Introduction

The size of an earthquake can be measured in many ways. The number of deaths and injuries and the value of property damaged and destroyed give measures of the seriousness of the event in human terms, but these statistics relate as much to population density at the epicenter as to the size of the shock. Lomnitz (1974) estimates 17,500 deaths per year worldwide on the average. The size of the area over which an earthquake is felt or within which damage to property occurs can be used but does not adequately represent the seriousness of the maximum effects. Early methods of quantifying earthquake size attempted to rate each event in terms of some kind of numerical scale based on a synthesis of observed effects. Such measures are called intensity scales. From an early date, attempts were made to relate intensity to the amplitude of ground motion. Most recently, recorded ground amplitudes are normalized for their variation with distance from the earthquake epicenter into a quantity called magnitude, of which there are many varieties, or they are used to estimate the energy released by the earthquake or the moment of the displaced ground.

Intensity scales

Accurate descriptions of earthquake phenomena accumulated steadily, especially in Europe, after the 1755 Lisbon earthquake. Early attempts to establish numerical scales for evaluating effects failed to achieve acceptance by any investigators other than the originators. Davison (1927) credits Domenico Pignataro, a physician of Monteleone, Italy, with the first attempt in 1788.