Geodynamics of the Makran subduction zone inferred from relocated seismicity

Abstract

The Makran Subduction Zone is unusual, with an ENE-WSW trending volcanic arc that is nonparallel to its ∼E-W trending accretionary prism. The discrepancy has been attributed to an eastward reduction of the subduction angle. To assess this theory, we relocated 146 moderate to large earthquakes in the region during the period 1968 to 2022. We found that in eastern Makran, the region with intermediate-depth earthquakes has a constant width of ∼180 km, with a trend sub-parallel with the trend of the volcanic arc. The fault planes of most of the normal-faulting intermediate-depth earthquakes are also parallel to the trend of the volcanic arc. These observations suggest an ENE-WSW trending subducting plate hinge and a constant subduction angle in eastern Makran. The ∼E-W trend of the intermediate-depth earthquakes in western Makran indicates an ∼E-W trending subducting plate hinge. We relate the different orientation of the plate hinges in eastern and western Makran to the different shape of the overriding plates. The cumulative Wadati-Benioff zone for eastern Makran shows a subduction angle of ∼15^o in the depth range of ∼45–80 km, increasing to ∼45^o at depths greater than 80 km. The significant component of strike-slip faulting of the intermediate-depth events in western Makran implies that the subducting plate in western and eastern Makran experience different stress regimes. Alignment of a considerable number of intermediate-depth earthquakes along narrow ENE-WSW lineaments implies a pre-existing weakness in the subducting plate.