Earthquake focal mechanism and stress tensor analysis along the central segment of the North Anatolian fault


Thesis Type: Postgraduate

Institution Of The Thesis: Middle East Technical University, Faculty of Engineering, Department of Geological Engineering, Turkey

Approval Date: 2010

Thesis Language: English

Student: EZGİ KARASÖZEN

Principal Supervisor (For Co-Supervisor Theses): Atilla Arda Özacar

Co-Supervisor: Nuretdin Kaymakcı

Abstract:

The North Anatolian Fault (NAF) is one of the world’s largest active continental strikeslip faults, and forms the northern margin of the Anatolian plate. Although its geologic and geomorphologic features are well defined, crustal deformation and associated seismicity around central segment of the NAF is relatively less-known. In this study, we analyzed locations and focal mechanisms of 172 events with magnitude ≥ 3, which are recorded by 39 broadband seismic stations deployed by the North Anatolian Passive Seismic Experiment (2005-2008). Distribution of the events shows that the local seismicity in the area is widely distributed, suggesting a widespread continental deformation, particularly in the southern block. For the entire data set, P- and S- arrival times are picked and events are relocated using the HYPOCENTER program. Then, relocated events which have a good azimuthal coverage with a maximum gap of 120° and at least 13 P- wave readings are selected and 1-D inversion algorithm, VELEST, is used to derive the 1-D seismic velocity model of the region. The final model with updated locations is later put together to the FOCMEC program, to obtain focal mechanisms solutions. In this step, an iterative scheme is applied by increasing the number of data errors. To obtain more unique solutions, first motions of P and SH v phases are used along with SH/P amplitude ratios. Resultant 109 well-constrained focal mechanisms later used to perform stress tensor inversion across the region. Our focal mechanisms suggest a dominant strike-slip deformation along two major fault sets in the region. In the east, E-W trending splays (Ezinepazarı, Almus, and Laçin Kızılırmak) show right-lateral strike-slip motion similar to the NAF whereas in the west, N-S trending faults (Dodurga, Eldivan) show left lateral strike-slip motion. Overall, stress orientations are found as: maximum principal stress, σ1, is found to be subhorizontal striking NW-SE, the intermediate principle stress, σ2, is vertically orientated and the minimum principal stress, σ3, is found to be NE –SW striking, consistent with the strike-slip regime of the region.