Detection of meteorologically triggered seiche oscillations, eigenanalysis, and implications for tsunami hazards in the Sea of Marmara

Seiches are resonant oscillations that occur when gravity waves in a basin are excited at a period coinciding with one of the periods of free oscillation of the basin. When triggered by earthquakes, seiches may influence the amplitude of tsunamis. They may also play an important role in shaping sedimentary deposits occurring during these events. Our study combines in situ monitoring (performed in the framework of EMSO-France and of Maregami Türkiye-France bilateral project)  and numerical modeling to characterize seiches in the Sea of Marmara, where the North Anatolian Fault system causes large earthquakes associated with turbidite-homogenite deposits and tsunamis. Pressure sensors deployed at five different locations at the seafloor in the Sea of Marama basins recorded bursts of small amplitude oscillations (< 1 hPa) with periods ranging from 5 to 200 minutes, apparently triggered by storms. Resonance spectra were extracted by cepstrum analysis, a method commonly used in speech recognition. Observed resonance modes were characterized by their period at peak amplitude and by their log amplitude at each deployment location. Theoretical free oscillation modes were calculated as eigenvalues and eigenvectors of the shallow water equation with the best available bathymetry (99 modes were calculated with periods ranging 17 to 183 minutes). These provide a better match of observed resonance frequencies than the shortcut calculation of Yalciner and Pelinovki (2002), especially at long periods (> 80 minutes). However, all calculated modes involve resonances in the shallow parts of the Sea of Marmara (shelves and bays) and most have low amplitudes in the deep basins, which may hinder their detection. Thus, it has not been possible to match observed and calculated modes one by one, but some observed-calculated pairs have fitting periods and fitting spatial variations in amplitude. Of specific interest, matching modes detected at periods of about 25 minutes have large theoretical amplitudes at the Istanbul coast, which may help explain historical reports and sedimentological evidence of tsunamis.