ALBERT

Description: This paper is concerned with the seismotectonics of the North Anatolian Fault in the vicinity of the OrtaCank[i]r[i] region, and consists of a study of a moderate-sized (Mw=6. 0) earthquake that occurred on 6 June 2000. The instrumental epicentre of this earthquake is far from the North Anatolian Fault Zone (NAFZ), and rapid focal mechanism solutions of USGSNEIC and Harvard-CMT also demonstrate that this earthquake is not directly related to the right-lateral movement of the North Anatolian Fault. This earthquake is the only instrumentally recorded event of magnitude (Mw) 〉5.5 since 1900 between Ankara and Cank[i]r[i], and therefore provides valuable data to improve our understanding of the neotectonic framework of NW central Anatolia. Field observations carried out in the vicinity of Orta town and neighbouring villages immediately after the earthquake indicated no apparent surface rupture, but the reported damage was most intense in the villages to the SW of Orta. We used teleseismic long-period P- and SH-body waveforms and first-motion polarities of P-waves, broadband P-waves, and InSAR data to determine the source parameters of the 6 June 2000 (OrtaCank[i]r[i], to=02:41:53.2, Mw=6. 0) earthquake. We compared the shapes and amplitudes of long-period P- and SH-waveforms recorded by GDSN stations in the distance range 3090{degrees}, for which signal amplitudes were large enough, with synthetic waveforms. The best-fitting fault-plane solution of the OrtaCank[i]r[i] earthquake shows normal faulting with a left-lateral component with no apparent surface rupture in the vicinity of the epicentre. The source parameters and uncertainties of this earthquake were: Nodal Plane 1: strike 2{degrees}{+/-}5{degrees}, dip 46{degrees}{+/-}5{degrees}, rake 29{degrees}{+/-}5{degrees}; Nodal Plane 2: strike 113{degrees}, dip 70{degrees}, rake 132{degrees}; principal axes: P=338{degrees} (48{degrees}), T=232{degrees} (14{degrees}), B=131{degrees} (39{degrees}); focal depth 8{+/-}2 km (though this does not include uncertainty related to velocity structure), and seismic moment Mo=(140185)x1016 N m. Furthermore, analysis of a coseismic interferogram also allows the source mechanism and location of the earthquake to be determined. The InSAR data suggest that the northsouth fault plane (Nodal Plane 1 above) was the one that ruptured during the earthquake. The InSAR mechanism is in good agreement with the minimum misfit solution of P- and SH-waveforms. Although the magnitude of slip was poorly constrained, trade-off with the depth range of faulting accurred such that solutions with a large depth range had small values of slip and vice versa. The misfit was small and the geodetic moment constant for fault slips greater than c. 1 m. The 6 June 2000 OrtaCank[i]r[i] earthquake occurred close to a restraining bend in the eastwest-striking rightlateral strike-slip fault that moved in the much larger earthquake of 13 August 1951 (Ms=6.7). The faulting in this anomalous earthquake could be related to the local geometry of the main strike-slip system, and may not be a reliable guide to the regional strain field in NW central Turkey. We tentatively suggest that one possible explanation for the occurrence of the 6 June 2000 OrtaCank[i]r[i] earthquake could be localized clockwise rotations as a result of shear of the lower crust and lithosphere.

Description: Historical tsunamis and tsunami propagation are synthesized in the Eastern Mediterranean Sea region, with particular attention to the Hellenic and the Cyprus arcs and the Levantine basin, to obtain a better picture of the tsunamigenic zones. Historical data of tsunami manifestation in the region are analysed, and compared with current seismic activity and plate interactions. Numerical simulations of potential and historical tsunamis reported in the Cyprus and Hellenic arcs are performed as case studies in the context of the nonlinear shallow-water theory. Tsunami wave heights as well as their distribution function are calculated for the Paphos earthquake of 11 May 1222 and the Crete earthquake of 8 August 1303 as illustrative examples depicting the characteristics of tsunami propagation, and the effects of coastal topography and near-shore amplification. The simulation studies also revealed that the long-normal distributions are compatible with reported damage. Furthermore, it is necessary to note that high-resolution bathymetry maps are a crucial component in tsunami wave simulations, and this aspect is rather poorly developed in the Eastern Mediterranean. The current study also demonstrates the role of bottom irregularities in determining the wave-height distribution near coastlines. Assuming the probability of occurrence of destructive tsunamigenic earthquakes, these studies will help us to evaluate the tsunami hazard for the coastal plains of the Eastern Mediterranean Sea region. We suggest that future oceanographic and marine geophysical research should aim to improve the resolution of bathymetric maps, particularly for the details of the continental shelf and seamounts.

Description: We develop and apply a full waveform inversion method that incorporates seismic data on a wide range of spatio-temporal scales, thereby constraining the details of both crustal and upper-mantle structure. This is intended to further our understanding of crust–mantle interactions that shape the nature of plate tectonics, and to be a step towards improved tomographic models of strongly scale-dependent earth properties, such as attenuation and anisotropy. The inversion for detailed regional earth structure consistently embedded within a large-scale model requires locally refined numerical meshes that allow us to (1) model regional wave propagation at high frequencies, and (2) capture the inferred fine-scale heterogeneities. The smallest local grid spacing sets the upper bound of the largest possible time step used to iteratively advance the seismic wave field. This limitation leads to extreme computational costs in the presence of fine-scale structure, and it inhibits the construction of full waveform tomographic models that describe earth structure on multiple scales. To reduce computational requirements to a feasible level, we design a multigrid approach based on the decomposition of a multiscale earth model with widely varying grid spacings into a family of single-scale models where the grid spacing is approximately uniform. Each of the single-scale models contains a tractable number of grid points, which ensures computational efficiency. The multi-to-single-scale decomposition is the foundation of iterative, gradient-based optimization schemes that simultaneously and consistently invert data on all scales for one multi-scale model. We demonstrate the applicability of our method in a full waveform inversion for Eurasia, with a special focus on Anatolia where coverage is particularly dense. Continental-scale structure is constrained by complete seismic waveforms in the 30–200 s period range. In addition to the well-known structural elements of the Eurasian mantle, our model reveals a variety of subtle features, such as the Armorican Massif, the Rhine Graben and the Massif Central. Anatolia is covered by waveforms with 8–200 s period, meaning that the details of both crustal and mantle structure are resolved consistently. The final model contains numerous previously undiscovered structures, including the extension-related updoming of lower-crustal material beneath the Menderes Massif in western Anatolia. Furthermore, the final model for the Anatolian region confirms estimates of crustal depth from receiver function analysis, and it accurately explains cross-correlations of ambient seismic noise at 10 s period that have not been used in the tomographic inversion. This provides strong independent evidence that detailed 3-D structure is well resolved.

Description: Here we present first-order results detailing the Anatolian crustal from receiver function analysis of data from approximately 300 stations within Turkey. Seismic data from the Kandilli Observatory array (KOERI; KO), the National Seismic Network of Turkey (AFAD-DAD; TU) and available IRIS data from the Northern Anatolian Fault experiment (YL) for the period between 2005 and 2010 is analysed. We calculate receiver functions in the frequency domain using water-level deconvolution. The results are analysed using a combination of H–K stacking and depth stacking to determine robust Moho conversion depths and V P / V s ratios across Anatolia. We detect a deep Moho in eastern Anatolia of up to ~55 km, a generally normal Moho in Central Anatolia of ~37–47 km and a thinned Moho in western Anatolia and Cyprus of ~30 km. The V P / V s ratio across the Anatolian Plate is generally slightly elevated; regions of extremely high V P / V s ratio (〉1.85) can be associated with recent volcanism in eastern and central Anatolia. High V P / V s ratio measurements (〉1.85) in western Anatolia may be indicative of partial melt in the lower crust associated with regional extension.

Description: The 17 November 2015 M w  6.6 earthquake in Leucas (Leukas, Lefkas, or Lefkada) Island in the Ionian Sea, western Aegean arc, was modeled using teleseismic long-period P and SH waveforms and Global Positioning System (GPS) slip vectors. Detailed fault modeling in this region, characterized by intense seismicity and deformation rates, usually assigned to the Cephalonia Transform fault, is a challenge because of the unfavorable observation system. To overcome this problem, we independently analyzed seismological and geodetic data and then jointly evaluated the results. The adopted model indicates that the 2015 earthquake can be assigned to a shallow strike-slip fault, with a minor component of thrusting, along the southwest coasts of Leucas and with relatively high slip for the area. Additionally, mostly low-angle fault solutions satisfying geodetic observations were identified but were not further investigated. The preferred fault model permits recognition that recent M w 〉6.0 earthquakes in the area, some marked by extreme peak ground accelerations, are associated with a string of strike slip (or oblique slip), occasionally overlapping fault segments with variable characteristics, along or close to the west coasts of Leucas and Cephalonia (Keffalinia, Kefalonia) Islands, whereas the catastrophic 1953 M w  7.2 Cephalonia and other previous major earthquakes were associated with thrust faulting. Electronic Supplement: Table of the Global Positioning System (GPS)-derived displacements and figures of P -wave first-motion polarities, comparison of earthquake source parameters, GPS time series, 2D projections of geodetic solutions, the variance–covariance matrix of the geodetic solution, and the geodetic variable slip model.

Description: A 3-D tomographic inversion of first arrival times of shot profiles recorded by a dense 2-D OBS network provides an unprecedented constraint on the P -wave velocities heterogeneity of the upper-crustal part of the North Marmara Trough (NMT), over a region of 180 km long by 50 km wide. One of the specific aims of this controlled source tomography is to provide a 3-D initial model for the local earthquake tomography (LET). Hence, in an original way, the controlled source inversion has been performed by using a code dedicated to LET. After several tests to check the results trade-off with the inversion parameters, we build up a 3-D a priori velocity model, in which the sea-bottom topography, the acoustic and the crystalline basements and the Moho interfaces have been considered. The reliability of the obtained features has been checked by checkerboard tests and also by their comparison with the deep-penetration multichannel seismic profiles, and with the wide-angle reflection and refraction modelled profiles. This study provides the first 3-D view of the basement topography along the active North Anatolian fault beneath the Marmara Sea, even beneath the deepest part of three sedimentary basins of NMT. Clear basement depressions reaching down 6 km depth below the sea level (bsl) have been found beneath these basins. The North Imrali Basin located on the southern continental shelf is observed with a similar sedimentary thickness as its northern neighbours. Between Central and Çinarcik basins, the Central High rises up to 3 km depth below (bsl). Its crest position is offset by 10 km northwestward relatively to the bathymetric crest. On the contrary, Tekirdag and Central basins appear linked, forming a 60-km-long basement depression. Beneath the bathymetric relief of Western High low velocities are observed down to 6 km depth (bsl) and no basement high have been found. The obtained 3-D Vp heterogeneity model allows the consideration of the 3-D supracrustal heterogeneity into the future earthquake relocations in this region. The topographic map of the pre-kinematic basement offers the possibility to take into account the locking depth variations in future geohazard estimations by geomechanical modelling in this region.

Description: We analyze S-receiver functions to investigate the variations of lithospheric thickness below the entire region of Turkey and surroundings. The teleseismic data used here have been compiled combining all permanent seismic stations which are open to public access. We obtained almost 12 000 S-receiver function traces characterizing the seismic discontinuities between the Moho and the discontinuity at 410 km depth. Common-conversion-points stacks yield well-constrained images of the Moho and of the lithosphere–asthenosphere boundary (LAB). Results from previous studies suggesting shallow LAB depths between 80 and 100 km are confirmed in the entire region outside the subduction zones. We did not observe changes of LAB depths across the North and East Anatolian Faults. To the east of Cyprus, we see indications of the Arabian LAB. The African plate is observed down to about 150 km depth subducting to the north and east between the Aegean and Cyprus with a tear at Cyprus. We also observed the discontinuity at 410 km depth and a negative discontinuity above the 410, which might indicate a zone of partial melt above this discontinuity.