Bulletin of the Seismological Society of America, 1997
The Kozani earthquake (Ms = 6.6) of 13 May 1995 is the strongest event of the decade in Greece an... more The Kozani earthquake (Ms = 6.6) of 13 May 1995 is the strongest event of the decade in Greece and occurred in a region of low seismic activity. Using regional data and the strong-motion record at the Kozani station, we relocate the mainshock at 40.183° N and 21.660° E, beneath the Vourinos massif at a depth of 14.2 km. We also compute a focal mechanism by body-waveform modeling at teleseismic distance, which confirms a normal mechanism. The most likely plane strikes 240° ± 1° N and dips 40° ± 1° N with a centroid depth of 11 ± 1 km. Modeling of the strong-motion record at Kozani confirms that nucleation started at the eastern termination of the bottom of the fault.Six days after the mainshock, we installed a network of 40 portable seismological stations for one week around the epicentral region. Several thousand aftershocks were recorded, among which we locate 622 with a precision better than 1 km. We compute 181 focal mechanisms that mostly show normal faulting. The aftershock sei...
We present the results of a multidisciplinary study of the M s = 6.2, 1995, June 15, Aigion earth... more We present the results of a multidisciplinary study of the M s = 6.2, 1995, June 15, Aigion earthquake (Gulf of Corinth, Greece). In order to constrain the rupture geometry, we used all available data from seismology (local, regional and teleseismic records of the mainshock and of aftershocks), geodesy (GPS and SAR interferometry), and tectonics. Part of these data were obtained during a postseismic field study consisting of the surveying of 24 GPS points, the temporary installation of 20 digital seismometers, and a detailed field investigation for surface fault break. The Aigion fault was the only fault onland which showed detectable breaks (<4 cm). We relocated the mainshock hypocenter at 10 km in depth, 38 21.7 0 N, 22 12.0 0 E, about 15 km NNE to the damaged city
During summer of 1995 local seismicity was recorded in the area around the Gulf of Arta in northw... more During summer of 1995 local seismicity was recorded in the area around the Gulf of Arta in northwestern Greece by a dense temporary seismic network. Of the 441 local events observed at 37 stations, 232 well locatable events with a total of 2776 P-phase readings were selected applying the criteria of a minimum of 6 P-observations and an azimuthal gap less than 180º. This data set is used to compute a minimum 1D velocity model for the region. Several tests are conducted to estimate model stability and hypocenter uncertainties, leading to the conclusion that relative hypocenter location accuracy is about 500 m in latitude and longitude and 1 km in depth. The minimum 1D velocity model serves as initial model in the non-linear inversion for three-dimensional P-velocity crustal structure by iteratively solving the coupled hypocenter-velocity problem in a least-squares sense. Careful analysis of the resolution capability of our data set outlines the well resolved features for interpretation. The resulting 3D velocity model shows generally higher average crustal velocities in the east, and the well resolved area of the eastern Gulf of Arta exhibits a homogeneous velocity around 6 km=s for the whole upper crust. A pronounced north-south trending zone of low velocities in the upper 5-10 km is observed in the area of the Katouna fault zone (KFZ). At greater depths (below 10 km) the KFZ is underlain by high-velocity material. E-W profiles suggest a horst-graben structure associated with the KFZ.
Data from a large-scale experiment which took place in Greece during the period January-July 1997... more Data from a large-scale experiment which took place in Greece during the period January-July 1997 have been used to investigate the structure of the Aegean area using surface waves. During this experiment, 30 seismic broadband instruments were deployed throughout the whole Greek area. Additional data during the period 1996-2000 from other temporary networks have been included in the dataset. One hundred eighty-five events with magnitudes 4.0 V M w V 5.5 recorded by these stations have been collected and processed. The individual dispersion curves of the group velocity of Rayleigh waves for each source-station path have been calculated, producing more than 700 paths covering the studied region. These curves have been used to determine Rayleigh group velocity maps using a 2D-tomography method. On the basis of a regionalization of the dispersion measurements, local averaged dispersion curves have been obtained and non-linearly inverted to obtain models of shear-wave velocity versus depth. Since the dispersion curves in the period range 5 s V T V 30 s are mostly affected by the crustal structure, the model velocities are estimated down to a depth of approximately 35-45 km. The results from the non-linear Hedhehog inversion as applied to a few local dispersion curves show a crustal thickness of approximately 32 km for the Northern Aegean Sea, and a relatively thin crust of approximately 22-24 km for the Southern Aegean Sea.
We have carried out experiments using a layered medium of sand and silicone to investigate the la... more We have carried out experiments using a layered medium of sand and silicone to investigate the lateral extrusion of a material which spreads over its own weight while being compressed by the advance of a rigid indenter. Boundary conditions in the box mimic those prevailing in the Anatolian‐Aegean system. Both shortening in front of the rigid piston, which models the northward motion of Arabia, and extension resulting from the gravity spreading of the sand‐silicone layer are necessary to initiate the lateral extrusion. Strike‐slip faults accommodate the lateral escape and link the normal faults accompanying gravity spreading with the thrust faults in front of the rigid indenter. Strike‐slip faults begin to accommodate extrusion at a late stage in the experiments after the normal and thrust faults have developed. Experiments also show that the initial geometry of the boundary of the spreading layer may result in the formation of two arcs behind which material extends, in a manner anal...
ABSTRACT In 1994, a resurvey of the geodetic control network in the seismic zone of Volvi, in Nor... more ABSTRACT In 1994, a resurvey of the geodetic control network in the seismic zone of Volvi, in Northern Greece (Mygdonian graben), was carried out using GPS techniques. The coordinates of the network stations as computed during the 1994 campaign were then compared with the original coordinates of the same stations as determined in 1979 by triangulation. The comparison between the two sets of data shows that this part of the Mygdonian graben experienced a N-S extension of about 8cm between 1979 and 1994 (5mm/year of horizontal extension).
Journal of Geophysical Research: Solid Earth, 2000
Between 1990 and 1995, we carried out seven Global Positioning System (GPS) campaigns in the Cori... more Between 1990 and 1995, we carried out seven Global Positioning System (GPS) campaigns in the Corinth rift area in order to constrain the spatial and temporal crustal deformation of this active zone. The network, 193 points over ∼10,000 km2, samples most of the active faults. In order to estimate the deformation over a longer period, 159 of those points are also Greek triangulation pillars previously measured between 1966 and 1972. Two earthquakes of magnitude 6.2 and 5.9 have occurred in the network since it was installed. The extension rate deduced from the analysis of the different GPS data sets is 14±2 mm/yr oriented N9° in the west, 13±3 mm/yr oriented S‐N in the center, and 10±4 mm/yr oriented N19°W in the east of the gulf. The comparison between GPS and triangulation gives higher rates and less angular divergence (25±7 mm/yr, N4°E; 22±7 mm/yr, S‐N; 20±7 mm/yr, N15°W, respectively). Both sets of data indicate that the deforming zone is very narrow (10–15 km) in the west, might ...
We present a detailed seismological study of the Kozani earthquake. We relocate the mainshock wit... more We present a detailed seismological study of the Kozani earthquake. We relocate the mainshock with regional data at depth of 14.2 km beneath the Vourinos massif. We compute the focal mechanism by body waveform modeling at teleseismic distance and find a normal fault striking N240" and dipping 40" toward the NW with a centro'id depth of 11 km. We installed a dense network of portable seismographs around the epicentral region and located several hundreds of aftershocks. The main cluster of aftershock seismicity defines a plane dipping north at an angle of about 35", consistent with the main-shock mechanism, while some seismic activity is also seen on an antithetic fault. Our results suggest the active fault plane to be the Deskati fault which dips at a constant angle and therefore branches on the Paleohori fault where surface breaks were observed. We also compute 18 1 focal mechanisms which mostly show normal faulting.
The rate of crustal deformation in Iran due to the Arabia-Eurasia collision is estimated. The res... more The rate of crustal deformation in Iran due to the Arabia-Eurasia collision is estimated. The results are based on new global positioning system (GPS) data. In order to address the problem of the distribution of the deformation in Iran, Iranian and French research organizations have carried out the first large-scale GPS survey of Iran. A GPS network of 28 sites (25 in Iran, two in Oman and one in Uzbekistan) has been installed and surveyed twice, in September 1999 and October 2001. Each site has been surveyed for a minimum observation of 4 days. GPS data processing has been done using the GAMIT-GLOBK software package. The solution displays horizontal repeatabilities of about 1.2 mm in 1999 and 2001. The resulting velocities allow us to constrain the kinematics of the Iranian tectonic blocks. These velocities are given in ITRF2000 and also relative to Eurasia. This last kinematic model demonstrates that (1) the north-south shortening from Arabia to Eurasia is 2-2.5 cm/year, less than previously estimated, and (2) the transition from subduction (Makran) to collision (Zagros) is very sharp and governs the different styles of deformation observed in Iran. In the eastern part of Iran, most of the shortening is accommodated in the Gulf of Oman, while in the western part the shortening is more distributed from south to north. The large faults surrounding the Lut block accommodate most of the subduction-collision transition.
In November 1994, a geodetic network situated on the Mygdonian graben, 30 km NE of Thessaloniki, ... more In November 1994, a geodetic network situated on the Mygdonian graben, 30 km NE of Thessaloniki, Greece, was remeasured using the GPS technique. This network was established in 1979, in the epicentral area of the 1978 earthquake (Ms = 6.5), and was measured using triangulation techniques seven times between 1979 and 1989. The comparison between 1979 and 1994 data shows that this part of the Mygdonian graben experienced about 8 cm of N-S horizontal extension. Most of the extension is concentrated in a narrow (one or two kilometers) E-W zone, located on the southern boundary of the graben. Furthermore, the analysis of repeated triangulation surveys suggest•'a rate of extension of 5.7 + 1.3 mm/y, that remained constant with time between 1979 and 1994. This extension could result from long-term postseismic relaxation processes, or from continous aseismic slip within the graben.
The Alborz Mountains of northern Iran form a belt of active crustal deformation along the souther... more The Alborz Mountains of northern Iran form a belt of active crustal deformation along the southern side of the Caspian Sea within the broad Arabian-Eurasia continental collision zone. Although the range has an average elevation of about 3000 m with the volcanic peak Damavand reaching an elevations of 5671 m, early gravity studies found that the crust beneath the range is no thicker than that beneath the surrounding region suggesting the range is not supported by a crustal root. We determine a model for the crust of the central Alborz Mountains using teleseismic receiver functions from data recorded on a network of broad-band seismographs temporarily deployed across the central part of the range. The receiver functions from these recordings have been inverted simultaneously with fundamental-mode Rayleigh wave group velocity measurements in the 10-100 s period range. Our analysis shows a thickening of the crust from ∼48 km beneath the northern part of the Central Iranian Plateau to 55-58 km below the central part of the Alborz Mountains, then a thinning of the crust to ∼46 km north of the Alborz Mountains beneath the coastal region of the South Caspian Sea. Our seismological results show that the central Alborz Mountains have a moderate crustal root but of insufficient thickness to compensate the elevation of the range. The analysis of free-air gravity shows that the elevation of the Alborz Mountains is largely supported by the elastic strength of the Iranian Plate, the South Caspian Plate, or both.
was installed in the epicentral area of the 2003 December 26 Bam earthquake to study the aftersho... more was installed in the epicentral area of the 2003 December 26 Bam earthquake to study the aftershock seismicity. We select the 331 earthquakes recorded at a minimum of 10 stations, with rms less than 0.1 s and uncertainties less than 1 km, to infer the precise geometry of the seismicity in the fault region. We also process the data with the Double Difference technique to confirm the results. The aftershock cluster is 25 km long, trends north-south, and is located 5 km west of the Bam-Baravat escarpment, exactly beneath the observed surface breaks. At depth, aftershocks are concentrated between 6 and 20 km, beneath the upper layer of relatively low velocity that experienced the maximum slip, and they dip slightly westward. The southernmost part of the aftershock cluster is narrow and defines the rupture zone that is likely the Bam-Baravat fault at depth. However, it is unlikely that it is connected at surface to the Bam-Baravat escarpment but more likely to the co-seismic ruptures south of Bam. On the contrary, the distribution of the northernmost aftershocks spread into a more complex pattern, which is consistent with a northward propagation of the rupture along the fault plane. The focal mechanisms are consistent with right-lateral strike-slip faulting on N-S trending faults, parallel to the Bam-Baravat escarpment.
The El Asnam (Algeria) earthquake of 1980 October is the largest earthquake to have occurred in N... more The El Asnam (Algeria) earthquake of 1980 October is the largest earthquake to have occurred in North Africa since instrumental records began (M, = 7.3). It was caused by movement on a segmented reverse fault which is part of the fault zone that separates the Chelif alluvial plains from the coastal range of the Atlas Mountains. The coastal range is the actively deforming plate boundary between Africa and Eurasia. In this paper, the results of an aftershock study carried out using a 28-station portable network in the epicentral area in the 5 weeks after the main shock are presented. A total of 4517 aftershocks have been located, using both Pand S-phases. Of these 4517 events, 1279 pass a set of quality criteria, and are used to discuss the detailed tectonics of the aftershock zone. The southwestern half of the aftershock zone has a relatively restricted band of seismicity running parallel to the main fault. In cross-section, this seismicity is observed to lie almost exclusively in the footwall of the main fault, and indicates both antithetic reverse faulting and extension parallel to the fault zone. The only exception to this pattern occurs at a junction of the main fault segments, where a distinct cluster of events with very variable focal mechanisms is located in the hangingwall. This cluster may be equated with a barrier that impeded the rupture of the main shock. Beyond the southwest end of the main fault, a mixture of strike-slip and reverse faulting occurred, and suggests a mechanism whereby this end of the fault can act as an asperity for earthquake generation. Aftershocks in the northeastern half of the study area indicate movement on a stack of listric reverse faults. The focal mechanisms of these events show that the N-dipping nodal planes (assumed to be the fault planes) flatten with depth. We suggest that the faults sole into a low-angle (20') decollement at about 8-10km depth, at the base of the aftershock zone. We argue that the northeastern part of the 1980 aftershock zone is typical of the actively deforming coastal belt of northwest Algeria. Subduction of the western Mediterranean basin beneath the north African margin does not seem to occur. Instead, the margin is absorbing the motion, with old normal faults being reactivated as thrusts.
A microearthquake survey was conducted in the central Andes of Peru, east of the city of Lima, to... more A microearthquake survey was conducted in the central Andes of Peru, east of the city of Lima, to study the seismicity and style of tectonic deformation of the Peruvian Andes. Although most of the stations forming the temporary seismographic network were located on the high Andes, the vast majority of the
SummaryThe nature of the transition between the Zagros intra-continental collision and the Makran... more SummaryThe nature of the transition between the Zagros intra-continental collision and the Makran oceanic subduction is a matter of debate: either a major fault cutting the whole lithosphere or a more progressive transition associated with a shallow gently dipping fault restricted to the crust. Microearthquake seismicity located around the transition between the transition zone is restricted to the west of the Jaz-Murian depression and the Jiroft fault. No shallow micro-earthquakes seem to be related to the NNW-SSE trending Zendan-Minab-Palami active fault system. Most of the shallow seismicity is related either to the Zagros mountain belt, located in the west, or to the NS trending Sabzevaran-Jiroft fault system, located in the north. The depth of microearthquakes increases northeastwards to an unusually deep value (for the Zagros) of 40 km. Two dominant types of focal mechanisms are observed in this region: low-angle thrust faulting, mostly restricted to the lower crust, and strik...
The catastrophic 2003 M w 6.6 Bam earthquake in southern Iran attracted much attention, and has b... more The catastrophic 2003 M w 6.6 Bam earthquake in southern Iran attracted much attention, and has been studied with an abundance of observations from synthetic aperture radar, teleseismic seismology, aftershock studies, strong ground motion, geomorphology, remote sensing and surface field work. Many reports have focused on the details of one or other data type, producing interpretations that either conflict with other data or leave questions unanswered. This paper is an attempt to look at all the available data types together, to produce a coherent picture of the coseismic faulting in 2003 and to examine its consequences for active tectonics and continuing seismic hazard in the region. We conclude that more than 80 per cent of the moment release in the main shock occurred on a near-vertical right-lateral strike-slip fault extending from the city of Bam southwards for about 15 km, with slip of up to 2 m but mostly restricted to the depth range 2-7 km. Analysis of the strong ground motion record at Bam is consistent with this view, and indicates that the extreme damage in the city can be attributed, at least in part, to the enhancement of ground motion in Bam because of its position at the end of the northward-propagating rupture. Little of the slip in the main shock reached the Earth's surface and, more importantly, aftershocks reveal that ∼12 km vertical extent of a deeper part of the fault system remained unruptured beneath the coseismic rupture plane, at depths of 8-20 km. This may represent a substantial remaining seismic hazard to the reconstructed city of Bam. We believe that some oblique-reverse slip (up to 2 m, and less than 20 per cent of the released seismic moment) occurred at a restricted depth of 5-7 km on a blind west-dipping fault that projects to the surface at the Bam-Baravat escarpment, an asymmetric anticline ridge that is the most prominent geomorphological feature in the area. This fault did not rupture significantly at shallow levels in 2003, and it may also represent a continuing seismic hazard. Widespread distributed surface ruptures north of the city are apparently unrelated to substantial slip at depth, and may be the result of enhanced ground motion related to northward propagation of the rupture. The faulting at Bam may be in the early stages of a spatial separation ('partitioning') between the reverse and strike-slip components of an oblique convergence across the zone. Such a separation is common on the continents, though in this case the slip vectors between the two faults differ only by ∼20 • as a substantial strike-slip component remains on the obliquereverse fault. The Bam earthquake is one in a series of large earthquakes involving faulting along the western edge of the Lut desert. In addition to the unruptured parts of the faults near Bam itself, continuing and substantial hazard is represented by unruptured neighbouring faults, particularly blind thrusts along the Jebel Barez mountains to the south and strike-slip faulting at Sarvestan to the west.
Surface wave dispersion measurements are interpreted jointly with the inversion of teleseismic P-... more Surface wave dispersion measurements are interpreted jointly with the inversion of teleseismic P-wave traveltime residuals along a dense 620-km long temporary seismic profile across the Zagros to investigate its upper-mantle structure. The S-wave model determined from Rayleigh wave dispersion in the Zagros fold and thrust belt has high velocities from 4.5 ± 0.2 km s −1 below the Moho to 4.9 ± 0.25 km s −1 at 200 km depth, which are comparable to a shield-like structure. Beneath the suture region from the Main Zagros Thrust (MZT) to the Urumieh-Dokhtar volcanic arc, S-wave velocities are lower than beneath the Zagros in the top 50 km of the upper mantle, with a minimum of 4.4 ± 0.2 km s −1 at 80 km depth. From 150 km and deeper, S velocities are as high as beneath the Zagros. We suggest that part of the velocity difference at shallow depth is due to higher mantle temperatures and/or higher fluid content beneath the northern half of the profile, but that velocities are too high to support the hypothesis of mantle lid delamination under the suture zone. Teleseismic P traveltime relative residuals display a long-wavelength variation along the transect, with a difference of 1.1 s between negative residuals in the Zagros Simple Folded Belt and positive residuals in Central Iran. This difference backprojects into a 6-7 per cent lateral variation of P-wave velocity in the shallow upper mantle, with higher V P beneath Zagros and lower V P beneath Central Iran. The main short wavelength variation of the residual is located in the suture region, with late P arrivals in the region of the MZT and early arrivals in the Sanandaj-Sirjan zone (SSZ). Using synthetic models of V P perturbations, we show that the high velocities of the Arabian platform have to extend laterally at least to the SSZ to fit the observed P delays. This model also predicts Rayleigh wave phase velocities, which are within the error bars of the observed dispersion. It supports the model of crustal-scale overthrusting at the MZT.
Bulletin of the Seismological Society of America, 1997
The Kozani earthquake (Ms = 6.6) of 13 May 1995 is the strongest event of the decade in Greece an... more The Kozani earthquake (Ms = 6.6) of 13 May 1995 is the strongest event of the decade in Greece and occurred in a region of low seismic activity. Using regional data and the strong-motion record at the Kozani station, we relocate the mainshock at 40.183° N and 21.660° E, beneath the Vourinos massif at a depth of 14.2 km. We also compute a focal mechanism by body-waveform modeling at teleseismic distance, which confirms a normal mechanism. The most likely plane strikes 240° ± 1° N and dips 40° ± 1° N with a centroid depth of 11 ± 1 km. Modeling of the strong-motion record at Kozani confirms that nucleation started at the eastern termination of the bottom of the fault.Six days after the mainshock, we installed a network of 40 portable seismological stations for one week around the epicentral region. Several thousand aftershocks were recorded, among which we locate 622 with a precision better than 1 km. We compute 181 focal mechanisms that mostly show normal faulting. The aftershock sei...
We present the results of a multidisciplinary study of the M s = 6.2, 1995, June 15, Aigion earth... more We present the results of a multidisciplinary study of the M s = 6.2, 1995, June 15, Aigion earthquake (Gulf of Corinth, Greece). In order to constrain the rupture geometry, we used all available data from seismology (local, regional and teleseismic records of the mainshock and of aftershocks), geodesy (GPS and SAR interferometry), and tectonics. Part of these data were obtained during a postseismic field study consisting of the surveying of 24 GPS points, the temporary installation of 20 digital seismometers, and a detailed field investigation for surface fault break. The Aigion fault was the only fault onland which showed detectable breaks (<4 cm). We relocated the mainshock hypocenter at 10 km in depth, 38 21.7 0 N, 22 12.0 0 E, about 15 km NNE to the damaged city
During summer of 1995 local seismicity was recorded in the area around the Gulf of Arta in northw... more During summer of 1995 local seismicity was recorded in the area around the Gulf of Arta in northwestern Greece by a dense temporary seismic network. Of the 441 local events observed at 37 stations, 232 well locatable events with a total of 2776 P-phase readings were selected applying the criteria of a minimum of 6 P-observations and an azimuthal gap less than 180º. This data set is used to compute a minimum 1D velocity model for the region. Several tests are conducted to estimate model stability and hypocenter uncertainties, leading to the conclusion that relative hypocenter location accuracy is about 500 m in latitude and longitude and 1 km in depth. The minimum 1D velocity model serves as initial model in the non-linear inversion for three-dimensional P-velocity crustal structure by iteratively solving the coupled hypocenter-velocity problem in a least-squares sense. Careful analysis of the resolution capability of our data set outlines the well resolved features for interpretation. The resulting 3D velocity model shows generally higher average crustal velocities in the east, and the well resolved area of the eastern Gulf of Arta exhibits a homogeneous velocity around 6 km=s for the whole upper crust. A pronounced north-south trending zone of low velocities in the upper 5-10 km is observed in the area of the Katouna fault zone (KFZ). At greater depths (below 10 km) the KFZ is underlain by high-velocity material. E-W profiles suggest a horst-graben structure associated with the KFZ.
Data from a large-scale experiment which took place in Greece during the period January-July 1997... more Data from a large-scale experiment which took place in Greece during the period January-July 1997 have been used to investigate the structure of the Aegean area using surface waves. During this experiment, 30 seismic broadband instruments were deployed throughout the whole Greek area. Additional data during the period 1996-2000 from other temporary networks have been included in the dataset. One hundred eighty-five events with magnitudes 4.0 V M w V 5.5 recorded by these stations have been collected and processed. The individual dispersion curves of the group velocity of Rayleigh waves for each source-station path have been calculated, producing more than 700 paths covering the studied region. These curves have been used to determine Rayleigh group velocity maps using a 2D-tomography method. On the basis of a regionalization of the dispersion measurements, local averaged dispersion curves have been obtained and non-linearly inverted to obtain models of shear-wave velocity versus depth. Since the dispersion curves in the period range 5 s V T V 30 s are mostly affected by the crustal structure, the model velocities are estimated down to a depth of approximately 35-45 km. The results from the non-linear Hedhehog inversion as applied to a few local dispersion curves show a crustal thickness of approximately 32 km for the Northern Aegean Sea, and a relatively thin crust of approximately 22-24 km for the Southern Aegean Sea.
We have carried out experiments using a layered medium of sand and silicone to investigate the la... more We have carried out experiments using a layered medium of sand and silicone to investigate the lateral extrusion of a material which spreads over its own weight while being compressed by the advance of a rigid indenter. Boundary conditions in the box mimic those prevailing in the Anatolian‐Aegean system. Both shortening in front of the rigid piston, which models the northward motion of Arabia, and extension resulting from the gravity spreading of the sand‐silicone layer are necessary to initiate the lateral extrusion. Strike‐slip faults accommodate the lateral escape and link the normal faults accompanying gravity spreading with the thrust faults in front of the rigid indenter. Strike‐slip faults begin to accommodate extrusion at a late stage in the experiments after the normal and thrust faults have developed. Experiments also show that the initial geometry of the boundary of the spreading layer may result in the formation of two arcs behind which material extends, in a manner anal...
ABSTRACT In 1994, a resurvey of the geodetic control network in the seismic zone of Volvi, in Nor... more ABSTRACT In 1994, a resurvey of the geodetic control network in the seismic zone of Volvi, in Northern Greece (Mygdonian graben), was carried out using GPS techniques. The coordinates of the network stations as computed during the 1994 campaign were then compared with the original coordinates of the same stations as determined in 1979 by triangulation. The comparison between the two sets of data shows that this part of the Mygdonian graben experienced a N-S extension of about 8cm between 1979 and 1994 (5mm/year of horizontal extension).
Journal of Geophysical Research: Solid Earth, 2000
Between 1990 and 1995, we carried out seven Global Positioning System (GPS) campaigns in the Cori... more Between 1990 and 1995, we carried out seven Global Positioning System (GPS) campaigns in the Corinth rift area in order to constrain the spatial and temporal crustal deformation of this active zone. The network, 193 points over ∼10,000 km2, samples most of the active faults. In order to estimate the deformation over a longer period, 159 of those points are also Greek triangulation pillars previously measured between 1966 and 1972. Two earthquakes of magnitude 6.2 and 5.9 have occurred in the network since it was installed. The extension rate deduced from the analysis of the different GPS data sets is 14±2 mm/yr oriented N9° in the west, 13±3 mm/yr oriented S‐N in the center, and 10±4 mm/yr oriented N19°W in the east of the gulf. The comparison between GPS and triangulation gives higher rates and less angular divergence (25±7 mm/yr, N4°E; 22±7 mm/yr, S‐N; 20±7 mm/yr, N15°W, respectively). Both sets of data indicate that the deforming zone is very narrow (10–15 km) in the west, might ...
We present a detailed seismological study of the Kozani earthquake. We relocate the mainshock wit... more We present a detailed seismological study of the Kozani earthquake. We relocate the mainshock with regional data at depth of 14.2 km beneath the Vourinos massif. We compute the focal mechanism by body waveform modeling at teleseismic distance and find a normal fault striking N240" and dipping 40" toward the NW with a centro'id depth of 11 km. We installed a dense network of portable seismographs around the epicentral region and located several hundreds of aftershocks. The main cluster of aftershock seismicity defines a plane dipping north at an angle of about 35", consistent with the main-shock mechanism, while some seismic activity is also seen on an antithetic fault. Our results suggest the active fault plane to be the Deskati fault which dips at a constant angle and therefore branches on the Paleohori fault where surface breaks were observed. We also compute 18 1 focal mechanisms which mostly show normal faulting.
The rate of crustal deformation in Iran due to the Arabia-Eurasia collision is estimated. The res... more The rate of crustal deformation in Iran due to the Arabia-Eurasia collision is estimated. The results are based on new global positioning system (GPS) data. In order to address the problem of the distribution of the deformation in Iran, Iranian and French research organizations have carried out the first large-scale GPS survey of Iran. A GPS network of 28 sites (25 in Iran, two in Oman and one in Uzbekistan) has been installed and surveyed twice, in September 1999 and October 2001. Each site has been surveyed for a minimum observation of 4 days. GPS data processing has been done using the GAMIT-GLOBK software package. The solution displays horizontal repeatabilities of about 1.2 mm in 1999 and 2001. The resulting velocities allow us to constrain the kinematics of the Iranian tectonic blocks. These velocities are given in ITRF2000 and also relative to Eurasia. This last kinematic model demonstrates that (1) the north-south shortening from Arabia to Eurasia is 2-2.5 cm/year, less than previously estimated, and (2) the transition from subduction (Makran) to collision (Zagros) is very sharp and governs the different styles of deformation observed in Iran. In the eastern part of Iran, most of the shortening is accommodated in the Gulf of Oman, while in the western part the shortening is more distributed from south to north. The large faults surrounding the Lut block accommodate most of the subduction-collision transition.
In November 1994, a geodetic network situated on the Mygdonian graben, 30 km NE of Thessaloniki, ... more In November 1994, a geodetic network situated on the Mygdonian graben, 30 km NE of Thessaloniki, Greece, was remeasured using the GPS technique. This network was established in 1979, in the epicentral area of the 1978 earthquake (Ms = 6.5), and was measured using triangulation techniques seven times between 1979 and 1989. The comparison between 1979 and 1994 data shows that this part of the Mygdonian graben experienced about 8 cm of N-S horizontal extension. Most of the extension is concentrated in a narrow (one or two kilometers) E-W zone, located on the southern boundary of the graben. Furthermore, the analysis of repeated triangulation surveys suggest•'a rate of extension of 5.7 + 1.3 mm/y, that remained constant with time between 1979 and 1994. This extension could result from long-term postseismic relaxation processes, or from continous aseismic slip within the graben.
The Alborz Mountains of northern Iran form a belt of active crustal deformation along the souther... more The Alborz Mountains of northern Iran form a belt of active crustal deformation along the southern side of the Caspian Sea within the broad Arabian-Eurasia continental collision zone. Although the range has an average elevation of about 3000 m with the volcanic peak Damavand reaching an elevations of 5671 m, early gravity studies found that the crust beneath the range is no thicker than that beneath the surrounding region suggesting the range is not supported by a crustal root. We determine a model for the crust of the central Alborz Mountains using teleseismic receiver functions from data recorded on a network of broad-band seismographs temporarily deployed across the central part of the range. The receiver functions from these recordings have been inverted simultaneously with fundamental-mode Rayleigh wave group velocity measurements in the 10-100 s period range. Our analysis shows a thickening of the crust from ∼48 km beneath the northern part of the Central Iranian Plateau to 55-58 km below the central part of the Alborz Mountains, then a thinning of the crust to ∼46 km north of the Alborz Mountains beneath the coastal region of the South Caspian Sea. Our seismological results show that the central Alborz Mountains have a moderate crustal root but of insufficient thickness to compensate the elevation of the range. The analysis of free-air gravity shows that the elevation of the Alborz Mountains is largely supported by the elastic strength of the Iranian Plate, the South Caspian Plate, or both.
was installed in the epicentral area of the 2003 December 26 Bam earthquake to study the aftersho... more was installed in the epicentral area of the 2003 December 26 Bam earthquake to study the aftershock seismicity. We select the 331 earthquakes recorded at a minimum of 10 stations, with rms less than 0.1 s and uncertainties less than 1 km, to infer the precise geometry of the seismicity in the fault region. We also process the data with the Double Difference technique to confirm the results. The aftershock cluster is 25 km long, trends north-south, and is located 5 km west of the Bam-Baravat escarpment, exactly beneath the observed surface breaks. At depth, aftershocks are concentrated between 6 and 20 km, beneath the upper layer of relatively low velocity that experienced the maximum slip, and they dip slightly westward. The southernmost part of the aftershock cluster is narrow and defines the rupture zone that is likely the Bam-Baravat fault at depth. However, it is unlikely that it is connected at surface to the Bam-Baravat escarpment but more likely to the co-seismic ruptures south of Bam. On the contrary, the distribution of the northernmost aftershocks spread into a more complex pattern, which is consistent with a northward propagation of the rupture along the fault plane. The focal mechanisms are consistent with right-lateral strike-slip faulting on N-S trending faults, parallel to the Bam-Baravat escarpment.
The El Asnam (Algeria) earthquake of 1980 October is the largest earthquake to have occurred in N... more The El Asnam (Algeria) earthquake of 1980 October is the largest earthquake to have occurred in North Africa since instrumental records began (M, = 7.3). It was caused by movement on a segmented reverse fault which is part of the fault zone that separates the Chelif alluvial plains from the coastal range of the Atlas Mountains. The coastal range is the actively deforming plate boundary between Africa and Eurasia. In this paper, the results of an aftershock study carried out using a 28-station portable network in the epicentral area in the 5 weeks after the main shock are presented. A total of 4517 aftershocks have been located, using both Pand S-phases. Of these 4517 events, 1279 pass a set of quality criteria, and are used to discuss the detailed tectonics of the aftershock zone. The southwestern half of the aftershock zone has a relatively restricted band of seismicity running parallel to the main fault. In cross-section, this seismicity is observed to lie almost exclusively in the footwall of the main fault, and indicates both antithetic reverse faulting and extension parallel to the fault zone. The only exception to this pattern occurs at a junction of the main fault segments, where a distinct cluster of events with very variable focal mechanisms is located in the hangingwall. This cluster may be equated with a barrier that impeded the rupture of the main shock. Beyond the southwest end of the main fault, a mixture of strike-slip and reverse faulting occurred, and suggests a mechanism whereby this end of the fault can act as an asperity for earthquake generation. Aftershocks in the northeastern half of the study area indicate movement on a stack of listric reverse faults. The focal mechanisms of these events show that the N-dipping nodal planes (assumed to be the fault planes) flatten with depth. We suggest that the faults sole into a low-angle (20') decollement at about 8-10km depth, at the base of the aftershock zone. We argue that the northeastern part of the 1980 aftershock zone is typical of the actively deforming coastal belt of northwest Algeria. Subduction of the western Mediterranean basin beneath the north African margin does not seem to occur. Instead, the margin is absorbing the motion, with old normal faults being reactivated as thrusts.
A microearthquake survey was conducted in the central Andes of Peru, east of the city of Lima, to... more A microearthquake survey was conducted in the central Andes of Peru, east of the city of Lima, to study the seismicity and style of tectonic deformation of the Peruvian Andes. Although most of the stations forming the temporary seismographic network were located on the high Andes, the vast majority of the
SummaryThe nature of the transition between the Zagros intra-continental collision and the Makran... more SummaryThe nature of the transition between the Zagros intra-continental collision and the Makran oceanic subduction is a matter of debate: either a major fault cutting the whole lithosphere or a more progressive transition associated with a shallow gently dipping fault restricted to the crust. Microearthquake seismicity located around the transition between the transition zone is restricted to the west of the Jaz-Murian depression and the Jiroft fault. No shallow micro-earthquakes seem to be related to the NNW-SSE trending Zendan-Minab-Palami active fault system. Most of the shallow seismicity is related either to the Zagros mountain belt, located in the west, or to the NS trending Sabzevaran-Jiroft fault system, located in the north. The depth of microearthquakes increases northeastwards to an unusually deep value (for the Zagros) of 40 km. Two dominant types of focal mechanisms are observed in this region: low-angle thrust faulting, mostly restricted to the lower crust, and strik...
The catastrophic 2003 M w 6.6 Bam earthquake in southern Iran attracted much attention, and has b... more The catastrophic 2003 M w 6.6 Bam earthquake in southern Iran attracted much attention, and has been studied with an abundance of observations from synthetic aperture radar, teleseismic seismology, aftershock studies, strong ground motion, geomorphology, remote sensing and surface field work. Many reports have focused on the details of one or other data type, producing interpretations that either conflict with other data or leave questions unanswered. This paper is an attempt to look at all the available data types together, to produce a coherent picture of the coseismic faulting in 2003 and to examine its consequences for active tectonics and continuing seismic hazard in the region. We conclude that more than 80 per cent of the moment release in the main shock occurred on a near-vertical right-lateral strike-slip fault extending from the city of Bam southwards for about 15 km, with slip of up to 2 m but mostly restricted to the depth range 2-7 km. Analysis of the strong ground motion record at Bam is consistent with this view, and indicates that the extreme damage in the city can be attributed, at least in part, to the enhancement of ground motion in Bam because of its position at the end of the northward-propagating rupture. Little of the slip in the main shock reached the Earth's surface and, more importantly, aftershocks reveal that ∼12 km vertical extent of a deeper part of the fault system remained unruptured beneath the coseismic rupture plane, at depths of 8-20 km. This may represent a substantial remaining seismic hazard to the reconstructed city of Bam. We believe that some oblique-reverse slip (up to 2 m, and less than 20 per cent of the released seismic moment) occurred at a restricted depth of 5-7 km on a blind west-dipping fault that projects to the surface at the Bam-Baravat escarpment, an asymmetric anticline ridge that is the most prominent geomorphological feature in the area. This fault did not rupture significantly at shallow levels in 2003, and it may also represent a continuing seismic hazard. Widespread distributed surface ruptures north of the city are apparently unrelated to substantial slip at depth, and may be the result of enhanced ground motion related to northward propagation of the rupture. The faulting at Bam may be in the early stages of a spatial separation ('partitioning') between the reverse and strike-slip components of an oblique convergence across the zone. Such a separation is common on the continents, though in this case the slip vectors between the two faults differ only by ∼20 • as a substantial strike-slip component remains on the obliquereverse fault. The Bam earthquake is one in a series of large earthquakes involving faulting along the western edge of the Lut desert. In addition to the unruptured parts of the faults near Bam itself, continuing and substantial hazard is represented by unruptured neighbouring faults, particularly blind thrusts along the Jebel Barez mountains to the south and strike-slip faulting at Sarvestan to the west.
Surface wave dispersion measurements are interpreted jointly with the inversion of teleseismic P-... more Surface wave dispersion measurements are interpreted jointly with the inversion of teleseismic P-wave traveltime residuals along a dense 620-km long temporary seismic profile across the Zagros to investigate its upper-mantle structure. The S-wave model determined from Rayleigh wave dispersion in the Zagros fold and thrust belt has high velocities from 4.5 ± 0.2 km s −1 below the Moho to 4.9 ± 0.25 km s −1 at 200 km depth, which are comparable to a shield-like structure. Beneath the suture region from the Main Zagros Thrust (MZT) to the Urumieh-Dokhtar volcanic arc, S-wave velocities are lower than beneath the Zagros in the top 50 km of the upper mantle, with a minimum of 4.4 ± 0.2 km s −1 at 80 km depth. From 150 km and deeper, S velocities are as high as beneath the Zagros. We suggest that part of the velocity difference at shallow depth is due to higher mantle temperatures and/or higher fluid content beneath the northern half of the profile, but that velocities are too high to support the hypothesis of mantle lid delamination under the suture zone. Teleseismic P traveltime relative residuals display a long-wavelength variation along the transect, with a difference of 1.1 s between negative residuals in the Zagros Simple Folded Belt and positive residuals in Central Iran. This difference backprojects into a 6-7 per cent lateral variation of P-wave velocity in the shallow upper mantle, with higher V P beneath Zagros and lower V P beneath Central Iran. The main short wavelength variation of the residual is located in the suture region, with late P arrivals in the region of the MZT and early arrivals in the Sanandaj-Sirjan zone (SSZ). Using synthetic models of V P perturbations, we show that the high velocities of the Arabian platform have to extend laterally at least to the SSZ to fit the observed P delays. This model also predicts Rayleigh wave phase velocities, which are within the error bars of the observed dispersion. It supports the model of crustal-scale overthrusting at the MZT.
Uploads
Papers by D. Hatzfeld