Earthquake: Magnitudes, Energy, and Moment

Abstract

Corner frequency The frequency fc at which the curve that represents the Fourier amplitude spectrum of a signal abruptly changes its slope (see Fig. 4). For earthquakes, this frequency is related to the seismic source properties such as fault size, stress drop in the source volume, rupture velocity, and related rupture duration. Also the frequency at which the magnification curve of a recording system changes its slope (cf. Fig. 1). Dispersion Frequency dependence of the wave propagation velocity. Whereas seismic body waves show virtually no dispersion, it is pronounced for seismic surface waves. It causes a significant stretching of the length of the surface wave record and the rather late arrival of its largest amplitudes (airy phases) from which the surface wave magnitude MS and the mantle magnitude Mm, respectively, are determined. Earthquake size A frequently used but not uniquely defined term. It may be related – more or less directly – to either the geometric-kinematic size or seismic moment of an earthquake in terms of area and average slip of the fault or to the seismic energy radiated as seismic waves from a seismic source and thus its potential to cause damage and casualty even at larger distance from the source (moment or energy magnitude). Earthquake source In general terms the whole area or volume of an earthquake rupture where seismic body waves are generated and radiated outward. More specifically one speaks either of the source mechanism or the source location. The latter is commonly given as earthquake hypocenter (i.e., the location at the source depth h from where the seismic rupture, collapse, or explosion begins) or as the point on the Earth’s surface vertically above the hypocenter, called the epicenter. Earthquakes at h < 70 km are commonly termed shallow and those at larger depth either intermediate (up to h = 300 km) or deep (h = 300–700 km). The determination of the origin time, geographical coordinates (latitude j and longitude l), and focal depth is the prime task of seismic source location. However, for extended seismic sources, fault ruptures of very large earthquakes in particular, the hypocenter is often not the location of the largest fault slip and/or seismic moment/energy release, and the epicenter is then also not the location where the strongest ground shaking is felt. These locations of the largest effects may be for great earthquakes dozens of km in space and many seconds to minutes in time away from the hypocenter or epicenter, respectively.