long period pulses in broadband records of near earthquakes broadband records of weak local earthquakes occasionally exhibit long period pulselike signals appearing exclusively on horizontal components such signals have been recorded at station sergoula in the corinth gulf by guralp cmg 3t 100 sec and wielandt streckeisen sts 2 120 sec instruments the pulses can be perfectly matched by the instrumental responses to a small step of acceleration similar pulses observed during moderate near earthquakes at stations of the greek national network equipped with lennartz le 3d 20s 20 sec sensors can be explained in the same way numerical simulation of the pulses allows their removal thus making the records fully utilizable for waveform inversion in seismic source studies prerequisite for that is precise knowledge of the instrument transfer function we found that poles and zeros of the cmg 3t and sts 2 sensors were consistent with those given by the manufacturer whereas those of the le 3d 20s were not we identified the latter by inversion of spontaneous pulses incidentally appearing in le 3d 20s records without relation to any earthquake what actually generates the earthquake related horizontal acceleration steps remains unclear rapid and permanently lasting tilt is a very likely candidate when we numerically model a coseismic near field static displacement we actually find a tilt step but its amplitude is several orders below the observed values we therefore rather speculate that it is a very local tilt triggered by high frequency ground vibrations

azimuth moveout for 3 d prestack imaging we introduce a new partial prestack migration operator called azimuth moveout amo that rotates the azimuth and modifies the offset of 3 d prestack data followed by partial stacking amo can reduce the computational cost of 3 d prestack imaging we have successfully applied amo to the partial stacking of a 3 d marine data set over a range of offsets and azimuths when amo is included in the partial stacking procedure high frequency steeply dipping energy is better preserved than when conventional partial stacking methodologies are used because the test data set requires 3 d prestack depth migration to handle strong lateral variations in velocity the results of our tests support the applicability of amo to prestack depth imaging problems amo is a partial prestack migration operator defined by chaining a 3 d prestack imaging operator with a 3 d prestack modeling operator the analytical expression for the amo impulse response is derived by chaining constant velocity dmo with its inverse equivalentl

reverse time migration from irregular surface by flattening surface topography abstract prestack reverse time migration rtm is a powerful tool to provide high resolution structure images of crust and upper mantle in wide angle seismic experiments as a standard migration algorithm assumes that data are recorded on a flat surface rtm meets severe challenge on the issue of complicated topography where many seismic surveys are carried out up till now there are two popular techniques for handling elevation changes i e elevation static correction or time shift and wave equation datuming numerous studies demonstrate that the elevation static correction is inaccurate for non vertically traveling energy and subsequently may migrate reflectors to incorrect positions wave equation datuming is more accurate but more sophisticated and too computationally demanding for routine use here we present an alternative scheme to deal with topography in prestack rtm that can migrate seismic data acquired in areas with irregular topography to the reflector positions directly without datuming or elevation static corrections in this scheme surface topography is flattened by a transformation of coordinates from cartesian to curvilinear such a transformation is equivalent to map a rectangular grid onto a curved one and is implemented by the introduction of boundary conforming grid then we use an explicit finite difference scheme with second order accuracy to discretize the elastic wave equations and perform the rtm in the curvilinear coordinate system moreover a flux corrected transport fct algorithm adapted from hydrodynamics is incorporated in the rtm implementation which can not only overcome problems of numerical dispersion that arise in finite difference algorithms but also offer the opportunity to use a coarse grid to obtain the same accuracy as conventional finite difference methods on a dense grid the fctrtm method presented here has no limitations on surface irregularity complexity of the subsurface structure or velocity function extensive numerical examples demonstrate the feasibility and effectiveness of our method

possible roles of atlantic circulations on the weakening indian monsoon rainfall enso relationship abstract since the 1970s the inverse relationship between the indian monsoon rainfall and the el nino southern oscillation enso has weakened considerably the cause for this breakdown is shown to be most likely the strengthening and poleward shift of the jet stream over the north atlantic these changes have led to the recent development of a significant correlation between wintertime western european surface air temperatures and the ensuing monsoon rainfall this western europe winter signal extended eastward over most of northern eurasia and remained evident in spring such that the effect of the resulting meridional temperature contrast was able to disrupt the influence of enso on the monsoon

application of the topographic datuming operator to a data setfrom the eastern arabian peninsula we apply a redatuming methodology designed to handle rugged topography and the presence of high velocity layers near the acquisition surface to a 2d land seismic data set acquired in saudi arabia this methodology is based on a recently developed prestack operator which we call the topographic datuming operator tdo the tdo unlike static corrections allows for the movement of reflections laterally with respect to their true locations corresponding to the new datum level thus it mitigates mispositioning of events and velocity bias introduced by the assumption of surface consistency and the time invariant time shifts brought about by static corrections using the shallow velocities estimated from refracted events the tdo provides a superior continuity of reflections and better focusing than that obtained from conventional static corrections in most parts of the processed 2d line the computational cost of applying the tdo is only slightly higher than static corrections the marginal additional computational cost and the possibility of estimating after tdo redatuming stacking velocities that are not affected by a spurious positive bias as in the case of static corrections are further advantages of the proposed methodology the likelihood of strong heterogeneities in the most complex part of the line limits the applicability of any approach based upon geometrical optics however the tdo produces results that are slightly better than those obtained from static corrections because of its ability to partially collapse diffractions generated in the near surface

upstream subtropical signals preceding the asian summer monsoon circulation in this study the authors address several issues with respect to the antecedent signals of the large scale asian summer monsoon that were earlier identified by webster and yang in particular they revisit the changes in the subtropical upper tropospheric westerlies preceding the monsoon depict the detailed structure of the monsoon s antecedent signals and investigate the physical processes from the signals to the monsoon they also explore the teleconnection of these signals to various large scale climate phenomena and emphasize the importance of the upstream location of the signals relative to the tibetan plateau and the monsoon before a strong weak asian summer monsoon the 200 mb westerlies over subtropical asia are weak strong during the previous winter and spring a significant feature of these signals is represented by the variability of the middle east jet stream whose changes are linked to the arctic oscillation north atlantic oscillation el

evidence for stability in coda q associated with the egion central greece earthquake of 15 june 1995 one hundred seventy two seismograms recorded at one seismological station located close to the source region of the egion ms 6 2 15 june 1995 earthquake were used to measure values of q from the decay of the earthquake coda the collected data were compared between events that occurred before and after the main event the analysis showed no significant temporal variation in the value of coda q for the region

evidence of nonlinear dynamics in the eastward shift of the nao 1 hilmer and jung 2000 have identified an eastward shift in the sea level pressure slp pattern associated with interannual variability of the north atlantic oscillation nao that took place around 1980 we investigate the nature of the eastward shift using a primitive equation dry atmospheric model driven by diabatic forcing diagnosed from observations the model results reveal the nonlinear dependence of the spatial pattern of the nao on the nao index the pattern being shifted to the east west for high low nao index general agreement is found between the model and observations we suggest that the eastward shift noted by hilmer and jung 2000 is a consequence of the trend towards higher nao index during the last several decades of the 20th century

the athens earthquake 7 september 1999 intensity distribution and controlling factors abstract the athens earthquake m s 5 9 that occurred on 7th september 1999 with epicenter located at the southern flank of mount parnitha greece attiki according to instrumental data is attributed to the reactivation of an ese wnw south dipping fault without surficial expression the earthquake caused a large number of casualties and extensive damage within an extended area damage displayed significant differentiation from place to place as well as a peculiar geographic distribution based on geological tectonic and morphological characteristics of the affected area and on the elaboration of damage recordings for intensity evaluation it can be safely suggested that intensity distribution was the result of the combination of a number of parameters both on macro and microscale on the macroscale the parameters are the strike of the seismogenic fault seismic wave directivity effects and to an old nne ssw tectonic structure and they are also responsible for the maximum intensity arrangement in two perpendicular directions ese wnw and nne ssw on the microscale site foundation formations old tectonic structures buried under recent formations and morphology are the parameters that differentiated intensities within the affected area

flooding the topography wave equation datuming of land data with rugged acquisition topography wave equation datuming overcomes some of the problems that seismic data recorded on rugged surface topography present in routine image processing the main problems are that 1 standard optimized migration and processing algorithms assume data are recorded on a flat surface and that 2 the static correction applied routinely to compensate for topography is inaccurate for waves that do not propagate vertically wave based processes such as stacking dip moveout correction normal moveout correction velocity analysis and migration after static shift can be severely affected by the nonhyperbolic character of the reflections to alleviate these problems i apply wave equation datuming early in the processing flow to upward continue the data to a flat datum above the highest topography this is what i refer to as flooding the topography this approach does not require detailed a priori knowledge of the near surface velocity and it streamlines subsequent processing because the data are regridded onto a regularly sampled datum wave equation datuming unravels the distortions caused by rugged topography and unlike the static shift method it does not adversely effect subsequent wave based processing the image obtained after wave equation datuming exhibits better reflector continuity and more accurately represents the true structural image than the image obtained after static shift

multifocusing imaging over an irregular topography if seismic data are acquired over an irregular topography standard elevation statics methods may be inaccurate because the assumption of vertical raypaths will no longer be valid an effective solution to the problem of irregular topography can be found through the use of the multifocusing method in which large supergathers of seismic traces are stacked each of which can span many common midpoint cmp gathers this can be done by extending the multifocusing moveout formula to explicitly account for nonzero elevations of the source and receiver as well as their horizontal coordinates implementation of this formula into the multifocusing algorithm is straightforward because estimating the necessary raypath information i e emergence angles is an integral part of the algorithm the extended multifocusing moveout correction can be applied directly to the data acquired in areas of irregular topography without the need for prior elevation static corrections synthetic tests on such data show that the proposed technique results in a better alignment of reflection events

solving the complex near surface problem using 3d data driven near surface layer replacement abstract in many land seismic situations the complex seismic wave propagation effects in the near surface area due to its unconsolidated character deteriorate the image quality although several methods have been proposed to address this problem the negative impact of 3d complex near surface structures is still unsolved to a large extent this paper presents a complete 3d data driven solution for the near surface problem based on 3d one way traveltime operators which extends our previous attempts that were limited to a 2d situation our solution is composed of four steps 1 seismic wave propagation from the surface to a suitable datum reflector is described by parametrized one way propagation operators with all the parameters estimated by a new genetic algorithm the self adjustable input genetic algorithm in an automatic and purely data driven way 2 surface consistent residual static corrections are estimated to accommodate the fast variations in the near surface area 3 a replacement velocity model based on the traveltime operators in the good data area without the near surface problem is estimated 4 data interpolation and surface layer replacement based on the estimated traveltime operators and the replacement velocity model are carried out in an interweaved manner in order to both remove the near surface imprints in the original data and keep the valuable geological information above the datum our method is demonstrated on a subset of a 3d field data set from the middle east yielding encouraging results

local high resolution passive seismic tomography and kohonen neural networks application at the rio antirio strait central greece a high resolution passive seismic investigation was performed in a 150 km 2 area around the rio antirio strait in centralgreeceusingnaturalmicroearthquakesrecordedduringthreemonthsbyadense temporaryseismicnetworkconsisting of 70 three component surface stations this work was part of the investigation for a planned underwater rail tunnel and it gives us the opportunity to investigate the potential of this methodology first 150 well located earthquake events were selected to compute a minimum 1d velocity model for the region next the 1d model served as the initialmodelfornonlinearinversionfora3dp ands velocity crustal structure by iteratively solving the coupled hypocenter velocity problem using a least squares method the retrieved vp and vp vs images were used as an input to kohonen self organizing maps soms to identify systematicallyandobjectively theprominentlithologiesintheregion soms are unsupervised artificial neural networks that map the input space into clusters in a topological form whose organization is related to trends in the input data this analysis revealed the existence of five major clusters one of which may be related to the existence of an evaporite body not shown in the conventional seismic tomography velocity volumes the survey results provide for the first time a 3d model of the subsurface in and around the rio antirio strait it is thefirst time that passive seismic tomography is used together with som methodologies at this scale thus revealing themethod spotential

teleconnections in the geopotential height field during the northern hemisphere winter abstract contemporaneous correlations between geopotential heights on a given pressure surface at widely separated points on earth referred to as teleconnections in this paper are studied in an attempt to identify and document recurrent spatial patterns which might be indicative of standing oscillations in the planetary waves during the northern hemisphere winter with time scales on the order of a month or longer a review of existing literature on the subject reveals the existence of at least four such patterns the north atlantic and north pacific oscillations identified by walker and bliss 1932 a zonally symmetric seesaw between sea level pressures in polar and temperature latitudes first noted by lorenz 1951 and what we will refer to as the pacific north american pattern which has been known to operational long range forecasters in this country since the 1950 s a data set consisting of nmc monthly mean sea level pressure and 500 mb height analyses for a 15 year period is used as a basis fo

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

upward continuation with topographic datuming operator the integrated wave equation datuming scheme revised the integrated wave equation datuming scheme is an integrated datuming method to implement the wave equation velocity replacement with a one way extrapolator for overthrust data the integrated wave equation datuming scheme is accomplished by downward continuing the data from the topography to a non planar base of the weathering layer followed by an upward continuation from the base of the weathering layer to a final planar datum both the downward and the upward continuation are accomplished by a one way extrapolator by a classical two step implementation the topographic datuming operator is a distinctive technique by which the shots and receivers are downward continued simultaneously although its inadaptability to non planar datum and the strong lateral variation of the near surface structure constrains its application to the downward continuation of the overthrust data we found that the one step feature of the topographic datuming operator is extremely suitable for the upward continuation in this paper the topographic datuming operator is incorporated into the integrated wave equation datuming scheme to perform upward continuation instead of a one way extrapolator thus a revised integrated wave equation datuming scheme is constructed compared with the previous scheme the computatiol cost of the revised integrated wave equation datuming scheme is greatly reduced thus the applicability of the integrated wave equation datuming scheme is further improved the synthetic and real data examples demonstrate its effectiveness and efficiency

complex kinematic rupture of the mw 5 9 1999 athens earthquake as revealed by the joint inversion of regional seismological and sar data summary slip distributions of the moderate magnitude mw 5 9 1999 athens earthquake inverted from surface waves and interferometric synthetic aperture radar sar images show very different characteristics the robustness analysis proposed in this study confirms the discrepancy between the well constrained features of each individual solution irrespective of the hypotheses we made data modeling errors slow deformation post or pre seismic slip the joint inversion of the two data sets led to a complex and heterogeneous rupture model this model is characterized by a short rise time 5s slip patch centred on the hypocentre extending bilaterally up to 4 km depth and down to 17 km and releasing approximately 70 per cent of the total moment located further to the wnw and releasing the remaining 30 per cent of the total moment a long rise time slip patch extends from 8 to 17 km depth if the short rise time slip patch propagated above and below the brittle zone delineated by the aftershocks the long rise time slip patch slow deformation appears to be mostly confined below the brittle zone this unified model satisfies the analysis of the seismic and geodetic slip distributions as well as the location of the aftershock sequence and attests to the diversity of the crustal response even for moderate size faults

predicting the atlantic multidecadal variability from initialized simulations abstractin contrast to dominant interannual time scale variability in other ocean basins the leading observed mode variability in the atlantic is characterized as a basinwide seesaw like sea surfa

a study of the 2 december 2002 m5 5 vartholomio western peloponnese greece earthquake and of its largest aftershocks broadband data from the greek national seismological network are used to study the moderate size m5 5 earthquake which occurred on 2 december 2002 near the town of vartholomio in western peloponnese greece time domain moment tensor inversion applied to retrieve the focal mechanism of the mainshock and of three of the larger aftershocks of the sequence revealed almost pure strike slip faulting along nw se or ne sw trending nodal planes the relative source time functions for the mainshock obtained from an empirical green s function analysis do not reveal any clear directivity to any of the stations a careful observer might suggest directivity towards nw if any optimum values are 0 4 s for the rise time and 2 7 km s for the rupture velocity the spatial and temporal distribution of fault slip showed that the major part of the resolved slip occurred beneath the mainshock s epicenter 20 km underneath the western coast of peloponnese this probably accounts for the considerable damage observed to the nearby towns the resolution between the two nodal planes does not permit an identification of the fault plane however the statistics on the slip distribution model the preliminary analysis of aftershock locations and macroseismic data favour the nw se trending plane as the fault plane which is connected with sinistral strikeslip motions these are the first implications for sinistral strike slip motions in this area and more data are needed in the future to get more reliable resolution of the motions d 2004 elsevier b v all rights reserved

source parameters of the 23 april 1992 joshua tree california earthquake its largest foreshock and aftershocks abstract seismic moments corner frequencies and stress drops have been calculated for the 23 april 1992 joshua tree california earthquake its largest foreshock and 12 aftershocks using recordings from terrascope stations source parameters are determined from nonlinear least squares best fits to ratios of p wave fourier amplitude spectra assuming an omega squared source model the spectral ratios are the mainshock spectra divided by the foreshock and aftershock spectra seismic moments range from 2 10 21 to 2 10 25 dyne cm corner frequencies range from 0 51 to 4 8 hz aftershock stress drops range from 4 2 to 41 bars with a log average stress drop of 15 bars the foreshock stress drop is relatively large 31 bars but is within the aftershock stress drop range the mainshock stress drop of 203 bars is significantly larger than any of the aftershock or foreshock stress drops

slip distribution of the mw 5 9 1999 athens earthquake inverted from regional seismological data 1 we studied the kinematic rupture process of the moderate magnitude mw 5 9 1999 athens earthquake using broadband seismic recordings at regional distances the apparent source time functions astf obtained by retrieving the wave propagation and site effects from the seismic recordings using an empirical green s function method indicate a rather complex rupture with a source directivity primarily pointing towards athens to further characterize the faulting we inverted the astfs for the kinematic history of the rupture using a damped least square inversion scheme with inequality constraints the rupture extent is delimited by a 10 km along strike 20 km along dip zone the mean slip amplitude on the observed rupture area is equal to 25 cm the slip concentrates in two elongated zones with amplitudes reaching locally 60 cm the total rupture lasted between 5 and 6 s

stochastic strong ground motion simulation of the 7 september 1999 athens greece earthquake the stochastic method for finite faults is applied to simulate strong ground motion from the 7 september 1999 moment magnitude m 5 9 athens earthquake the method includes descritization of the fault plane into a certain number of subfaults each of which is assigned an 2 spectrum a slip distribution model derived from previous studies of this earthquake is used to specifically account for the source effect contributions from all subfaults are then empirically attenuated to the observation sites where they are summed to produce the synthetic acceleration time history the method is first calibrated against its ability of reproducing the recordings at 19 strong motion stations at epicentral distances ranging from 16 to 61 km the calibrated model is then applied to calculate synthetics at a large number of grid points covering the area around the fault plane simulated peak values are subsequently used to produce synthetic peak ground acceleration and spectral acceleration maps at hard rock both peak ground acceleration and spectral acceleration maps imply energy directivity toward the east where most of the damage was concentrated the directivity effect is most prominent at large periods 2 sec and in the period range 0 2 to 0 3 sec independent geotechnical studies showed considerable site effect at periods 0 5 sec within the meizoseismal area this result coupled with the results of the present study imply that the damage distribution pattern of the 1999 athens earthquake can be explained by the destructive combination of two factors the source directivity and the site effect

the zero velocity layer migration from irregular surfaces seismic data acquired in areas with irregular topography are usually corrected to a flat datum before migration a time honored technique for handling elevation changes is to time shift the data before application of migration this simple time shift or elevation static correction cannot properly represent wide angle or dipping reflections as they would have been recorded at the datum as a result when elevation varies significantly accuracy in event positioning may be compromised for migration and other wave equation processes such as dip moveout processing dmo traditionally such over and under migration artifacts have been dealt with by increasing or decreasing the migration velocity however simple adjustment of the migration velocity cannot undo the wave field distortions induced in seismic data acquired over varying elevations more sophisticated and accurate solutions such as wave equation datuming are too computationally demanding for routine use here we propose an efficient and accura

displacement field and fault model for the september 7 1999 athens earthquake inferred from ers2 satellite radar interferometry on september 7 1999 a moderate mw 5 9 normal faulting earthquake occurred in the northwest of athens hellas causing heavy damages and casualties using interferometric combinations of ers2 sar images we analyzed the coseismic deformation field two fringes are observed south of the fill mountain up to the coastline of the elefsis gulf they correspond to 56 mm increase in slant range modeling the earthquake as a dislocation in an elastic half space we inverted the interferometric data to assess the fault location and geometry and the amplitude of the coseismic slip the model suggests 300 mm slip on an 18 km long blind fault composed of two pieces the intersection of the fault plane with the earth surface is located in the fill mountain with a n 120 o orientation

source pulse enhancement by deconvolution of an empirical green s function observations of the earthquake source time function are enhanced if path recording site and instrument complexities can be removed from seismograms assuming that a small earthquake has a simple source its seismogram can be treated as an empirical green s function and deconvolved from the seismogram of a larger and or more complex earthquake by spectral division when the deconvolution is well posed the quotient spectrum represents the apparent source time function of the larger event this study shows that with high quality locally recorded earthquake data it is feasible to fourier transform the quotient and obtain a useful result in the time domain in practice the deconvolution can be stabilized by one of several simple techniques in this paper the method is implemented and tested on high quality digital recordings of aftershocks of the jan 9 1982 miramichi new brunswick earthquake in particular seismograms from a jan 17 aftershock 017 13 33 gmt local mag 3 5 exhibit path or site effects which complicate the determination of source parameters after deconvolution the apparent far field source of this event is a simple pulse in displacement with duration 0 07 second for both p and s

a review of the discrete wavenumber method we present a review of the discrete wavenumber dwn method the method introduced by bouchon and art 1977 allows the simple and accurate calculation of the complete green s functions for many problems in elastodynamics

evidence for a recent change in the link between the north atlantic oscillation and arctic sea ice export evidence for a recent change in the link between the north atlantic oscillation nao and arctic sea ice export through fram strait during wintertime djfm is presented from the analysis of simulated arctic sea ice and observations for the period 1958 1997 whereas no correlation between the two time series is found from 1958 to 1977 r 0 1 the correlation increased significantly thereafter r 0 7 1978 1997 the increased coherency between the nao and arctic sea ice export through fram strait during the last two decades resulted from anomalous meridional wind components near fram strait which were associated with the more easterly position of the nao s centers of interannual variability compared to 1958 1977

the egf method for dissimilar focal mechanisms the athens 1999 earthquake abstract the empirical green s function method egf is innovated to examine the rupture nucleation and propagation during the disastrous athens earthquake of september 7 1999 ml 5 4 waveforms recorded at seven regional broadband stations are studied one of the two strongest aftershocks ml 4 was selected as the egf but its focal mechanism differs from the mainshock mechanism therefore the paper suggests an innovation of the classical egf method the assumption of the similarity of the mainshock and aftershock focal mechanisms is relaxed as follows the mainshock is modeled by an egf like method using synthetic weak events computed by discrete wave number method dw two times once with the focal mechanism of the mainshock and again with the assumed focal mechanism of the aftershock these computations are used to determine the subset of the stations at which the disparate focal mechanism results in a station dependent multiplicative factor only with minimum waveform distortion real data from that station subset are then inverted as if the mainshock and aftershock mechanisms were the same the egf synthetics are produced for constant velocity radial rupture propagation starting at 36 trial grid points regularly distributed on the fault plane and the grid point providing the best fit to the observed waveforms is assumed to be the nucleation point synthetic tests show that success of the method strongly depends on the exact knowledge of the mainshock true fault plane orientation but that is fairly well known from the aftershocks distribution in this case the aftershock sequence suggests two possible sizes of the fault a large fault 20 16 km along strike and dip respectively and a small fault 8 10 km that fills in the gap identified during the first 12 observation days between two aftershock clusters the egf modeling does not resolve a preferred fault dimension however for both sizes the method locates the nucleation point at the western part of the fault plane thus clearly indicating the rupture propagation toward athens

an integrated method for resolving the seismic complex near surface problem we describe an integrated method for solving the complex near surface problem in land seismic imaging this solution is based on an imaging approach and is obtained without deriving a complex near surface velocity model we start by obtaining from the data the kinematics of the one way focusing operators i e time reversed green s functions that describe propagation between the acquisition surface and a chosen datum reflector using the common focus point technology the conventional statics solutions obtained from prior information about the near surface are integrated in the initial estimates of the focusing operators the focusing operators are updated iteratively until the imaging principle of equal traveltime is fulfilled for each subsurface gridpoint of the datum reflector therefore the seismic data is left intact without any application of time shifts which makes this method an uncommitted statics solution the focusing operators can be used directly for wave equation redatuming to the respective reflector or for prestack imaging if determined for multiple reflecting boundaries the underlying velocity model is determined by tomographic inversion of the focusing operators while also integrating any hard prior information e g well information this velocity model can be used to perform prestack depth imaging or to calculate the depth of the new datum level we demonstrate this approach on 2d seismic data acquired in saudi arabia in an area characterized by rugged topography and complex near surface geology

depth dependent seismic attenuation in western greece abstract coda q estimates for the seismically active region of western greece are obtained by analyzing earthquakes occurring within the borders of the recently established patras seismological network the data set is composed of 97 low magnitude local events coda q values obtained show a clear dependence of frequency according to the relation q q 0 f n where n ranges from 0 77 to 0 82 the data fit indicated an increase of q 0 with increasing lapse time which is interpreted as being due to attenuation variation with depth

observed atlantic sst anomaly impact on the nao an update abstract the pan atlantic sea surface temperature sst anomaly pattern that was found in a previous study to have a significant impact on the north atlantic oscillation nao in early winter seemed to reflect the nearly uncorrelated influence of a horseshoe sst anomaly in the north atlantic and an sst anomaly in the eastern equatorial atlantic a lagged rotated maximum covariance analysis of a slightly longer dataset shows that the horseshoe sst anomaly influence is robust but it deemphasizes the center of action southeast of newfoundland canada on the other hand it suggests that the link between equatorial sst and the nao was artificial and due both to enso teleconnections and the orthogonality constraint in the maximum covariance analysis

characteristics of the recent eastward shift of interannual nao variability recent observational studies have shown that the centers of action of interannual variability of the north atlantic oscillation nao were located farther eastward during winters of the period 1978 97 compared to previous decades of the twentieth century in this study which focuses on the winter season december march new diagnostics characterizing this shift are presented further the importance of this shift for nao related interannual climate variability in the north atlantic region is discussed it is shown that an nao related eastward shift in variability can be found for a wide range of different parameters like the number of deep cyclones near surface air temperature and turbulent surface heat flux throughout the north atlantic region by using a near surface air temperature dataset that is homogenous with respect to the kind of observations used it is shown that the eastward shift is not an artifact of changes in observational practices that took place around the late 1970s finally an eof based monte carlo test is developed to quantify the probability of changes in the spatial structure of interannual nao variability for a relatively short 20 yr time series given multivariate white noise it is estimated that the likelihood for differences in the spatial structure of the nao between two independent 20 yr periods which are similar as measured by the angle and pattern correlation between two nao patterns to the observed differences to occur just by chance is about 18 from the above results it is argued that care has to be taken when conclusions about long term properties of nao related climate variability are being drawn from relatively short recent observational data e g 1978 97

common shot fresnel beam migration based on wave field approximation in effective vicinity under complex topographic conditions gaussian beam migration is a versatile imaging method for geologically complex land areas which overcomes the limitation of kirchhoff migration in imaging multiple arrivals and has no steep dip limits of one way wave equation migration however its imaging accuracy depends on the geometry of gaussian beam that is determined by the initial parameter of dynamic ray tracing as a result its applications in exploration areaswith strong variations in topography and near surface velocity are limited combined with the concept of fresnel zone and the theory of wave field approximation in effective vicinity we present a more robust common shot fresnel beam imaging method for complex topographic land areas in this paper compared with the conventional gaussian beam migration for irregular topography our method improves the beam geometry by limiting its effective half width with fresnel zone radius moreover through a quadratic travel time correction and an amplitude correction that is based on the wave field approximation in effective vicinity it gives an accurate method for plane wave decomposition at complex topography which produces good imaging results in both shallow and deep zones trials of two typical models and its application in field data demonstrated the validity and robustness of our method

estimation and application of near surface full waveform redatuming operators abstract land seismic data quality can be severely affected by near surface anomalies the imprint of a complex near surface can be removed by redatuming the data to a level below the surface from where the subsurface structures are assumed to be relatively smooth however to derive a velocity depth model that explains the propagation effects of the near surface is a non trivial task therefore an alternative approach has been proposed where the redatuming operators are obtained in a data driven manner from the reflection event related to the datum in the current implementation the estimation of these redatuming operators is done in terms of traveltimes only based on a high frequency approximation the accompanying amplitudes are usually derived from a local homogeneous medium which is obviously a simplification of reality such parametrization has produced encouraging results in the past but cannot completely remove the near surface complexities leaving artefacts in the redatumed results in this paper we propose a method that estimates the redatuming operators directly from the data i e without using a velocity model in a full waveform manner such that detailed amplitude and phase variations are included the method directly outputs the inverse propagation operators that are needed for true amplitude redatuming based on 2d synthetic data it is demonstrated that the resulting redatuming quality is improved and artefacts are reduced

postseismic displacement of the 1999 athens earthquake retrieved by the differential interferometry by synthetic aperture radar time series 1 in september 1999 a moderate mw 5 9 earthquake struck the attica plain causing unexpected and extensive damage to athens and its population in this work we exploit the potential of multitemporal differential interferometry by synthetic aperture radar dinsar analysis using about a hundred european remote sensing ers 1 2 images to calculate the displacement time series from 1992 to 2002 this analysis allows us to clearly separate a strictly coseismic signal from a postseismic gradual subsidence reaching a maximum value of about 3 cm in the following 2 5 years we model this signal in terms of afterslip on the seismogenic fault the afterslip distribution retrieved by linear inversion reflects the coseismic slip distribution and occurs mainly downdip of the area that ruptured during the main shock the analysis of the static stress transfer suggests that the afterslip was triggered by the main shock then it propagated aseismically through the fault plane a partial overlap between the coseismic and aseismic slip area at the hypocentral region indicates that the 1999 rupture surface was not healed at least until the date of the last postseismic image april 2002 the results obtained with a time series approach for this moderate magnitude earthquake suggest that multitemporal dinsar analysis should become an important methodology for the study of large earthquake ruptures

decadal variations of the relationship between the summer north atlantic oscillation and middle east asian air temperature 1 the relationship between the summer july september north atlantic oscillation snao and simultaneous east asian air temperature is investigated the results show that the snao is related to middle east asian summer air temperature however this linkage varies with time on decadal timescale a strong connection appears after the late 1970s but a weak connection before the late 1970s further analysis indicates that this instable relationship may have resulted from the shift of the snao mode around the late 1970s in the period of 1979 2003 the centers of the snao mode are located more eastward a positive phase negative phase snao produces a strong lower level divergence convergence over the asian jet entrance region in turn stimulates a strong upper level convergence divergence via the ekman pumping such a convergence divergence then excites a zonally oriented quasi stationary barotropical rossby wave train along the asian upper level jet thus the snao signal is transported eastward to east asia resulting in an anomalous summer air temperature over middle east asia however in the period of 1951 1975 the centers of the snao mode are located more westward the associated upper level divergence convergence is away from the asian jet entrance region and the snao signal cannot be transported eastward to east asia hence the connection is broken

rupture directivity during the september 7 1999 mw 5 9 athens greece earthquake inferred from forward modeling of strong ground motion the empirical green s functions technique is applied to simulate strong ground motion records from the september 7 1999 athens earthquake information on the fault parameters from previous independent studies has been used and several scenarios were examined in regard to the location of the starting point of the rupture by comparing the synthetic records with the corresponding observed ones through a residual function and a correlation function the results show that the rupture started at the deepest 4 5 km part of the fault from its western edge this hypocenter was then used in combination with the initial fault model to stochastically simulate the strong ground motion during the athens main shock in terms of peak ground acceleration at hard rock the results show that directivity might have significantly contributed to the destructiveness of this earthquake at specific parts of the athens metropolitan area

possible mechanisms for the coupling between late spring sea surface temperature anomalies over tropical atlantic and east asian summer monsoon the relationship between the preceding late spring sea surface temperature sst over the tropical atlantic and the east asian summer monsoon easm is investigated based on the observational data and coupled model intercomparison project phase 5 cmip5 historical simulations the results show that warm cold tropical atlantic sst tasst during may tends to be followed by a strong weak easm with positive negative precipitation anomalies over the subtropical frontal area evidence is also provided that the atmospheric teleconnections propagating in both east and west directions are the key mechanisms linking the easm with the preceding may tasst that is the warm tasst anomaly during late spring can persist through the subsequent summer which in turn induces the gill type rossby wave response in the eastern pacific exciting the westward relay of the atlantic signal as well as the eastward propagation of the rossby wave along the jet stream furthermore the westward eastward propagating teleconnection signal may induce the anomalous anticyclone in the lower troposphere over the philippine sea anomalous tropospheric anticyclone with barotropic structure over the okhotsk sea the anomalous anticyclonic circulation over the philippine sea okhotsk sea brings warm and humid cold air to higher latitudes lower latitudes these two different types of air mass merge over the baiu meiyu changma region causing the enhanced subtropical frontal rainfall to support the observational findings cmip5 historical simulations are also utilized most state of the art cmip5 models can simulate this relationship between may tasst and the easm

role of the tropical atlantic sea surface temperature in the decadal change of the summer north atlantic oscillation 1 recent observational studies have shown that the southern center of the summer north atlantic oscillation snao shifted eastward after the late 1970s in this study this phenomenon and its causes are further explored it is found that the decadal spatial shift of the snao southern center is related to the decadal variability of the tropical atlantic sea surface temperature tasst in the past half century the tasst experienced an abrupt change around the late 1970s with a rapid warming in the recent 2 decades thus in the period after the late 1970s when the tasst is relatively warmer the tasst released more energy into the atmosphere then stimulated strong convection over the tropical atlantic and further excited anomalous wave like patterns this strengthened the circulation in the region of the mediterranean sea namely the east part of the snao southern center consequently leading to an eastward shift of the snao southern center meanwhile in the period before the late 1970s when the tasst is relatively cooler the tasst released less energy into the atmosphere so its impact on the overlying atmosphere was significantly weakened and confined to the lower level atmosphere of the tropical atlantic thus the tasst possibly did not influence the variability of the snao southern center and the snao pattern exhibited a traditional distribution with two centers over the north atlantic

asymmetry in the response of central eurasian winter temperature to amo the asymmetry in the teleconnection of the central eurasian winter surface air temperature sat with the atlantic multidecadal oscillation amo is discussed using observations and model simulations observations indicate that the winter sat over central eurasia 30 70 e 30 50 n shows significant positive anomalies during the warm amo period but weak and insignificant anomalies in the cold amo period in general the warm winters in central eurasia are associated with large scale negative sea level pressure anomalies in europe anomalous southwesterly winds at 850 hpa over europe the geopotential height anomalies at 500 hpa in the south of greenland northern europe western asia and the slant north south pattern jet stream anomalies at 200 hpa in the north and south of the caspian sea reverse patterns occur during cold winters these statistically significant features are observed in the warm phase of amo reversed circulation anomalies are observed during the cold phase of amo however these anomalies are weak and not statistically significant furthermore the asymmetry in the atmospheric response to amo is well supported by simulations with a suite of gfdl atmospheric model idealized experiments and four cmip5 models historical experiments both observations and simulations indicate that rossby waves propagating from the north atlantic eastward to eurasia emerge in the warm amo and disappear in the cold amo thus the different propagations of rossby waves induced by the different surface thermal conditions of the warm and cold amo are the potential connection between the north atlantic ocean and central eurasian climate and may explain the asymmetry

empirical relationships between aftershock area dimensions and magnitude for earthquakes in the mediterranean sea region fault dimension estimates derived from the aftershock area extent of 36 shallow depth v31 km earthquakes that occurred in the mediterranean sea region have been used in order to establish empirical relationships between length width area and surface wave moment magnitude this dataset consists of events whose aftershock sequence was recorded by a dense local or regional network and the reported location errors did not exceed on average 3 5 km surface wave magnitudes for these events were obtained from the neic database and or published reports while moment magnitudes as well as focal mechanisms were available from the harvard usgs catalogues contrary to the results of some previously published studies we found no evidence in our dataset that faulting type may have an effect on the fault dimension estimates and therefore we derived relationships for the whole of the dataset comparisons by means of statistical f tests of our relationships with other previously published regional and global relationships were performed in order to check possible similarities or differences most such comparisons showed relatively low significance levels b95 since the differences in source dimension estimates were large mainly for magnitudes lower than 6 5 becoming smaller with increasing magnitude some degree of similarity however could be observed between our fault length relationship and the one derived from aftershock area lengths of events in greece while a difference was found between our regional and global fault length relationships a calculation of the ratio defined as the fault length derived from our relationships to the length estimated from regional empirical relationships involving surface ruptures showed that it can take a maximum value of about 7 for small magnitudes while it approaches unity at ms 7 2 when calculating the same ratio using instead global empirical relationships we see the maximum value not exceeding 1 8 while unity is reached at mw 7 8 indicating the existence of a strong regional variation in the fault lengths of earthquakes occurring in the mediterranean sea region also a relationship between the logarithms of the rupture area and seismic moment is established and it is inferred that there is some variation of stress drop as a function of seismic moment in particular it is observed that for magnitudes

coulomb stress triggering of earthquakes along the atalanti fault central greece two april 1894 m6 events and stress change patterns abstract two m6 events occurred 15 20 km apart in central greece on april 20 and april 27 1894 we identify the april 27 1894 rupture 2nd in the sequence with the atalanti segment of the atalanti fault zone because of unequivocal surface rupturing evidence reported by skouphos skouphos t 1894 die swei grossen erdbeben in lokris am 8 20 und 15 27 april 1894 zeitschrift ges erdkunde zu berlin vol 24 pp 409 474 coulomb stress transfer analysis and macroseismic evidence suggest that the april 20 1894 event 1st in the sequence may be associated with the martinon segment of the same fault zone our stress modelling suggests that this segment may have ruptured in an m 6 4 event producing a 15 km long rupture which transferred 1 14 bar in the epicentral area of the april 27th 1894 event thus triggering the second m 6 6 earthquake along the atalanti segment and producing a 19 km long rupture we also examined three alternative fault sources for the first event however all these produce smaller stress stresses for triggering the second event the proposed slip model for the second earthquake is capable of producing coastal subsidence of the order of centimetres to decimetres which fits the geological data the 1894 earthquake sequence was followed by a difference in the timing of subsequent m 5 events in each of the relaxed areas stress shadows a negative change in coulomb failure stress 0 6 bar which terminated between 22 37 years north and 80 years south

the egion june 15 1995 6 2m l earthquake western greece on june 15 1995 at 00 15 gmt a devastating earthquake 6 2m l occurred in the western end of the gulf of corinth this was followed 15 min later by the largest aftershock 5 4m l the main event was located by the university of patras seismological network patnet at the northern side of the gulf of corinth graben the second event 5 4m l was located also by patnet near the city of egion on a fault parallel to the eliki major fault that defines the south bound of the gulf of corinth graben a seismogenic volume that spans the villages of akrata se and rodini nw and extends to eratini ne was defined by the aftershock sequence which includes 858 aftershocks of magnitude greater than 2m l that occurred the first seventeen days the distribution of hypocentres in cross section does not immediately suggest a planar distribution but rather defines a volume about 15 km depth by 35 km nw se and by 20 km ne sw

dynamics of the extratropical response to a tropical atlantic sst anomaly abstract previous atmospheric general circulation model agcm experiments revealed that atmospheric responses to a tropical atlantic sea surface temperature anomaly ssta were asymmetric with respect to the sign of the ssta a positive ssta produced a south north dipole in geopotential heights much like the north atlantic oscillation nao while a negative ssta yielded an eastward propagating wave train with the northern lobe of the nao absent here these height responses are decomposed into components that are symmetric or antisymmetric with respect to the sign of the ssta the symmetric or notionally linear component is a nearly south north dipole projecting on the nao while the antisymmetric or notionally nonlinear component is a different dipole experiments with a diagnostic linear baroclinic model lbm suggest that both components are maintained primarily by transient eddy forcing dynamical mechanisms for the formation of the two components are explored using the lbm and a nonlinear baro

northwestern turkey earthquakes and the crustal structure inferred from surface waves observed in western greece records of several earthquakes occurring in turkey in 1999 obtained at broadband seismic stations in western greece have been used to study the dispersion of surface waves mainly love waves the observed group velocity dispersion curves have been inverted into horizontally layered models of the earth s crust by a modified method of the single parameter variation as compared with a previous model for the territory of greece the dispersion data require significantly lower velocities in the uppermost crust and smaller crustal thickness in particular the resulting model displays s wave velocities between 1 3 and 2 4 km sec in the upper 2 km and a crustal thickness of about 33 km manuscript received 31 july 2000

effects of local soil conditions on the topographic aggravation of seismic motion parametric investigation and recorded field evidence from the 1999 athens earthquake during the 1999 athens earthquake the town of adames located on the eastern side of the kifissos river canyon experienced unexpectedly heavy damage despite the particular geometry of the slope that caused significant motion amplification topography effects alone cannot explain the uneven damage distribution within a 300 m zone parallel to the canyon s crest which is characterized by a rather uniform structural quality in this article we illustrate the important role of soil stratigraphy and material heterogeneity on the topographic aggravation of surface ground motion for this purpose we first conduct an extensive time domain parametric study using idealized stratified profiles and gaussian stochastic fields to characterize the spatial distribution of soil properties and using ricker wavelets to describe the seismic input motion the results show that both topography and local soil conditions significantly affect the spatial variability of seismic motion we next perform elastic two dimensional wave propagation analyses based on available local geotechnical and seismological data and validate our results by comparison with aftershock recordings

decadal variation of summer precipitation over china and associated atmospheric circulation after the late 1990s abstractin this study the authors found that the summer precipitation over china experienced different decadal variation features from north to south after the late 1990s in northeastern and north china and the lower middle reaches of the yangtze river precipitation decreased after 1999 while precipitation experienced a significant reduction over south and southwestern china and a significant increase over the southern parts of hetao region and huaihe river valley after 2003 the authors next analyzed the associated decadal variation of the atmospheric circulation and attempted to identify the mechanisms causing the two decadal variations of precipitation the wind anomalies for the former exhibit a barotropic meridional dipole pattern with anticyclonic anomalies over mongolia to northern china and cyclonic anomalies over the southeastern chinese coast to the northwestern pacific for the latter there is a southeast northwest oriented dipole pattern in the middle and lower troposphere with cyclonic

role of the north pacific sea surface temperature in the east asian winter monsoon decadal variability in this study a possible mechanism for the decadal variability in the east asian winter monsoon eawm is proposed specifically the north pacific sea surface temperature sst may play an important role an analysis of the observations shows that the north pacific sst has a remarkable decadal pattern whose phase shifted around the mid 1980s this north pacific sst decadal pattern can weaken the east asian trough and enhance the north pacific oscillation through changing air sea interactions over the north pacific the weak east asian trough enhances the zonal circulation and weakens the meridional circulation over east asia consequently leading to a weaker southward cold surge and east asia warming around the mid 1980s the numerical experiment further confirms the pronounced physical processes in addition over the longer period of 1871 2012 the indices of the eawm and north pacific sst decadal pattern are also highly consistent on the decadal timescale which further confirms the impact of the north pacific sst decadal pattern on the eawm decadal variability

effects of the january 2018 seismic sequence on shear wave splitting in the upper crust of marathon ne attica greece abstract on the 15th of january 2018 a moderate mw 4 2 earthquake occurred in the area of marathon ne attica this event is of great interest given that it is located in a region characterized by low seismic activity even though there are previously identified faults e g penteli fault the sequence started 7 days before the mainshock and lasted until the end of the january 2018 in the present study waveforms from stations of the hellenic unified seismological network were utilized to explore the seismotectonic regime using data recorded since 2012 while the focal mechanism of the mainshock is in agreement with known local faults relocated hypocenters indicate the existence of blind faults in addition the proximity of two stations dion and ptl to the january 2018 sequence provided a unique opportunity to investigate seismic anisotropy of the upper crust in the study area for the first time analysis of local events between 2012 and 2018 led to the revelation of an anisotropic layer beneath marathon however a complex anisotropy regime is identified the polarization direction of the sfast in dion presents stable characteristics and is oriented according to the regional maximum horizontal stress component however ptl exhibits rotation to a direction perpendicular to the one that conforms to stress orientation possibly associated with local faults of similar direction time delays through the whole period are uncommonly low for such studies in both stations nevertheless this changes for the january 2018 sequence in dion higher values of time delays permit the establishment of trends that could be attributed to stress accumulation and relaxation related to the mw 4 2 event in agreement with the anisotropic poro elasticity model

the athens earthquake of 7 september 1999 based on detailed aftershock monitoring the first model of the athens earthquake is formulated which is consistent with global regional and local strong motion data of the mainshock and fits with geological setting the 30 station temporary network located 450 aftershocks during the first 20 observation days the aftershocks identified the mainshock fault plane dipping 52 and striking 117 consistently with the teleseismic fault plane solution a formal upward continuation of the fault plane intersects the surface close to the fili fault numerical modeling of the broadband regional data at 10 stations epicentral distances 140 to 370 km estimates the centroidal source depth of 10 km and yields an average source duration of 5 to 6 sec the interstation variability of the apparent duration indicates source directivity the empirical green9s function modeling at the closest broadband station suggests a fault length of 20 or 10 km both the numerical and empirical modelings give a very short rise time of 0 1 to 0 3 sec the short rise time seems to favor the nearly complete stress release of an asperity a 10 km asperity stress drop of 2 7 m pa is in agreement with a gap identified during the first 12 observation days between two aftershock clusters the strong motion accelerograms in athens also indicate a short apparent duration due to directivity about 3 sec and confirm an abrupt rupture beginning there is no evidence for an abrupt stopping the short rise time and short apparent source duration were two principal factors determining the damaging ground motions in athens

slip distribution of the 7 september 1999 athens earthquake inferred from an empirical green s function study the source process during the 7 september 1999 athens greece earth quake is investigated using broadband seismograms recorded at regional distances source time functions are estimated through an empirical green s function approach and their shapes are inverted to reveal the spatial and temporal distribution of fault slip the resulting slip distribution pattern implies that about 50 of the total slip occurred at a depth greater than the hypocentral depth indicating downward rupture propagation whereas 25 of the total slip was concentrated at a shallower patch forward calculations of the displacement field at the surface show that the shallower slip patch is possibly responsible for the asymmetry observed in the displacement field and it may have contributed significantly to the distribution of damage