Ekrem Zor

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Papers by Ekrem Zor

Crust and Upper Mantle Structure of the Northern Portion of the Arabia-Eurasia Collisional Belt

East Anatolian high plateau as a mantle-supported, north–south shortened domal structure

The East Anatolian High Plateau is a region of average ~2 km elevation a.s.l. exhibiting active d... more The East Anatolian High Plateau is a region of average ~2 km elevation a.s.l. exhibiting active diffuse N-S shortening and widespread Pliocene to recent volcanicity. Its elevation was hitherto thought to result from a presumed crustal thickness of ± 55 km. Seismic data collected by a new network of 29 seismograph stations have shown, however, that its crustal thickness is only some 45 km. Combined with observations on Pn and Sn phases, this shows that most of the East Anatolian High Plateau is devoid of mantle lithosphere. Areas of no mantle lithosphere is inferred to coincide broadly with the extent of the East Anatolian Accretionary Complex, a subduction-accretion prism of late Cretaceous to earliest Oligocene age. The absence of mantle lithosphere is ascribed to slab breakoff beneath the prism and the widespread vulcanicity to melting its lower levels because of direct contact with hot asthenosphere. The East Anatolian High Plateau is thus supported not by thick crust, but by hot...

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Rayleigh wave dispersion curve inversion by using particle swarm optimization and genetic algorithm

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Crustal and Upper Mantle Seismic Velocity Structure in Eastern Anatolia: Results From The Eastern Turkey PASSCAL Seismic Experiment

A temporary 29 station broadband PASSCAL array was deployed in eastern Turkey from October 1999 u... more A temporary 29 station broadband PASSCAL array was deployed in eastern Turkey from October 1999 until August 2001. The Eastern Turkey Seismic Experiment (ETSE) was designed to image the crustal and upper mantle seismic velocity structure beneath the northernmost Arabian plate and Anatolian Plateau. These images will constrain the current geodynamic models for young continent continent collision in eastern Turkey.

Hızın derinlikle değişiminin (p - t) yaklaşımı ile hesaplanması

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Sismolojide ters çözüm ile üç boyutlu görüntüleme (seismic imaging)

Measurements of Regional Phase Q in the Middle East

In order to construct reliable frequency-dependent Q models for both Lg and Pg, we have used appr... more In order to construct reliable frequency-dependent Q models for both Lg and Pg, we have used approximately 5,000 waveforms from approximately 200 events recorded by 10 permanent and temporary networks throughout the Middle East. Waveforms with unstable values that had higher standards were separated from the final data file used in our model. Using these waveforms, we have developed a tomographic model with frequency-dependent Q using direct Lg waves in this region. Our tomographic model of Lg Qo is consistent with previous, more-qualitative Lg attenuation models that showed inefficient or blocked Lg across the Eurasian-Arabian plate boundary. We have also found, similar to the previous models, efficient Lg propagation throughout much of the Arabian Plate. The lowest Qo values found were located in the East Anatolian Plateau (~70 to 100) and East Anatolian Fault Zone (~80 to 120). The systematic variations appear to correlate with regions where there is evidence of Sn-to-Lg converte...

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The seismotectonics of the Marmara region (Turkey): results from a microseismic experiment

Tectonophysics, 2000

The Marmara region is an active tectonic zone characterised by the transition between the dextral... more The Marmara region is an active tectonic zone characterised by the transition between the dextral strike-slip regime of the North Anatolian Fault (NAF) and the extension regime of the Aegean Sea. Strong historical earthquakes (M>7) and the presence of known seismic gaps imply a high level of seismic hazard. A synthesis of recent studies of active tectonics in the region is presented, including inland and underwater observations. The branching of the NAF is explained in terms of increasing influence of the extension. Historical information and instrumental seismicity are reinterpreted in order to have a critical appraisal of the existence of large seismic gaps in the central and eastern Marmara Sea. Focal mechanisms of strong earthquakes are used to obtain orientation and shape factor of the deviator of the stress tensor. The resulting tensor is in shear regime (s 2 vertical) but close to extension (R=0.93) with s 1 oriented N145°0. A microseismic experiment with 48 stations distributed around the Marmara Sea was carried out in October-December 1995. A total of 137 microearthquakes were located and 23 of those were selected to obtain focal mechanism solutions. The epicentral distribution indicates activity along the system of pull-apart basins north of the Marmara Sea. The segment between Marmara Sea and the Saros Bay, activated in 1912, and the Gulf of Izmit, site of the 1754 earthquake, are now silent. Seismic activity is very linear along the northern branch of the NAF, but it is more diffused on the Bursa and Iznik branches, southeast of the Marmara Sea. The stress tensor obtained from the focal mechanisms of the micro-earthquakes is compared to the one inverted from teleseismic data. The microseismic stress is compatible with a shear (intermediate) regime like the one obtained from strong earthquakes, though not as well constrained, one of the acceptable solutions having the same orientation but different shape (R=0.5).

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Shear wave splitting in a young continent-continent collision: An example from Eastern Turkey

Geophysical Research Letters, 2003

We have determined the shear wave splitting fast polarization direction and delay time using data... more We have determined the shear wave splitting fast polarization direction and delay time using data from the ETSE broadband experiment (Eastern Turkey Seismic Experiment), a deployment of 29 broadband seismic stations across the collision zone of the Arabian, Eurasian, and Anatolian plates. Our results show that the fast polarization directions are relatively uniform and they exhibit primarily NE-SW orientations. No abrupt changes in anisotropy directions are observed across the main tectonic units in the region: the Bitlis Suture (BS) and the North and Eastern Anatolian Fault zones. The fast polarization directions are determined to be sub-parallel to the Anatolian, Arabian, and Eurasian absolute plate velocities, except for those stations in the northeastern corner of the Anatolian Plateau. Observed delay times range from 0.7 to 2.0 seconds with an average value of 1.0 second; the largest values are within the northern Anatolian Plateau which is underlain by an exceptionally low velocity zone in the uppermost mantle. We interpret shear wave splitting as the vector difference of the Eurasian lithosphere and northeastern or southwestern directed flow of the asthenospheric mantle. Comparisons of the polarization anisotropy with measurements of Pn azimuthal anisotropy suggest vertical anisotropic layering except in those areas which are underlain by partially molten uppermost mantle.

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The crustal structure of the East Anatolian plateau (Turkey) from receiver functions

Geophysical Research Letters, 2003

The crustal structure of the Anatolian plateau in Eastern Turkey is investigated using receiver f... more The crustal structure of the Anatolian plateau in Eastern Turkey is investigated using receiver functions obtained from the teleseismic recordings of a 29 broadband PASSCAL temporary network, i.e., the Eastern Turkey Seismic Experiment [ETSE]. The S-wave velocity structure was estimated from the stacked receiver functions by performing a 6-plane layered grid search scheme in order to model the first order features in the receiver functions with minimum tradeoff. We found no significant crustal root beneath the western portion of the network, but there is some evidence of crustal thickening in the northern portion of the network. We found an average crustal thickness of 45 km and an average crustal shear velocity of 3.7 km/s for the entire eastern Anatolian plateau. Within the Anatolian plateau we found evidence of a prominent low velocity zone where the crust thickness is approximately 46 km. These results suggests that the 2 km high topography across the Anatolian plateau is dynamically supported because most of the plateau appears to be isostatically under-compensated. Also, there appears to be a region of thin crust at the easternmost edge of the Anatolian plateau that may be a relic from the accretion of island arcs to the Eurasian plate.

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Seismogenic zones in Eastern Turkey

Geophysical Research Letters, 2003

A 29-station temporary broadband PASSCAL network was operated from late October 1999 to August 20... more A 29-station temporary broadband PASSCAL network was operated from late October 1999 to August 2001 in eastern Turkey in order to decipher the geodynamics of one of the youngest continent-continent collision zones in the world. This paper focuses on the hypocentral distribution of local earthquakes located during the operation of the network and provides new insights into the active faulting in the Anatolian plateau. A total of 1165 earthquakes were located and classified into four different categories based on the reliability of the locations as established by the data coverage. The accuracy of the locations ranked in the best two categories is estimated to be less than approximately 5 km. The results show that seismic activity in Eastern Turkey is higher than previously documented and there were no subcrustal earthquakes beneath the Arabian-Eurasian collision zone or beneath the Anatolian plateau during our deployment. This result suggests no or very little underthrusting of the Arabian plate beneath Eurasia. Our results also suggest that the North Anatolian Fault zone extends farther toward the southeast, well beyond the Karliova triple junction, and that a number of unmapped active, seismogenic faults exist in the region. We also observed a possible difference in the seismogenic thickness of the East Anatolian fault zone (EAFZ) and the North Anatolian fault zone (NAFZ).

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Shear wave velocity structure of the İzmit Bay area (Turkey) estimated from active-passive array surface wave and single-station microtremor methods

Geophysical Journal International, 2010

To provide quantitative information on the shear wave velocity structure of the İzmit Bay area, w... more To provide quantitative information on the shear wave velocity structure of the İzmit Bay area, we conduct active-passive array surface wave and single-station microtremor measurements at 60 sites. We process these array measurements to produce combined Rayleigh wave dispersion curves that span a broad-frequency range (0.5-20 Hz). We also make use of horizontal-to-vertical spectral ratio (HVSR) curves obtained from the single-station microtremor measurements to benefit from their close relation to the ellipticity of Rayleigh waves. Using the dispersion curve sensitivity to the absolute velocities and the shape of the HVSR curve sensitivity to the velocity contrasts of the velocity-depth model, we have applied a combined inversion technique to derive better constrained shear velocity models at each site. The derived shear velocity models are used to provide a V s 30 site classification map and information on the sediment-bedrock structure for the İzmit Bay area. The V s 30 map shows that the entire shoreline regions including artificial infill areas of the İzmit Bay have V s 30 values less than 200 m s -1 , locally as low as 80 m s -1 . The older sediment areas of the İzmit basin have V s 30 values ranging between 250 and 350 m s -1 . The highest V s 30 values are associated with rock sites reaching about 950-1300 m s -1 . We also present a high correlation between the V s 30 values and the phase velocity of Rayleigh wave corresponding to the 40 m wavelength (C40). The important implication of this correlation is that V s 30 may be estimated from C40 without inverting the dispersion curve. The inferred sediment-bedrock interface along a crosssection shows an antisymmetric V shaped basin with a sedimentary cover thickness reaching about 1200 m at the deepest part of the İzmit basin. This deepest part coincides in the map view where the North Anatolian fault zone crosses the basin in the east-west direction. The sedimentary cover thickness is found to be 750 m in the Gölcük-Derince basin.

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Tomographic evidence of slab detachment beneath eastern Turkey and the Caucasus

Geophysical Journal International, 2008

Teleseismic phase readings from the Eastern Turkey Seismic Experiment (ETSE) have been inverted u... more Teleseismic phase readings from the Eastern Turkey Seismic Experiment (ETSE) have been inverted using teleseismic tomography in order to create a 3-D image of the underlying mantle beneath Eastern Turkey. The aim was to investigate the existence of an upper mantle negative velocity anomaly that is used to explain the uplift of Eastern Anatolian plateau and the potential pieces of detached oceanic slabs related to Neo-Tethyan subduction suggested by previous studies. Using teleseismic waveforms from the 29 stations of the Eastern Turkey Seismic Experiment, 2926 direct P phases from 146 events were picked by using adaptive stacking techniques. In order to increase the station coverage and resolve the surrounding area, the phase readings from the International Seismological Center (ISC) Bulletin have also been added. The data set consists of 9571 P and PKP phase readings of 79 stations from 503 teleseismic events. This study develops the first highresolution 3-D upper mantle P-wave tomographic model for this region. The tomographic results show the existence of an upper mantle negative velocity anomaly to a depth of ∼200 km beneath the eastern Anatolian accreationary complex (EAAC) as commonly observed in the previous studies that suggest a shallow partially molten asthenosphere. The slab-like high velocity anomalies beneath the EAAC, Pontides and Caucasus are interpreted as the detached southern and northern Neo-Tethys slabs.

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The crustal structure of the eastern Marmara region, Turkey by teleseismic receiver functions

Geophysical Journal International, 2006

We study the crustal structure of eastern Marmara region by applying the receiver function method... more We study the crustal structure of eastern Marmara region by applying the receiver function method to the data obtained from the 11 broad-band stations that have been in operation since the 1999 İzmit earthquake. The stacked single-event receiver functions were modelled by an inversion algorithm based on a five-layered crustal velocity model to reveal the firstorder shear-velocity discontinuities with a minimum degree of trade-off. We observe crustal thickening from west (29-32 km) to east (34-35 km) along the North Anatolian Fault Zone (NAFZ), but we observe no obvious crustal thickness variation from north to south while crossing the NAFZ. The crust is thinnest beneath station TER (29 km), located near the Black Sea coast in the west and thickest beneath station TAR (35 km), located inland in the southeast. The average crustal thickness and S-wave velocity for the whole regions are 31 ± 2 km and 3.64 ± 0.15 km s -1 , respectively. The eastern Marmara region with its average crustal thickness, high heat flow value (101 ± 11 mW m -2 ) and with its remarkable extensional features seems to have a Basin and Range type characteristics, but the higher average shear velocities (∼3.64 km s -1 ) and crustal thickening from 29 to 35 km towards the easternmost stations indicate that the crustal structure shows a transitional tectonic regime. Therefore, we conclude that the eastern Marmara region seems to be a transition zone between the Marmara Sea extensional domain and the continental Anatolian inland region.

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Investigation of 3-D basin structures in the İzmit Bay area (Turkey) by single-station microtremor and gravimetric methods

Geophysical Journal International, 2011

We investigate 3-D basin structures and site resonance frequencies in theİzmit Bay area of Turkey... more We investigate 3-D basin structures and site resonance frequencies in theİzmit Bay area of Turkey by new geophysical surveys based on 239 single station microtremor and 405point gravity measurements. A fundamental resonance frequency map of theİzmit Bay was produced from the main peak in the horizontal-to-vertical component spectral ratio (HVSR) of microtremors. The HVSR analysis of the microtremor data reveals single, double, broad peaked or no peak type HVSR curves varying in accordance with the surface geology and spatial extent of the three basins present in theİzmit Bay area. In the deepest sections of thė Izmit, Gölcük and Derince basins of theİzmit Bay, the fundamental resonance frequencies are dominantly 0.24-0.30 Hz. These resonance frequencies should be taken into consideration along with higher mode frequencies to construct earthquake resistant structures in theİzmit Bay area. The 3-D gravimetric bedrock depth map of theİzmit basin shows that the basin has an asymmetric shape with its deepest section coinciding with the surface trace of the North Anatolian Fault. The northern shoulder of the basin has a gentle dip on the Kocaeli Peneplain side and the southern shoulder is much steeper, and it is bounded by the Samanlı Mountains. We derive a power-law relationship that correlates the fundamental site resonance frequencies with the sedimentary cover thickness obtained from the gravity and shear wave velocity data in theİzmit Bay. We use this relationship to estimate bedrock depths beneath Gölcük and Derince basins. Our estimation of maximum basin depths is 1400 m for theİzmit and is 800 m for the Gölcük and Derince basins. Finally, we have analysed a converted Sp phase from a local earthquake recording made at site CMP to calculate and verify the sediment thickness estimations obtained from our gravimetric and microtremor analyses. This calculation shows close agreement with that of the gravimetric and microtremor results. Our results show that the basins in theİzmit Bay area have a very thick sedimentary cover with very low shear velocities underlined by hard bedrock, forming a sharp impedance contrast. We anticipate that these results will be a key contribution to the quantitative assessment of seismic hazard for thė Izmit Bay area before the occurrence of strong earthquakes in the Marmara region.

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Seismogenic zones in Eastern Turkey: The Turkey seismic experiment: The study of a young continent-continent collision

Geophysical research …, 2003

... TURKELLI Niyazi (1) ; SANDVOL Eric (2) ; ZOR Ekrem (1) ; GOK Rengin (1) ; BEKLER Tolga (1) ; ... more

Crustal Attenuation within the Turkish Plateau and Surrounding Regions

Bulletin of the Seismological Society of America, 2007

We have imaged lateral variations in the seismic attenuation in the crust from the inversion of i... more We have imaged lateral variations in the seismic attenuation in the crust from the inversion of inter-station Lg Q measurements in Turkey and the surrounding regions. We used data collected from three temporary arrays (ETSE, WTRN, ASN) and three permanent networks (KOERI,LLNL-JSO, GSN). We used approximately 2300 waveforms from ~200 events recorded at 75 stations in the region. Our tomographic

Pg Attenuation Tomography within the Northern Middle East

Bulletin of the Seismological Society of America, 2011

Abstract A seismic attenuation map of regional phase Pg is constructed for the northern Middle Ea... more

Lg and Pg Attenuation in the Middle East

The objective of this study is to obtain laterally varying Q models for multiple regional waves, ... more

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Upper Mantle Velocity Structure Beneath Eastern Anatolian Plateau and Surrounding Regions

The Eastern Turkey Seismic Experiment (ETSE) was designed to image the crustal and upper mantle v... more The Eastern Turkey Seismic Experiment (ETSE) was designed to image the crustal and upper mantle velocity structure beneath the northernmost Arabian plate and beneath the Anatolian Plateau, constraining geodynamic models for young continent-continent collision. ETSE was composed of a 29 station broadband PASSCAL array which was deployed from October 1999 until August 2001. We have measured Rayleigh wave phase velocities across the Anatolian plateau using 45 teleseismic and regional events. We used a method that approximates the wavefield from each earthquake as the sum of two interfering plane waves to map Rayleigh wave phase velocities for periods between 30 and 140 seconds. The optimal one dimensional velocity model suggests that there is little, if any, lithospheric mantle beneath the Eastern Anatolian plateau. The Rayleigh wave phase velocity maps are consistent with the Bitlis suture?s marking an abrupt change in the lithospheric mantle velocity structure. Tomographic mapping of the Rayleigh wave phase velocities suggests that the uppermost mantle ultra-low velocity zone is underlain by relatively normal to slightly low velocity mantle. This finding is consistent with our preliminary three dimensional P-wave velocity model that was obtained using teleseismic body wave tomography. The preliminary P-wave model does not show a clear contrast in mantle properties across the Bitlis suture. Teleseismic shear wave splitting indicates that there is a relatively uniform northeast fast direction throughout the plateau. Across the eastern portion of the Anatolian plateau, there appears to be some correlation between very slow mantle lid velocity and large splitting lag times. We are investigating the depth extent of the azimuthal anisotropy beneath the eastern Anatolian plateau using Rayleigh wave phase velocities.

Ekrem Zor

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