Browsing by Author "Pinar, Ali"

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An Integrated Critical Approach To Off-Fault Strike-Slip Motion Triggered by the 2011 Van Mainshock (Mw 7.1), Eastern Anatolia (Turkey): New Stress Field Constraints on Subcrustal Deformation
(Pergamon-elsevier Science Ltd, 2021) Toker, Mustafa; Pinar, Ali; Hoskan, Nihan
In this study, we retrieved the finite source characteristics of the October 23, 2011 Van earthquake (Mw 7.1) using the teleseismic waveforms to focus on the source location. The outstanding off-fault aftershock sequence of the Van mainshock was readily explained by calculating the Coulomb stress changes imparted to the surrounding crust. This may be accomplished through finite source modelling to examine the stress interaction between the fault, ruptured by the Van mainshock, and the surrounding fault(s) triggered by the same mainshock. In addition, to provide further support for the Coulomb failure stress changes in the off-fault area, centroid moment tensor (CMT) inversion of the off-fault aftershocks was performed and stress tensors were derived from their focal solutions. This identified the dominant fault slip, the constraints of the crustal stress fields and illuminated the crustal nature of the stress interaction. The off-fault aftershocks showed a strike-slip stress regime in rotational (to NW) and non-rotational (to N) stress fields of the upper and lower crusts, respectively. However, this was inconsistent with a horizontal compressional stress direction striking to the north. This suggests that a local source and/or rotation of lateral variation in stress magnitudes in crustal and sub-crustal structures strongly perturbed the regional stress field. It was also evident that these strike-slip aftershocks increased the intensity of stress in an off-fault area, NE of the source rupture. This revealed a uniquely triggered strike-slip motion, activated and rooted in the weak lower crust. We conclude that the Van mainshock rupture source area, associated with the stress changes imparted to the surrounding crust, had undergone anomalous modifications to generate distinctive off-fault aftershock responses in the entire crust, and also triggered and loaded the weak lower crust. We hypothesize that the strike-slip motion, the so called "transfer fault", as a distinctly triggered slip event, was generated or selectively activated by subcrustal ductile processes in the absence of mantle lid beneath the study area. However, local slab fragmentation, tearing and cold mantle beneath the study area lead to paradigm changes in interpreting the strike-slip motion and subcrustal deformation. The presence of a small piece of oceanic lithosphere, consistent with fragmented, torn slab and cold mantle, may be an alternative hypothesis that remains to be tested. The Van earthquake, combined with careful examination of associated off-fault aftershocks, revealed new information about stress field constraints on subcrustal deformation. This investigation also provided insights into an important role of stress interaction, with a newly discovered transfer fault within the offfault area, which extends through the entire crust beneath Lakes Van and Ercek areas.
New Strain Configuration of Convergence in the Arabia-Eurasia Collisional Zone: a Comprehensive Analysis of Large Shear Zones Inferred From Geodetic and Seismic Observations
(Pergamon-elsevier Science Ltd, 2024) Toker, Mustafa; Pinar, Ali
A new strain configuration is presented concerning the ongoing convergence deformation field in the TurkishIranian High Plateau (TIHP), E Anatolia-NW Iran. The strain configuration is derived from the contribution of geodetic strain-rate fields using GPS velocity data to reveal new constraints not documented previously in the region. This provides good evidence for novel "highland shear strain zones" by analysis of the combination of interpolated GPS velocity fields and seismological observations with maximum principal strain axes. Both the strain-rate fields and rotational elements of the estimated geodetic deformation exhibit juxtaposed, patchy strain rate partitioning in E Anatolia-NW Iran. These comprise both compressional strain lobes combined with local extensional areas and large shear strain fields in the core of the collision where the rotation rate fields present an unexpected propagation zone of continuous, counterclockwise rotation towards the W. The main results define approximately N-S- and W-E-trending, large, continuous, diagonal shear strain patterns with high total strain rate and maximum shear strain rate, consistent with right-lateral strike-slip and normal focal mechanisms. These are the Longitudinal Shear Zone (LSZ) extending from S to N and dividing the TIHP into the E Anatolia and NW Iran blocks, and the right-lateral Transcurrent Shear Zone (TSZ) extending from E (NW Iran) to W (E Anatolia). These differential movements of the inconsistent LSZ and TSZ shape both the highly buoyant plateau topography and the area, which undergoes migration to the W with respect to stable Eurasia by means of plateau push. Geodetic deformation compatible with fault focal data indicates that the LSZ and TSZ, in the framework of thermal and mechanical changes in the structure of the lithospheric thickness beneath the TIHP, are invaded and shaped by flowing mantle material that triggers and drives lateral escape of convergent crustal blocks, and configures the convergence with a shear-dominated deformation regime. The shear regime is primarily driven by NE- and NWdirected mantle flow fields and associated buoyancy forces due to slab fragmentation and tears in the region, exerting dominant control over the differential evolution of the LSZ and TSZ patterns. The new strain configuration for this convergence illuminates "the highland tectonics of large shear zones" resulting from slab thinning or its absence, and challenges conventional tectonic strain models in the TIHP.