial system with predominantly southward drainage. Footwall uplift on the southern basin margin at an initially relatively slow rate led to the development of an overfilled basin, within which an alluvial fan prograded to the south‐west, south and south‐east over a hangingwall dip slope. Deposition of the alluvial system sourced from the north coincided with the establishment of small‐scale alluvial fans sourced from the newly uplifted footwall in the south. Deposits of non‐cohesive debris flows close to the proposed hangingwall fan apex pass gradationally downstream into predominantly bedload conglomerate deposits indicative of sedimentation via hyperconcentrated flows laden with sand‐ and silt‐grade sediment. Subsequent normal faulting in the hangingwall resulted in the establishment of further barriers to stream drainage, blocking flow routes to the south. This culminated in the termination of sediment supply to the basin depocentre from the north, and the onset of underfilled basin conditions as signified by an associated lacustrine transgression. The evolution of the fluvial system described in this study records transitions between three possible end‐member types of interaction between active rifting and antecedent drainage systems: (a) erosion through an uplifted footwall, (b) drainage diversion away from an uplifted footwall and (c) deposition over the hangingwall dip slope. The orientation of antecedent drainage pathways at a high angle to the trend of a developing rift axis, replete with intrabasinal faulting, exerts a primary control on the timing and location of development of overfilled and underfilled basin states in evolving depocentres.

Characteristics of earthquake clusters: Application to western Corinth Gulf (Greece)

The western Corinth Gulf, one of the most seismically active areas in Greece with frequent earthquake sequences and a dense seismological network, is an ideal region for investigating the properties of earthquake clusters. We study 47 earthquake clusters identified using a high-resolution earthquake catalog and the CURATE clustering algorithm. Based on the skewness and kurtosis of their moment release history, 31 sequences are successfully distinguished as earthquake swarms and 13 as mainshock-aftershock sequences (three sequences could not be classified). The relationship between skewness and kurtosis leads to a parabola where mainshock–aftershock sequences are located to the right part (high values of skewness and kurtosis), whereas earthquake swarms to the center–left (low values of skewness and kurtosis). This discrimination is validated by taking into consideration the occurrence time of the largest event in the sequence, as well as the difference in magnitude between the largest event and other larger events. Twelve clusters, which consist of several burst-like repeaters, show evidence of fluid driven activity with diffusivity ranging from 0.5 to 1.8 m2/s for the triggering front and 0.1 to 0.4 m2/s for the back front. The observed temporal evolution of earthquake swarms compared to that expected from the ETAS stochastic model shows that they have distinct values for the model parameters α and μ. Finally, interevent time distributions for four clusters having at least 100 events show that a Lognormal distribution fits best in all cases, suggesting a degree of triggering between subsequent events.

Repeating earthquakes in western Corinth Gulf (Greece): implications for aseismic slip near locked faults

An extensive search for repeating earthquakes was performed in the western Corinth Gulf by applying waveform cross correlation on 22,000 earthquakes that occurred in 2008–2014. Event pairs with high correlation coefficient (CC ≥ 0.95) recorded by two or more stations are classified as multiplets of repeating events. The highly similar event pairs have typically interevent distances less than a quarter wavelength (∼150 m for a dominant frequency of 10 Hz) and are used to estimate the accuracy of the relocated catalog. A detailed analysis of the spatiotemporal properties of the repeating sequences revealed two types of repeating events, namely, burst like and continuous type repeaters. Burst like repeaters are widespread in the entire study area, mostly associated with seismic excitations located at depths between 5 and 9 km, triggered either by fluid intrusion or stress transfer. Their duration is short with high values of coefficient of variation in recurrence intervals (COV > 1) and high slip rates. The continuous type repeaters, which last 1 to 7 years, with COV ∼ 1 and slip rates almost 0.26 cm/yr, form a very narrow shallow north dipping seismic zone at 10 km depth along the Psathopyrgos and Aigion faults. That kind of activity provides strong evidence for aseismic slip in the western Corinth Gulf and defines the boundaries between brittle-ductile transition in the area.

LOCAL EARTHQUAKE TOMOGRAPHY IN THE BROADER AREA OF WESTERN CORINTH GULF

In this study, we applied Local Earthquake Tomography in order to investigate the detailed 3-D structure within and around the broader region of Western Corinth Gulf which is one of the most seismically active regions in the world. We use data from the 2012-2014 time-period, selecting about 2,000 seismic events recorded by the local seismic stations of Hellenic Unified Seismological Network (HUSN) and the Corinth Rift Laboratory Network (CRLN). Applying Tomographic Inversion, we produce and interpret 3-D models of Vp, Vs and Vp/Vs ratio in the study area. The obtained results shows several distinct structures, namely areas of high and low Vp/Vs ratio correlating the hypocenter distribution with changes in lithology or fluid concentration. In the area of Patraikos Gulf, an ascending velocity volume was traced that could possibly be connected to salt tectonics in the alpine basement, effecting the fluid circulation as well as the behavior of local faults.

Upper crust seismic anisotropy study and temporal variations of shear-wave splitting parameters in the western Gulf of Corinth (Greece) during 2013

During 2013, the Western Gulf of Corinth (WGoC, Central Greece) experienced a period of increased seismicity, with a total of over 4700 earthquakes. This fact in combination with the existence of dense seismological networks provided an excellent opportunity for the study of crustal seismic anisotropy. Of special note is the seismic crisis period of May–October, during which the main feature was the occurrence of the Helike seismic swarm. Polarigrams and hodograms were employed to analyze local waveforms. This method resulted in 659 measurements of shear-wave splitting parameters, namely the direction of the fast shear-wave (Sfast), the time-delay (Td) between the two split shear-waves and the source polarization direction. A pattern of a general WNW–ESE anisotropy direction, parallel to the GoC’s fault systems’ strike, is established, with the exception of two stations located in adjacent areas at the north. This is in agreement with the existence of fluid-filled microcracks, oriented according to the regional stress field. The obtained splitting parameters are compared to the results of other anisotropy studies performed in the WGoC. A detailed analysis of the temporal evolution of the normalized time-delay (Tn) was performed to associate temporal stress changes to seismicity fluctuations. Increase in normalized time-delays and drop before the occurrence of the first significant event belonging to the “July Cluster”, which occurred between the 13th and the 16th of the same month, was observed for most of the analyzed stations.

Spatio-temporal earthquake clustering in the Western Corinth Gulf

Corinth Gulf has been studied, thoroughly using multidisciplinary approaches (geological, seismological, geodetic etc), which revealed its complicated tectonic behaviour. In the last five years or more, an intense continuous microseismic activity is observed in the westernmost part of the gulf covering the area from west of Aigio tothe area west of Rio Antirrio strait, near the city of Patras. Aiming to study in detail the properties of this microseismic manifestation, the recordings of the Hellenic Unified Seismological Network (HUSN) are used to accurately determine the focal coordinates of earthquakes with magnitudes M=1.5 or more. Relocation was performed using the HYPOINVERSE program, for all available data for the period 2010-2011 when earthquakes up to magnitude M=5.5 occurred. The space time plot of the epicentres shows that seismicity is not random in the area but formed distinctive clusters, indicating an E-W striking seismic zone whose patches are successively activated for certain periods. The present work deals with the investigation of spatio-temporal evolution of seismicity with the intention to examine the migration ofseismicity and multi-segment activation.

Sedimentary impacts of recent moderate earthquakes from the shelves to the basin floor in the western Gulf of Corinth

In seismically active areas, long term records of large earthquakes are indispensable to constrain reccurence patterns of large earthquakes. In the western Corinth Rift, one of the most active areas in Europe in terms of seismicity, data about ancient earthquakes are still insufficient, despite historical records covering the last two millenia and several studies in onshore paleoseimology. In this paper, we test the use of offshore sediments from the Gulf of Corinth to identify sediment failures and tsunamis that have been triggered by historical earthquakes. Two shelves (40–100m deep), one sub-basin (180m) and the basin floor (330m) have been sampled by short gravity cores. The cores were analyzed in order to identify and characterize event deposits. The age control has been provided by 137Cs and 210Pb activity measurements showing that the cores represent 2 to 4 centuries of sedimentation. In each site, sandy event deposits are interbeded in the muddy, hemipelagic sedimentation. The age of event deposits has been compared to the record of historical earthquakes using new and published macroseismic data. This comparison shows temporal coincidence of some event deposits and documented earthquakes with a macroseismic intensity≥VII in the area, e.g. in 1861CE, 1888CE and 1909CE. In nearshore, shallow-water settings, the record of event deposits does not exactly fit with the historical record of large earthquakes because too few event deposits are present. This may be due to the absence of sediment failures or to a lower preservation of the deposits in such settings. In the deepest site, in the basin floor, the correspondence is better: a sandy turbidite probably corresponds to each large earthquake since 1850CE, except one aseismic sediment density flow that occurred at the end of the 20th century. Surprisingly, the Ms=6.2, June 15, 1995 Aigion earthquake is only possibly recorded in one nearshore site on the Aigion Shelf, in the form of a tsunami back-wash flow deposit. This study showed that moderate earthquakes (M 5.8–6.5) can significantly impact marine sediments. Regarding the evaluation of seismic hazard in the area, the basin floor is proposed as a promising site for long term paleoseismology in the Gulf of Corinth, while shallower settings need to be considered more carefully.

The seismogenic fault of the 2010 Efpalion moderate-size seismic sequence (western Corinth gulf, Central Greece)

The 2010 Efpalion seismic sequence with two main moderate earthquake events occurred in the northwestern part of the Corinth Gulf (Central Greece)—a region that has been intensely stretched due to an on-going N–S extensional stress regime. Previous studies assign these two events to activations of (a) two faults dipping to the north with low angles; (b) two faults dipping at high angles, the first dipping to the south, and the second to the north; and (c) two faults dipping at high angles, but the first dips to the north, and the second to the south. The recently proposed TR method for focal mechanisms that identifies the seismogenic fault of an earthquake sequence is applied on the available focal mechanisms of the sequence, and its results are interrelated with the geology of the region, and previous contradictory interpretations. The focal mechanisms constructed with MT inversion define a steep north-dipping normal fault, whereas those constructed with first motions of P-waves define the activation of two adjoining faults that dip with high angles to the SSE and south, respectively, and which are characterized by strain (slip) compatibility. The latter option fits well with the geology of the region that is dominated by a SE to S-dipping horse-tail splay fault zone which exists at the eastern tip of the Nafpaktos Mountain Front. The application of the TR method reveals that the usage itself of different methods for the construction of the focal mechanisms complicates the problem of correctly identifying the seismogenic fault.