P-Wave Amplitude Decay and Magnitude Estimation Uncertainty in Regional Earthquake Early Warning Systems: Analysis of 8,412 Pacific Northwest Events

This study investigates P-wave amplitude decay characteristics and magnitude estimation uncertainty in the ShakeAlert Pacific Northwest earthquake early warning system using 8,412 catalogued seismic events within the context of seismology and earthquake hazard engineering, an area of growing scientific importance given its implications for ShakeAlert network design optimization, station density gap identification, and alert threshold tuning for Pacific Northwest communities. Using P-wave amplitude-distance attenuation analysis using 1D and 3D regional velocity models, magnitude estimation bias quantification, and warning time versus false alert rate optimization, we examine geometric spreading and anelastic attenuation governing P-wave amplitude decay with distance, introducing magnitude estimation uncertainty that scales with event depth and source-station azimuthal distribution in 8,412 seismic events (M1.5-7.1) from 428 PNSN stations with hypocentral distances 10-400 km during the 2018-2020 operational ShakeAlert period drawn from Pacific Northwest Seismic Network (PNSN) station array across Washington, Oregon, and northern California. Results indicate that magnitude estimation RMSE of 0.42 units for events within 100 km of sufficient station density, degrading to 0.84 units beyond 200 km, with false alert rate of 1.8% achievable at 4.8-second median warning time for M5+ (p < 0.001), with RMSE 0.42 at <100 km vs. 0.84 at >200 km; 4.8s median warning for M5+ as the primary quantitative benchmark. Concordance between primary and confirmatory measurement approaches exceeded 93%, validating the analytical framework. These findings contribute empirically to seismology and earthquake hazard engineering and carry actionable implications for the design of programs and policies targeting ShakeAlert network design optimization, station density gap identification, and alert threshold tuning for Pacific Northwest communities.