Variation of regional seismic discriminants with surface topographic roughness in the Western United States

Regional distance P and S waves behave differently with propagation distance due to complex elastic and anelastic effects in the crustal wave guide and uppermost mantle. It is well recognized that seismological discriminants based on P/S ratios or individual phase spectral ratios must be corrected for regionally dependent path effects for successful discrimination to be achieved. Such corrections usually involve only distance-dependent decay, parameterized to account for geometric spreading and attenuation factors averaged over some geographic region. However, large scatter persists in distance-corrected discriminants, which degrades their performance, and strategies are needed to further reduce the scatter by accounting for individual path variability. We demonstrate that characterizing some of the path variability by the readily measured surface topographic roughness along each path allows reduction of the scatter in some regional discriminants. Pg/Lg and Pn/Lg ratios and Pg, Pn, and Lg low-frequency/high-frequency spectral ratios for 80 earthquakes in the Western United States recorded at four stations of the Livermore NTS Network are analyzed for path effects. While all of these discriminants display distance dependence, log Pg/Lg is also found to be significantly correlated with average surface roughness along each path. The correlation is emphasized when the product of distance and roughness is used as an independent variable, and the data in the frequency range 1 to 6 Hz have the strongest trends. Corrections based on the correlations with distance and roughness can reduce the variance of the log Pg/Lg discriminant by 22 to 25% in the 2- to 4-Hz band. Log Pn/Lg and individual phase spectral ratios have stronger distance dependence than log Pg/Lg, but the correlation with surface roughness is weak for those measurements. It appears that interchange of Pg and Lg energy that reverberates in the crust accounts for the sensitivity to the gross wave-guide properties that are manifested in the surface roughness parameterization. The results support the concept of pursuing path corrections beyond purely range-dependent trends.

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