Geodynamics of the Makran Subduction Zone inferred from Relocated Seismicity

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 ~15o in the depth range of ~45-80 km, increasing to ~45o 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 consid-erable number of intermediate-depth earthquakes along narrow ENE-WSW lineaments implies a pre-existing weakness in the subducting plate.马克兰俯冲带（Makran Subduction Zone）具有独特性：其呈东东北-西西南（ENE-WSW）走向的火山弧，与近东西向（E-W）展布的增生楔（accretionary prism）并不平行。这种构造不协调现象被归因于俯冲角向东逐渐减小。为验证该假说，我们对1968年至2022年间该区域的146次中强以上地震开展了重新定位工作。研究结果显示，马克兰东部区域的中深源地震分布带宽度恒定约180 km，其走向与火山弧走向近乎平行；多数正断层型中深源地震的断层面也与火山弧走向一致。上述观测结果表明，马克兰东部存在东东北-西西南走向的俯冲板片枢纽，且俯冲角度保持恒定。马克兰西部的中深源地震呈近东西向展布，指示此处俯冲板片枢纽为近东西走向。我们将马克兰东西部板片枢纽的取向差异，归因于上覆板块形态的不同。马克兰东部的累积和达-贝尼奥夫带（Wadati-Benioff Zone）数据显示，在约45~80 km的深度区间内，俯冲角约为15°，当深度超过80 km后，俯冲角增至约45°。马克兰西部中深源地震显著的走滑断层分量表明，其东西部俯冲板片所处的应力环境存在显著差异。大量中深源地震沿狭窄的东东北-西西南向构造线展布，这暗示俯冲板片内部存在预先存在的软弱带。