Seismic zonation of Makran using Seismotectonic Approach

Earthquake hazard of the Makran subduction zone have been studied as a part of national and/or small-scale earthquake hazard maps. Due to the ever-rising strategic and economic importance of Makran, calculation of a reliable seismic hazard map of Makran has already become a necessity. Seismic zonation is the most important step in any seismic hazard analysis. This paper uses a seismotectonic approach to present a relaible seismotectonic zonation for Makran where the earthquake catalogs show very low level of seismicity. The sparsity of the observed earthquakes is compensated by geophysical, geological, and geodetic data and also the assumption of similarity and analogy of tectonic zones. Recently, extensive geological, seismological and geodetical field studies have been conducted in Makran. Using the new information about segmentation of Makran megathrust, geometry and dip of the subducting plate, crustal structure of the wide accretionary prism, Jaz Murian and Mashkal Depression, improved seismicity and GPs velocity vectors, Makran and its surroundings were divided into 14 seismic zones. To take into account the Makran subducting plate, three-dimensional seismic zones are defined in different depth ranges. For example, normal earthquakes with intermediate-depth focal depth (40-75 km) are related to bending of the subducting oceanic lithosphere and megathrust earthquakes are related to the Makran megathrust zone lying in the depth range of 20-40 km. One seismic zone at depth range of 40-75 km is considered to represent the seismic hazard arising from the intermediate-depth earthquakes. The southern younger part of Makran megathrust zone is divided into two eastern and western zones where the western part has a much smaller locking ratio than the eastern part. The northern and older part of the wide accretionary prism of Makran is considered as a separate sesismic zone because it is consisted of very thick high seismic-velocity sedimentary cover. The eastern and western ends of Makran are regions with large rate of geodetic strain accomodated by almost N-S left-lateral strike-slip motion on the Chaman and right-lateral strike-slip motion on the Minab-Zendan-Palami fault systems, respectively. The transfer zones are represented by two seismic zones. Two seismic zones are related to aseismic Jaz Murian and Mashkel depressions. Lut block and Helmand blocks, and Sistan Suture Zone acts a backstop of Makran accretionary wedge. Three separate seismic zones are considered to account for different kinematics and crustal structure of the Lut and Helmand blocks, and Sistan Suture Zone. Due to the large observed geodetic strain rate, the new seismic zonation would predict the highest seismic hazard in Minab-Zendan-Palami and Ornachnal-Chaman zones. Due to much larger locking ratio, the earthquake and tsunami hazard in eastern Makran would be much higher than that of western Makran. Iranshahr is expected to have large seismic hazard level because it is located at the intersection of N-S trending fault system along the western edge of Sistan Suture Zone and the E-W trending fault system south of Jaz Murian Depression and additionally located on top of the seismic zone responsible for production of the intermediate-depth earthquakes.