Complex active deformation along southwestern part of the East Anatolian Fault Zone: Insights from 2023 Türkiye earthquake doublet

The southwestern continuation of the East Anatolian Fault Zone (EAFZ), specifically its relationship with Iskenderun Basin, the Dead Sea Fault Zone (DSFZ) and Cyprus Slap is still enigmatic. In 2023, nearby splays of EAFZ in the southwest are ruptured by two large earthquakes that are nine hours apart. At first, Pazarcık earthquake (M7.8) initiated at a secondary fault, later jump to the main strand of EAFZ and propagated bilaterally producing a surface rupture exceeding 315 km in length. The Ekinözü earthquake (M7.7) triggered nine hours later also displayed bilateral rupture propagation and produced a 140 km long surface rupture. Surface deformations associated to both events that ruptured multiple fault segments with left-lateral strike-slip mechanism, are mapped in detail using satellite images and field observations. Surface offsets of both events are highly variable, reaches up to 8 m, and controlled mainly by subsurface slip. The accuracy of mapped active faults prior to the doublet, reduce significantly along plains where distributed deformations are common and occasionally surface rupture follows mapped inactive faults suggesting reactivation of old faults or unrecognized active faulting in the area.

Large 19th century earthquakes previously associated to the faults ruptured during this doublet, are likely mislocated and these segments were accumulating stress at least for 500 years. Earthquake mechanisms recorded before and after the doublet revealed strike-slip regime corresponding well with EAFZ but towards south, extensional events become abundant. Based on the computed stress field, east-west striking Çardak fault ruptured during the second event, is not optimally oriented for left-lateral failure but suffered from noticeable static stress increase and rate-and-state friction based simulations including both static and dynamic effects suggested that it was at the end of its seismic cycle. Static stress changes resulted from the doublet also indicate pronounced increases, especially along Malatya, Savrun, Türkoğlu and Antakya fault segments which are remained as seismic gaps.

GPS based slip models along multiple profiles constrained left-lateral slip rates of ruptured faults and suggested an increase in slip rate from south to north across EAFZ. During Pazarcık earthquake, rupture made a sharp bend towards south rather than following parallel fault segments towards Adana which are previously proposed as the western continuation of EAFZ. Our field observations indicated a fault traversing the Amanos mountains parallel to EAFZ along which fault kinematics and compiled GPS data together suggest left-lateral motions. Based on these findings, alternative regional kinematic models assuming Iskenderun and Maras blocks as independent or intact are established and later utilized for probabilistic seismic hazard analysis throughout the Adana basin by considering variable site conditions and basin effect in long spectral periods.