Explosion Source Models and the Scattering Origin of Regional Phases From SPE Phase 1 Coda Spectral Ratios

Date: 4/24/2019

Time: 02:45 PM

Room: Cascade I

The Source Physics Experiment (SPE) chemical explosions provide comprehensive ground truth information, and near-source recordings that are rarely available in typical monitoring scenarios. Here, we analyze these data together with local and regional distance recordings to isolate source and propagation effects, and to potentially improve monitoring at distance. Manual inspection of SPE waveforms shows strong P and Rg phases in the near source region, with distinct crustal shear phases emerging at far-local distances (100 km). Near-local distance secondary phases are indistinct at high frequencies (e.g. 4-8 Hz), likely due to intense Rg-to-Rg scattering. We examine source spectral ratios, which can be measured to high precision by taking advantage of many SPE and independently operated stations, as well as the redundancy available from coda waves. Classical source models (Mueller and Murphy, 1971, MM71; Denny and Johnson, 1991, DJ91) predict the source ratios poorly; however, a hybrid model (MM71 with DJ91 cavity radii) performs better, and by varying cavity and elastic radii we obtain models that fit spectral ratios to within measurement precision. Finally, we observe a distinct spectral modulation at 6-9 Hz that is not predicted by classical models, most likely caused by short period surface waves (Rg) interfering with those produced by spallation of near surface layers. The same modulation is observed for compressional (P) and shear (S) waves at distance, indicating that local and regional phases originate as near-source Rg that is scattered into body waves. The scattering process must be accounted for quantitatively to best use remotely recorded signals to monitor treaty compliance, discriminate event types, and estimate explosion yields over broad regions.

Presenting Author: William S. Phillips

Authors