Seismicity Before and After the 2023 <i>M</i>7.7 and <i>M</i>7.5 Turkey Earthquakes

A magnitude (M) 7.7 earthquake struck on February 6, 2023 in Turkey. Nine hours later, a M7.5 earthquake occurred near the initial M7.7 quake. We studied seismicity before and after these doublet quakes, integrating physics-based and statistical approaches. We first used the statistical Epidemic-Type Aftershock Sequence (ETAS) and the Bayesian Gu-tenberg-Richter b-value models to confirm previously-reported seismicity transients (seismic activation and low b-values) prior to the future M7.7 quake. We then showed that the low b-value area coincided with a high-slip area on the strand segment from which the M7.7 rupture started, a similar result to that obtained for the 2011 Tohoku megaquake case in Japan. We next used the physics-based Coulomb and statistical b-value models to find which locations of the largest and second largest events in the post-doublet-quake se-quence were in relatively high-stress regions and became closer to failure, as a result of the doublet quakes. We further used the ETAS model to show that this sequence is currently active, but is decaying with time. The duration of the sequence was estimated at 2.7-5.5 years, longer than previously proposed (1-2.5 years). Our result was stable because it was based on earthquake data in about 600 days, six times longer than the study period used in a previous study.