Adalar at the Eastern Marmara Seismic Gap: The August 2026 Earthquake Cluster and Its Implications for Seismic Hazard near Istanbul

Shaheen Mohammed Saleh Ahmed 

The eastern Marmara Sea and Adalar sector lie close to İstanbul and the western North Anatolian Fault, where earthquake-source processes interact with strong basin and site effects. This study integrates a reconciled earthquake catalog, waveform-quality measures, spatial statistics, trial hypocentral relocation, reconnaissance focal mechanisms, point-source Coulomb stress screening, and published engineering-geological evidence for 1–20 August 2026. The catalog contains 127 regional events, including 79 events within 40 km of Adalar. Sixty-two of the Adalar-window events occurred on 19–20 August, and the largest event was ML 3.2 at a reported depth of 7 km. A 10-km spatial neighborhood produced the strongest standardized Moran result (z = 21.69), and 13 cells remained in the 99% Getis–Ord Gi* hot class after false-discovery-rate correction. Bootstrap standard deviational ellipses confirm an elongated distribution but do not define a rupture plane. The waveform archive contains 549 automatic P picks from 49 stations; however, 547 picks are from one network, and only four automatic picks were retained for the ML 3.2 event. Twelve of 22 trial relocations passed permissive gates, but only four satisfied a stricter joint 10-km horizontal and depth-uncertainty criterion. The focal-mechanism sample is too sparse and dispersed to define a new Adalar stress tensor, so the published regional maximum compression near N35°E remains the most defensible first-order constraint. A point-source model for the 23 April 2025 Mw 6.2 event gives a small positive median Coulomb stress change of +0.204 kPa in the Çınarcık receiver corridor; this does not demonstrate triggering. The combined evidence supports denser offshore monitoring, reviewed phase picking, improved relocation, and site-specific geotechnical investigation. It does not predict the time, magnitude, or location of a future earthquake.