Western Hellenic Arc Research Articles

Assessment of Run-Up Height and Arrival Time of 365 AD Tsunami at Lampedusa Island, Italy

The eastern Mediterranean Sea is a subduction zone between the African plate and the Eurasian plate that has high seismicity. The tsunami of July 21, 365 AD was one of the most severe in the region, and its impact was estimated to have extended to the islands around the Aegean Sea, Cyprus, southern Calabria, Sicily, Malta, and Lampedusa. However, an analysis of tsunami waves reaching Lampedusa Island has not been conducted. Therefore, this study will present the run-up height and arrival time of tsunami waves near the Lampedusa coast. The methods applied are linear and non-linear shallow water equations by inputting the rupture parameters of the 365 AD earthquake. We used Delft Dashboard and Delft3D software to simulate tsunami wave propagation offshore. The simulation results indicate that an earthquake with a magnitude of approximately Mw 8.5 triggered tsunami waves with a maximum run-up height of 2.02 m in nearshore Lampedusa. The travel time for the maximum run-up height was 3.5 hours after the generation of the first tsunami wave in the western Hellenic Arc, Greece. We infer that seawater most likely inundated the coast of Lampedusa during the tsunami earthquake in 365 AD.

InSAR Time-Series Analysis for Monitoring Ground Displacement Trends in the Western Hellenic Arc: The Kythira Island, Greece

Kythira Island is situated at the western Hellenic Arc, which is a region of very high seismicity and tectonic activity. On 8 January 2006, a large seismic event of Mw = 6.7 occurred close to Kythira, in association with the Hellenic subduction zone. We present an extensive multi-temporal interferometry study of ground displacements in Kythira Island exploiting SAR data of a decade, from 2003 to 2009, and from 2015 to 2019. The line-of-sight displacement field for the 2003–2009 time period presents small-scale displacements that do not exceed −3 mm/y, identified to the East of Kythira’s airport, and 3 mm/y in the central part of the island. We exploit then Sentinel-1 data from 2015 to 2019, of both descending and ascending Sentinel-1 SAR imaging modes to decompose the line-of-sight measured deformation to vertical and east–west motion components. Higher vertical displacement rates characterize the central part of the island, with a maximum value of 5 mm/y. This short-term uplift rate exceeds by an order of magnitude the long-term geologic uplift rate of ~0.13 mm/y found in the island during the Quaternary. We discuss possible regional geophysical explanations for this discrepancy. Strike-slip components are detected in the N–E coast of the Potamos village, where horizontal displacements occur, with an east-ward trend and a maximum value of −3 mm/y. These insights are valuable input for the systematic monitoring of this high seismic risk island and the dynamic assessment of its hazard potential.

The 25 October 2018 Mw = 6.7 Zakynthos earthquake (Ionian Sea, Greece): A low-angle fault model based on GNSS data, relocated seismicity, small tsunami and implications for the seismic hazard in the west Hellenic Arc

We present a joint analysis of seismological and Global Navigation Satellite System (GNSS) data of the seismic sequence that started on October 25, 2018 with a shallow Mw = 6.7 earthquake offshore Zakynthos (Ionian Sea, Greece). The inversion of the GNSS data shows the activation of a N-S striking thrust/oblique-slip fault at the African-Aegean plate interface with a length of ∼26 km, and depth shallower than 15 km. The fault-plane geometry is well constrained by GNSS with a low-dip angle (23°), and dip-direction towards east. This is consistent with the distribution of the relocated aftershocks (1811 events). Our analysis indicates that the October 25, 2018 event ruptured the Hellenic megathrust. This event highlights the high degree of seismic coupling in the western region of the Hellenic Arc. It also highlights the “strong” nature of the subducting slab with the occurrence of “locked” patches under the Ionian seafloor that fail during large, reverse/oblique-slip earthquakes. The latest, shallow seismic events of the western Hellenic arc (1976 Zakynthos, 1997 Strofades, 2018 Zakynthos) point to a sequence of low-angle events along the plate interface with most of co-seismic deformation taken up by the upper (Aegean) plate. The GNSS data also point to a similar pattern between the co-seismic strain release and the long-term (interseismic) strain accumulation along the west Hellenic Arc.

The Tsunami Inundation Hazard of the Maltese Islands (Central Mediterranean Sea): A Submarine Landslide and Earthquake Tsunami Scenario Study

The tsunami hazard for the Maltese Islands is poorly defined, and historic records are available for only two recent events. Most of the population and touristic infrastructure of the archipelago is concentrated along the eastern low-lying coastline, which is exposed to tsunamis from near-field and far-field sources. In this study we present a scenario-based tsunami inundation study to assess the impact of potential significant cases. We simulated four scenarios—two submarine landslide sources (outer Malta Plateau slide and Gela Basin slide) and two earthquake sources mimicking events comparable to the 365 A.D. western Hellenic Arc event and the 1693 south-east of Sicily event. We find that all scenarios cause inundation in densely populated low-lying bays or rias of Mellieha Bay, Xemxija, Salini, Gzira, Msida, Marsaskala, St Thomas Bay, Marsaxlokk and Birzebbuga. The largest inundation extents and flow depths (> 10 m) are produced by the two landslide sources and the western Hellenic Arc earthquake. Of interest is the role of the Malta Escarpment and Sicily in amplifying and reflecting tsunami waves, and in generating consistent hot spots along the eastern coastline of Malta.

Seismic anisotropy in the Hellenic subduction zone: Effects of slab segmentation and subslab mantle flow

The segmentation and differentiation of subducting slabs have considerable effects on mantle convection and tectonics. The Hellenic subduction zone is a complex convergent margin with strong curvature and fast slab rollback. The upper mantle seismic anisotropy in the region is studied focusing at its western and eastern edges in order to explore the effects of possible slab segmentation on mantle flow and fabrics. Complementary to new SKS shear-wave splitting measurements in regions not adequately sampled so far, the source-side splitting technique is applied to constrain the depth of anisotropy and to densify measurements. In the western Hellenic arc, a trench-normal subslab anisotropy is observed near the trench. In the forearc domain, source-side and SKS measurements reveal a trench-parallel pattern. This indicates subslab trench-parallel mantle flow, associated with return flow due to the fast slab rollback. The passage from continental to oceanic subduction in the western Hellenic zone is illustrated by a forearc transitional anisotropy pattern. This indicates subslab mantle flow parallel to a NE–SW smooth ramp that possibly connects the two subducted slabs. A young tear fault initiated at the Kefalonia Transform Fault is likely not entirely developed, as this trench-parallel anisotropy pattern is observed along the entire western Hellenic subduction system, even following this horizontal offset between the two slabs. At the eastern side of the Hellenic subduction zone, subslab source-side anisotropy measurements show a general trench-normal pattern. These are associated with mantle flow through a possible ongoing tearing of the oceanic lithosphere in the area. Although the exact geometry of this slab tear is relatively unknown, SKS trench-parallel measurements imply that the tear has not reached the surface yet. Further exploration of the Hellenic subduction system is necessary; denser seismic networks should be deployed at both its edges in order to achieve a more definite image of the structure and geodynamics of this area.

The M w6.7 12 October 2013 western Hellenic Arc main shock and its aftershock sequence: implications for the slab properties

The 12 October 2013 M w6.7 earthquake offshore Crete Island is one of the few strong earthquakes to have occurred in the last few decades in the southwestern part of the Hellenic subduction zone (HSZ), providing the opportunity to evaluate characteristics of the descending slab. The HSZ has experienced several strong (M ≥ 7.0) earthquakes in historical times with the largest one being the 365 AD, M w = 8.4 earthquake, the largest known ever occurred in the Mediterranean region. The 2013 main shock occurred in close proximity with the 365 event, on an interplate thrust fault at a depth of 26 km, onto the coupled part of the overriding and descending plates. GCMT solution shows a slightly oblique (rake = 130°) thrust faulting with downdip compression on a nearly horizontal (dip = 3°) northeast-dipping fault plane with strike (340°) parallel to the subduction front, with the compression axis being oriented in the direction of plate convergence. The subduction interface can be more clearly resolved with the integration of aftershock locations and CMT solution. For this scope, the aftershocks were relocated after obtaining a v p/v s ratio equal to 1.76, a one-dimensional velocity model and time delays that approximate the velocity structure of the study area, and the employment of double-difference technique for both phase pick data and cross-correlation differential times. The first-day relocated seismicity, alike aftershocks in the first 2 months, shows activation of an area at the upper part of the descending slab, with most activity being concentrated between 13 and 27 km, where the main shock is also encompassed. Aftershocks are rare near to the main shock, implying homogeneous slip on a large patch of the rupture plane. Based on the aftershock distribution, the size of the activated area estimated is about 24 km long and 17 km wide. Coulomb stress changes resolved for transpressive motion reveal negligible off-fault aftershock triggering, evidencing a comparatively stable regime in the downdip part of the slab or different fault mechanism.

Scenario-based assessment of buildings' damage and population exposure due to earthquake-induced tsunamis for the town of Alexandria, Egypt

Abstract. Alexandria is the second biggest city in Egypt with regards to population, is a key economic area in northern Africa and has very important tourist activity. Historical records indicate that it was severely affected by a number of tsunami events. In this work we assess the tsunami hazard by running numerical simulations of tsunami impact in Alexandria through the worst-case credible tsunami scenario analysis (WCTSA). We identify three main seismic sources: the western Hellenic Arc (WHA – reference event AD 365, Mw = 8.5), the eastern Hellenic Arc (EHA – reference event 1303, Mw = 8.0) and the Cyprus Arc (CA – hypothetical scenario earthquake with Mw = 8.0), inferred from the tectonic setting and from historical tsunami catalogues. All numerical simulations are carried out in two sea level conditions (mean sea level and maximum high-tide sea level) by means of the code UBO-TSUFD, developed and maintained by the Tsunami Research Team of the University of Bologna. Relevant tsunami metrics are computed for each scenario and then used to build aggregated fields such as the maximum flood depth and the maximum inundation area. We find that the case that produces the most relevant flooding in Alexandria is the EHA scenario, with wave heights up to 4 m. The aggregate fields are used for a building vulnerability assessment according to a methodology developed in the framework of the EU-FP6 project SCHEMA and further refined in this study, based on the adoption of a suitable building damage matrix and on water inundation depth. It is found that in the districts of El Dekhila and Al Amriyah, to the south-west of the port of Dekhila, over 12 000 (13 400 in the case of maximum high tide) buildings could be affected and hundreds of them could sustain damaging consequences, ranging from critical damage to total collapse. It is also found that in the same districts tsunami inundation covers an area of about 15 km2, resulting in more than 150 000 (165 000 in the case of maximum high tide) residents being exposed.

The structural framework of the Peloponnese continental margin from Zakynthos to Pylos from seismic reflection and morpho-bathymetric data

One of the objectives of the European project SEAHELLARC (SEismic and tsunami risk Assessment and mitigation scenarios in the western HELLenic ARC) was to identify the key elements controlling the geological structure of the Peloponnese continental margin in order to better understand the distribution of earthquakes in the area. Vintage multichannel seismic lines were reprocessed, interpreted and integrated with detailed morpho-bathymetric data and high-resolution single channel seismic data. According to the interpretation, most of the upper continental margin has been created within the former Alpine Gavrovo, Ionian and Apulian thrust units, including their Miocene to Quaternary cover and their Triassic salt-bearing formations. The Ionian thrust complex seems to have been continuously overthrusting the Apulian Ridge, itself characterised by strongly fractured acoustic basement features. Ductile and mobile Triassic evaporitic sequences are present in most of the major Ionian zone morpho-structures. The westward directed thrusts of the Ionian zone reached their final positions in Lower Pliocene and were followed by uplift of the ridges that is continuing in the present time. Previous compressional structures are being reactivated as normal faults with associated diapirism in the Triassic evaporites. NNE-SSW shear displacements related to trans-extensional motion may have activated pull apart basins with ENE-WSW normal faulting at their heads. This NNE-SSW dextral strikeslip motion agrees with the present anticlockwise rotation of the Peloponnese. The focal mechanism and the NNE-SSW distribution of aftershocks of a major earthquake in June 2008 along the Andravida fault define a major lineament that can be followed past the Katakolo Peninsula to the diapiric morpho-structures defining the eastern boundary of the Strophades basin.

Geophysical and geological imprints of southern Neotethyan subduction between Cyprus and the Isparta Angle, SW Turkey

The present-day eastern Mediterranean region is characterized by two main arc systems: the western Hellenic arc and the eastern Cyprian arc, the latter having no significant trench unlike the former. Therefore, plate interactions in the western-northwestern side of the island of Cyprus is a matter of debate concentrating mainly on subduction or wrench systems. In order to understand the plate interactions within the area between the Isparta Angle and western segment of the Cyprus arc, the epicenter distribution of earthquakes, b-values and gravity anomalies for 5 depth intervals (0–35 km, 35–55 km, 55–75 km, 75–95 km, >95 km) have been analyzed on NE-SW trending regional profiles, and compared with geological structures. Although there are no earthquakes with depths greater than 50 km (i.e. crustal scale earthquakes) in the northern part of the Isparta Angle, all earthquake epicenters lie on a linear zone from southwest of Cyprus to the northwest, nearly parallel to the NW-SE trending Florence Rise. The b-value distribution shows a very good consistency with the epicentral and tectonic maps of Antalya Bay. The b-value maps, deeper than 55 km, show that low b-values depicting a NW-SE trending linear pattern correspond to seismic zones. Furthermore, this pattern shifts to the northeast and resembles the epicenters of the deep-focus earthquakes on the northeast dipping plane supporting the existence of a subduction zone. The gravity profiles show low gravity anomalies along the Florence Rise, high gravity anomalies at a certain distance interval to the NE direction which presumably resulted from subducting oceanic lithosphere, and again low gravity anomalies corresponding to the end of subducting slab and continental eastern limb of the Isparta Angle. On the other hand, the observed gravity anomalies are found to be consistent with the earthquake distribution patterns in the area. As a conclusion, for the area between the apex of the Isparta Angle and the western Cyprian arc, the distribution of earthquake epicenter locations, b-values, and gravity data suggest the presence of a subduction zone inclined to the northeast beneath the eastern limb of the Isparta Angle.

Earthquake‐generated tsunamis in the Mediterranean Sea: Scenarios of potential threats to Southern Italy

We calculated the impact on Southern Italy of a large set of tsunamis resulting from earthquakes generated by major fault zones of the Mediterranean Sea. Our approach merges updated knowledge on the regional tectonic setting and scenario‐like calculations of expected tsunami impact. We selected three potential source zones located at short, intermediate and large distance from our target coastlines: the Southern Tyrrhenian thrust belt; the Tell‐Atlas thrust belt; and the western Hellenic Arc. For each zone we determined a Maximum Credible Earthquake and described the geometry, kinematics and size of its associated Typical Fault. We then let the Typical Fault float along strike of its parent source zone and simulated all tsunamis it could trigger. Simulations are based on the solution of the nonlinear shallow water equations through a finite difference technique. For each run we calculated the wavefields at desired simulation times and the maximum water elevation field, then produced traveltime maps and maximum wave‐height profiles along the target coastlines. The results show a highly variable impact for tsunamis generated by the different source zones. For example, a large Hellenic Arc earthquake will produce a much higher tsunami wave (up to 5 m) than those of the other two source zones (up to 1.5 m). This implies that tsunami scenarios for Mediterranean Sea countries must necessarily be computed at the scale of the entire basin. Our work represents a pilot study for constructing a basin‐wide tsunami scenario database to be used for tsunami hazard assessment and early warning.

Seismic coupling and structure of the Hellenic subduction zone in the Ionian Islands region

The western Hellenic arc has been commonly considered as a largely aseismic subduction zone, from the comparison of a small rate of shortening derived from the seismic moment release, with a large rate of convergence inferred from geology. Complete seismic coupling would instead be expected from models that consider a control by plate tectonic forces, because of the trenchward velocity of the Hellenic–Aegean upper plate now confirmed with GPS measurements. In the region of the Ionian Islands, a subduction interplate boundary has been recently imaged and its seismogenic downdip width suggested to be moderate, from reflection seismic profiling and local earthquake tomography. In the appropriate model for such an earthquake source region, which considers a single interplate fault and takes into account these features, the moderate seismic moment release is found consistent with complete seismic coupling of this subduction. The shallow downdip limit of the seismogenic zone can be interpreted as due to the interplate boundary being overlain there by the ductile deeper crust of the orogenically thickened Hellenides.

Earthquake mechanisms of the Adriatic Sea and Western Greece: implications for the oceanic subduction-continental collision transition

SUMMARY We present 21 focal solutions (magnitude > 5.5) reliably computed by body-wave modelling for the western Hellenic arc from Yugoslavia to the southern Peloponnese. Mechanisms located within the Aegean show normal faulting, the T-axis trending N-S in the centre and parallel to the active boundary in the external part. Mechanisms associated with the Keffalinia fault are consistent with dextral strike-slip motion. Reverse mechanisms located along the active boundary are remarkably consistent and do not depend on the nature of the active boundary (continental collision or oceanic subduction). The consistency in azimuth of the slip vectors and of the GPS velocity relative to Africa, all along the active boundary, suggests that the deformation is related to the same motion. The discrepancy between seismic-energy release and the amount of shortening confirms that the continental collision is achieved by seismic slip on faults but the oceanic subduction is partially aseismic. The northward decrease in velocity between continental collision and oceanic subduction suggests the continental collision to be a recent evolution of the active subduction.

The 1953 earthquake in Cephalonia (Western Hellenic Arc): coastal uplift and halotectonic faulting

SUMMARY Geomorphological, marine biological and radiometric data in combination with earlier reports reveal that the M, =7.2, 1953 Cephalonia earthquake, the most destructive but least studied earthquake in Greece this century, was associated with a 0.3-0.7m quasi-rigid-body uplift and westward tilting of the central part of the island. Another palaeoseismic event, around 1500 yr BP, associated with coastal uplifts was also identified. Structural data indicate that the 1953 uplift is bounded by two subparallel, east-dipping major reverse faults and corresponds to a piston-like motion. This structurally unusual pattern of seismic deformation is detached from the deformation of the basement (conspicuously a thrust) and different from the long-term deformation pattern of the area; it is probably due to the particularities of salt tectonics: a -1500m thick salt layer acts as a regional dCcollement, while thinner layers are sandwiched between the carbonate thrust sheets that compose the overburden and reduce their friction during fault movement. This crustal anisotropy is responsible for the observed anomalous attenuation of seismic waves during the 1953 and historical earthquakes. The 1953 seismic surface deformation mimics long-term halotectonic patterns, but is not directly indicative of the regional stress-field, for it reflects uplift-induced stresses only.

On the shape of the subducting slab beneath the Peloponnese, Greece

The Mediterranean sea floor is subducted beneath the western Hellenic arc at a gentle angle for about 200 km and then steepens beneath the Gulf of Corinth. The seismic zone extends to a depth of 150 km, but the subducted intact lithosphere could reach 600 km. The present relative motion between Africa and Aegea can be considered as the sum of 2 motions: 1) slow (1 cm/y) relative motion between Africa and Europe, which describes the rate at which the deep slab penetrates the asthenosphere and which started during the upper Cretaceous time when Africa started to converge toward Europe and 2) fast (5 cm/y) relative motion between the southern Aegea and Europe, manifest by internal deformation within the Aegean, which may have been active at present rates only since Pliocene time. I suggest that the increase in the total convergent rates which began about 5 my ago is responsible for the wide, flat slab beneath the Peloponnese.

The strain pattern in the western Hellenic arc deduced from a microearthquake survey

SUMMARY than loo0 earthquakes recorded during 7 weeks in 1986 in the Peloponnese and surrounding areas show shallow seismicity spread over a wide area bounded by the Hellenic trench. The highest level in the energy release is for clusters located along the Hellenic trench, where changes in the morphology are seen. A few clusters are also observed, as at the intersection between the Gulf of Corinth and the Gulf of Patras, near the Lake Trikhonis, or between Kythira and Crete. In the Peloponnese, the shallow earthquakes do not define single faults, but are diffusely distributed. A higher concentration and a deepening of the foci (within the whole crust) towards the west seem to reflect a higher strain rate there. Fault plane solutions exhibit a scattered pattern for earthquakes shallower than 11 km, but for earthquakes deeper than 11 km, they show some consistency in the orientation of P-, T-, and B-axes. Gently dipping nodal planes are seen all over the Peloponnese, with the conjugate vertical plane striking in various directions, and with no consistency in the sense of motion. These earthquakes are located between 8 and 18 km deep and could reflect a decoupling between the lower and the upper crust. Reverse faulting is seen in the Gulf of Kefallinia and the western Peloponnese. The P-axes trend NE-SW to E-W. Normal faulting is seen in the Gulf of Corinth and in central Peloponnese with the T-axes trending N-S, and in the southern Peloponnese, where the T-axes trend NW-SE. The deformation over the western Hellenic arc, revealed by fault plane solutions of earthquakes with depth between 11 and 40 km, appears to be the superposition of two phenomena. First, throughout the Aegean, crustal extension with orientation roughly parallel to the trench dominates, with crustal shortening and subduction along the Aegean arc. Superimposed on this is a more local effect of the collision between the Aegea and Apulia which seems to induce horizontal compression roughly perpendicular to the Aegean arc west of the Peloponnese and Epirus. The compression due to this decays eastward, at a distance comparable with the width of the collision zone. The deepening of the brittleductile transition is likely to be due to an increase of the strain field towards the Ionian islands.

The Hellenic subduction beneath the Peloponnesus: first results of a microearthquake study

A preliminary examination of the 1070 earthquake locations, determined from 6 weeks of recording in 1986 by 46 stations, show that the seismicity is spread over a wide area of the Peloponnesus and the western Hellenic arc and throughout the whole crust. No clear individual faults can be identified from the seismicity, but clusters of activity are observed in some places. Seismicity is concentrated above 40 km and deeper earthquakes were not numerous. Only 28 of the 466 events with uncertainties in depth less than 5 km occurred deeper than 40 km. Seismicity deeper than 30 km defines a flat zone at a depth between 40 km and 70 km, starting from the trench to about 200 km towards the northeast. Further northeast, the dip of the seismic zone abruptly changes to 45°. Fault plane solutions for the deeper events, generally indicate T-axes plunging northeast, within the subducted slab. Therefore, we interpret the seismicity deeper than 30 km as due to the superposition of two different causes: (1) the steep zone is due to the subduction of the African litospheric plate beneath the Aegean, and (2) the shallow flat zone located between the trench and the Argolide is partly due to the loading of the overriding Aegean plate which is deforming above the African plate.

Long-term accelerating foreshock activity may indicate the occurrence time of a strong shock in the Western Hellenic Arc

In the decades prior to the occurrence of the 1899 ( M s = 6.6) and 1947 ( M s = 7.0) main shocks in the Western Hellenic Arc (WHA) the rate of occurrence of foreshocks ( M s ⩾ 5.2) within a radius of 100 km around the epicenters can generally be said to have accelerated. After a long period of very low foreshock activity (stage I), the process culminates in a final rapid acceleration of foreshocks some months before the main shocks (stage II), while the last two months are quiescent (stage III). These three stages are in good correspondence with the three stages of crustal deformation and several precursors to the main shock as predicted by the dilatancy model. The data fit very well power law equations similar to those found for short-term foreshocks of ordinary earthquakes and those associated with the creation of artificial lakes. The position of the rupture zones of past WHA strong shocks implies that the eastern part of the segment is the most probable location for the next strong shock in the arc. A process of accelerating seismic activity, similar to that which preceded the 1899 and 1947 shocks, has been under way since 1966 around this part of the segment. A comparison between changes in the power law curves for the earlier earthquakes and the one now expected, indicates that the latter is now 3–8 months “overdue”. Assuming that the long-term accelerating foreshock activity is a seismotectonic peculiarity of the WHA segment, I suggest that the preparation of the next rupture in the WHA has entered a highly mature stage, and that there will very probably be an earthquake in this area within the next few months.

Structural development of Neogene basins in western Greece

Abstract An account is given of the structural setting of the various Neogene sedimentary basins of western Greece. Compressional basins are attributable to foreland loading by the Alpine fold and thrust belt of the Outer Hellenides, and to active subduction in the adjacent western Hellenic arc. Late extensional basins are related to N‐S crustal extension in the Aegean marginal basin and, in western Greece, are superimposed on the earlier compressional structures. The local seismicity provides evidence that the main E‐W‐trending basin‐bounding faults of the extensional basins form a linked system that includes NW‐SE‐ and NE‐SW‐trending transfer zones of transtension. The transfer zones are themselves the sites of small extensional basins.

Earthquake locations in the western Hellenic arc relative to the plate boundary

AbstractFour vertical-component seismograph stations were operated in western Crete, southeastern Peloponnesus, the island of Milos, and south of Corinth on Mount Didimon from 1981 to 1984. Combining arrival times from this network, the Greek National Network, and VOLNET, we relocated hypocenters within Greece and the surrounding area. The difference between the relocations and the locations published in the National Observatory of Athens Bulletin in most instances were minor, except for earthquakes located in the vicinity of the western Hellenic arc. In this area, we found that epicenters thought to lie south of the plate boundary, within the African plate, relocated an average of approximately 60 km to the N30°E, which places them at or north of the plate boundary. We suggest that this difference is a minimum estimate of the location error and the true locations may lie still further to the NNE. We propose that anomalously slow seismic velocities in the Aegean back-arc upper mantle cause travel-time delays to the seismograph stations located to the NE of the source volumes, resulting in location errors in a direction away from these stations. Extrapolating from our observations to other segments of the Hellenic plate boundary and to periods not covered by our operation, we hypothesize that most or all earthquakes thought to have originated on the African plate, within about 100 km of the Hellenic arc, were actually located at the plate boundary or on the Aegean plate.

Seismic moments of earthquakes at the western Hellenic arc and their application to the seismic hazard of the area

Seismic moments of 45 large earthquakes ( M s ⩾ 5.2) which occurred in the southwestern part of the Hellenic arc-trench system between 1950 and 1981 were used to express the frequency of occurrence of earthquakes in terms of the subduction rate, and the largest seismic moment likely to occur in the region. For this purpose, a moment versus magnitude relationship for earthquakes characteristic of the region was derived, and the subduction rate was estimated from seismic moment rates.