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The M, Rayleigh wave magnitude formula is revised for purposes of eliminating the heretofore variable effects of near distances and propagation paths on the values computed from standard long-period seismograms. The improved formulation employs a revised distance correction function and a period-dependent path correction that normalizes M, to large teleseismic distance 20-s values. For purposes of earthquake-explosion discrimination, an empirical focal depth correction is derived on the basis of Rayleigh wave frequency content as a function of focal depth, which normalizes M , values to the surface focus equivalent, i.e. aids discrimination when it can be applied by increasing earthquake M , values and moving them away from the equivalent explosion population on M, : m, plots.The revised M , improves on previously achieved discrimination of North American events, and provides reliable discrimination between suites of Eurasian earthquakes and explosions. Having removed the dominant propagation path effects on M,, the residual differences in M , : mb among events are generally attributed to source environment and regional effects on m,,.The 42 Eurasian WWSSN stations employed are shown to have a discrimination threshold at the M,3.2 level. With the improved M , scale now equivalent to first order for North American and Eurasian continental propagation, available Nevada Test Site explosion yields are extrapolated to the Eurasian sites to illustrate that this M,3.2 discrimination threshold is equivalent to an Eurasian explosion of about 20 kt in hard rock. Given improved long-period instrumentation to reduce the Rayleigh wave detection threshold, the principal restriction on further studies of discrimination to lower levels of magnitude and yield will be the availability of earthquake occurrence information at the low magnitudes.

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New probabilistic seismic ground motion maps of Canada, displaying peak horizontal acceleration and peak horizontal velocity at a probability of exceedence of 10 percent in 50 years, have been recommended as the replacement for the 1970 Seismic Zoning Map in National Building Code applications. This report presents a comprehensive description of the basic earthquake data and the methods employed in deriving the new maps.

Canadian seismograph network mean body-wave and surface-wave magnitudes are computed for 28 earthquakes and 28 nuclear explosions in south-western North America to test the effectiveness of the surface-vs body-wave discriminant between earthquakes and explosions for purely continental paths. For the present Canadian network, the magnitude threshold of discrimination is about nz 4.5. A comparison is made between these and other (intercontinental) surface-vs body-wave relationships by normalizing all data to standard magnitudes. Surface-wave attenuation for intercontinental paths limits the effectiveness of the discriminant to magnitudes about 1 .O higher than the same recording techniques can achieve for intracontinental paths.