Lithospheric waveguide beneath the Midwestern United States; massive low-velocity zone in the lower crust

Abstract

Variations in seismic velocities are essential in developing a better understanding of continental plate tectonics. Fortunately, the USArray has provided an excellent set of regional phases from the recent M5.6 Oklahoma earthquake (6 November 2011, Table 1) that can be used for such studies. Its strike-slip mechanism produced an extraordinary set of tangential recordings extending to the northern edge of the USArray. The crossover of the crustal slow S to the faster S_n phase is well observed. S_mS has a critical distance of around 2° and its first multiple, SmS^2, reaches critical angle near a distance of about 4°, and so on, until S_mS^n merges with the stronger crustal Love waves. These waveforms are modeled in the period band of 2–100 s by assuming a simple three-layer crust and a two-layer mantle, which allows a grid-search approach. Our results favor a 15 km thick low-velocity zone (LVZ) in the lower crust with an average shear velocity of less than 3.6 km/s. The short-period Lg waves (S waves, at periods of 0.5–2 s) travel with velocities near 3.5 km/s and decay with distance faster than high-frequency S_n (>5.0 Hz) which travels at a velocity of 4.6 km/s and persists to large distances. Although these short-period waveforms are not modeled, their amplitude and travel times can be explained by adding a small velocity jump just below the Moho with essentially no attenuation. P_n is equally strong but is complicated by the interference produced by the depth phase sP, but well modeled. The P velocities appear normal with no definitive LVZ. While these observations of S_n and P_n are common beneath most cratons, the lower crustal LVZ appears to be anomalous and maybe indicative of hydrous processes, possibly caused by the descending Farallon slab.

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