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(a) Cumulative number of earthquakes of 5.0 M 5.4 that occurred during 1900-2002 within annuli of 100-150 km, as a function of time. (b) Same as (a) but for events of 5.5 M 5.9. (c) Same as (a) for annuli of 150-200 km. (d) Same as (b) for annuli of 150-200 km.
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The largest earthquake (M0=4.91027 dyncm) of the 20th century in the territory of Greece occurred south of Amorgos Island, causing extensive destruction in the southern Aegean area. It occurred on an ENE–trending normal fault that is seated parallel to the Islands southern coastline. Changes in the rates of moderate–size earthquakes (M 5.0) that oc...
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... which agrees with the finding of PAPAZACHOS and PAPAZACHOS (2000) as mentioned earlier. The data inside the annuli of 100-150 km and 150-200 km are now considered and their occurrence frequency is plotted in Figure 3. The excitation period 1933-1955 is more evident for the annuli 100-150 km and for all magnitude ...
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A series of dynamic rupture events under constant tectonic loading is simulated on a fault with multiscale heterogeneity and a stochastic rupture initiation process. The fracture energy of the fault plane is assumed to have multiscale heterogeneous distribution using fractal circular patches. The stochastic rupture initiation process as a function...
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Citations
... Southeastern Aegean Sea is one of the most seismically active areas in the Eastern Mediterranean, with a distinctive seismic zone extending from western Turkey, and characterized mainly by normal faulting and diffuse crustal seismicity. Frequent strong (M ≥ 6.0) main shocks are known from historical information (Papazachos and Papazachou, 2003) and instrumental recordings (http://geophysics.geo.auth.gr/ss/) in this extensional zone, displaying clustering behavior (Papadimitriou et al., 2005). The largest event which took place in the region occurred in 1956 with M w 7.5 (hereafter, we drop the subscript w and refer to the earthquake magnitude as the moment magnitude, unless otherwise noted), associated with a normal fault bounding the southern coastline of the Amorgos Island. ...
On 20 July 2017 (22:31:10 UTC), an Mw6.6 earthquake took place offshore Kos Island, in the southeastern (SE) Aegean Sea, producing severe damage and loss of life in the city of Kos and several smaller cities and villages both in Kos Island (Greece) and in the Bodrum peninsula (Turkey). All available seismological data until the end of October 2017 were gathered from seismological stations located in Greece and Turkey for the relocation process. The relocated aftershocks are clustered at least in three distinctive patches, creating a zone reaching a total length of about 40 km, elongated in a nearly east-west direction, and are mainly concentrated at depths 8–15 km, with the main shock hypocenter placed at ~13 km, implying a seismogenic layer of 7 km thickness, indicative for normal faulting earthquakes with Mmax ~ 6.5. The aftershock fault plane solutions are predominantly normal faulting in response to the north–south extension of the back arc Aegean area and consistent with the broader regional stress field. We also applied the satellite radar interferometry (InSAR) technique to define the coseismic surface displacements, revealing the range of deformation on the Island and the surrounding mainland. We combined this deformation field along with the available vectors of displacement measured by the Global Navigation Satellite System (GNSS) technique with the seismological data to determine the fault geometry and rupture process. The resulted variable slip model indicates a rather elliptical rupture area of 306 km², with the coseismic slip ranging between 0.5 and 2.3 m. The peak moment release occurred in the depth interval of 9–11 km, consistent with the depth distribution of seismicity in the study area. We used the variable slip model to calculate Coulomb stress changes and investigate possible triggering due to stress transfer to the nearby fault segments.
... Subcrustal earthquakes coexist with crustal seismicity in the SAVA, with the major seismicity rates observed east of the SVC, mainly due to the recent unrest of Santorini volcano during 2011-2012 [54], and the M6. 6 Kos earthquake on 20 July 2017 [95]. A prominent feature highlighted from seismicity is a 55 km long concentration of shallow events along the Amorgos fault that produced a catastrophic tsunamigenic event in 1956 with M7.7, the largest magnitude observed in the Aegean during the instrumental era [96,97]. To the West of the SVC, seismicity is sparse. ...
Knowledge and visualization of the present-day relationship between earthquakes, active tectonics and crustal deformation is a key to understanding geodynamic processes, and is also essential for risk mitigation and the management of geo-reservoirs for energy and waste. The study of the complexity of the Greek tectonics has been the subject of intense efforts of our working group, employing multidisciplinary methodologies that include detailed geological mapping, geophysical and seismological data processing using innovative methods and geodetic data processing, involved in surveying at various scales. The data and results from these studies are merged with existing or updated datasets to compose the new Seismotectonic Atlas of Greece. The main objective of the Atlas is to harmonize and integrate the most recent seismological, geological, tectonic, geophysical and geodetic data in an interactive, online GIS environment. To demonstrate the wealth of information available in the end product, herein, we present thematic layers of important seismotectonic and geophysical content, which facilitates the comprehensive visualization and first order insight into seismic and other risks of the Greek territories. The future prospect of the Atlas is the incorporation of tools and algorithms for joint analysis and appraisal of these datasets, so as to enable rapid seismotectonic analysis and scenario-based seismic risk assessment.
... They compared their findings to what they observed in New Zealand and Japan, suggesting that earthquake swarms can be long-range precursors of strong main events. Papadimitriou et al. (2005) investigated the seismicity patterns before and after the 1956 M7.7 Amorgos earthquake, one of the strongest events in the Aegean region during the instrumental era. The spatiotemporal variation of seismicity, combined with the estimated changes in the stress field of the area, gave evidence of a doughnut pattern, i.e. a quiescent period in the focal region and increasing seismicity (for several years) on the surrounding structures. ...
Precursors related to seismicity patterns are probably the most promising phenomena for short-term earthquake forecasting, although it remains unclear if such forecasting is possible. Foreshock activity has often been recorded but its possible use as indicator of coming larger events is still debated due to the limited number of unambiguously observed foreshocks. Seismicity data which is inadequate in volume or character might be one of the reasons foreshocks cannot easily be identified. One method used to investigate the possible presence of generic seismicity behavior preceding larger events is the aggregation of seismicity series. Sequences preceding mainshocks chosen from empirical data are superimposed, revealing an increasing average seismicity rate prior to the mainshocks. Such an increase could result from the tendency of seismicity to cluster in space and time, thus the observed patterns could be of limited predictive value. Randomized tests using the empirical catalogues imply that the observed increasing rate is statistically significant compared to an increase due to simple clustering, indicating the existence of genuine foreshocks, somehow mechanically related to their mainshocks. If network sensitivity increases, the identification of foreshocks as such may improve. The possibility of improved identification of foreshock sequences is tested using synthetic data, produced with specific assumptions about the earthquake process. Complications related to background activity and aftershock production are investigated numerically, in generalized cases and in data-based scenarios. Catalogues including smaller, and thereby more, earthquakes can probably contribute to better understanding the earthquake processes and to the future of earthquake forecasting. An important aspect in such seismicity studies is the correct estimation of the empirical catalogue properties, including the magnitude of completeness (Mc) and the b-value. The potential influence of errors in the reported magnitudes in an earthquake catalogue on the estimation of Mc and b-value is investigated using synthetic magnitude catalogues, contaminated with Gaussian error. The effectiveness of different algorithms for Mc and b-value estimation are discussed. The sample size and the error level seem to affect the estimation of b-value, with implications for the reliability of the assessment of the future rate of large events and thus of seismic hazard.
... The northern Aegean is dominated by dextral strike-slip faulting whereas the southern Aegean by normal faulting, characteristic for back arc areas. The largest earthquake in the territory of Greece and its surroundings during the instrumental era, occurred in this latter area in 1956 with M w = 7.7 (see [18,21,23]). Further historical information and instrumental data can be found in [24]. ...
The real stress field in an area associated with earthquake generation cannot be directly observed. For that purpose we apply hidden semi-Markov models (HSMMs) for strong earthquake occurrence in the areas of North and South Aegean Sea considering that the stress field constitutes the hidden process. The advantage of HSMMs compared to hidden Markov models (HMMs) is that they allow any arbitrary distribution for the sojourn times. Poisson, Logarithmic and Negative Binomial distributions as well as different model dimensions are tested. The parameter estimation is achieved via the EM algorithm. For the decoding procedure, a new Viterbi algorithm with a simple form is applied detecting precursory phases (hidden stress variations) and warning for anticipated earthquake occurrences. The optimal HSMM provides an alarm period for 70 out of 88 events. HMMs are also studied presenting poor results compared to these obtained via HSMMs. Bootstrap standard errors and confidence intervals for the parameters are evaluated and the forecasting ability of the Poisson models is examined.
... The respective widths W were estimated from the dip angle of the fault and the distance measured down-dip from the surface to the upper (h 1 ) and lower (h 2 ) edges of the rectangular dislocation plane, respectively, as jh 1 − h 2 j= sindip. The upper bound h 1 of the seismogenic layer in the Aegean region is considered to begin from 3 km depth (e.g., Papadimitriou et al., 2005). To estimate the rupture widths associated with low angle thrust faulting, the constraint L > W was adopted (Lin and Stein, 2004;Messini et al., 2007). ...
... Sample size is also a crucial precondition for such kinds of analysis; in all the tested cases in which data was sparse due to the short time window and/or high corresponding completeness magnitude, the correlation was negligible. Additional uncertainties arise from the former strong event influence (e.g., 1956 M w 7.7 Amorgos earthquake, Papadimitriou et al., 2005) that was not taken into account because of the data insufficiency for a robust seismicity rate investigation before 1970. Influence of other physical processes such as viscoelastic relaxation, postseismic deformation, and rheological properties were also neglected. ...
Seismicity rate changes in the Hellenic subduction zone (southern Aegean Sea) were studied by applying the Dieterich (1994) rate/state-dependent friction model combined with static Coulomb stress changes (ΔCFF). The coseismic slip of the strongest earthquakes (with moment magnitude,Mw ≥6:0) was considered to contribute to the stress field evolution along with the continuous tectonic loading. Stress changes were calculated just after each strong event, and their influence was examined in connection with the smaller magnitude earthquake occurrence rates. Qualitative and quantitative comparison between the observed seismicity rates (smoothed by the means of a probability density function) and the expected ones, as they were forecasted by the rate/state model, were investigated for the interseismic periods (study periods) between subsequent strong earthquakes. The calculations aim to identify areas of expected increased seismicity rates as candidates to accommodate enhanced seismic activity. Results strongly depend on the determination (smoothing) of the unperturbed (reference) seismicity rates and data adequacy. Seismicity rate results were filtered by certain criteria and constraints, in an attempt to overcome model uncertainties (epicentral errors, rupture models, parameter values) and to provide reliable results for specific areas of major interest, that is, in areas with increased positive Coulomb stress changes values. The modeling approach resulted in satisfactory correlation between observed and synthetic seismicity rates and, in particular, the two strong (Mw ≥6) earthquakes that occurred in 2013 are located in areas of increased expected seismicity rates. © 2014, Bulletin of the Seismological Society of America. All rights reserved.
... FERRARI et al. (2000) calculated the stress field that resulted from the coseismic slips of events of M 6 or greater since 1700 and the secular interseismic stress changes to show that the 1999 events were anticipated. For the southeastern Aegean area, part of which coincides partially with our study area, the evolutionary model satisfactorily explained the clustering of strong (M C 6.5) normal faulting earthquakes (PAPADIMITRIOU et al., 2005). In all these studies possible future occurrences are suggested, which will be discussed in the following sections along with the results obtained in the present study. ...
Western Turkey has a long history of destructive earthquakes that are responsible for the death of thousands of people and which caused devastating damage to the existing infrastructures, and cultural and historical monuments. The recent earthquakes of Izmit (Kocaeli) on 17 August, 1999 (M
w
= 7.4) and Düzce (M
w
= 7.2) on 12 November, 1999, which occurred in the neighboring fault segments along the North Anatolian Fault (NAF), were catastrophic ones for the Marmara region and surroundings in NW Turkey. Stress transfer between the two adjacent fault segments successfully explained the temporal proximity of these events. Similar evidence is also provided from recent studies dealing with successive strong events occurrence along the NAF and parts of the Aegean Sea; in that changes in the stress field due to the coseismic displacement of the stronger events influence the occurrence of the next events of comparable size by advancing their occurrence time and delimiting their occurrence place. In the present study the evolution of the stress field since the beginning of the twentieth century in the territory of the eastern Aegean Sea and western Turkey is examined, in an attempt to test whether the history of cumulative changes in stress can explain the spatial and temporal occurrence patterns of large earthquakes in this area. Coulomb stress changes are calculated assuming that earthquakes can be modeled as static dislocations in elastic half space, taking into account both the coseismic slip in large (M ≥ 6.5) earthquakes and the slow tectonic stress buildup along the major fault segments. The stress change calculations were performed for strike-slip and normal faults. In each stage of the evolutionary model the stress field is calculated according to the strike, dip, and rake angles of the next large event, whose triggering is inspected, and the possible sites for future strong earthquakes can be assessed. A new insight on the evaluation of future seismic hazards is given by translating the calculated stress changes into earthquake probability using an earthquake nucleation constitutive relation, which includes permanent and transient effects of the sudden stress changes.
... smic slips of events of M 6 or greater since 1700 and the secular interseismic stress changes to show that the 1999 events were anticipated . For the southeastern Aegean area , part of which coincides partially with our study area , the evolutionary model satisfactorily explained the clustering of strong ( M C 6 . 5 ) normal faulting earthquakes ( PAPADIMITRIOU et al . , 2005 ) . In all these studies possible future occurrences are suggested , which will be discussed in the following sections along with the results obtained in the present study . This study differs from the previously mentioned ones , as regards calculations of static stress changes , in that it aims to integrate the stress evolution history ...
... FERRARI et al. (2000) calculated the stress field that resulted from the coseismic slips of events of M 6 or greater since 1700 and the secular interseismic stress changes to show that the 1999 events were anticipated. For the southeastern Aegean area, part of which coincides partially with our study area, the evolutionary model satisfactorily explained the clustering of strong (M C 6.5) normal faulting earthquakes (PAPADIMITRIOU et al., 2005). In all these studies possible future occurrences are suggested, which will be discussed in the following sections along with the results obtained in the present study. ...
Western Turkey has a long history of destructive earthquakes that are responsible for the death of thousands of people and which caused devastating damage to the existing infrastructures, and cultural and historical monuments. The recent earthquakes of Izmit (Kocaeli) on 17 August, 1999 (M
w
= 7.4) and Düzce (M
w
= 7.2) on 12 November, 1999, which occurred in the neighboring fault segments along the North Anatolian Fault (NAF), were catastrophic ones for the Marmara region and surroundings in NW Turkey. Stress transfer between the two adjacent fault segments successfully explained the temporal proximity of these events. Similar evidence is also provided from recent studies dealing with successive strong events occurrence along the NAF and parts of the Aegean Sea; in that changes in the stress field due to the coseismic displacement of the stronger events influence the occurrence of the next events of comparable size by advancing their occurrence time and delimiting their occurrence place. In the present study the evolution of the stress field since the beginning of the twentieth century in the territory of the eastern Aegean Sea and western Turkey is examined, in an attempt to test whether the history of cumulative changes in stress can explain the spatial and temporal occurrence patterns of large earthquakes in this area. Coulomb stress changes are calculated assuming that earthquakes can be modeled as static dislocations in elastic half space, taking into account both the coseismic slip in large (M ≥ 6.5) earthquakes and the slow tectonic stress buildup along the major fault segments. The stress change calculations were performed for strike-slip and normal faults. In each stage of the evolutionary model the stress field is calculated according to the strike, dip, and rake angles of the next large event, whose triggering is inspected, and the possible sites for future strong earthquakes can be assessed. A new insight on the evaluation of future seismic hazards is given by translating the calculated stress changes into earthquake probability using an earthquake nucleation constitutive relation, which includes permanent and transient effects of the sudden stress changes.
... The accelerating moment release model (Bufe and Varnes 1993, Bowman et al. 1998) has been proved to hold in areas almost ten times the fault length. In areas of smaller dimensions (three to four times the fault length) precursory decelerated moment release has been detected either from the temporal variation of the Benioff strain release (Papazachos et al. 2005), or from the seismicity rates (Papadimitriou et al. 2005 ). A basic issue concerns the identification of the precursory area where these changes are expected to occur. ...
The spatial distribution of the stress field around a fault, before its failure, depends on the focal mechanism of the ensuing rupture. In the preseismic stage, the main fault is locked and background seismicity is distributed in the surrounding area along small faults due to the raise of the stress level. This distribution is well correlated with changes of the stress field, when these are calculated considering the fault slipping on the opposite sense (back-slip model), before the incoming strong event. Studies of the seismicity around faults with known geometry, dimensions and slip properties could contribute in the seismic hazard assessment.
In the present work, the spatio-temporal distribution of the smaller magnitude seismic activity before the occurrence of the four most recent (1995–2008) strong (M ≥ 6.4) shallow earthquakes in Greece is compared with the stress pattern necessary for the generation of the strong earthquakes. Studying the annual rate of occurrence it is found that in areas of positive pre-stress changes a sharp increase of the number of earthquakes is observed several years before the occurrence of the main shock. This increase lasts for a few years and then for several years before the main shock the occurrence rate declines but usually remains higher than it was before the sharp increase. Seismicity distribution in the respective areas of negative pre-stress changes shows a rather stable occurrence rate.
... Coulomb stress changes are widely used in the literature to seek for fault interaction between large magnitude earthquakes as well as to model aftershock patterns and seismicity rate changes over long-time windows (Harris 1998; Stein 1999 among others). Such studies exploring fault interaction have been compiled for the broader Aegean area during the last years for strike-slip events (Nalbant et al. 1998; Papadimitriou and Sykes 2001; Papadimitriou 2002), normal faulting (Papadimitriou and Karakostas 2003; Papadimitriou et al. 2005) as well as between strike-slip and subduction earthquakes (Messini et al. 2005). The results of the above studies revealed that the vast majority of the earthquakes whose triggering was inspected, were located inside areas of positive static stress changes. ...
Activation of major faults in Bulgaria and northern Greece presents significant seismic hazard because of their proximity
to populated centers. The long recurrence intervals, of the order of several hundred years as suggested by previous investigations,
imply that the twentieth century activation along the southern boundary of the sub-Balkan graben system, is probably associated
with stress transfer among neighbouring faults or fault segments. Fault interaction is investigated through elastic stress
transfer among strong main shocks (M≥6.0), and in three cases their foreshocks, which ruptured distinct or adjacent normal fault segments. We compute stress
perturbations caused by earthquake dislocations in a homogeneous half-space. The stress change calculations were performed
for faults of strike, dip, and rake appropriate to the strong events. We explore the interaction between normal faults in
the study area by resolving changes of Coulomb failure function (ΔCFF) since 1904 and hence the evolution of the stress field in the area during the last 100years. Coulomb stress changes were
calculated assuming that earthquakes can be modeled as static dislocations in an elastic half-space, and taking into account
both the coseismic slip in strong earthquakes and the slow tectonic stress buildup associated with major fault segments. We
evaluate if these stress changes brought a given strong earthquake closer to, or sent it farther from, failure. Our modeling
results show that the generation of each strong event enhanced the Coulomb stress on along-strike neighbors and reduced the
stress on parallel normal faults. We extend the stress calculations up to present and provide an assessment for future seismic
hazard by identifying possible sites of impending strong earthquakes.
... Coulomb stress changes are widely used in the literature to seek for fault interaction between large magnitude earthquakes as well as to model aftershock patterns and seismicity rate changes over long-time windows (Harris 1998; Stein 1999 among others). Such studies exploring fault interaction have been compiled for the broader Aegean area during the last years for strike-slip events (Nalbant et al. 1998; Papadimitriou and Sykes 2001; Papadimitriou 2002), normal faulting (Papadimitriou and Karakostas 2003; Papadimitriou et al. 2005) as well as between strike-slip and subduction earthquakes (Messini et al. 2005). The results of the above studies revealed that the vast majority of the earthquakes whose triggering was inspected, were located inside areas of positive static stress changes. ...
Recent strong (M ≥ 6.6) earthquakes in Greece are examined from the
point of view of two current, but disparate, approaches to long-term
seismogenesis. These are the evolving stress field (ESF) approach, in
which earthquakes are considered to be triggered by accumulated stress
changes from past earthquakes and tectonic loading on the major faults,
and the precursory scale increase (Ψ) approach, in which a major
earthquake is preceded in the long term by an increase in minor
earthquake occurrences, with the magnitude of the precursory
earthquakes, and the precursor time and area all scaling with the major
earthquake magnitude. The strong earthquakes are found to be consistent
with both approaches, and it is inferred that both approaches have a
relevant role to play in the description of the long-term generation
process of major earthquakes. A three-stage faulting model proposed
previously to explain the Ψ phenomenon involves a major crack, which
eventually fractures in the major earthquake, being formed before the
onset of precursory seismicity. Hence we examine whether ESF can account
for the formation of the major crack by examining the accumulated stress
changes at the time of the onset of Ψ for each strong earthquake. In
each case, the answer is in the affirmative; there is enhanced stress in
the vicinity of the main shock at the time of the onset. The same is
true for most, but not all, of the locations of precursory earthquakes.
