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

Abstract

We carried out 3D finite‐difference (FD) simulations (&lt;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 small‐scale 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.

Ground Motion Residuals, Path Effects, and Crustal Properties: A Pilot Study in Southern California

“…Our interpretation is that with increasing distance, more or greater velocity contrasts along a ray's path yields more energy lost to scattering (refractions or reflections), and less energy transmitted. Recent numerical modeling studies (Olsen et al, 2018) have found similar results. At closer distances, we observe a weak positive correlation between path residuals and dVI, which is perhaps unsurprising.…”

A Machine Learning Approach to Developing Ground Motion Models From Simulated Ground Motions

“…In recent years, significant progress has been made in modeling synthetic ground motions. With the advance of modern computing power, studies such as Bydlon et al (), Graves and Pitarka (), Mai et al (), Taborda et al (), Imperatori and Mai (), Frankel et al (), Wirth et al (), Withers, Olsen, Day, and Shi,(), Withers, Olsen, Shi, and Day, (), Olsen et al (), Graves and Pitarka (), Andrews and Ma (), Hartzell et al (), Pitarka et al (), Pitarka et al (), and Moschetti et al () have incorporated increasingly accurate physics into simulations. Some of these improvements include explicitly accounting for complex rupture processes and the propagation of waves through realistic 3‐D crustal structure to simulate ground motions at a wider range of frequencies for a variety of end‐use cases (from assessing regional seismic hazard related to basin amplification to better understanding the role of anelasticity in nuclear monitoring).…”

Achievements and Prospects of Global Broadband Seismographic Networks After 30 Years of Continuous Geophysical Observations

“…The challenges of that approach are daunting, as indicated for example, by K. B. Olsen et al (2018) in their finite difference simulations of regional seismic waves at stations in the Republic of Korea (South Korea) from the DPRK nuclear test of 2009.…”

“…B. Olsen et al (2018) pushed simulation of wave propagation in state-of-the-art 3D crustal models, using a fairly smooth 3D model with 24 billion points at uniform spacing (100 m), noting that modern capabilities enable synthetics up to 4 Hz out to distances of several hundred km. They included anelastic attention parameterized by a variety of Q models, and achieved some success with synthetics that matched the timing of observed regional wave arrivals; but they at first grossly overpredicted Rayleigh-wave amplitudes and under-predicted the coda.…”