Atilla Arda Ozacar

The 2002 Denali Fault and 2001 Kunlun Fault Earthquakes: Complex Rupture Processes of Two Large Strike-Slip Events

We studied the source processes of two large continental earthquakes, the 3 November 2002 Denali fault earthquake and the 14 November 2001 Kunlun fault earthquake, associated with strike-slip faulting along ancient sutures. We inverted teleseismic P waveforms using a pulse-stripping method for multiple time windows with different focal mechanisms and derived composite source models. According to our results, the 2002 Denali fault earthquake began with initial thrusting ( M w 7.3) along a 40-km-long segment of the north-northwest-dipping Susitna Glacier fault and later ruptured a 300-km-long segment along the Denali and Totschunda faults with a right-lateral strike-slip mechanism ( M w 7.7). In contrast, the 2001 Kunlun fault earthquake nucleated near an extensional step-over with a subevent pair consisting of 30-km-long strike-slip ( M w 6.9) event and 40-km-long normal ( M w 6.8) faulting event and later ruptured a 350-km-long segment along the Kunlun fault with a left-lateral strike-slip mechanism ( M w 7.7). Both earthquakes propagated primarily unilaterally to the east and released most of their energy along slip patches (asperities) far from the hypocenter locations. We find that both the Denali fault and Kunlun fault earthquakes had high-average rupture velocities of 3.2 km/sec and 3.4 km/sec, respectively. We also compared the source properties of these two earthquakes with other strike-slip earthquakes. For scaling purposes, large strike-slip earthquakes were classified as interplate, oceanic intraplate, or continental intraplate events. By using this classification the Denali fault and Kunlun fault earthquakes have an interplate signature that suggests overall weak faulting.

Seismic images of crustal variations beneath the East Anatolian Plateau (Turkey) from teleseismic receiver functions

Abstract We used teleseismic P-wave receiver functions recorded by the Eastern Turkey Seismic Experiment to determine the crustal structure across an active continent–continent collision zone. Moho depth and Vp/Vs variations in the region are mapped by incorporating crustal multiples and later two-dimsional (2-D) seismic profiles are produced using a common conversion point technique with our crustal Vp/Vs estimates. Moho depths do not correlate with surface topography and reveal a relatively thin crust consistent with the high plateau being supported by hot asthenosphere near the base of the crust. Under the Arabian plate, the crust is thinnest (c. 35 km) and exhibits high Vp/Vs (≥1.8) associated with mafic compositions. In the east, the crust gradually becomes thicker towards the north and exceeds 45 km in the northeastern side whereas in the west, the crust thickens sharply near the Bitlis suture and displays pronounced Moho topography within the Anatolian plate that suggests the presence of multiple fragments. Vp/Vs variations show an anomalously high Vp/Vs corridor (≥1.85) along the North Anatolian Fault and near the youngest volcanic units (c. 3 Ma) and support the presence of partial melt. This corridor is spatially limited from both north and south by low Vp/Vs regions implying a change in crustal composition. Near the Bitlis suture, a layered Vp/Vs model points to the source of low Vp/Vs in the lower crust that may be rich in quartz. Furthermore, the seismic profiles indicate a prominent low velocity zone in the lower crust across a large area beneath the plateau that may act as a decoupling zone between the crust and upper mantle.

Structure of the crust and African slab beneath the central Anatolian plateau from receiver functions: New insights on isostatic compensation and slab dynamics

The central Anatolian plateau in Turkey is a region with a long history of subduction, continental collision, accretion of continental fragments, and slab tearing and/or breakoff and tectonic escape. Central Anatolia is currently characterized as a nascent plateau with widespread Neogene volcanism and predominantly transtensional deformation. To elucidate the present-day crustal and upper mantle structure of this region, teleseismic receiver functions were calculated from 500 seismic events recorded on 92 temporary and permanent broadband seismic stations. Overall, we see a good correlation between crustal thickness and elevation throughout central Anatolia, indicating that the crust may be well compensated throughout the region. We observe the thickest crust beneath the Taurus Mountains (>40 km); it thins rapidly to the south in the Adana Basin and Arabian plate and to the northwest across the Inner Tauride suture beneath the Tuz Gölü Basin and Kırşehir block. Within the Central Anatolian Volcanic Province, we observe several low seismic velocity layers ranging from 15 to 25 km depth that spatially correlate with the Neogene volcanism in the region, and may represent crustal magma reservoirs. Beneath the central Taurus Mountains, we observe a positive amplitude, subhorizontal receiver function arrival below the Anatolian continental Moho at ~50–80 km that we interpret as the gently dipping Moho of the subducting African lithosphere abruptly ending near the northernmost extent of the central Taurus Mountains. We suggest that the uplift of the central Taurus Mountains (~2 km since 8 Ma), which are capped by flat-lying carbonates of late Miocene marine units, can be explained by an isostatic uplift during the late Miocene–Pliocene followed by slab breakoff and subsequent rebound coeval with the onset of faster uplift rates during the late Pliocene–early Pleistocene. The Moho signature of the subducting African lithosphere terminates near the southernmost extent of the Central Anatolian Volcanic Province, where geochemical signatures in the Quaternary volcanics indicate that asthenospheric material is rising to shallow mantle depths. INTRODUCTION Central Anatolia displays a typical plateau-like morphology that appears similar to other collision-related plateaus but on a smaller scale. Elevation increases in the interior of the central Anatolian plateau to the north and the Taurus Mountains to the south (Fig. 1). The crustal architecture of the central Anatolian plateau comprises the amalgamation of continental fragments that coalesced during the closure of the Neo-Tethyan Ocean system between AfricaArabia and Eurasia and records both subduction and collisional-related processes (Şengör and Yılmaz, 1981). To the east of the central Anatolian plateau, compression related to the Arabia-Eurasia continental collision dominates the formation and development of tectonic structures, while regional extension due to the rollback of the African slab has dominated the west since the Miocene (Bozkurt, 2001; Ring et al., 2010). These processes have led to the development of the Anatolian plate, which has been extruding westward since the Miocene as a result of African slab rollback and Arabia-Eurasia collision (Şengör et al., 1985; Reilinger et al., 2006). Thanks to the expansion of seismic station coverage in Turkey, a number of regional-scale studies of the eastern Mediterranean have provided researchers with a broad understanding of the seismic structure of the Anatolian system (e.g., Biryol et al., 2011; Mutlu and Karabulut, 2011; Salaün et al., 2012; Fichtner et al., 2013; Vanacore et al., 2013; Delph et al., 2015; Govers and Fichtner, 2016). Smaller scale regional studies have also been performed using temporary seismic deployments to investigate the seismic structure of this system in higher detail, such as the North Anatolian Fault Experiment (Beck and Zandt, 2005) and the Eastern Turkey Seismic Experiment (Sandvol et al., 2003). This has resulted in dense seismic station coverage throughout most of Turkey, when combined with the extensive backbone network of the Kandilli Observatory and Earthquake Research Institute (KOERI). However, until recently seismic station coverage and associated studies investigating central Anatolia have been largely neglected due to its relative stability and assumed low potential of large-magnitude earthquakes. GEOSPHERE GEOSPHERE; v. 13, no. 6 doi:10.1130/GES01509.1 8 figures; 1 supplemental file CORRESPONDENCE: bizhan .abgarmi@metu .edu.tr CITATION: Abgarmi, B., Delph, J.R., Ozacar, A.A., Beck, S.L., Zandt, G., Sandvol, E., Turkelli, N., and Biryol, C.B., 2017, Structure of the crust and African slab beneath the central Anatolian plateau from receiver functions: New insights on isostatic compensation and slab dynamics: Geosphere, v. 13, no. 6, p. 1774–1787, doi:10.1130/GES01509.1. Received 4 February 2017 Revision received 16 August 2017 Accepted 27 September 2017 Published online 27 October 2017 For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.

The effects of subduction termination on the continental lithosphere: Linking volcanism, deformation, surface uplift, and slab tearing in central Anatolia

(1) Department of Earth Science, Rice University, Houston, Texas, United States (jrdelph@rice.edu), (2) Faculty of Engineering, Middle East Technical University, Cankaya, Ankara, Turkey, (3) The Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah, USA, (4) Department of Geosciences, The University of Arizona, Tucson, Arizona, United States, (5) Department of Geological Sciences, University of Missouri, Columbia, Missouri, United States, (6) Department of Geophysics, Bogazici University (KOERI), Cengelkoy, Istanbul, Turkey

Subduction termination through progressive slab deformation across Eastern Mediterranean subduction zones from updated P-wave tomography beneath Anatolia

National Science Foundation [EAR-1109336]; Wiess Postdoctoral Research Fellowship at Rice University

Probabilistic Seismic‐Hazard Assessment for East Anatolian Fault Zone Using Planar Fault Source ModelsProbabilistic Seismic‐Hazard Assessment for East Anatolian Fault Zone Using Planar Fault Source Models

The objective of this article is to provide state-of-the-art probabilistic seismic-hazard assessment maps for the East Anatolian fault zone (EAFZ) based on planar seismic-source models and up-to-date ground-motion models. Development of fault-based seismic-source models requires the definition of source geometry in terms of fault length, fault width, fault-plane angles, and segmentation points for each segment of the EAFZ, building rupture systems that consider fault-to-fault ruptures and associating the observed seismicity with defined rupture systems. This complicated task was performed by compiling the seismotectonic characteristics of the EAFZ using available geological information and the instrumental earthquake catalogs of Turkey. Recently published global Next Generation Attenuation (NGA)-West2 groundmotion models (Bozorgnia et al., 2014) and Turkey-adjusted NGA-West1 models (Gülerce et al., 2016) are used in the ground-motion logic tree with equal weights. The results are presented in terms of the seismic-hazard maps for hazard levels in design codes for different spectral periods and for rock-like reference site conditions (VS30 760 and 1100 m=s).

M‐Split: A Graphical User Interface to Analyze Multilayered Anisotropy from Shear‐Wave Splitting

ABSTRACT Shear‐wave splitting analysis is commonly used to infer deep anisotropic structure. For simple cases, obtained delay times and fast‐axis orientations are averaged from reliable results to define anisotropy beneath recording seismic stations. However, splitting parameters show systematic variations with back azimuth in the presence of complex anisotropy and cannot be represented by average time delay and fast‐axis orientation. Previous researchers had identified anisotropic complexities at different tectonic settings and applied various approaches to model them. Most commonly, such complexities are modeled using multiple anisotropic layers with priori constraints from geologic data. In this study, a graphical user interface called M‐Split is developed to easily process and model multilayered anisotropy with capabilities to properly address the inherited nonuniqueness. M‐Split program runs user‐defined grid searches through the model parameter space for two‐layered anisotropy using formulation of Silver and Savage (1994) and creates sensitivity contour plots to locate the local maxima and analyze all possible models with parameter trade‐offs. To minimize model ambiguity and identify the robust model parameters, various misfit calculation procedures are also developed and embedded in M‐Split and can be used depending on the quality of the observations and their back azimuthal coverage. M‐Split is an open‐source program and can be extended by users for additional capabilities or for other applications.