Tea Godoladze, Rengin Gok, Manana Dzmanashvili, Irakli Gunia, Nino Tumanova, Tuna Onur, Gurban Yetermishl and Hektor Babayanr.

Institute of Earth Sciences and NSMC, Ilia State University;

Lawrence Livermore National Laboratory;

Onur Seeman Cosnulting, Inc;

Republican Seismic Survey Center of Azerbaijan National Academy of Sciences;

Institute of Geological Sciences, RA

The Caucasus is one of the most active segments of Alpine-Himalayan continent-continent collision. High seismicity of the area reflects general tectonics of the region. The main seismo-tectonic feature is the junction between Arabian and Eurasian plates. The Caucasus has a documented historical catalog stretching back to the beginning of the Christian era. Most of the largest historical earthquakes prior to the 19th century are assumed to have occurred on active faults of the Greater Caucasus, but size and location of those events have high uncertainties and can not be directly assigned to one or another active tectonic structures in the area. Instrumental seismic observation in the Caucasus began in 1899, when the first seismograph was installed in Tbilisi (Capitol of Georgia). Seismic network grew over the years. During Soviet era number of stations increased in the region, providing better network coverage and valuable data set for the research. Data of many thousand of earthquakes recorded by the regional network were stored as the seismic bulletins in a paper form.

The Southern Caucasus countries started joint project “Probabilistic Seismic Hazard Assessment in the Caucasus ” supported by LLNL, USA (Lawrence Livermore National Laboratory). The major goal of the project was to compile regional seismic data in order to provide reliable input for the Hazard map calculation.

First arrival and amplitude data for 15 000 earthquakes were digitized from the old bulletins. Participant countries provided data for the events with magnitude more than 3.7 of both: analogue and digital seismic networks. Data was stored in the web database, developed particularly for the project.

In this study we present relocated 15 000 regional events of 118 years of observations time in the Caucasus as the major input for hazard analyses. Compiling Soviet magnitude data K, MLH, MPV, we derived new relation formulas of Soviet and modern magnitude scales. Newly compiled catalogues has unified Mw magnitude scale.

The results of this study will tremendously improve National Seismic Hazards maps of the Southern Caucasus
Countries.

Giorgi Akhalaia, Tea Godoladze, Zurab Tavadze, David Tsiklauri, Zurab Javakhishvili, and Luka Tsiskarishvili

Ilia State University, Tbilisi, Georgia

Continental collisions are a fundamental part of the Wilson Cycle and play a significant role in the evolution of the Earth’s continents, including being responsible for many major mountain belts. Only two active continentcontinent collisions occur today, India-Eurasia, and Arabia-Eurasia (AR-EU). Because of its young age (less then 30 Ma), limited spatial extent (600 km across the entire collision zone), and less then 20 years of geodetic studies, the AR-EU continental collision zone offers the opportunity to determine the detailed kinematics of active deformation for the entire region of plate interaction, from the stable Arabian Plate in the south to the stable Eurasian Plate in the north. The Caucasus region is a broad zone of convergence that forms part of the Alpine–Himalayan collision belt. The Greater and Lesser Caucasus mountains roughly extend between the Caspian and Black seas, and are separated by an inter-mountain depression.
The region is tectonically and structurally complex. Thus, quantifying the distribution of crustal strain within the collision zone, a principal objective of this research, is important both for clarifying our understanding of the dynamics of continental deformation, and for developing an improved physical basis for estimating and mitigating earthquake hazards in this rapidly developing region.

We used the available GPS data throughout the collision zone obtained throughout the Caucasus region and enhanced the existed GPS network by means of installing new permanent stations and performing trans-section GPS surveys from the western part of the Caucasus mountains to the very eastern edge of the main Caucasus thrust. This effort utilized and build upon a new GPS velocity field (1994-2018) including all GPS sites of the Caucasus ountries (Armenia, Azerbaijan, Georgia). and further constrained by geodetic observations available in Turkey, the northern part of the Arabian Plate, the northern Caucasus in Russia, and Iran. Our study provided new constraints on, 1 – convergence across the Greater Caucasus (spatial distribution of active faults and their associated slip rates and locking depths) from the Black Sea to the Caspian Sea, 2 – faulting and block rotation in the Lesser Caucasus.

I.Shengelia, N.Jorjiashvili, T.Godoladze, N.Tumanovi

Ilia State University, Tbilisi, Georgia

Among the seismic areas of Georgia, Racha region is notable for its high level of seismicity. This region is situated in the western Greater Caucasus and is characterized by southward-directed thrusting of folded Paleozoic, Mesozoic, and paleogene volcanic and sedimentary rocks. During the instrumental period, the strongest earthquake in the Caucasus (M 7.0) occurred in Racha in 1991. The historical earthquake of 1350 (M 7.0) is also known. The Georgian catalog of seismicity for 1955 – 1990 shows some sparse activity within Racha range; but after 1991 earthquake the seismic rate increased in this territory, and seismic activity continues into present. Here, from 1991 to the present, small earthquakes occur almost every day.

The main goal of the study was  to calculate the quality factor (Q_C)   using the single-scattering model in the frequency range of 1-32 Hz,  comparing received results and connecting them to the tectonics and seismicity of the study region.  Seventy  local earthquakes in 2007-2012  were analyzed and Qc values were estimated by applying  three  different methods in time and frequency domains. Earthquakes magnitudes varied from 1.2 to 3.7; epicentral distances and depth were smaller than 45 km and 15 km, respectively; coda window ranged from 10 sec to 60 sec.  These earthquakes were recorded by five digital seismic stations equipped with broadband  Guralp CMG40T and Trillium 40  seismometers.

The Q_C   values were fitted to a  power-law,    Q_C (f)= Q₀ (f)ⁿ , where Q₀ is the quality factor at 1Hz and n is the frequency parameter, which depends on  the heterogeneity of the medium. In our study Q_C  increases both with respect to lapse time and frequency for all methods. The frequency dependent Q_C relations obtained for different coda windows are the following estimates:

Qc = (8.2±0.3)f^{(1.27±0.045)}   –  (10sec);       Qc = (20.4±1.6)f^(1.24±0.54)   –   (20sec);

Qc =(32.5±3.6)f^{(1.16±0.040)}  –  (30sec);      Qc = (39.6±4.5)f^{(1.13±0.053)} –   (40sec);

Qc =(65.9±4.0)f^{(0.99±0.062)}  –  (50sec);      Qc = (82.4±9.9)f^{(0.95±0.077)}  –  (60sec).

These empirical relations represent the average attenuation properties of the region obtained by all seismic station data.   We also evaluated those volumes of the earth where studied coda waves formed.  Observed Q_c, Q₀ and n values indicate that the studied region is seismic and tectonically active with high heterogeneities.

N. Jorjiashvili, M.Gigiberia, I.Shengelia, M.Otinashvili

Ilia State University, Tbilisi, Georgia

Georgia is situated in the Caucasus region, which is one of the most seismically active regions in the Alpine-Himalayan collision belt. Analysis of the historical and instrumental seismology of this region shows that it is still of moderate seismicity. The seismicity of the area reflects the general tectonics of the region. The main seismo-tectonic feature is the junction between the Arabian and Eurasian plates. The northern movement and counterclockwise rotation of the Arabian plate causes westward movement of the Turkish block, eastward movement of the Iranian block along the strike-slip faults and the creation of thrust faulting systems in the Caucasus region.

The main goal of the study was to find out lithofacies structure of basic and contemporary sediments (about 25-30 meter depth) for Georgia using geophysical survey and taking into consideration existing geological and geophysical data to find out the influence of the estimated parameters on seismic hazard assessment.

The study areas were selected where geophisical and local geological survey are done. Using geophysical survey direct and shear wave velocities were obtained, using which the following parameters are calculated: µ_d – viscosity  (Poisson coefficient), E_d – elasticity dynamic (Young’s) modulus, G_d – shear modulus, K_d – bulk modulus, D – modulus of common deformation and  – ultimate compression strength, according to known theoretical and experimental relationships.

Maps of the values of these parameters up to 30 meter depth are obtained. Distributions of physical-mechanical parameters are analyzed for local zones. Then data were divided by rock type and 2D and 3D statistical analysis were done for each rock type.

As a result, finally, site-specific seismic hazards were assessed for PGA (Peak Ground Acceleration) for different probabilities and 50 years exposure time period. For the assessment new ground motion prediction models were used specially done for Caucasus region. Also, local soil conditions were included for the study and results were compared with ordinary results obtained without taking into account local soil conditions.

Finally, results were visualized as maps in GIS (geo-information systems).

Giorgi Merebashvili, Zurab javakhishvili, Lasha Sukhishvili, Giorgi Boichenko

Ilia State University, Tbilisi, Georgia

Gombori Range (GR) is a part of Kura Thrust Fault Belt Structure (KFTB). The range is separated from the Greater Caucasus to the north by the 10- to 25-km-wide Alazani Basin. Regional geologic and stratigraphic data and balanced cross sections suggest that the Kura fold-thrust belt has accommodated the majority of Arabia-Eurasia convergence since the early Pliocene. KFTB is characterized by seismic activity, (1967 Khashmi M=6.1 and M=4.8-5.3 in 1973, etc). In Pliocene, prior to GR formation, the area was covered by Paratethys Ocean. Initiation of deformation of KFTB was simultaneous of Ak-Ap sedimentation caused by transgression-regression phases (3.4 – 0.7_Ma).

The purpose of the research is to derive tectonic and paleogeographic information from paleocurrent analyses of Ak-Ap layers along the range. Field observations, lithostratigraphic cross sections and existing Geological maps were used to correlate paleocurrent layers of the Northern slopes to the supposedly same ones in the south. We used photogrammetric modelling and measurements in virtual reality for the areas hard to access in the field.

The final result of the project was complex analyzing of our investigation in frames of existing studies about structural and tectonic settings of the KFTB.

Irakli Skhirtladze, Avtandil Okrostsvaridze

Ilia State University, Tbilisi, Georgia

The Late Miocene Goderdzi formation in the Lesser Caucasus forms a volcanic highland of andesitic-dacitic composition, which occupies more than 4500 km² in Georgia. Its total thickness reaches 1100 m [1]. In the Mtkvari river canyon which cuts through the formation, thick (40-50 m) ignimbrite deposits crop out at 35 km distance. Isotopic εNd parameter of the ignimbrites varies between +2 – +4, and ⁸⁷Sr/⁸⁸Sr parameter – between 0,7034 – 0.7045. These parameters point to the mantle origins of these ignimbrites. U-Pb dating of zircons show that the Goderdzi formation ignimbrites were erupted about 7.5 Ma – in the Late Miocene [2].

In Mtkvari river canyon, near Vardzia cave complex pyroclastic fallout deposits (250 m) are overlain by above-mentioned thick (60 m) ignimbrite deposits (Vardzia horizon), which, in turn are capped by heterolithic breccias. Sizes of individual breccia fragments exceed 1 meter in diameter. These breccias record caldera collapse event, when blocks of destroyed volcano vent were emplaced on top of pyroclastic density currents (ignimbrites). Near Chachkari village, breccias are covered by 2.5-3 m thick volcanic ash layers – typical for silicic calderas. Interestingly, similar layer of ash can be found on the opposite side of Mtkvari river, on the Javakheti highland.

Presumably, all explosive eruptions that erupt more than 5 km³ of volcanic material produce a caldera. The sequence, described above, was noticed recently; it is obviously very similar to so-called caldera-forming eruption (CFE) sequence that is characteristic for large collapse calderas and it records different stages of caldera forming events [3].  Large overall thickness and peculiarities of sequence of pyroclastics indicate that at least some part of the Goderdzi formation was formed by strong caldera forming explosive eruption. The caldera itself has not yet been identified due to intense deformation, erosion and concealment of volcanics.

Observed thickness of ignimbrites in Mtkvari river canyon increases from south to north, whereas changes in color and degree of welding, point to the temperature decrease; thus, they are indicative of the movement direction of pyroclastic density currents and the ignimbrites can be assumed to be extracaldera formations.

Recognizing CFE sequence and analyzing the spatial-temporal distribution of volcanic products of the Goderdzi formation are important steps for identifying the Late Miocene caldera. Calderas in their turn, can provide us with useful information about magma evolution, major tectonic and magmatic events, and they can be significant source of several types of ore deposits.