What causes seiches in narrow isolated gulfs?

Seiches in narrow, isolated gulfs are rhythmic oscillations of water caused primarily by rapid changes in atmospheric pressure and tectonic movements, such as earthquakes.12

When the water surface is disturbed by these forces, gravity acts to restore the horizontal surface, resulting in simple vertical harmonic motion. This impulse travels the length of the basin and reflects off the ends, creating standing waves. Because these gulfs are partially enclosed and have a simple shape, they are particularly susceptible to these free oscillations. While the initial cause may disappear, the resulting oscillation can persist for some time, gradually decreasing in intensity.12