VARIATION OF REGIONAL SEISMIC DISCRIMINANTS WITH SURFACE TOPOGRAPHIC ROUGHNESS IN THE WESTERN UNITED-STATES

Regional distance P and S waves behave differently with propagation di stance due to complex elastic and anelastic effects in the crustal wav e 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 successfu l discrimination to be achieved. Such corrections usually involve only distance-dependent decay, parameterized to account for geometric spre ading and attenuation factors averaged over some geographic region. Ho wever, large scatter persists in distance-corrected discriminants, whi ch degrades their performance, and strategies are needed to further re duce the scatter by accounting for individual path variability. We dem onstrate that characterizing some of the path variability by the readi ly measured surface topographic roughness along each path allows reduc tion of the scatter in some regional discriminants. P-g/L(g) and P-n/L (g) ratios and P-g, P-n, and L(g) low-frequency/high-frequency spectra l ratios for 80 earthquakes in the Western United States recorded at f our stations of the Livermore NTS Network are analyzed for path effect s. While all of these discriminants display distance dependence, log P -g/L(g) is also found to be significantly correlated with average surf ace 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 trend s. Corrections based on the correlations with distance and roughness c an reduce the variance of the log P-g/L(g) discriminant by 22 to 25% i n the 2- to 4-Hz band. Log P-n/L(g) and individual phase spectral rati os have stronger distance dependence than log P-g/L(g), but the correl ation with surface roughness is weak for those measurements. It appear s that interchange of P-g and L(g) energy that reverberates in the cru st accounts for the sensitivity to the gross wave-guide properties tha t are manifested in the surface roughness parameterization. The result s support the concept of pursuing path corrections beyond purely range -dependent trends.