seismicity of the aegean and surrounding area abstract seismicity parameters based on instrumental and historical data are given for 36 seismic sources of shallow earthquakes and for 6 seismic sources of intermediate depth earthquakes in the aegean and surrounding area 34 n 43 n 18 e 30 e these parameters are of two categories parameters of the first category are useful for seismic hazard evaluation a and b values of the frequency magnitude relation maximum earthquake magnitude number of shocks larger than certain magnitude per year while parameters of the second category are proper for describing the spatial distribution of seismicity mean return period most probable magnitude in a certain time period moment rate shallow seismicity is highest along the convex side of the hellenic arc but close to the coast ionian islands south peloponnesus south of crete south of karpathos and rhodos in central greece patraikos corinthiakos evoikos gulfs thessalia and along a seismic belt which includes the northwestern anatolia fault zone the northernmost part of the aegean sea and the serbomacedonia zone the intermediate depth seismic activity is distributed in two parts of the benioff zone which dips from the convex side eastern mediterranean to the concave side aegean sea of the hellenic arc the one part shows a low dip angle and includes earthquakes with focal depths between 70 km and 100 km this part has a high seismicity earthquake magnitudes up to 8 0 and is situated below the inner slope of the sedimentary part of the hellenic arc eastern peloponnesus north of crete rhodos based on the fact that this part of the benioff zone is a continuation of the seismic zone of shallow earthquakes along the hellenic arc and that both zones have the same seismicity it is concluded that they constitute a continuous seismic belt on which coupling occurs between the subducted eastern mediterranean lithosphere and the overthrusted aegean lithosphere the other inner part of the benioff zone shows low seismicity maximum earthquake magnitude up to 7 1 with earthquakes of focal depths between 100 km and 180 km it dips at a mean angle of 38 below the volcanic arc of the southern aegean sea and is the result of the subduction of the front part of the eastern mediterranean lithosphere without coupling with the aegean lithosphere

crustal strain in central greece from repeated gps measurements in the interval 1989 1997 summary a 66 station gps network spanning central greece first observed in 1989 has been occupied fully on three occasions june 1989 october 1991 and may 1993 subsets of this network bounding the gulf of korinthos have also been occupied in june 1995 october 1995 may 1996 and september october 1997 the first three occupations were processed using a fiducial gps methodology whereas later surveys were processed using code precise orbits combination of data from different surveys to yield smooth site velocities requires global network translations at each epoch to compensate for errors in the realization of the reference frame this method provides a posteriori estimates of the relative coordinate errors and reference frame noise only one earthquake the 1995 june 15 egion event has caused significant local coseismic displacement and its effects on the interseismic velocity field are removed using an elastic dislocation model we constrain the orientation of the 100 yr triangulation gps velocity estimates of davies et al 1997 using 14 sites common to the two networks the goodness of fit of this transformation indicates that the short term and 100 yr geodetic estimates of deformation are highly compatible we infer that short term geodetic studies are capable of determining longer term deformation rates provided that transient local effects can be modelled from the combined velocity field we estimate principal strains and rigid body rotation rates at points on a regular grid using data from neighbouring sites strain rates are high within the gulf of korinthos and much lower elsewhere the extension rate across the gulf of korinthos increases from east to west comparison of the extension rate with historical and recent rates of seismic release of strain reveals significant medium term seismic hazard in the western gulf of korinthos and may also indicate long term aseismic strain

paleoseismological trenching across the atalanti fault central greece evidence for the ancestors of the 1894 earthquake during the middle ages and roman times the atalanti fault bounds to the southwest the evoikos gulf one of the major extensional basins of central greece this fault ruptured during the 1894 earth quakes producing at the surface a complex ca 30 km long rupture paleoseismo logical trenching performed at three sites along this fault provided the first insights on its seismogenic behavior unfavorable trench stratigraphy and scarcity of datable material made the identification and characterization of individual paleoearthquakes quite difficult however by integrating paleoseismological geological historical and archaeoseismological data we defined three surface faulting earthquakes the most recent event is the 1894 earthquake the penultimate occurred during the middle ages between a d 770 and 1160 whereas the third event back occurred in roman times between 50 b c and a d 230 and is interpreted to be the opus earthquake of a d 105 these results suggest that 1894 type earthquakes repeat each 660 1120 yr the average minimum slip per event and vertical slip rates are of the order of 45 cm and 0 4 1 6 mm yr respectively these values are in agreement with other geological estimates and with geodetic measurements because of the short time elapsed since the 1894 earthquake the atalanti fault does not appear to contain an important seis mogenic potential on the other hand these results may shed light on the potential of other seismogenic sources threatening the area

periodic black auroral patches at the dawnside dipolarization front during a substorm 1 periodic black auroral patches were observed in the canadian postmidnight sector by ground based all sky cameras during an auroral substorm the aurora consisted of several black patches of 50 km 1 in longitude scales that appeared along the equatorward edges of the proton auroras over a 3 h local time segment from gillam to sanikiluaq ground stations 15 min after the auroral intensification occurred in alaska to the best of our knowledge this type of black auroras has never been reported before although the surrounding luminous auroras evolve into the well known torch structures later in situ observations by the themis satellites three of which were close to being magnetically conjugate in the plasma sheet as well as ground based proton auroral observations show that a dipolarization front and associated dawnward ion flows propagated eastward at poleward of the black patches coinciding with their appearance the flow motions of auroras observed poleward of the black patches are possibly generated by decelerations of dawnward ion flow because of the consistency of both durations and positions we speculate based on manifestations of high pressure regions in dipolarized flux tubes the pressure driven interchange instability formed a beading shape periodic black patches of field aligned currents the black auroral patches developed into torch structures after the flow disappearance indicating the dawnward flows might stabilize and limit the instability within a narrow l shell where the pressure gradient is directed earthward and flows cannot enter we conclude that the periodic black patches arise as a consequence of a relaxation process of enhanced plasma pressure near the inner boundary of the plasma sheet during a substorm

viscoelastic gravitational deformation by a rectangular thrust fault in a layered earth previous papers in this series have been concerned with developing the numerical techniques required for the evaluation of vertical displacements which are the result of thrust faulting in a layered elastic gravitational earth model this paper extends these methods to the calculation of fully time dependent vertical surface deformation from a rectangular dipping thrust fault in an elastic gravitational layer over a viscoelastic gravitational half space the elastic gravitational solutions are used together with the correspondence principle of linear viscoelasticity to give the solution in the laplace transform domain the technique used here to invert the displacements into the time domain is the prony series technique wherein the transformed solution is fit to the transformed representation of a truncated series of decaying exponentials purely viscoelastic results obtained are checked against results found previously using a different inverse transform method and agreement is excellent the major advantage in using the prony series technique is that deformations can be computed for arbitrary time intervals a series of results are obtained for a rectangular 30 dipping thrust fault in an elastic gravitational layer over viscoelastic gravitational half space time dependent displacements are calculated out to 50 half space relaxation times a or 100 maxwell times 2 m a significant effects due to gravity are shown to exist in the solutions as early as several a the difference between the purely viscoelastic solution and the viscoelastic gravitational solutions grows as time progresses typically the solutions with gravity reach an equilibrium value after 10 20 relaxation times when the purely viscoelastic solutions are still changing significantly additionally the length scaling which was apparent in the purely viscoelastic problem breaks down in the viscoelastic gravitational problem two independent length scales one of which changes with time are now seen to characterize the problem

horizontal crustal motion in the central and eastern mediterranean inferred from satellite laser ranging measurements four campaigns to acquire satellite laser ranging slr measurements at sites in the mediterranean region have been completed these measurements to the lageos satellite made largely by mobile systems cover a time span beginning in november 1985 and ending in june 1993 the range data from 18 sites in the central and eastern mediterranean have been simultaneously analyzed with data acquired by the remainder of the global laser tracking network estimates of horizontal motion were placed into a regional northern europe fixed kinematic reference frame uncertainties are on the order of 5 mm yr for sites having at least four occupations by mobile systems and approach 1 mm yr for permanently located sites with long histories of tracking the resulting relative motion between sites in the aegean exhibit characteristics of broadly distributed pattern of radial extension but at rates that are about 50 larger than those implied from studies of seismic strain rates based on seismicity of magnitude 6 or greater or across the region the motion estimated for sites in turkey exhibit velocity components associated with the westward motion of the anatolian block relative to eurasia these results provide a present day snapshot of ongoing deformational processes as experienced by the locations occupiedmore by slr systems less

trajectories of crustal deformation of western greece from gps observations 1989 1994 the geodynamic processes in the alpine mediterranean area are reflected in the recent crustal movements determined by satellite geodesy there is counterclockwise rotation of africa relative to eurasia with complex deformation in between we report repeated gps measurements carried out between se italy and the island of crete in the period 1989 to 1994 the data reveal distinct crustal motion of the central ionian islands and sw greece over the time span of five years western greece has moved 200mm 10mm to the sw relative to se italy and epiros the trajectories constructed from the repeated observations show that the boundary zone against the sites of no significant motion relative to south italy coincides with the kephalonia fault zone kfz this intense deformation accounts in part for the earthquakes that have frequently devastated the central ionian islands

active tectonics of the north and central aegean sea summary in this paper we examine the connection between the westward motion of turkey relative to europe and the extension in and around the aegean sea the principal new data available since the last attempt to synthesize the tectonics of this region by mckenzie 1978 are much improved focal mechanisms of earthquakes constrained by p and sh body wave modelling as well as by first motions these mechanisms show that the faulting in the western part of the aegean region is mostly extensional in nature on normal faults with a nw to wnw strike and with slip vectors directed nnw to nne there is evidence from palaeomagnetism that this western region rotates clockwise relative to stable europe in the central and eastern aegean and in nw turkey distributed right lateral strike slip is more prevalent on faults trending ne to ene and with slip vectors directed ne palaeomagnetic data in this eastern region is more ambiguous but consistent with very small or no rotations in the northern part and possibly anticlockwise rotations relative to europe in the south the strike slip faulting that enters the central aegean from the east appears to end abruptly in the sw against the nw trending normal faults of greece the kinematics of the deformation is controlled by three factors the westward motion of turkey relative to europe the continental collision between nw greece albania and the apulia adriatic platform in the west and the presence of the hellenic subduction zone to the south as the right lateral slip on the north anatolian fault enters the aegean region it splays out becoming distributed on several parallel faults the continental shortening in nw greece and albania does not allow the rotation of the western margin of the region to be rapid enough to accommodate this distributed e w right lateral shear and thus leads to e w shortening in the northern aegean which is compensated by n s extension as the southern aegean margin can move easily over the hellenic subduction zone the dynamics of the system once initiated is self sustaining being driven by the high topography in eastern turkey and by the roll back of the subducted slab beneath the southern aegean the geometry of the deformation resembles the behaviour of a system of broken slats attached to margins that rotate in spite of its extreme simplicity a simple model of such broken slats is able to reproduce quantitatively most of the features of the instantaneous velocity field in the aegean region including the slip vectors and nature of the faulting in the eastern and western parts the senses and approximate rates of rotation the overall extensional velocity across the aegean and the distribution of strain rates as seen in the seismicity and topography or bathymetry and using geodetic measurements as part of this study we re examined the relation between the surface faulting and the focal parameters determined seismologically for the three large 1981 gulf of corinth earthquakes and reassessed the evidence for associating particular earthquakes in the sequence with observed surface faulting an important result is that in one of the events there is a resolvable discrepancy between the slip vector measured at the surface and that determined from the seismic body waves

the impact of refraction correction on leveling interpretations in southern california recent investigations show that the refraction corrections in leveling are large but can be adequately modeled the corrections are proportional to the temperature differences and the square of the sight lengths changes in sight length with time have resulted in apparent crustal movements when non refraction corrected data were used applying refraction corrections to leveling data in southern california significantly alters the size and configuration of the southern california uplift uplift at palmdale and gorman lebec relative to los angeles bm v32 during the 1953 1978 time period is no greater than 10 cm the bulk of the motion about 6 cm occurs as localized motion along the san gabriel fault during the 1953 1964 time period this motion along the san gabriel fault and uplift of 8 cm directly north of the san fernando fault between 1955 and 1968 may represent precursors to the 1971 san fernando earthquake

the motion of crustal blocks driven by flow of the lower lithosphere and implications for slip rates of continental strike slip faults geodetic measurements in actively deforming areas of the continents reveal the pattern of deformation in the lithosphere if the dominant forces acting on crustal blocks are tractions at their bases then the long term motion of each block will be given by the average velocity of the underlying lithosphere slip rates between blocks estimated in this way from recent geodetic measurements across fault zones in the south island of new zealand and southern california are in good agreement with slip rates estimated geologically

reflection refraction seismics in the gulf of corinth hints at deep structure and control of the deep marine basin abstract the gulf of corinth is a natural laboratory for the study of seismicity and crustal deformation during continental extension seismic profiling along its axis provides a 24 fold normal incidence seismic reflection profile and wide angle reflection refraction profiles recorded by sea bottom seismometers obs and land seismometers at wide angle incidence the land receivers document the moho at 40 km depth under the western end of the gulf north of aigion rising to 32 km depth under the northern coast in the east of the gulf both refraction and normal incidence reflection sections image the basement under the deep marine basin that has formed by recent extension the depth to the base of the sedimentary basin beneath the gulf constrained by both methods is no more than 2 7 km with 1 km of water underlain by no more than 1 7 km of sediment less than what was expected from past modeling of uplift of the south coast in the east of the gulf unlike the flat sea bottom the basement and sedimentary interfaces show topography along this axial line several deeps are identified as depocenters which suggest that this axial line is not a strike line to the basin it appears instead to be controlled by several faults oblique to the s60 e overall trend of the south coast of the gulf their more easterly strikes being consistent with the instantaneous direction of extension measured by earthquake slip vectors and by gps

saugus palmdale california field test for refraction error in historical leveling surveys a field test designed to measure atmospheric refraction error in historical and modern leveling was conducted in may june 1981 on a 50 km long grade from saugus to palmdale california during 1955 1971 the length of sights made between the level instrument and rods systematically decreased from 60 m to 26 m the difference in height near palmdale measured by single run long sight 42 m and short sight 22 m leveling during the test was 6 times larger than expected random error correction for refraction by using either the observed or modeled vertical temperature gradient in kukkamaki s balanced sight equation reduced the height difference to the level of random error uncertainty the observed temperature gradient obeyed a power law relation t a bzc where z is the height and a b and c are constants that depended on atmospheric conditions and the ground surface beneath the line of sight the refraction corrected leveling satisfies the specifications and meets all standards of first order control surveys the six historical surveys of the saugus palmdale grade were corrected for refraction error using the results of the experiment and for rod scale errors and nontectonic subsidence considered in previous investigations the corrected uplift near palmdale reached 56 16 mm with respect to saugus during the period 1955 1965 this amount of uplift is about one third that obtained before removal of refraction error the corrected displacement profiles also reveal previously unrecognized deformation in the epicentral region of the 1971 san fernando ml 6 4 earthquake during the decade before the main shock

a microseismic study in the western part of the gulf of corinth greece implications for large scale normal faulting mechanisms summary we present the results of a dense seismological experiment in the western part of the gulf of corinth psathopyrgos aigion area one of the most active rifts in the aegean region for which we have precise tectonic information the network included 51 digital stations that operated during july and august 1991 covering a surface of 40 x 40 km2 among the 5000 recorded events with ml ranging between 1 0 and 3 0 we precisely located 774 events we obtained 148 well constrained focal mechanisms using p wave first motions of these 60 also have mechanisms obtained by combining the p wave first motions with the s wave polarization directions the observed seismicity is mainly located between 6 and 11 km depth most of the fault plane solutions correspond to e w striking normal faulting in agreement with the geological evidence most of the well determined mechanisms indicate a nodal plane dipping 10 25 due north and a steep south dipping plane a similar asymmetry is also seen in the seismicity distribution and in the overall geological structure of the corinth rift we discuss this evidence and the inference of a deep detachment zone a structure where the major faults seen at the surface appear to root a large part of the microseismic activity appears to cluster in regions near the junctions of the main faults with the proposed detachment zone this feature of the microseismicity is interpreted in terms of stress transfer and stress concentration in regions of probable nucleation of future large earthquakes

active faulting within the offshore western gulf of corinth greece implications for models of continental rift deformation discrimination between different lithospheric extension models focusing on the roles of low angle vs high angle faulting and how strain is distributed requires high fidelity imaging of brittle deformation high resolution seismic reflection and multibeam bathymetric data in the western gulf of corinth continental rift were collected to establish the contribution of offshore faults to extension onshore fault slip here is significantly less than expected from geodetic strain rates the rift at this location is a half graben tilted to the north by s dipping faults within the uppermost crust a basement horst on the northern margin is uplifted by the north and south eratini faults and the axial channel is fault controlled subsided lowstand shorelines in the hanging wall of the north eratini and the well studied aigion fault suggest that the faults have similar displacements summed extension from the four major faults across this part of the rift eliki subchannel south eratini north eratini is 8 16 mm yr thereby reconciling geologic and geodetic data sets distributed deformation across isolated multiple faults can model this part of the rift without recourse to and potentially incompatible with an underlying low angle detachment

recent plate motions and crustal deformation reports by u s workers on geodetic measurements of recent plate motions or crustal deformation published in 1987 1990 are reviewed the review begins with global plate motions proceeds through plate boundaries in california alaska and the pacific northwest and finishes with volcanic phenomena monument stability and longevity and gps relative position measurements 184 refs

a comparison of satellite laser ranging and seismicity data in the aegean region in this study we compare the strain rates obtained from earthquake moment tensors in the time interval 1911 92 in greece with measured velocities of slr stations relative to eurasia we find that the slr measurements do not require strain different in orientation from that seen in the earthquakes but simply more of it the clearest deficit is in the se aegean where earthquakes this century can account for virtually none of the measured slr strain rates by contrast in the northern aegean and nw turkey the earthquakes can account for a substantial part 50 of the measured strain rates by using both geodetic and seismic data to localize apparent deficits in seismically released strain this study represents a first step towards using space based geodesy for crude regional seismic risk assessment

aftershock relocation and frequency size distribution stress inversion and seismotectonic setting of the 7 august 2013 m 5 4 earthquake in kallidromon mountain central greece abstract on august 7 2013 a moderate earthquake noa ml 5 1 noa mw 5 4 occurred in central kallidromon mountain in the pthiotis region of central greece 2270 aftershocks were relocated using a modified 1 d velocity model for this area the b value of the aftershock sequence was b 0 85 for a completeness magnitude of mc 1 7 the rate of aftershock decay was determined at p 0 63 the spatial distribution of the aftershock sequence points towards the reactivation of a n70 10 e striking normal fault at crustal depths between 8 and 13 km a nnw sse cross section imaged the activation of a steep south dipping normal fault a stress inversion analysis of 12 focal mechanisms showed that the minimum horizontal stress is extensional at n173 e no primary surface ruptures were observed in the field however the earthquake caused severe damage in the villages of the kallidromon area the imaged fault strike and the orientation of the long axis of the aftershock sequence distribution are both at a high angle to the strike of known active faults in this area of central greece we interpret the kallidromon seismic sequence as release of extensional seismic strain on secondary steep faults inside the fokida viotia crustal block

assimilated observations of thermospheric winds the aurora and ionospheric currents over alaska we present simultaneous measurements of thermospheric winds auroral emissions and ionospheric currents over alaska obtained from four separate instruments thermospheric f region wind maps were recorded by an all sky imaging fabry perot spectrometer located at poker flat and observing at 630 0 nm auroral images at 557 7 nm were obtained from the low resolution visible imager on board the polar satellite white light all sky auroral images were recorded by ground based all sky cameras located in alaska at poker flat 65 07 n 212 34 e and at kaktovik 70 06 n 217 24 e finally the east west component of the ionospheric f region plasma convection was inferred using the alaskan meridian chain of magnetometers montage images of these four data sets are presented projected onto a geographic map of the alaskan region we examine a 10 hour period during the alaskan local nighttime of february 10 1997 these montages illustrate a close relationship between spatial structures occurring in the aurora in the ionospheric plasma convection and in the f region wind field latitudinal shear of the geomagnetic zonal wind often observed in the premidnight time sector was seen to be associated with both the equatorward and poleward boundaries of the discrete aurora we focus particularly on a period commencing just after 0900 ut when a strong shear in the zonal wind was observed to sweep southward across alaska after magnetic midnight the wind field was dominated by the emergence of the cross polar jet from the polar cap this overwhelmed any wind features associated with local auroral processes

the tectonic development of the hellenic arc and the sea of crete a synthesis abstract we use field analyses together with interpretation of aerial photographs orbital imagery and sea beam data to define more accurately the geometry of fault systems in the hellenic arc the tectonics of the area are dominated by extensional processes related to normal faulting we then use sea beam data previous oceanographic studies and extrapolation of field analyses on surrounding islands to describe the fault pattern of the cretan sea basin which is found consistent with the land fault pattern the mechanisms of the late neogene and quaternary deformation of the central and southern aegean region are analysed in terms of stress trajectories as revealed by field studies of fault populations we then examine the subsidence of the sea of crete in terms of a model of uniform stretching of the lithosphere the opening of the sea of crete was cylindrical and did not result in bending of crete this leads us to modify the southern portion of an earlier kinematic reconstruction of aegea in agreement with recent paleomagnetic results the implications of the opening of the sea of crete on the tectonic evolution of the straits to the west and east of crete are discussed finally the uplift of the outer arc is explained by underplating due to subduction

monitoring changes in mean sea level to millimeters using gps the fiducial gps technique is now well established in geodesy and geophysics for determining horizontal and vertical land movements in particular the technique is used in crustal dynamics and in monitoring changes in mean sea level the main limiting factor to achieving accuracies of a few millimeters is the reference frame or datum as defined by the adopted coordinates of the fiducial stations this paper summarises a series of tests which have been carried out to assess the quality of alternative global reference frameworks in order to achieve the highest accuracies for tide gauge heights in the uk

comments on saugus palmdale california field test for refraction error in historical leveling surveys by ross s stein charles t whalen sanford r holdahl william e strange and wayne thatcher

local and regional components of western aegean deformation extracted from 100 years of geodetic displacement measurements summary our objectives are as follows first we wish to develop a methodology to recover the long term component of deformation from any set of distributed time averaged geodetic strain measurements that were subject to seismic disturbance given a catalogue of local seismicity that occurred during the measurement period second using seismic and geodetic data sets that span approximately 100 years we apply this technique in the western aegean to assess the role of local seismicity in regional deformation the methodology is developed using a model for crustal deformation constructed from a long term smooth regional strain field combined with instantaneous local perturbations from upper crustal earthquakes approximated by static elastic dislocations by inverting geodetic displacements for the smooth field while simultaneously floating influential but uncertain earthquake source parameters an estimate of the regional component of deformation that is approximately independent of the seismicity can be made in the western aegean we find that the horizontal component of regional deformation can be described with minor inaccuracy by a quadratic relative displacement field the principal horizontal extensional axes calculated from the regionally smooth displacement field agree in orientation with the t axes of earthquakes in the region these observations indicate that the instantaneous elastic strain of the 10 km thick seismogenic layer is driven by a stress field that is smooth on the scale of the geodetic network as a whole 200 300 km

the kephalonia transform fault and the rotation of the apulian platform evidence from satellite geodesy this paper describes a collaborative gps project to determine recent crustal movements and deformations across the ne ionian sea this region is characterized by rifting in the african foreland pelagian sea subduction along the calabrian and west hellenic arcs rotation of the italian peninsula and distributed normal faulting in central greece from gps campaigns on the ionian islands and in nw greece performed in 1989 and 1991 a kinematic field is deduced which is controlled by the deep trough following the nw coast of kephalonia island it is concluded that this prominent bathymetric feature resembles an active fault system kephalonia fault which has the signature of a right lateral transform fault

record of the younger part of the pringle falls excursion at long valley california summary the younger of two closely spaced palaeomagnetic excursions at pringle falls oregon is recorded in lacustrine silts that crop out in long valley california assigned an age of about 220 000 years the virtual geomagnetic poles of the younger excursion form a clockwise loop that reached 35 s latitude east of south america before returning to the northern hemisphere in the pacific ocean west of central america the poles then form a narrow band across north america while moving to high northern latitudes this record matches extremely well feature b of the original excursion record from pringle falls reported by herrero bervera et al 1994 and is similar to this excursion at summer lake oregon negrini et al 1994 in that the pole path is confined primarily to the east central pacific ocean on the basis of an assumed sedimentation rate of 30 cm per thousand years the younger excursion feature b at pringle falls spans an estimated 1200 years and followed by about 1000 years a larger excursion feature a at pringle falls previously discovered at the same long valley site at a second long valley site 30 m away the younger excursion feature b is only partially recorded because of a presumed small hiatus in the sedimentary section

the horizontal velocity field in the deforming aegean sea region determined from the moment tensors of earthquakes we use the spatial distribution of seismic moment tensors of earthquakes in the aegean region over the time interval 1909 1983 to recover a continuous horizontal velocity field that describes the overall deformation of the lithosphere at large length scales the calculated velocity field is dominated by two effects 1 an e w right lateral shear of the eastern aegean related to motion on the north anatolian fault becoming distributed as it enters the aegean and 2 a n s extension probably related to the sinking of the slab in the hellenic trench the southern part of the central aegean is found to be moving in a sw direction relative to europe at a rate of about 30 mm yr probably a lower bound with an error of around 10 mm yr and rotating clockwise in the seismogenic upper crust this velocity field is accommodated by right lateral strike slip faults in the eastern aegean and by normal faults that rotate clockwise in central greece a comparison of paleomagnetic declination data with the expected rotation of rigid elongate inclusions in the velocity field shows in most places an agreement in sense and approximate agreement in rate of rotation expected rotation rates of line elements are sometimes too low probably because our derived velocity field is smoothed and unable to match locally high strain rates there is only one part of the region where line elements are predicted to rotate in either clockwise or counterclockwise directions depending on their orientation this is in western turkey which coincident ally is the only place where paleomagnetic rotations in both directions have been observed this coincidence in particular suggests to us that the analogy of rigid elongate inclusions in the velocity field responding to forces on their bases may be useful in predicting the senses and approximate rates of rotation of crustal blocks in deforming continental regions the velocity field we obtain preserves the strike directions of the major faults as directions of zero length change in spite of considerable smoothing we use this observation to speculate that the interaction between the upper crust and the rest of the lithosphere beneath it may involve an interplay of effects on one hand the variation of strength with direction in the crust may control the strike directions of faults that form or become reactivated and may also limit the velocity fields that are allowable on the other hand the fault bounded blocks may rotate in the velocity field in response to forces on their bases which would require the velocity field to change with time if the directions of zero length change are fixed to the blocks

the strain field in northwestern greece and the ionian islands results inferred from gps measurements abstract recent crustal movements detected by the analysis of repeated satellite geodetic measurements reflect the ongoing geodynamic processes in the alpine mediterranean area superimposed on the large scale counterclockwise rotation of the african plate complex dynamic processes are affecting the lithospheric fragments between the african and eurasian plates key features to better understand the driving forces and associated seismic activity in the africa eurasia collision zone are the calabrian and hellenic arcs in this paper geodynamic investigations along the west hellenic arc are discussed they are based on two epochs 1989 and 1993 of satellite geodetic measurements carried out using the us global positioning system gps the results are presented in terms of relative displacements and strain rates within the time span of 4 years southwestern greece has moved to the southwest relative to southeastern italy by an average of 120 mm increasing from 80 mm at lefkada in the center of the ionian islands to 160 mm at the peloponnesus the maximum strain rate is 0 18 strain a located in the vicinity of lefkada where anomalously high earthquake activity is observed the data provide strong evidence for dextral strike slip motion on the order of 25 mm a along the kephalonia fault zone kfz the deformation field of the kfz is interpreted as a transition zone between the kinematics of the apulian platform and the west hellenic fold and thrust belts

quaternary evolution of the corinth rift and its implications for the late cenozoic evolution of the aegean summary we present geological and morphological observations at different scales to constrain rates of faulting and the distribution of deformation in the seismically active aegean region we focus first on the 130 km long corinth rift an asymmetric graben where a flight of terraces of marine origin are uplifted we show that the edges of the terraces lie in the footwall of the normal fault bounding the corinth rift and correspond to sea level highstands of laic pleistocene age using a detailed analysis of aerial and spot imagery supported by field observations we have mapped 10 terrace platforms and strandlines ranging in elevation from 10 to 400 m over distances of 2 to 20 km from the fault the elevation of the terraces inner edges was estimated at 172 sites with an error of 5m this data set contains a precise description of the uplift and flexure of 10 different palaeohorizontal lines with respect to the present sea level to date the deformation we correlate the corinth terraces with late pleistocene oxygen isotope stages of high sea level stands and with global sea level fluctuations using a thick elastic plate model consistent with our current understanding of the earthquake cycle and a boundary element technique we reproduce the geometry of the shorelines to constrain both mechanical parameters and the slip on the fault we show that the seismogenic layer behaves over the long term as if its elastic modulus were reduced by a factor of about 1000 all the terraces are fitted for fault slip increasing in proportion to terrace age and the component of regional uplift is found to be less than 0 3 mm yr 1 the best fits give a slip rate of 11 3 mm yr 1 on the main rift bounding fault over the last 350 kyr other geological and morphologic information allows us to estimate the total age of the main fault 1 ma and to examine the mechanical evolution of the corinth rift the minimum observed sediment thickness in the gulf places an extreme check on the results of the modelling and a lower bound on slip rate of 6 7 mm yr 1 40 per cent less than estimated with modelling even this slip rate is nearly 10 times higher than for comparable features in most of the aegean and elsewhere in the world at a larger scale the spacing and asymmetry of the rift systems in the aegean suggest strain localization in the upper mantle with slow extension starting 15 myr ago or earlier the more recent 1 myr rapid phase of rifting in corinth partly reactivated this earlier phase of extension the younger faulting in corinth appears to result from its present location in the inhomogeneous stress field process zone of the south westward propagating tip of the southern branch of the north anatolian fault we extend these relations to propose a mechanical model for the late cenozoic evolution of the aegean as the arabia europe collision progressed in eastern turkey it caused anatolia to move to the west and the north anatolian fault to propagate into the aegean where the early slow extension started to be modified about 5 ma ago the process of propagation dramatically increased the activity of some but not all of the earlier rifts the model we present is compatible with tectonic observations as well as with the seismicity the palaeomagnetic rotations and the displacement field now observed with gps and slr

north aegean crustal deformation an active fault imaged to 10 km depth by reflection seismic data three multichannel seismic profiles imaged a normal fault to at least 10 km depth in the north aegean trough and thermaikos basin the fault is active and recent forming a scarp at sea bottom and crossing the quaternary deltaic front on the northern slope of the trough controlled to the south by the north anatolian fault prestack depth migration imaged the fault as a seismic reflector cutting steeply across the sedimentary rocks and flattening in the basement from the seismic image the n100 e strike of the fault scarp and the orientations of the three profiles the true fault dip is constrained to an average 20 in the basement a low angle dip the throw and age estimated from the geometry in the sedimentary rocks document recent onset of the motion that must have occurred at a high rate both the direction and the rate of slip are consistent with the instantaneous motion as measured by space based geodesy which shows the fault to be forming by pure normal slip large earthquakes that have occurred in the basin may relate to such normal faults whereas the north anatolian fault with its current strike slip earthquakes appears to slip here under low resolved shear stress

geodetic estimate of seismic hazard in the gulf of korinthos the recent 15 june 1995 m0 6 0 1018 n m aigion earthquake in the western gulf of korinthos has focussed attention on the seismic hazard of the region although there have been few large earthquakes in the region during this century the historical record suggests that there may have been many large earthquakes there in the interval 1750 1900 we present geodetic data that give estimates of the rate of extension of the gulf of korinthos during this century and which suggest that less than half of the elastic strain in the central and western gulf of korinthos has been released by earthquakes during this century in contrast the seismic and geodetic strains in the eastern gulf of korinthos are in agreement with each other if the discrepancy between seismic and geodetic strains in the western gulf of korinthos that has accumulated during this century is removed in earthquakes the moment release will be equivalent to several ms 6 5 earthquakes

an extensive survey of dayside diffuse aurora based on optical observations at yellow river station by using 7 years optical auroral observations obtained at yellow river station magnetic latitude 76 24 n at ny alesund svalbard we performed the first extensive survey for the dayside diffuse auroras ddas and acquired observational results as follows 1 the ddas can be classified into two broad categories i e unstructured and structured ddas the unstructured ddas are mainly distributed in morning and afternoon but the structured ddas predominantly occurred around the magnetic local noon mln 2 the unstructured ddas observed in morning and afternoon present obviously different properties the afternoon ones are much stable and seldom show pulsating property 3 the ddas are more easily observed under geomagnetically quiet times 4 the structured ddas mainly show patchy stripy and irregular forms and are often pulsating and drifting the drifting directions are mostly westward with speed 5 km s but there are cases showing eastward or poleward drifting 5 the stripy ddas are exclusively observed near the mln and most importantly their alignments are confirmed to be consistent with the direction of ionospheric convection near the mln 6 a new auroral form called throat aurora is found to be developed from the stripy ddas based on the observational results and previous studies we proposed our explanations to the ddas we suggest that the unstructured ddas observed in the morning are extensions of the nightside diffuse aurora to the dayside but that observed in the afternoon are predominantly caused by proton precipitations the structured ddas occurred near the mln are caused by interactions of cold plasma structures which are supposed to be originated from the ionospheric outflows or plasmaspheric drainage plumes with hot electrons from the plasma sheet we suppose that the cold plasma structures for producing the patchy ddas are in lumpy and are more likely from the plasmaspheric drainage plumes the cold plasma structure for producing the stripy ddas should be in wedge like and is generated by conveying the cold plasmas from lower l shell toward higher l shell with magnetospheric convection and that for producing the irregular ddas is resulted from deforming the wedge like structure by disturbance the throat aurora is supposed to be projection of a newly opened flux of reconnection in addition we also found that structured ddas correspond to structured electron precipitations in the ionosphere which implies that the cold plasma structures in the magnetosphere are magnetically mapped to the ionosphere and act as a duct for producing the structured ddas we argue that we have presented some new observational results about dda in this paper which will be useful for fully understanding the ddas

no net rotation model of current plate velocities incorporating plate motion model nuvel 1 we present nnr nuvel1 a model of plate velocities relative to the unique reference frame defined by requiring no net rotation of the lithosphere while constraining relative plate velocities to equal those in global plate motion model nuvel 1 demets et al 1990 differences between nnr nuvel1 and no net rotation plate motion model am0 2 minster and jordan 1978 are as large as 15 mm yr in nnr nuvel1 the pacific plate rotates in a right handed sense relative to the no net rotation reference frame at 0 67 m y about 63 s 107 e this rotation nearly parallels but is 32 slower than the pacific hotspot euler vector of hs2 nuvel1 which is a global model of plate hotspot velocities constrained to consistency with nuvel 1 gripp and gordon 1990 at hawaii the pacific plate moves relative to the no net rotation reference frame at 70 mm yr which is 25 mm yr slower than the pacific plate moves relative to the hotspots differences between nnr nuvel1 and hs2 nuvel1 are described completely by a right handed rotation of 0 33 m y about 49 s 65 e the 99 confidence ellipsoids for plate hotspot motion in model hs2 nuvel1 exclude the corresponding euler vectors from model nnr nuvell thus the no net rotation reference frame differs significantly from the hotspot reference frame if the difference between reference frames is caused by motion of the hotspots relative to a mean mantle reference frame then hotspots beneath the pacific plate move with coherent motion towards the east southeast alternatively the difference between reference frames may show that the uniform drag no net torque reference frame which is kinematically equivalent to the no net rotation reference frame is based on a dynamically incorrect premise possible exceptions to the assumption of uniform drag include a net torque on the lithosphere due to attached subducting slabs and greater resistance to plate motion beneath continents than beneath oceans

geodetic determination of tectonic deformation in central greece from 1900 to 1988 the global positioning system has been used to measure the relative displacements of fifteen monuments in a hundred year old triangulation network spanning part of the aegean extensional basin these displacements reflect the tectonic deformation of the region over the past century showing more than one metre of north south extension across the network the crust in this region appears to contain a few slowly deforming blocks separated by more rapidly deforming zones

preconsolidation stress of aquifer systems in areas of induced land subsidence aquifer systems in the eloy picacho area arizona the houston galveston area texas and the tulare wasco area and santa clara valley california appear to have been overconsolidated by an amount that ranged approximately from 1 6 to 6 2 bars 16 to 63 m of water before man began to withdraw groundwater from them the relation between land subsidence and water level decline in these areas consists of two linear segments in these areas subsidence per unit water level decline was approximately constant until water levels had declined an amount that ranged from 16 to 63 m when water levels declined past these values subsidence per unit water level decline increased to larger constant values although slow drainage from aquitards may have contributed to this response it is interpreted here to be caused primarily by natural overconsolidation of the compacting part of the aquifer system the water level decline at which the ratio of subsidence to unit water level decline changed indicates approximately the amount by which the preconsolidation stress exceeded the overburden stress on the aquifer system that existed before groundwater withdrawals began

episodic occurrence of strong mw 6 2 earthquakes in thessalia area central greece abstract between 1954 and 1957 three seismic sequences struck the southern margin of thessalia basin central greece the proximity in time 3 yr and space 100 km between these three seismic manifestations suggests a possible link between them their interconnection is then investigated by resolving changes of coulomb failure function cff during the 20th century coulomb stress changes were calculated assuming that earthquakes can be modelled as static dislocations in an elastic half space and taking into account both the coseismic slip in strong m w 6 2 earthquakes and the slow tectonic stress buildup associated with major fault segments the study area constitutes one of the most active fault zones of the greek mainland where several strong events have occurred in the past as it is inferred from both historical and instrumental information however no earthquake with m w 6 2 occurred there for about two centuries before the last activity this fact led us to investigate the occurrence mode of strong events in the thessalia area since the 16th century when adequately reliable historical information exists for the stronger events forasmuch as they occur in clusters both in space and time as it is the case during the 20th century the episodic occurrence of strong events in this area is substantiated when these episodes are separated by longer quiescent periods since during the 20th century all the earthquakes have occurred in stress enhanced areas by extending the stress calculations to 2025 and provided that no additional large m w 6 2 earthquake occurs between 2002 and 2025 assessment for the future seismic hazard is given

seismicity and associated strain of central greece between 1890 and 1988 summary we examined the seismicity of central greece between 1890 and 1988 using macroseismic and instrumental data to ask two questions 1 does the seismicity of this period reveal all the major tectonic structures that are known to be active and 2 what are the likely strains associated with the seismicity over this period many known active structures have been effectively aseismic for the last hundred years and even the inclusion of all known large events earlier than 1890 reveals no activity associated with the ne coast of evia gulf of argos or graben ne of mt parnassos it is clear that even 100 years data are inadequate for either a reasonable assessment of seismic risk or for a confident estimation of maximum magnitude however we are aware of no earthquakes in central greece during the last 200 yr that were larger than ms 7 0 it is probable that the maximum magnitude is restricted by the maximum length of fault segments which appears to be around 15 20 km the earthquakes of ms 5 8 during 1890 1988 can account for a n s displacement of around 45 70 cm with maximum and minimum estimates a factor of two greater and smaller than this across part of a 1890 1900 triangulation network in central greece that was resurveyed in 1988 the contribution of smaller events may increase this displacement by about 50 per cent this cumulative seismic displacement is similar to that estimated from the geodetic work about 100 cm but a detailed comparison of the two sets of observations will be reported elsewhere a re evaluation of all the important earthquakes of 1890 1988 in central greece is presented in the appendix which summarizes information of use to both earth scientists and engineers

reply to comments on saugus palmdale california field test for refraction error in historical leveling surveys by ross s stein charles t whalen sanford r holdahl william e strange and wayne thatcher

geodetic strain of greece in the interval 1892 1992 a first order triangulation of greece was carried out in the 1890s reoccupation using global positioning system receivers of 46 of the 93 original markers yielded estimates of the deformation of the region over the intervening interval broad regions have similar geodetic strain over the 100 year time span strain north of the gulf of korinthos is predominantly north south extension though with a significant east west component the central peloponnisos is relatively stable whereas the gulfs of the southern peloponnisos are all characterized by uniaxial east west extension the seismic expression of strain for the entire region calculated from the seismic moment tensors of earthquakes of m s 5 8 during the past 100 years accounts for only 20 50 of the geodetically determined strain at a scale of 50 100 km the fraction of the strain that is expressed seismically varies much more than this range in particular whereas seismic strain in the eastern gulf of korinthos over the past 100 years is commensurate with the geodetic strain there is rapid extension across the western gulf of korinthos 0 3 strain yr 1 with negligible seismic strain for the 100 year period prior to 1992 the egion earthquake of june 1995 in the western gulf of korinthos released only a small proportion 20 of the elastic strain that had accumulated in that region the observed distribution of displacements can be explained by the relative rotation of two plates with a broad accommodation zone between them but it is equally consistent with the deformation that would be expected of a sheet of fluid moving toward a low pressure boundary at the hellenic trench a simple calculation implies that if the region does behave as a fluid then its effective viscosity is 10 22 10 23 pa s such viscosities are consistent with the deformation of a lithosphere obeying a rheological law similar to that obtained for olivine in the laboratory

geometry and kinematics of active faults and their seismotectonic significance in the western corinth patras rift greece abstract the western part of the corinth patras rift is formed in an area which is being uplifted rifted domains have subsided relative to the adjacent areas in nw peloponnesus and sterea hellas but subsidence rates rarely exceed uplift rates extension is mainly accomplished by wnw trending active faults with lengths up to 25 km most of these faults show an along strike segmentation and often terminate against transfer faults displacement analysis along 170 mesoscopic faults and 36 mappable faults has been carried out to assess slip rates and to estimate the magnitudes and recurrence time intervals of earthquakes occurring along them earthquake magnitude ranges between 5 and 6 7 only eight faults appear to be capable of producing earthquake magnitudes larger than 6 recurrence times of these large earthquakes vary between 80 and 1690 years upper crustal deformation in the western corinth graben can be described by six large asymmetric grabens which are inferred to bottom out to a n dipping curved ramp the asymmetric shape of most wnw trending rifts in the hellenic peninsula including the corinth patras rift promotes the simple shear model of extension against the pure shear model in the upper crustal layer

the aegean sea we summarize briefly the main features of a kinematic model of evolution of the aegean area since 13 ma the formation of the aegean sea by extensional tectonics is related to the subduction of the mediterranean floor below the hellenic arc we then make a quantitative estimate of vertical movements in the aegean area on the basis of geological data and demonstrate that the outer arc was built since the beginning of this episode of subduction by vertical uplift the easiest way to explain the uplift is by underplating finally we discuss briefly the dynamics of the subduction marginal sea formation processes in this continental collision framework

observations of black auroral patches and of their relationship to other types of aurora a morphological study of black patches and newly reported black rings black patches with islands of embedded diffuse aurora was conducted using an archive of low light level all sky and narrow field television camera data the overwhelming majority of black patches and rings drifted eastward poleward of westward drifting pulsating aurora in the early recovery phase of the auroral substorm occasionally black aurora would overlap with pulsating aurora sometimes the pulsations would add to the intensity in the black aurora more than in the surrounding diffuse aurora called veiling one example was documented where the pulsation was purely additive adding to the intensity of the black aurora and the surrounding diffuse aurora equally cases of total veiling suggest an intensity limit possibly associated with the strong pitch angle diffusion limit

strain probe best fitting a homogeneous strain to the motions of scattered points abstract in laboratory experiments or e g in studies of glacier motion one sometime wants to estimate a finite strain from images or surveys of scattered marker points taken at successive times similarly crustal geodetic strain can be estimated by measuring velocities of geodetic stations the strain probe presented here is a simple formula that calculates a best fit strain from the motions of n marker points distributed over an area or a volume and monitored in a cartesian reference frame specifically it is the product of two 2 2 or 3 3 in 3 d symmetric matrices whose components are populated from the known positions and the measured displacements of the markers two examples implemented in excel workbooks illustrate how to combine the strain probe with the processing of the data and of its results speck motions in a means urai see through experiment are an example of a finite strain calculation published geodetic site velocities 120 stations in central anatolia are an example of infinitesimal strain

the evolution of the gulf of corinth greece an aftershock study of the 1981 earthquakes summary a preliminary study of the aftershocks of three earthquakes that occurred near to corinth greece in 1981 is combined with observations of the morphology and faulting to understand the evolution of the eastern gulf of corinth the well located aftershocks form a zone 60km long and 20km wide they do not lie on the main fault planes and are mostly located between the north dipping faulting on which the first two earthquakes occurred and the south dipping faulting associated with the third event a cluster of aftershocks also lies in the footwall of the eastern end of the south dipping fault of the third event morphologically it is observed that in the evolution of the eastern gulf of corinth antithetic faulting apparently predates the appearance of the main faulting at the surface this evolution can be explained by motion on a deep seated shallow angle aseismic normal fault a model based on such a fault also accounts for the aftershock distribution of the 1981 earthquakes

geodetic determination of the kinematics of central greece with respect to europe implications for eastern mediterranean tectonics we use a new satellite laser ranging global positioning system slr gps solution at seven sites in anatolia and aegea to obtain a better definition of the extrusion motion of the anatolian aegean block with respect to europe we show that this motion can be described in a first approximation by a counterclockwise rotation which transfers most of the motion of arabia to anatolia we combine 78 displacement vectors obtained at common points of two triangulation nets measured in central greece in 1895 and 1975 with the slr gps measurements to compute the velocity field over greece with respect to europe these measurements indicate that central greece is a zone of extension between the anatolian aegean counterclockwise rotation to the south and the northern greece clockwise rotation to the north this extension is principally localized within the gulf of corinth to the east but is distributed to the west we then extrapolate this velocity field to the whole aegea and western anatolia using recently published gps results as well as the slr results the narrow dextral north anatolian fault which limits the velocity field to the north progressively gives way to a much wider boundary zone where extension becomes dominant we show that the collision between the mediterranean ridge and africa began 3 6 ma and we describe the modifications that this collision has produced on the kinematic pattern both in aegea and on the mediterranean ridge