Non-extensive statistical physics approach to fault population distribution. A case study from the southern Hellenic arc (Central Crete)

Fault population statistics play a key role in the understanding of any statistical seismicity approach. In the present work a non-extensive statistical physics approach is formulated and tested for the local fault length distribution. The approach is composed of the following parts: (i) Tsallis entropy, S _q , (ii) maximization of the Tsallis entropy under appropriate constrains, and (iii) derivation of the cumulative distribution function (CDF) of the fault length population. This model is tested using fault length data from the Central Crete graben in front of the Hellenic arc and estimated a thermodynamic q parameter equal to 1.16, which supports the conclusion that the fault system in Central Crete graben is a sub-extensive one.     

Seismicity at the convergent plate boundary offshore Crete,Greece, observed by an amphibian network

We investigate microseismic activity at the convergent plate boundary of the Hellenic subduction zone on- and offshore south-eastern Crete with unprecedented precision using recordings from an amphibian seismic network. The network configuration consisted of up to eight ocean bottom seismometers as well as five temporary short-period and six permanent broadband stations on Crete and surrounding islands. More than 2,500 local and regional events with magnitudes up to M _L = 4.5 were recorded during the time period July 2003–June 2004. The magnitude of completeness varies between 1.5 on Crete and adjacent areas and increases to 2.5 in the vicinity of the Strabo trench 100 km south of Crete. Tests with different localization schemes and velocity models showed that the best results were obtained from a probabilistic earthquake localization using a 1-D velocity model and corresponding station corrections obtained by simultaneous inversion. Most of the seismic activity is located offshore of central and eastern Crete and interpreted to be associated with the intracrustal graben system (Ptolemy and Pliny trenches). Furthermore, a significant portion of events represents interplate seismicity along the NNE-ward dipping plate interface. The concentration of seismicity along the Ptolemy and Pliny trenches extends from shallow depths down to the plate interface and indicates active movement. We propose that both trenches form transtensional structures within the Aegean plate. The Aegean continental crust between these two trenches is interpreted as a forearc sliver as it exhibits only low microseismic activity during the observation period and little or no internal deformation. Interplate seismicity between the Aegean and African plates forms a 100-km wide zone along dip from the Strabo trench in the south to the southern shore-line of Crete in the north. The seismicity at the plate contact is randomly distributed and no indications for locked zones were observed. The plate contact below and north of Crete shows no microseismic activity and seems to be decoupled. The crustal seismicity of the Aegean plate in this area is generally confined to the upper 20 km in agreement with the idea of a ductile deformation of the lower crust caused by a rapid return flow of metamorphic rocks that spread out below the forearc. In the region of the Messara half-graben at the south coast of central Crete, a southward dipping seismogenic structure is found that coalesces with the seismicity of the Ptolemy trench at a depth of about 20 km. The accretionary prism south of Crete indicated by the Mediterranean Ridge showed no seismic activity during the observation period and seems to be deforming aseismically.     

Seismicity of Western Crete and the destruction of the town of Kisamos at AD 365: Archaeological evidence

The study of numerous archaeological excavations permits us to concludethat shortly after AD 355–361, the wealthy Roman town of Kisamos inwestern Crete was affected by a devastating earthquake (minimum intensityXI), which left many of the town's inhabitants buried under the ruins. Thisearthquake can be related to the July 21, 365 earthquake (M > 8),which was associated with a great tsunami, and was probably responsible forup to 9 m uplift of Western Crete; it probably resulted from thereactivation of a major thrust fault along the Hellenic Arc. Thearchaeological stratigraphy of Kisamos between circa AD 50 and AD650 testifies to two other, small-scale stratigraphic discontinuities that maybe related to two other smaller earthquakes, which produced 10–20 cmof coastal subsidence in AD 46–66 and in circa AD 270. There isevidence for a tsunami associated with the AD 46–66 earthquake, whichagrees with sedimentological data from the nearby ancient harbour ofPhalasarna, which was uplifted about 6.5 m in AD 365.The following evidence indicates that Western Crete is not seismicallyquiescent, as previously believed on the basis of historical data, but that ithas been affected by very strong, AD 365-type earthquakes followed byrelatively quiescence periods, that were at least several thousand years long:(1) Major earthquakes of Western Crete produced coastal uplift orsubsidence, while coastal stability indicates the absence of strongearthquakes; such stability characterizes this part of the island after AD365; (2) No evidence of earthquakes exist in the archaeological record ofKisamos between AD 365 and circa AD 650 and of Chania since AD1400/1500.     

Comparison of gravimetric and seismic constraints on the structure of the Aegean lithosphere in the forearc of the Hellenic subduction zone in the area of Crete

The island of Crete is located in the forearc of the Hellenic subduction zone, where the African lithospheric plate is subducting beneath the Eurasian one. The depth of the plate contact as well as the internal structure of the Aegean plate in the area of Crete have been a matter of debate. In this study, seismic constrains obtained by wide-angle seismic, receiver function and surface wave studies are discussed and compared to a 3D density model of the region.The interface between the Aegean continental lithosphere and the African one is located at a depth of about 50 km below Crete. According to seismic studies, the Aegean lithosphere in the area of Crete is characterised by strong lateral, arc–parallel heterogeneity. An about 30 km thick Aegean crust is found in central Crete with a density of about 2850 kg/m³ for the lower Aegean continental crust and a density of about 3300 kg/m³ for the mantle wedge between the Aegean crust and the African lithosphere. For the deeper crust in the area of western Crete two alternative models have been proposed by seismic studies. One with an about 35 km thick crust and another one with crustal velocities down to the plate contact. A grid search is performed to test the consistency of these models with gravimetric constraints. For western Crete a model with a thick lower Aegean crust and a density of about 2950 kg/m³ is favoured. The inferred density of the lower Aegean crust in the area of Crete correlates well with S-wave velocities obtained by surface wave studies.Based on the 3D density model, the weight of the Aegean lithosphere is estimated along an E–W oriented profile in the area of Crete. Low weights are found for the region of western Crete.     

Shallow bias in Mediterranean paleomagnetic directions caused by inclination error

A variety of paleomagnetic data from the Mediterranean region show a strong bias toward shallow inclinations. This pattern of shallow inclinations has been interpreted to be the result of (1) major northward terrane displacement, (2) large nondipole components in the Earth's magnetic field, and (3) systematic inclination flattening of the paleomagnetic directions. Here, we use the observation that, in addition to the well-known variation of magnetic inclination with latitude, the N-S elongation of directional dispersion also varies, being most elongate at the equator and nearly symmetric at the poles. Assuming that inclination shallowing follows the relationship long known from experiment, we invert the inclinations using a range of “flattening factors” to find the elongation/inclination pair consistent with a statistical model for the paleosecular variation. We apply the so-called “elongation/inclination” method to the extensive paleomagnetic data sets from the Miocene sediments of the Calatayud basin (Spain) and the island of Crete (Greece). After correction, the Spanish data are in good agreement with the expected middle Miocene latitude of the region. The data from Crete suggest that it occupied a position in the late Miocene about 275 km north of the predicted location. This is in agreement with the geological and geodynamical models for the east Mediterranean region, which indicate that slab rollback processes in combination with Anatolian push generated southward migration of Crete. The 7.5 million year average displacement rate of Crete estimated by the E/I method is 37 mm/yr to the south, which closely coincides with present-day rates based on global positioning system (GPS) and model measurements. We also show that inappropriate tilt corrections lead to a shallow inclination bias as well, explaining that observed in studies of lava flows of the region. We conclude that the east Mediterranean inclination anomaly is caused by sedimentary inclination error and not by a persistent octupolar contribution to the geomagnetic field, or northward transport.     

Comparison of gravimetric and seismic constraints on the structure of the Aegean lithosphere in the forearc of the Hellenic subduction zone in the area of Crete

The island of Crete is located in the forearc of the Hellenic subduction zone, where the African lithospheric plate is subducting beneath the Eurasian one. The depth of the plate contact as well as the internal structure of the Aegean plate in the area of Crete have been a matter of debate. In this study, seismic constrains obtained by wide-angle seismic, receiver function and surface wave studies are discussed and compared to a 3D density model of the region.The interface between the Aegean continental lithosphere and the African one is located at a depth of about 50 km below Crete. According to seismic studies, the Aegean lithosphere in the area of Crete is characterised by strong lateral, arc–parallel heterogeneity. An about 30 km thick Aegean crust is found in central Crete with a density of about 2850 kg/m³ for the lower Aegean continental crust and a density of about 3300 kg/m³ for the mantle wedge between the Aegean crust and the African lithosphere. For the deeper crust in the area of western Crete two alternative models have been proposed by seismic studies. One with an about 35 km thick crust and another one with crustal velocities down to the plate contact. A grid search is performed to test the consistency of these models with gravimetric constraints. For western Crete a model with a thick lower Aegean crust and a density of about 2950 kg/m³ is favoured. The inferred density of the lower Aegean crust in the area of Crete correlates well with S-wave velocities obtained by surface wave studies.Based on the 3D density model, the weight of the Aegean lithosphere is estimated along an E–W oriented profile in the area of Crete. Low weights are found for the region of western Crete.     

A long-term rock uplift rate for eastern Crete and geodynamic implications for the Hellenic subduction zone

The island of Crete in the forearc of the Hellenic subduction zone has a rugged topography with local relief exceeding 2 km. Based on the elevation of marine shorelines, rates of rock uplift during the Late Holocene were previously estimated to range between 1 and 4 mm/a in different parts of the island. These rates may, however, not be representative for longer timescales, because subduction earthquakes with up to 9 m of vertical coseismic displacement have affected Crete in the Late Holocene. Here we use a well preserved sequence of marine terraces near Kato Zakros in eastern Crete to determine the rate of rock uplift over the last ∼600 ka. Field investigations and topographic profiles document a flight of more than 13 marine bedrock terraces that were carved into limestones of the Tripolitza unit. Preliminary age constraints for the terraces were obtained by ¹⁰Be exposure dating of rare quartz-bearing sandstone clasts, which are present on some terraces. The ¹⁰Be ages of these samples, which have been corrected for an inherited nuclide component, yielded exposure ages between ∼100 ka and zero. Combined with geomorphologic evidence the two oldest ¹⁰Be ages suggest that the terraces T₄ and T₅, with shoreline angles at an elevation of ∼68 and ∼76 m above sea level, respectively, formed during the marine isotope stage 5e about 120 ka ago. The correlation of the higher terraces (T₆ to T₁₃) with regional sea-level highstands indicates sustained rock uplift at a rate of ∼0.5 m/ka since at least ∼600 ka. As normal faulting has dominated the tectonics of Crete during the last several million years, upper crustal shortening can be ruled out as a cause for rock uplift. We argue that the sustained uplift of the island results from the continuous underplating of sediments, which are transferred from the subducting African plate to the base of the crust beneath Crete.     

CONTRIBUTION TO THE TRACER KINETICS THEORY

Abstract

This paper presents a new integrated GIS modelling methodology for assessing groundwater contamination risk. Analytical and numerical tools within a GIS framework were used to define the raster maps of various factors interfering along the contaminant pathway from source to groundwater. In the proposed methodology, these factors were introduced into a unified GIS model for groundwater risk assessment that incorporates all the necessary information to improve the accuracy of the results. Fifteen factors were considered in order to estimate the spatial distribution of the groundwater contamination risk areas. Some of these factors, such as artificial drainage and seepage velocity, had not been used previously in GIS groundwater risk mapping. The study area, the island of Crete in Greece, was divided into five regions characterized by different degrees of groundwater risk ranging from very low to very high. A sensitivity analysis was performed and the developed methodology was validated for different contaminants that were detected in groundwater of the island of Crete.

Editor Z.W. Kundzewicz     

Deformation and stress regimes in the Hellenic subduction zone from focal Mechanisms

Fault plane solutions for earthquakes in the central Hellenic arc are analysed to determine the deformation and stress regimes in the Hellenic subduction zone in the vicinity of Crete. Fault mechanisms for earthquakes recorded by various networks or contained in global catalogues are collected. In addition, 34 fault plane solutions are determined for events recorded by our own local temporary network on central Crete in 2000–2001. The entire data set of 264 source mechanisms is examined for types of faulting and spatial clustering of mechanisms. Eight regions with significantly varying characteristic types of faulting are identified of which the upper (Aegean) plate includes four. Three regions contain interplate seismicity along the Hellenic arc from west to east and all events below are identified to occur within the subducting African lithosphere. We perform stress tensor inversion to each of the subsets in order to determine the stress field. Results indicate a uniform N-NNE direction of relative plate motion between the Ionian Sea and Rhodes resulting in orthogonal convergence in the western forearc and oblique (40–50^∘) subduction in the eastern forearc. There, the plate boundary migrates towards the SE resulting in left-lateral strike-slip faulting that extends to onshore Eastern Crete. N110^∘E trending normal faulting in the Aegean plate at this part is in accordance with this model. Along-arc extension is observed on Western Crete. Fault plane solutions for earthquakes within the dipping African lithosphere indicate that slab pull is the dominant force within the subduction process and responsible for the roll-back of the Hellenic subduction zone.     

Manifestation and measurement of the fractal characteristics of karst hydrogeological formations

A new method of estimating the fractal dimension of the percolation backbone of karst systems, which are discharged through karst springs, is presented. This method is based on the simulation of the spring by the MODKARST deterministic mathematical model. Application has been made to the Psiloritis karst formation in Crete, which feeds the periodically brackish karst spring “Almiros” in Crete. Furthermore, the estimated dimension justifies an independently determined power law that quantifies the sea intrusion into the karst system.     

Improvement of groundwater level prediction in sparsely gauged basins using physical laws and local geographic features as auxiliary variables

This work aims to present new modeling tools that help to better monitor and predict the groundwater level in sparsely gauged basins. The working area is the Mires basin of Mesara valley in the island of Crete (Greece). Efficient groundwater management in the basin is crucial in light of regional climate change model estimates showing a substantial risk of desertification for Crete. We propose that the prediction of the hydraulic head spatial variability in Mires basin can be improved by incorporating in the trend the distance of the prediction points from a temporary river crossing the basin and a component based on the generalized Thiem’s equation for multiple wells as well as using the flexible Spartan semivariogram family to perform Residual Kriging. Our proposal is supported by the results of cross validation analysis. Our results are applicable to other unconfined aquifers.     

The impact of recent climate change on flooding and sediment supply within a Mediterranean mountain catchment,southwestern Crete,Greece

This paper presents work from a geomorphological investigation carried out in the Aradena Gorge, southwestern Crete, Greece. The gorge is typical of many steepland fluvial systems in the Mediterranean, with steep relief, coarse‐gravel sediments and high rates of sedimentation generated during intense winter storm events. Hillslope deposits and coarse‐gravel flood units within a 5 km section of the gorge have been mapped, dated (using lichenometry and dendrochronology), and their sedimentological characteristics recorded to establish a c. 200‐year record of flood frequency/magnitude and hillslope/channel sediment supply variability. This record has been compared with instrumented and previously published records of climate change from Crete and the Mediterranean region and used to establish the major controls on flooding and sediment dynamics within the Aradena Gorge. Rates of colluviation and sediment delivery to the channel appear to have been greater than the present sometime before c. AD 1800 and may be related to cooler climates with a more seasonal precipitation regime during the Little Ice Age (c. AD 1450 to 1850). In gorge sections where the present rate of sediment supply from hillslope colluvium is very low, the channel has incised into older alluvial and colluvial deposits. Conversely, in the few sections where sediment supply is currently very high, the channel is aggrading with a braided pattern. Major rock‐fall deposits at certain locations in the gorge have restricted any major downstream sediment transfer. Twelve periods of increased flooding during the last 150 years have been identified and these correlate quite well with negative or declining phases of the North Atlantic Oscillation (NAO). Analysis of daily precipitation data from Crete suggests negative phases of the winter NAO are characterized by an increase in the number of long‐duration, high‐intensity storms. These storms, particularly those with five‐day and greater duration, appear to be significant in triggering major floods in the Aradena Gorge. During the last 40 years the NAO index has been increasing and become locked into a positive phase. As a consequence of this, major flooding appears to have declined during the same period. Copyright © 2002 John Wiley & Sons, Ltd.     

The LF radio anomaly observed before the Mw = 6.5 earthquake in Crete on October 12, 2013

On October 12, 2013, an earthquake with M_w = 6.5 occurred in the southern Hellenic Arc, approximately 20 km off the west coast of Crete. The main shock, the focal depth of which is on the order of 40 km, was followed by aftershocks felt in the nearby cities and villages, although the aftershock sequence was poor. The epicentre was located at approximately 60 km from a radio receiver in Crete (CRE), which belongs to the European VLF/LF Radio Network. Several days before the earthquake, a clear disturbance occurred in one of the ten radio signals that the CRE receiver sampled. The disturbance, which can be considered an anomaly, appeared in the 216 kHz radio signal radiated by the Radio Monte Carlo (MCO) transmitter. The radio path MCO-CRE crossed directly over the epicentre area of the aforementioned earthquake. In this work, we present a detailed analysis of the MCO signal anomaly using spectral tools. We also investigate the behaviour of other radio signals sampled by the CRE receiver and consider other possible causes of disturbances on the MCO radio signal. We conclude that the disturbance in the MCO radio signal is a convincingly possible precursor of the earthquake in Crete. Emission of electromagnetic waves with a frequency band that includes 216 kHz from the focal zone of the earthquake can provide a satisfactory explanation of the radio anomaly.     

Die Gezeiten des Meeres und die Rotation der Erde

Summary The problem of tidal friction as a cause of the secular deceleration of earth rotation was higherto approached on the basis of estimating the kinetic energy dissipated by bottom friction. It would appear however, that in any analysis of the influence of oceanic tides on earth rotation the varying directions of the torques due to tidal streams and acting on the solid earth, must be taken into account. As examples for this, the torques due to tidal friction are calculated for the North Sea and for the 10°-world ocean, applying hydrodynamical-numerical methods. The results show that earth rotation is not retarded at all points of the world ocean, but that tidal areas exist exerting either decelerating or accelerating forces.     

A reanalysis of the AD 365 tsunami impact along the Egyptian Mediterranean coast

The historical tsunamigenic event of 21 July AD 365 destroyed several coastal locations in the Eastern Mediterranean region. The foremost destructive impacts were recorded in Crete and Egypt. The present study re-examines the effect of location, direction, height and time of travel of the tsunami towards the Egyptian coast. Evidently, this tsunamigenic event is related to an earthquake which is identified with a Hellenic Arc subduction-zone event of great magnitude, M > 8, as manifested by up to 9 m uplift in western Crete. The maximum run-up height distribution in the front of the Nile Delta was about 9.5 m in Alexandria, while those of the neighboring cities were 7.1 m, 4.9 m, and 1.9 m at Rashid, Damietta and El-Arish, respectively. Data obtained from this study is essential to evaluate the tsunami hazards along the Egyptian coast.     

Climate change impact on the hydrological budget of a large Mediterranean island

ABSTRACT

Crete is a Mediterranean, karst-dominated island, characterized by long drought periods. The Karst-SWAT model, combined with 11 climate change scenarios, was run to assess climate change impacts on the island under two set-ups, both using the auto-irrigation function of the model: (1) with water drawn from the shallow or deep aquifer, and (2) with irrigated water derived from an unlimited outside source. The first set-up provided insight into the fluctuation of future irrigation needs, and when compared to the second set-up, enabled quantification of the future water deficit. The Water Exploitation Index was used to describe the spatial variability of future water stress on Crete. A decrease in both surface and karstic spring flows is foreseen, especially after 2060 (24.2 and 16.5%, respectively). Simulated irrigation water demand and water deficit show continuous increase throughout the projection period (2020–2098).     

Temporal and spatial variation of annual rainfall on the island of Crete,Greece

Annual rainfall records from the island of Crete in Greece were used with the aid of a geographical information system (GIS) to study the temporal and spatial rainfall characteristics. The GIS was used to produce a digital elevation model, delineate watersheds and estimate the areal rainfall from a network of raingauges by using different interpolation schemes. The rainfall–elevation correlation was significant, suggesting an orographic type of precipitation for the island. The rainfall records for the majority of the stations were found to fit the normal distribution. Deviation from normal for the rest of the records was attributed to the wettest year of 1977–1978. The year 1989–1990 was the driest, and most rainfall records showed a decrease in rainfall over 30 years with higher negative rainfall gradients at the higher elevations. Frequency analysis of the rainfall records was used to estimate areal rainfall for the island of Crete and its main watersheds for return periods of 2, 5 and 10 years. Copyright © 2003 John Wiley & Sons, Ltd.     

The large earthquake of 8 August 1303 in Crete: seismic scenario and tsunami in the Mediterranean area

By conducting a historical review of this large seismic event in the Mediterranean, it has been possible to identify both the epicentral area and the area in which its effects were principally felt. Ever since the nineteenth century, the seismological tradition has offered a variety of partial interpretations of the earthquake, depending on whether the main sources used were Arabic, Greek or Latin texts. Our systematic research has involved the analysis not only of Arab, Byzantine and Italian chronicle sources, but also and in particular of a large number of never previously used official and public authority documents, preserved in Venice in the State Archive, in the Marciana National Library and in the Library of the Museo Civico Correr. As a result, it has been possible to establish not only chronological parameters for the earthquake (they were previously uncertain) but also its overall effects (epicentral area in Crete, Imax XI MCS). Sources containing information in 41 affected localities and areas were identified. The earthquake also gave rise to a large tsunami, which scholars have seen as having certain interesting elements in common with that of 21 July 365, whose epicentre was also in Crete. As regards methodology, this research made it clear that knowledge of large historical earthquakes in the Mediterranean is dependent upon developing specialised research and going beyond the territorial limits of current national catalogues.     

Magnetic field structure in differentially rotating discs

Abstract

In order to gain a better understanding of the processes that may give rise to non-axisymmetric magnetic fields in galaxies, we have calculated field decay rates for models with a realistic galactic rotation curve and including the effects of a locally enhanced turbulent magnetic diffusivity within the disc. In all cases we have studied, the differential rotation increases the decay rate of non-axisymmetric modes, whereas axisymmetric ones are unaffected. A stronger magnetic diffusivity inside the disc does not lead to a significant preference for non-axisymmetric modes. Although Elsasser's antidynamo theorem has not yet been proved for the present case of a non-spherical distribution of the magnetic diffusivity, we do not find any evidence for the theorem not to be valid in general.