ABSTRACT
SIZE AND SCALE OF EARTHQUAKES Ground movements are measured on seismographs. Magnitude (M) defines the size of an earthquake on the Richter scale: log10 of the maximum wave amplitude in microns on a Wood Anderson seismograph 100 km from the epicentre (point on the surface above the focus). Moment magnitude (MW) relates to fault area, movement and rock rigidity; it gives a better indication of an earthquake’s energy, and is numerically close to M. Intensity is the scale of earthquake damage at any one point, described on the modified Mercalli scale, and declining away from the epicentre. (MSK and EMS-98 scales are similar to Mercalli, but with more detail.) Damage relates largely to peak ground acceleration, also to peak velocity, frequency and duration. Duration usually 10 secs for magnitude 5, may last for 40 secs at magnitude 8; increases away from epicentre.
