Very long water waves in oceans or seas, tsunamis (or seismic sea waves), sweep inshore following certain earthquakes. They sometimes reach great heights and may be extremely destructive. The immediate cause of a tsunami is a disturbance in an adjacent seabed sufficient to cause the sudden raising or lowering of a large body of water. This disturbance may be centred in the focal region of an earthquake or it may be a submarine landslide arising from an earthquake.

Following the initial disturbance to the sea surface, water waves spread out in all directions. Their speed of travel in deep water is given by (gh)1/2, where h is the sea depth and g is the acceleration of gravity. This speed may be considerable; e.g., 100 metres per second (224 miles per hour) when h is 1,000 metres (3,280 feet). The amplitude at the surface does not exceed a few metres in deep water, but the principal wavelength may be on the order of hundreds of kilometres; correspondingly, the principal wave period may be on the order of tens of minutes. Because of these features, the waves are not noticed by ships far out at sea.

When tsunamis approach shallow water, the wave amplitude increases. The waves may occasionally reach a height of 20 to 30 metres in U- and V-shaped harbours and inlets. They sometimes do a great deal of damage in low-lying ground around such inlets. Frequently the wave front in the inlet is nearly vertical, as, for example, in a tidal bore, and the speed of onrush may be on the order of 10 metres per second. In some cases there are several great waves separated by intervals of several minutes or more. The first of these waves is often preceded by an extraordinary recession of water from the shore, which may commence several minutes or even half an hour beforehand.

Organizations, notably in Japan, Siberia, Alaska, and Hawaii, have been set up to provide tsunami warnings. A key development is the Seismic Sea Wave Warning System (SSWWS), an internationally supported system designed to reduce loss of life in the Pacific Ocean. Centred in Honolulu, it issues alerts based on reports of earthquakes from circum-Pacific seismographic stations.

These are rhythmic motions of water in nearly landlocked bays or lakes that are sometimes induced by earthquakes and by tsunamis (in the case of the former). Oscillations of this sort may last for hours or even for a day or two.

The great Lisbon earthquake of 1755 caused the waters of canals and lakes in areas as far away as Scotland and Sweden to go into observable oscillations. Seiche surges in Texas in the southwestern United States commenced between 30 and 40 minutes after the 1964 Alaska earthquake and were produced by seismic surface waves passing through the area.

Of course, P waves from an earthquake may pass through the sea following refraction through the seafloor. The speed of these waves is about 1.5 kilometres per second, the speed of sound in water. If such waves meet a ship with sufficient intensity, they give the impression that the ship has struck a submerged object. This phenomenon is called a seaquake.