Magnitude and Q determinations in southern Africa using Lg wave amplitudes

Abstract

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-' 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 20.1 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.

Determination ofLg-wave attenuation using single-station seismograms: a case study for Zimbabwe

“…Furthermore, we considered different propagation paths over Zimbabwe as the seismic waves travelled to BUL and found a low Q value of 350 for the Deka fault zone. The Q value of 650 obtained in this study agrees well within error with that of 603 reported by Chow et al (1980) using the multistation, multi-event method, and lies within the range of Q values (454-759) reported by Xie & Mitchell (1990) using a 'back-project' method to image large-scale lateral variations of Lg coda Q. The results obtained are important as the Q value constitutes part of the input data in seismic hazard calculations.…”

“…where A is the amplitude of the maximum Lg-wave trace, Tis the phase period at the point of maximum amplitude and B(A) is an empirically determined term that is a function of angular distance, A. Details on the determination of B ( A ) and mb (BUL) are given by Henderson (1974) andChow et al (1980). For events occurring within the chosen radius, the most prominent seismic phase on the seismogram is the Lg-wave complex.…”

“…Attenuation of regional shear waves (Lg waves) at 1 Hz for distances greater than 100 km is higher in tectonically active regions than in stable zones (see e.g. Aki 1980). Chow et al (1980 report for southern Africa a Q value of 603.…”

“…Such values of y and Q are useful in estimating, for example, seismic hazard and in formulating building codes for given regions. Chow et al (1980) used the Bulawayo (BUL) records and the International Seismological Centre (ISC) magnitudes to determine the mb (Lg) (BUL) magnitude relation. They determined Lg-wave attenuation for southern Africa using 575 events recorded by six seismograph stations.…”

Seismic hazard in central southern Africa

“…Earthquake magnitudes in this catalogue are homogenized to BUL magnitudes. BUL reports magnitudes of m,(Lg) type that are on average 0.16 units higher than ISC mh values (Chow et al 1980). Considering the 0.16 magnitude bias as lying within acceptable magnitude error limits, the BUL m,(Lg) magnitudes may be accepted as comparable to ISC reported values.…”