Magnitude and Q determinations in southern Africa using Lg wave amplitudes

The amplitude of vertical, short period (1 s) Lg-waves from 575 shallow earthquakes recorded within the distance range 0°–30° by the Rhodesian seismograph network during the period 1968–77 are analysed to separate the effects of earthquake size, epicentral distance and station structure. When corrected for geometrical spreading and Airy phase dispersion the decay of amplitude with distance yields an estimate of anelastic attenuation of 0.160 deg−1 which gives an average value of Q (the specific quality factor) of 603 ± 50 for propagation paths that lie along and across the East African Rift System. Inversion of the amplitude-distance curve gives the calibration or distance normalizing function. Thereby the amplitude of Lg can be used to provide an estimate of the size of small, local earthquakes in terms of the teleseismic body wave magnitude mb (after Henderson). The station effects of the six seismograph stations making up the network all lie within ±0.1 magnitude units. Since three of the stations lie on the Rhodesian craton while the remaining three lie on Precambrian mobile belts adjacent to the craton, the Precambrian basement geology does not significantly affect the amplitude of Lg