How an extensional pull-apart basin modulates fault slip and earthquake rupture on the Main Marmara Fault

Pull-apart basins introduce extensional bends and strong along-strike heterogeneity into otherwise strike-slip systems, potentially altering slow slip, earthquake nucleation, and multi-segment rupture. The Princes Islands segment of the Main Marmara Fault (MMF) hosts a ~30-40 km pull-apart basin within the Marmara seismic gap south of Istanbul, where geodetic and geological observations suggest partial unlocking and complex rupture behavior. Yet the mechanical role of this extensional geometry in controlling fault slip and rupture remains poorly constrained. We perform three-dimensional quasi-dynamic simulations using PyQuake3D (Tang et al., 2025) to quantify the impact of the Princes Islands pull-apart basin on interseismic loading, slow-slip transients, dynamic rupture propagation, and earthquake recurrence along the MMF. We model a continuous fault surface with variable dip, bends, and segmentation, and prescribe depth-dependent effective normal stress, spatial frictional contrasts, and stress heterogeneity representative of extensional basin environments, guided by published geophysical constraints. Our results show that the basin exerts a first-order control on slip style and rupture outcomes through the competition between geometric unclamping, frictional heterogeneity, and stress structure. Extensional bends favor localized unlocking and recurrent aseismic or slow-slip episodes, which in turn modulate where dynamic ruptures nucleate. At the event scale, the basin can behave either as a rupture barrier or a rupture accelerator: in many realizations, ruptures do not continue smoothly across the basin but instead produce triggered seismicity via static stress transfer and re-nucleation near segment boundaries. Stress concentrations at geometric transitions primarily govern nucleation locations, while frictional contrasts regulate rupture persistence and arrest. These findings highlight that explicitly representing extensional fault structures is critical for assessing multi-segment rupture potential and time-dependent seismic hazard on the MMF near Istanbul.

 

Tang, R., Gan, L., Li, F., & Dal Zilio, L. (2025). PyQuake3D: A Python tool for 3‐D earthquake sequence simulations of seismic and aseismic slip. Journal of Geophysical Research: Machine Learning and Computation, 2(4), e2025JH000871.