Regional mb: Ms, Lg/Pg amplitude ratios and Lg spectral ratios as criteria for distinguishing between earthquakes and explosions: a theoretical study

Theoretical seismograms for point explosion and double-couple sources in a uniform plane-layered earth are used to evaluate the likely effectiveness of three criteria that have been proposed for distinguishing between earthquakes and explosions. The criteria are: regional ms: Ms, Lg/Pg amplitude ratios, and Lg spectral ratios. For explosions in material with a high wave-speed the near regional mb:Ms discriminant gives only marginal separation between the two types of source, ranging from zero for strike-slip earthquakes, to on average 0.4 magnitude units for 45° dip-slip mechanisms. Only for explosions in material with a low wave-speed do the theoretical data show an overall separation. Lg/Pg amplitude ratios successfully separate the source types except for strike-slip earthquakes where there is overlap with shallow explosions. Theoretical Lg spectra for deep (⩾1.5 km) explosions are depleted in high frequencies when compared with those for earthquakes and shallow explosions. Discrimination between earthquakes and these deep explosions is therefore possible using the spectral-ratio method. An important mechanism underlying these Lg-based discriminants is the interaction of the spherical P-wavefront from an explosion with the free surface which generates shear waves with amplitude depending both on frequency and source depth of burial.