Slip Rates on the Main Marmara Fault from Bayesian Inversion of Dense GNSS and InSAR Velocity Fields

The Main Marmara Fault lies at the western termination of the North Anatolian Fault. While the North Anatolian Fault ruptured from its eastern termination to the Izmit segment over the 20th century through a westward‐propagating sequence of Mw ≥ 7 earthquakes, the underwater Marmara segment has not experienced a large earthquake in recent times. However, due to its proximity to the megacity of Istanbul, this segment represents one of the most hazardous fault systems in the Middle East. In particular, no historical earthquake has been identified on the Central Basin segment since at least 1766, potentially making it a major seismic gap.

To better assess seismic hazard along the Main Marmara Fault, we estimate the slip rate by jointly using a dense GNSS velocity field and four Sentinel-1 InSAR tracks. The GNSS velocity field consists of 111 measurements, including newly acquired densified sites along the northern shore of the Marmara Sea. The InSAR velocity field was processed automatically within the framework of the FLATSIM project, covering the period from October 2016 to April 2021. InSAR velocities are referenced to a Eurasia-fixed plate using the GNSS velocity field. We then perform a joint Bayesian inversion of slip rates using both datasets, allowing us to quantify uncertainties on the estimated slip rates.

Our results indicate that the Main Marmara Fault is predominantly creeping between longitudes 27.5 and 28.6, implying that the Central Basin segment is largely aseismic. Uncertainty estimates and forward modeling demonstrate that our datasets are capable of resolving slip behavior on this segment with good accuracy. However, the shallow portion of the Central Basin segment is still accumulating up to ~10 mm/yr of slip deficit, which could permit earthquakes of up to Mw 6.0 every few decades, similar to the 2025 sequence. West of longitude 27.5 and east of longitude 28.6, including the Prince Islands segment, the fault appears to be mostly locked down to 12 km depth. On the Prince Islands segment, close to Istanbul, the accumulated strain has the potential to generate an earthquake with Mw > 7.