Local travel-time curves and their geologic implications for the Pacific Northwest states

Abstract

Travel-time curves were constructed for the Pacific Northwest states based on recordings of recent local earthquakes. Average velocities of P_n and S waves were found to be 4 per cent lower in the region west of the Cascade Mountains than they are to the east of the Cascades, while velocities of P* and $P¯$ waves are essentially the same in the two provinces. West of the Cascades the velocities obtained are 7.67 km/sec for P_n, 4.37 for S_n, 6.61 for P*, and 5.48 for $P¯$⁠. East of the Cascades they are 7.96 km/sec for P_n, 4.56 for S_n, 6.60 for P*, and 5.53 for $P¯$⁠. Intercept times indicate that the crust (above the Mohorovicic discontinuity) is 5 to 10 km thinner to the west than east of the Cascades. P_n velocities are found to be independent of epicentral distance to distances of 1000 km, implying that the corresponding mantle materials and densities are uniform down to the low-velocity layer in each province. In both provinces Poisson's ratio in the mantle is 0.26. From the travel-time curves and gravity data, it appears that the section above the low-velocity layer east of the Cascades is approximately typical or “normal,” while the corresponding mantle west of the Cascades consists of a different or anomalous material. The likelihood of crystal alignment, temperature anomalies, and normal velocity-density relations in the upper mantle sections are discussed.