On the potential seasonality of seismicity along the North Anatolian Fault, Marmara region

Seasonal modulation of seismicity has been reported in several regions worldwide, suggesting that earthquake occurrence may be sensitive to small, time-dependent stress perturbations. Such observations point to a range of hydro-meteorological processes that can generate seasonal stress changes, including variations in groundwater storage, rainfall, snow accumulation and melt, and sea-level fluctuations. Although the associated stress amplitudes are typically small, often of the order of a few kilopascals, they may influence the temporal distribution of seismicity. The mechanical response of faults to such forcings may involve different processes, including elastic loading and unloading, as well as poro-elastic and thermo-elastic effects.

Recently, seismicity associated with an active hydro-thermal system in the eastern Marmara Sea has been shown to respond to temporal variations in sea level. In this setting, sea-level changes induce small vertical loading variations that generate stress perturbations of a few kilopascals, sufficient to modulate seismicity timing in a critically stressed, fluid-rich crust. Here, we extend the study area to examine whether seasonal variations in the Marmara Sea level are associated with seismicity variations across the entire Marmara region, with a particular focus on seismic activity along the North Anatolian Fault Zone.

We analyze seismicity using an earthquake catalog covering the Marmara region for the period 2006–2024. The catalog is declustered using an adaptable Random Forest–based approach to isolate background seismicity and reduce the influence of aftershock sequences. Temporal variations in background seismicity are then examined using Multichannel Singular Spectrum Analysis (MSSA) and Multi-Seasonal Trend decomposition using Loess (MSTL), enabling the identification of independent seasonal components in seismicity rates. The resulting seasonal signals are compared with independent observations of surface loading, including GRACE-derived mass variations and Marmara Sea level changes derived from satellite altimetry and local tide-gauge records. We use these comparisons to assess the mechanisms controlling the seasonal variability observed in the seismicity catalog.