Also historical records have shown that a tsunami was generated by the earthquake of 1867 which affected St. Thomas and Tortola in the Virgin Islands. By measuring the strength of these waves, seismographs record the characteristics of these waves and, from the measurements, scientists can determine their location. The strength of an earthquake can be measured by magnitude and intensity. Magnitude is a measure of the physical energy released or the vibrational energy of the shock. It is commonly measured on the Richter scale which is an open-ended logarithmic scale.
Intensity is the effect of an earthquake on the earth's surface.
The intensity scale consists of certain key responses such as people awakening, movement of furniture, structural damage and finally, total destruction. The version of the modified Mercalli Intensity scale that is currently used is a 1956 modification by Richter of the 1931 version by the American seismologists Harry Wood and Frank Neumann. This scale, composed of 12 increasing levels of intensity which range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. Not felt except by a very few under especially favourable conditions detected mostly by Seismography. Felt only by a few persons at rest, especially on upper floors of buildings.