Dynamic activation of near-orthogonal conjugate faults during earthquakes:Insights from the 2023 Türkiye Mw 7.6 earthquake
Conjugate faults represent one or several pairs of faults,which are located close to one another but exhibit opposite senses of slip.The geometric configuration of conjugate faults,such as their intersection angle,depends on stress conditions and rock rheological properties,and may even change during the course of fault evolution.Classical fault-mechanics theory,such as the Anderson's theory of faulting or the Mohr-Coulomb failure criterion,predicts that conjugate faults should form or become reactivated at～60° under dry,brittle(with typical friction coefficient of 0.6)and small-strain conditions,which has been widely verified by many field and experimental observations.However,in the last decades an increasing number of natural observations have shown that near-orthogonal conjugate faults can be activated during earthquakes,which is in stark contrast with the prediction of the classical fault-mechanics theory.Although several special mechanisms,such as high pore fluid pressure,low intrinsic rock friction and ductile deformation,are later proposed to explain the near-orthogonal configuration of conjugate faults,these mechanisms still do not provide a satisfactory explanation for the detailed process of fault activation during earthquakes.The 2023 Türkiye Mw 7.6 earthquake,which ruptured multiple fault segments including a pair of high-angle(～75°)conjugate ones to the west,provides a good opportunity to probe the activation process of near-orthogonal conjugate faults during earthquakes.Based on the available information from surface rupture trace,finite source inversion for the mainshock and relocated aftershocks,we build a simplified two-dimensional reference fault model,taking into account the diverse orientation of different segments and the connectivity condition between them.First,we use this reference model to numerically simulate a scenario of rupture process that is largely consistent with observations,including supershear rupture and depleted off-fault inelastic deformation along the western half of the main fault.Then,we vary several model parameters for the near-orthogonal conjugate pair to evaluate the sensitivity and generality of the results.The simulation results show the following features:(1)Different fault segments,including the near-orthogonal conjugate pair,are activated sequentially rather than simultaneously;(2)conjugate fault segments with a range of intersection angles can be activated,but the activation result depends on the detailed fault geometry and frictional properties;(3)rupture termination by a barrier along a former segment can facilitate the triggering of a conjugate segment that intersects the former near the barrier.These results provide useful clues for understanding the mainshock rupture process and the aftershock pattern of the 2023 Türkiye Mw 7.6 earthquake.Furthermore,we find that the above features are also valid for many other strike-slip earthquakes,including the 2021 Yangbi(Mw 6.1)earthquake in Yunnan(China),the 2019 Ridgecrest(Mw 6.4 and 7.1)earthquake sequence in California(USA),the 2000 Wharton Basin(Mw 7.8)earthquake,the 2014 Yutian(Mw 6.9)earthquake in Xinjiang(China)and the 2020 Monte Cristo Range(Mw 6.5)earthquake in Nevada(USA).In all these examples,the activation of the secondary fault occurs in the extensional quadrant of the primary fault,and is associated with an opposite sense of slip.We therefore infer that the dynamic reduction of normal stress must play a critical role and the effective fault friction coefficient cannot be too low,otherwise we should have observed successfully activated secondary fault in both compressional and extensional quadrants of the primary fault.Combining all of the above,we conclude that for many(perhaps most)shallow strike-slip earthquakes,rupture-induced stress transfer can serve as a viable mechanism to activate near-orthogonal conjugate faults,with no need to invoke high pore fluid pressure,low intrinsic fault friction or ductile deformation.
Türkiye earthquakeconjugate faultsfault reactivationearthquake rupturestress transfer
万方数据
