Constraints of Crustal Heterogeneity and Q(f) from Regional (<4 Hz) Wave Propagation for the 2009 North Korea Nuclear TestConstraints of Crustal Heterogeneity and Q(f) from Regional (<4 Hz) Wave Propagation

We carried out 3D finite-difference (FD) simulations (< 4 Hz) of regional wave propagation for the 2009 North Korea nuclear explosion and compared the synthetics with instrument-corrected records at stations INCN and TJN in South Korea. The source is an isotropic explosion with a moment magnitude of 4.1. Synthetics computed in the relatively smooth Sandia/Los Alamos National Laboratory SALSA3D (SAndia LoS Alamos 3D) velocity model significantly overpredict Rayleigh-wave amplitudes by more than an order of magnitude while underpredicting coda amplitudes. The addition to SALSA3D of a von Karman distribution of smallscale heterogeneities with correlation lengths of ∼1000 m, a Hurst number of 0.1, and a horizontal-to-vertical anisotropy of ∼5 produces synthetics in general agreement with the data. The best fits are obtained from models with a gradient in the strength of the velocity and density perturbations and strong scattering (10%) limited to the top 7.5–10 km of the crust. Deeper scattering tends to decrease the initial P-wave amplitudes to levels much below those for the data, a critical result for methods discriminating between explosive and earthquake sources. In particular, the amplitude at the onset of Pn can be affected by as little as 2% small-scale heterogeneity in the lower crust and upper mantle. Simulations including a constant Q of 200 (INCN) to 350 (TJN) below 1 Hz and a power-law Q f formulation at higher frequencies, with an exponent of 0.3, generate synthetics in best agreement with the data. In our simulations, very limited scattering contribution from the near-source area accumulates along the regional path. Electronic Supplement: Description of the source time function used in the simulations, additional waveform comparisons (stations INCN and TJN), snapshots of wave propagation, and vertical cross sections and horizontal slices of the SAndia LoS Alamos 3D (SALSA3D) model with and without small-scale heterogeneities. Introduction The primary aim of nuclear-explosion monitoring is to be able to detect nuclear explosions, which includes discrimination between seismic records generated by different sources. The seismic signature in the records used in explosion monitoring is a combination of source, path, and local site effects. The origin of the source generally provides a constraint on the relative amount of P-, S-, and surface-wave phases included in the recorded waves. Although explosive sources typically give rise to records with a relatively large ratio of P-to-S waves, details of such trends depend on the length and character of the crustal path encountered by the waves. In many cases, the closest available records are obtained from stations several hundred kilometers from the source, typically causing the addition of severe path effects to the recorded seismic waves. These path effects are generated by a combination of (frequency-dependent) anelastic attenuation, crustal scattering, focusing, and multipathing. Unless detailed modeling can be carried out to sufficiently high frequencies, path effects can lead to misinterpretation of the origin of the wavetrains. It is therefore imperative to be able to replicate the general characteristics of the seismic records from regional wave propagation. Conventionally, regional modeling of crustal wave propagation has been carried out using either 1D or 2D approximations, due to the large computational cost associated with fully 3D deterministic models. However, recent advances in available supercomputing resources have facilitated full-waveform simulations in 3D velocity models for 1369 Bulletin of the Seismological Society of America, Vol. 108, No. 3A, pp. 1369–1383, June 2018, doi: 10.1785/0120170195