Development of Regional Phase Tomographic Attenuation Models for Eurasia

摘要

We are developing regional-phase (Pn, Pg, Sn, Lg) tomographic attenuation models for Eurasia. The models will be integrated into the National Nuclear Security Administration (NNSA) Knowledge Base and used in the Magnitude and Distance Amplitude Correction (MDAC) station calibration for the development of regional seismic discriminants. Our current focus is on Pn, an extremely important phase in seismic event identification. Accurately accounting for regional-phase geometric spreading is critical for the development of useful attenuation models. It is particularly important for Pn and Sn waves because the propagation mode of these waves makes them more susceptible to upper mantle velocity structures and the Earth's sphericity, which in turn causes the geometric spreading of Pn (and Sn) to be dependent on frequency as well as on range in a complicated way. We conduct numerical simulations to quantify Pn and Sn geometric spreading in a spherical Earth model with constant mantle velocities. Based on our simulation results, we have presented new Pn and Sn geometric spreading models in the form G(r,) = 10(n/3)(f)/r(0) (r(0)/r)(n/1)(f)(log)(r(0)/r) + (n/2)(f) and n(i)(f)=n(i) log(f/f(0))(2) + n(i2) (log)(f/f(0)) + n(i3), where i = 1, 2, or 3; r is epicentral distance; f is frequency; r(0) = 1 km and f(0) = 1 Hz. We derive values of coefficients n(ij) by fitting the model to computed Pn and Sn amplitudes for a spherical Earth model having a 40-km-thick crust, generic values of P and S velocities and a constant-velocity uppermost mantle. We applied the new spreading model to observe data in Eurasia to estimate average Pn attenuation, obtaining more reasonable results compared to using a standard power-law model. Our new Pn and Sn geometric-spreading models providing generally applicable reference behavior for spherical Earth models with constant uppermost-mantle velocities.