The invention as disclosed is a system for real time detection of tsunami waves. A tsunami is considered a “shallow water wave”, having a large wavelength compared to the ocean depth. A tsunami can have wavelengths ranging from 100 to 500 km, and amplitudes up to 60 cm (i.e., a pressure signal of up to 1 psi). At least one telecommunications grade optical fiber cable fitted with repeaters spanning the length of an ocean, a laser and a signal processor are used to measure pressure signals over spatial cells as small as 0.5 meters in width based on Rayleigh scattering effects and employing optical time delay reflectometry. The tsunami pressure signals are extracted from the pressure signals generated by wind-generated water waves (having much shorter wavelengths) through signal processing to distinguish between the substantial differences in wavelength, period and propagation speed.