Some analyses of P- and Rayleigh wave amplitudes observed at North American stations

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Summary P- and Rayleigh wave amplitudes observed by the Long Range Seismic Measurements network of stations in North America are analysed into source, distance and station effects. The main results of the P-wave analyses are: (a) there is a significant correlation of station effect and αn the Pn wave speed in the vicinity of the recording station; (b) there are no contributions to the station effect from differences in the noise levels at the recording stations; (c) the short-period (SP) station effects are weakly dependent on period, but the correlation may be partially masked by systematic errors in the measurement of period; (d) the variation in crustal structure makes little or no contribution to the SP and long-period (LP) station effects. The main results of the Rayleigh wave analyses are: (a)the gradient h of the decay of amplitude with distance A is nearly unity. This is in agreement with earlier estimates obtained assuming the decay has the form h log Δ+ constant; (b)the attenuation coefficient k as a function of period in the range 20 to 37.5 s in agreement with other estimates for paths across North America and for various paths from epicentres in China. The study confirms that the surface wave decay across North America is much slower than that implied by the standard calibration functions used for computing surface wave magnitude. Combined with other published estimates the values of Qγ-1 suggest that the average Qγ-1 is lower (Qγ is higher) for North America than on many other paths.