The depth distribution of seismicity parameters estimated for the South American area

Two techniques are applied in order to estimate the seismicity parameters a nd their depth distribution in South America, an area of high seismic activ ity. These techniques are: (1) Gumbel's third asymptotic (GIII) distributio n of extremes and (2) the maximum likelihood approach. The obtained seismic ity parameters through GIII distribution of extremes, are the omega value w hich is considered as the upper bound magnitude and is related to the finit e maximum stresses and strains which are currently accumulated and released by the rocks, as earthquake, in a region, the shape parameter lambda, and the characteristic value u, with P(u) = 1/e and P(omega) = 1. The most prob able maximum magnitude, RM, of earthquakes for 95 years is also computed. T he seismicity parameters obtained by the application of the maximum likelih ood approach are the activity rate, lambda, of the seismic events, the mean return period, RP, of earthquakes above a cutoff magnitude M greater than or equal to m and the parameter b of the magnitude-frequency relation. The parameter beta which is related to b with b = beta log(10)(e), is also comp uted. The regional maximum magnitude, M-max (this is considered as the maxi mum possible magnitude in a specific region), is also provided under some c onstraints. Six depth-range subdivisions are defined for the examined area. All the seismicity parameters are computed and especially for GIII distrib ution a covariance analysis of its three parameters (omega, u and lambda) i s performed for each individual depth range. In all cases omega values are higher than the regional M-max estimates. The seismic activity rate decreas es with depth, while the beta parameter, which ranges between 1.18 to 2.06, does not show a clear regularity in its depth distribution. An important n otification is that the parameter lambda obtained by the two methods, expre sses totally different quantities. (C) 2000 Elsevier Science B.V. All right s reserved.