- GEOPHYSICAL LABORATORY
ARISTOTLE UNIV. THESSALONIKI
PO BOX 352-1, GR-54124,
THESSALONIKI, GREECE - +30 2310998510
C. Papazachos
Aristotle University of Thessaloniki, Geology, Faculty Member
Research Interests:
Research Interests:
Research Interests:
An alternative method is proposed for the robust estimation of a and b values of the Gutenberg-Richter relation. The main hypothesis is that b values depend on material properties and the seismotectonic setting and therefore should vary... more
An alternative method is proposed for the robust estimation of a and b values of the Gutenberg-Richter relation. The main hypothesis is that b values depend on material properties and the seismotectonic setting and therefore should vary relatively smoothly in space. As far as the a values are concerned, more sharp variations are allowed because these values determine the seismicity level, once the b value is fixed. The study area is organized into a grid, and the a and b values are simultaneously determined for the whole grid by solving an appropriate linear system. Smooth b variations are imposed by introducing additional linear constraints, similar to the Occam's inversion used in tomography studies. The method is applied to Greece and the surrounding area, which is a high seismicity area. The results are in very good agreement with previous studies and further enhance our knowledge for the study area. Moreover, additional seismicity measures (return periods, probabilities, et...
Research Interests:
Within the framework of this study the complicated fault system of Western Crete was napped in detail and its kinematic and dynamic setting was analysed in order to distinguish 13 major active and possible active fault zones, the seismic... more
Within the framework of this study the complicated fault system of Western Crete was napped in detail and its kinematic and dynamic setting was analysed in order to distinguish 13 major active and possible active fault zones, the seismic potential of which was assessed. Moreover, kinematic data and striations were used to estimate the corresponding stress field geometry. Two stress phases were recognized: 1st the N-S extension phase (D1) in Mid-Upper Miocene to Lower Pliocene times forming E-W normal faults that bound the Neogene basins; 2nd the E-W extension phase (D2) in Late Pliocene-recent times forming N-S trending active normal faults. Smaller, mainly NE-SW trending faults, with significant strike-slip component, indicate a kinematic compatibility to the D2 phase, acting as transfer faults between larger N-S fault zones. The faults were incorporated in a detailed seismic hazard analysis together with the available seismological data, involving both probabilistic and determinis...
Research Interests: Geology and Seismology
In the present work, a combination of single station and multi-sensors array measurements of ambient noise have been performed in the urban area of Chania and its southern basin, in order to determine the main subsurface geophysical model... more
In the present work, a combination of single station and multi-sensors array measurements of ambient noise have been performed in the urban area of Chania and its southern basin, in order to determine the main subsurface geophysical model properties and assess the local site-effect on seismic motions. Furthermore, Horizontal to Vertical Spectral Ratio on Earthquake recordings (HVSRE) has been employed to validate the ambient noise results. Simulation of ambient seismic noise has been also used in an effort to cross-validate the experimental methods applied. The results indicate a highly-varying subsurface model due to the complex geological setting. Horizontal to Vertical Spectral Ratio (HVSR) reveals a low frequency resonance peak associated with the seismic contrast of the overall Quaternary/Neogene sediment cover with the underlying bedrock. A second higher HVSR frequency peak, usually disregarded in standard HVSR analysis, is observed for most HVSR data. This peak is found to be associated with the strong velocity contrast of the low velocity Quaternary sediments and the underlying stiffer Neogene deposits, using both empirical correlations and appropriate modelling. The available array measurements verify the clear separation of these Quaternary deposits and the underlying Neogene sediments in the southern Chania basin, as well as the more loose surficial Neogene formations and the underlying stiffer layers within the main city complex. Simulated ambient noise signals processed with the same HVSR settings verify the findings of the experimental approaches. Earthquake recordings processed with the HVSRE method show a good agreement with the expected amplification pattern of the final geological-geophysical structural model of the broader Chania area.
Research Interests:
Research Interests:
ABSTRACT A study of the microearthquake activity observed at the Kaniani-Parnassos mines has performed in order to determine the origin of the several "unknown" shocks observed at the area of the mine. Preliminary... more
ABSTRACT A study of the microearthquake activity observed at the Kaniani-Parnassos mines has performed in order to determine the origin of the several "unknown" shocks observed at the area of the mine. Preliminary analysis of the recordings indicated that the shocks were of tectonic origin. A portable network of three accelerographs (located inside the mine -no absolute time) and five three-component digital seismographs (located in surface locations on top of the mine -absolute time available) were installed for a period of nine months. The analysis of impulsive, high frequency (>20 Hz), short duration (2-3 sec) recordings, showed that the observed seismicity is of very local scale, located at a specific section of the mine. The microseismic activity exhibited a very strong time-correlation with the exploitation level, as this is revealed from the very significant variation of the earthquake rate of occurrence after the start of the operation of the mine, as well as the clear coupling on the rate of microearthquake occurrence with the rate of mining explosions. The focal mechanisms of the ruptures and the local stress field were evaluated using the first arrivals of the P-waves along with the P/S spectral ratios. The focal mechanisms are all of thrust type, indicating reverse ruptures caused by a compressional stress field with ESE-WSW direction. This field is almost perpendicular to the strike of the local old syncline-anticline, where the mine deposit is located; hence it can be safely assumed that is the "remnant" stress field that created this compressional mega-structure. The determined ruptures are in excellent agreement with the main tectonic discontinuities mapped inside the mine. A typical dynamic stress drop of 10-20 bars was also calculated for the biggest events (M~1) which is relatively comparable with the typical expected value of the mean effective isotropic stress (~50 bars).
A new model is derived for the P and S velocity structure of the lithosphere continen- tal collision area of the Dalmatian, Albania and NW Greece coasts by the inversion of travel times of local events. The inversion technique applied is... more
A new model is derived for the P and S velocity structure of the lithosphere continen- tal collision area of the Dalmatian, Albania and NW Greece coasts by the inversion of travel times of local events. The inversion technique applied is non-linear with appro- priate three-dimensional ray tracing and preconditioning in order to reduce ray density effects on the results.
Research Interests:
Research Interests:
... The earthquakes of Greece. Ziti Publ., Thessalo-niki, Greece, 317 pp. Papazachos, BC, Kiratzi, AA and Karakostas, VG (1997). ... J. Geophys. Res., 99, 20,203–20,233. Zanchi, Α., Garzanti, E., Larghi, C., Angiolini, L. and Gaetani, M.... more
... The earthquakes of Greece. Ziti Publ., Thessalo-niki, Greece, 317 pp. Papazachos, BC, Kiratzi, AA and Karakostas, VG (1997). ... J. Geophys. Res., 99, 20,203–20,233. Zanchi, Α., Garzanti, E., Larghi, C., Angiolini, L. and Gaetani, M. (2003). ...
Research Interests:
ABSTRACT In this work a new algorithm for the efficient and fast two dimensional (2D) inversion of long Electrical Resistivity Tomography (ERT) sections is introduced. The algorithm is based on 2.5D Finite Element Method (FEM) scheme to... more
ABSTRACT In this work a new algorithm for the efficient and fast two dimensional (2D) inversion of long Electrical Resistivity Tomography (ERT) sections is introduced. The algorithm is based on 2.5D Finite Element Method (FEM) scheme to solve the Poisson’s equation that describes the current flow into the earth’s subsurface. The adjoint equation technique was incorporated into the FEM framework to estimate the sensitivity values. The reconstructed 2D resistivity models are recovered through an iterative, non-linear smoothness constrained least-squares approach. The algorithm further incorporates an experimental procedure to avoid the calculation and storage of the entire Jacobian matrix. The basic concept of this new algorithm relies on the fact that for every measurement there is a number of model parameters which are located in parts of the 2-D model at distant locations from potential and current electrodes. The corresponding absolute Jacobian matrix values in such cases are very small (almost zero) and can be omitted by the Jacobian calculation. Around every measurement a fixed rectangular threshold region is defined a-priori based on geometrical criteria. The algorithm calculates only Jacobian matrix values for the model parameters that are included in this threshold area omitting the calculation of the Jacobian entries related to model parameters outside this region. This approach speeds up the Jacobian matrix calculations while the efficient storage of the sparse Jacobian and Smoothness matrices and the inversion using an iterative routine like LSQR method increase significantly the inversion speed and reduce the memory requirements. The new algorithm is almost more than one order of magnitude (~ 30 times) faster and consumes one order of magnitude (~ 90%) less storage memory than the original one based on full Jacobian calculations for typical applications. The application of the new algorithm to synthetic and real data sets shows that the reconstructed models exhibit comparable accuracy to the standard inversion approach.
Research Interests:
Research Interests:
Research Interests:
The use of 1D or pseudo- 3D ray tracing techniques in linearized tomographic problems leads to solutions for which it is difficult to assess the true resolution and error distribution. For this reason, we employ a revised 3D bending... more
The use of 1D or pseudo- 3D ray tracing techniques in linearized tomographic problems leads to solutions for which it is difficult to assess the true resolution and error distribution. For this reason, we employ a revised 3D bending algorithm (Moser et al., 1992) and show that it can be used efficiently for a non-linear inversion in a stepwise scheme. Initial paths are determined from graph theory in order to avoid local minima in bending. The importance of 3D ray tracing in inversion studies and the limitations of the standard 1D approach are demonstrated through synthetic examples. The speed of the ray tracing and the simple scaling scheme allow for an implementation in large-scale tomographic problems.
Research Interests:
We examine the behavior of the intermediate magnitude preshock activity in the fault region of a mainshock during the critical period, that is, in the period when accelerating seismic activity is observed in a broader region (critical... more
We examine the behavior of the intermediate magnitude preshock activity in the fault region of a mainshock during the critical period, that is, in the period when accelerating seismic activity is observed in a broader region (critical region). For this purpose, data concerning 10 recent strong mainshocks (M=6.6-8.3) occurred since 1981 in Greece, Anatolia, Japan and California are used. It is observed that during the critical preshock period, decelerating seismic crustal deformation (Benioff strain) is observed in the fault region (within the fault zone and its close vicinity), whereas accelerating deformation is observed in the broader (critical) region. The dimension of the fault region where decelerating deformation is observed scales positively with the mainshock magnitude and negatively with the mean seismicity rate of this region. The duration of this decelerating deformation scales also negatively with the mean seismicity rate. The physical explanation and importance of these...
Research Interests: Geology and Seismology
Research Interests:
Research Interests:
Research Interests:
Research Interests:
The present work focuses on the study of the main ophiolite complex ofNorthern Greece, which is one of the dominant geological features in thebroader Aegean area, by the use of geophysical (gravity and magnetic)data. This ophiolite... more
The present work focuses on the study of the main ophiolite complex ofNorthern Greece, which is one of the dominant geological features in thebroader Aegean area, by the use of geophysical (gravity and magnetic)data. This ophiolite complex, which trends in a NW-SE direction, startsat the eastern part of the borders of Greece with F.Y.R.O.M. and continuesup to the southern part
Research Interests:
In the present work we examine the active tectonics setting and related deformation scheme in the Mygdonia basin (N. Greece), on the basis of the joint interpretation of seismological and neotectonic data. In order to determine its... more
In the present work we examine the active tectonics setting and related deformation scheme in the Mygdonia basin (N. Greece), on the basis of the joint interpretation of seismological and neotectonic data. In order to determine its spatial distribution we studied the stress-field derived from fault plane solutions of small-magnitude events from a local seismological experiment, as well as neotectonic
Research Interests:
The space distribution of decelerating and accelerating preshocks is examined for six samples of forty-three mainshocks which have occurred recently in the western Mediterranean, the Aegean Sea, Anatolia, California, Japan and central... more
The space distribution of decelerating and accelerating preshocks is examined for six samples of forty-three mainshocks which have occurred recently in the western Mediterranean, the Aegean Sea, Anatolia, California, Japan and central Asia. It is observed that the frequency of shocks of a decelerating or accelerating preshock seismic sequence decays rapidly with distance from an easily located distinct geographical point according to a power-law. This point is defined as the physical center of the decelerating Pf, and the accelerating Pq, preshocks, respectively. On the basis of the mean distance and the corresponding uncertainty of the forty-three mainshock epicenters taken from the corresponding physical centers of the decelerating and accelerating preshocks, respectively, a simple procedure is proposed for a possible contribution of the locations of the two physical centers of preshocks to the location of an ensuing mainshock.
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
On the basis of growing evidence thatstrong earthquakes are preceded by a periodof accelerating seismicity of moderatemagnitude earthquakes, an attempt is madeto search for such seismicity pattern in NWAegean area. Accelerating seismic... more
On the basis of growing evidence thatstrong earthquakes are preceded by a periodof accelerating seismicity of moderatemagnitude earthquakes, an attempt is madeto search for such seismicity pattern in NWAegean area. Accelerating seismic crustaldeformation has been identified in the areaof southern Albanides mountain range(border region between Greece, formerYugoslavia and Albania). Based on certainproperties of this activity and on itssimilarity with accelerating
Research Interests:
Research Interests:
Northern Greece is an intracontinental region behind the Hellenic subduction zone, with widespread seismic activity (ranging from low to high), with strong destructive earthquakes of M ≥ 6.0 in historical to recent times. Geological and... more
Northern Greece is an intracontinental region behind the Hellenic subduction zone, with widespread seismic activity (ranging from low to high), with strong destructive earthquakes of M ≥ 6.0 in historical to recent times. Geological and seismological data indicate that recent seismic activity is mainly localized along large, inherited, fault zones, which have transected Northern Greece since Oligocene-Miocene times. The main active fault zones in Thrace, and Eastern and Central Macedonia strike approximately east-west, with lengths of 40–120 km. Fault segments strike WNW-ESE to ENE-WSW and range from 10 to 30 km in length. In Western Macedonia the main active fault zones strike NE-SW to ENE-WSW with lengths of 40–60 km and consist of 10–30 km segments. The region’s strong earthquakes are usually associated with reactivation of these fault segments and are estimated at M = 5.6–6.5. Focal mechanisms and fault-slip data from the fault zones indicate a change in the trend of extension a...
Research Interests:
ABSTRACT A geochemical survey of fumarolic and submerged gases from fluid discharges located in the Nea Kameni and Palea Kameni islets (Santorini Island, Greece) was carried out before, during, and after the unrest related to the... more
ABSTRACT A geochemical survey of fumarolic and submerged gases from fluid discharges located in the Nea Kameni and Palea Kameni islets (Santorini Island, Greece) was carried out before, during, and after the unrest related to the anomalously high seismic and ground deformation activity that affected this volcanic system since January 2011. Our data show that from May 2011 to February 2012, the Nea Kameni fumaroles showed a significant increase of H2 concentrations. After this period, an abrupt decrease in the H2 contents, accompanied by decreasing seismic events, was recorded. A similar temporal pattern was shown by the F−, Cl−, SO42−, and NH4+ concentrations in the fumarolic condensates. During the sharp increase of H2 concentrations, when values up to 158 mmol/mol were measured, the δ13C–CO2 values, which prior to January 2011 were consistent with a dominant CO2 thermometamorphic source, have shown a significant decrease, suggesting an increase of mantle CO2 contribution. Light hydrocarbons, including CH4, which are controlled by chemical reactions kinetically slower than H2 production from H2O dissociation, displayed a sharp increase in March 2012, under enhanced reducing conditions caused by the high H2 concentrations of May 2011–February 2012. The general increase in light hydrocarbons continued up to July 2012, notwithstanding the contemporaneous H2 decrease. The temporal patterns of CO2 concentrations and N2/Ar ratios increased similarly to that of H2, possibly due to sealing processes in the fumarolic conduits that diminished the contamination related to the entrance of atmospheric gases in the fumarolic conduits. The compositional evolution of the Nea Kameni fumaroles can be explained by a convective heat pulse from depth associated with the seismic activation of the NE–SW-oriented Kameni tectonic lineament, possibly triggered by either injection of new magma below Nea Kameni island, as apparently suggested by the evolution of the seismic and ground deformation activity, or increased permeability of the volcanic plumbing system resulting from the tectonic movements affecting the area. The results of the present study demonstrate that the geophysical and geochemical signals at Santorini are interrelated and may be precursory signals of renewed volcanic activity and encourage the development of interdisciplinary monitoring program to mitigate the volcanic risk in the most tourist-visited island of the Mediterranean Sea.
Research Interests:
Research Interests:
ABSTRACT Before performing dense ambient noise measurements in the historical center of the city of Thessaloniki (Northern Greece), a number of tests and evaluation noise recordings were acquired in selected sites. This data set was... more
ABSTRACT Before performing dense ambient noise measurements in the historical center of the city of Thessaloniki (Northern Greece), a number of tests and evaluation noise recordings were acquired in selected sites. This data set was processed and compared with previously site effect results for the same sites in terms of horizontal-to-vertical (h/v) spectral ratio. The noise recording system to be used for massive measurements (Cityshark velocimeter &Lennartz 3D/5s sensor) was tested against a reference one (Reftek accelerometer &Guralp CMG40T sensor). Noise recordings were performed at selected sites of the city where earthquake weak and strong motion data were available. In addition, systematic noise measurements in eight sites of the historical center of the city were performed, in order to evaluate diurnal and seasonal variation (summer -- winter) of ambient noise for a frequency range between 0.1Hz to 20Hz. It was concluded that the recording system to be used for massive measurements gives reliable results within the aforementioned frequency range and (h/v) spectral ratio of noise recordings at selected sites were found to be in very good agreement with relevant results based on earthquake weak and strong motion data. Study of diurnal variation of (h/v) spectral ratio showed that it is generally preferable to perform noise measurements during the calm hours of a day, that is, when man made noise within urban environment does not exceed a desirable level. Moreover, it was found that there is no systematic seasonal fluctuation effect on (h/v) spectral ratio.
Research Interests:
Research Interests:
SIMBAAD (Seismic Imaging of the Mantle in the Aegean-Anatolian Domain) is a temporary seismic experiment which aims at investigating the crustal and mantle structure beneath Western Turkey, the Aegean Sea, and continental Greece. This... more
SIMBAAD (Seismic Imaging of the Mantle in the Aegean-Anatolian Domain) is a temporary seismic experiment which aims at investigating the crustal and mantle structure beneath Western Turkey, the Aegean Sea, and continental Greece. This tectonically very active region has experienced a variety of geodynamic processes and its geology and kinematics have been extensively studied. It is thus a good place to test competing hypotheses on how the surface kinematics is related to mantle structure and dynamics. In ...