Purpose. The purpose of the study is to assess the potential of coastal laser strainmeters for recording and quantitatively identifying seiche oscillations in small coastal water areas using the example of Alekseev Bay (Popov Island, Peter the Great Bay).

Methods and Results. Micro-displacements of the Earth's crust were recorded by a 20-metre coastal laser strainmeter installed at a distance of approximately 20 meters from the coastline. To independently monitor sea level fluctuations, video recording of a marker object in the bay was performed, followed by metric image calibration (with a scale factor of 0.69 cm/px). Comparison of the signals required identical bandpass filtering of the time series within the range of the main seiche mode. A linear relationship between the micro-displacements of the strainmeter base and the amplitude of sea level fluctuations was revealed. The linear transformation coefficient K = 0.66±0.03 μm/cm at |r| > 0.9 was determined based on a series of synchronous observation intervals. Analysis of time series, dynamic spectrograms, and amplitude spectra made it possible to detect stable periods of approximately 10.5 min, 2.6–2.7 min, and 4.1–4.3 min, which correspond to the fundamental longitudinal seiche of the whole bay, the first eigenmode, and the localised mode of the inner sub-basin, respectively. It was shown that intensification of wind forcing results in an increase in the oscillation amplitude without changing the set of dominant periods.

Conclusions. A coastal laser strainmeter provides reliable recording of seiche oscillations in a small bay and makes it possible to quantify their amplitudes based on deformation measurements. The obtained results confirm the decisive role of the water area geometry and bathymetry in the formation of the eigenmode spectrum, as well as demonstrate the applicability of remote deformation methods to monitoring coastal hydrodynamic processes.