Estimation of size of megathrust zone in the Makran subduction system by thermal modelling

Abstract

SUMMARY To estimate the maximum possible size of megathrust earthquakes, we calculate the thermal structure along two profiles in west and east Makran subduction zone by solving the steady-state 2-D energy equation. For the western profile, we derive the slab geometry from a recent receiver function study along IASBS (Institute for Advanced Studies in Basic Sciences) seismic profile in the onshore part of the Iranian Makran. For the eastern profile, the slab geometry is derived from a recent relocation of seismicity of Makran. Using the improved slab geometry and a force balance establishment in the accretionary wedge, the effective coefficient of friction, $\mu ^{\prime}$, is assumed to be equal to 0.03. We estimate the updip and downdip of the megathrust zone by simultaneously considering the seismicity related to the events with thrust and normal mechanisms and intersection between 100–150 and 350–450 °C isotherms and the subducting slab interface. Along the western profile, the megathrust updip locates ∼95 km north of the deformation front (DF) at the depth of ∼20 km and the downdip locates ∼300 km north of the DF at the depth of ∼35 km. Presence of normal mechanism events at deeper depths indicates that the downdip limit of the megathrust zone is consistent with the 350 °C isotherm. The megathrust width is ∼205 km along the western profile. Along the eastern profile, the megathrust updip locates ∼60 km north of the DF at the depth of ∼15 km and the downdip locates ∼280 km north of the DF at the depth of ∼35 km. The downdip limit of the megathrust zone is closely related to the 350 °C isotherm. The megathrust width is ∼220 km along the eastern profile. Assuming a segmentation of the thrust zone into the western and eastern parts, the areal size of the megathrust zones in west and east Makran is ∼82 000 and 88 000 km2, respectively. We estimate the magnitude of the largest possible megathrust earthquakes in the west and east Makran to be 8.65 ± 0.26 and 8.75 ± 0.26 Mw, respectively.