Rayleigh wave
Is a seismic surface wave which causes the ground to shake in an elliptical motion with no perpendicular motion involved.

Recession/Receeding waters
The process by which the water withdraws from the shoreline sometimes by a kilometer or more exposing the sea bottom, rocks and marine creatures. This recession of the sea is a natural warning sign that a tsunami is approaching. It may not always occur before a tsunami inundates an area.

Recurrence interval
See return period

Regional tsunami
A tsunami capable of creating destruction  in a particular geographic region with a maximum travel time of 1-3 hours or within 1000 km form its source. Effects are usually localized to that area.

Return period
Is the average time span between earthquake occurrences on a fault or in a seismic zone.

Richter Scale
This scale was developed in 1935 by Charles F. Richter as a mathematical way to compare the size of earthquakes. Most earthquakes are measured using the local magnitude that it sometimes still referred to as the ‘Richter magnitude’.

Ring of Fire
Is the plate boundary of the Pacific plate. This zone of earthquakes and volcanic activity surrounds the Pacific Ocean and about 90% of the world’s seismic activity occurs here.

Risk is the possibility of a loss (e.g. of life, property, infrastructure) within the area subject to the hazard(s). Risk is evaluated by the relation: risk = value x vulnerability x hazard. Value includes everything threatened by the hazard, whereas vulnerability is the percentage of the value likely to be lost in a given hazardous event.