Simulating High-Frequency Seismograms in Realistic Earth Models to Better Understand Source Discrimination Based on Differential Magnitudes (ML−Mc)

Journal Article · Thu May 08 04:00:00 EDT 2025 · Seismological Research Letters

DOI:https://doi.org/10.1785/0220240378· OSTI ID:2585295

Hutchings, Sean J. ^[1]; Pitarka, Arben ^[2]; Koper, Keith D. ^[1]; Burlacu, Relu ^[1]; Voyles, Jonathan R. ^[3]

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1. Univ. of Utah, Salt Lake City, UT (United States)
2. Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
3. Chevron Technical Center, Houston, Texas (United States)

Discriminating low-yield underground nuclear explosions from small earthquakes is a key task in monitoring nuclear test ban treaties. P/S amplitude ratios have been an effective discriminant for moderate-sized events recorded at regional distances, but it is unclear if they are as effective in discriminating small seismic events recorded at local distances (<150 km). The difference between local magnitude (M_L) and coda duration magnitude (M_c) has been proposed as a new discriminant that may complement P/S amplitude ratios at local distances. Here, in this work, we calculate high-frequency (up to ∼4 Hz) synthetic seismograms at epicentral distances of 0–30 km in realistic models of the Salt Lake basin (Utah, United States) to better understand how variations in source type and depth affect M_L−M_c values. The Earth models incorporate simplified 1D and deterministic 3D structures, small-wavelength stochastic velocity perturbations, and surface topography. Coda waves are enhanced for the more complicated models compared to the base 1D model, but still underpredict observed durations by about a factor of two, which results in overprediction of amplitude to duration ratios (i.e., M_L−M_c values) for a near-surface explosion and a 7 km deep earthquake. For both source types, the predicted M_L and M_c values decrease as source depth increases, and M_L−M_c shows only minor variation with depth; however, M_L−M_c is on average ∼0.5 units smaller for explosions than earthquakes. This finding may imply that M_L−M_c has sensitivity to source type, in addition to being a depth discriminant, but more modeling is needed given the limitations of the current study. Future modeling should incorporate higher-frequency (≳5 Hz) simulations over a larger distance range (0–150 km), where M_L and M_c are commonly measured, while honoring low shear velocities (<300 m/s) near the surface and sampling a wider range of earthquake and explosion source mechanisms.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: AC52-07NA27344
• OSTI ID: 2585295
• Report Number(s): LLNL--JRNL-2002049
• Journal Information: Seismological Research Letters, Journal Name: Seismological Research Letters Journal Issue: 5 Vol. 96; ISSN 0895-0695; ISSN 1938-2057
• Publisher: Seismological Society of America (SSA)
• Country of Publication: United States
• Language: English