Dynamic rupture simulations generate synthetic waveforms that account for nonlinear source and path complexity. Here, we analyze millions of spatially dense waveforms from 3D dynamic rupture simulations in a novel way to illuminate the spectral fingerprints of earthquake physics. We define a Brune‐type equivalent near‐field corner frequency to analyze the spatial variability of ground‐motion spectra and unravel their link to source complexity. We first investigate a simple 3D strike‐slip setup, including an asperity and a barrier, and illustrate basic relations between source properties and variations. Next, we analyze >13,000,000 synthetic near‐field strong‐motion waveforms generated in three high‐resolution dynamic rupture simulations of real earthquakes, the 2019 7.1 Ridgecrest mainshock, the 6.4 Searles Valley foreshock, and the 1992 7.3 Landers earthquake. All scenarios consider 3D fault geometries, topography, off‐fault plasticity, viscoelastic attenuation, and 3D velocity structure and resolve frequencies up to 1–2 Hz. Our analysis reveals pronounced and localized patterns of elevated, specifically in the vertical components. We validate such variability with observed near‐fault spectra. Using isochrone analysis, we identify the complex dynamic mechanisms that explain rays of elevated and cause unexpectedly impulsive, localized, vertical ground motions. Although the high vertical frequencies are also associated with path effects, rupture directivity, and coalescence of multiple rupture fronts, we show that they are dominantly caused by rake‐rotated surface‐breaking rupture fronts that decelerate due to fault heterogeneities or geometric complexity. Our findings highlight the potential of spatially dense ground‐motion observations to further our understanding of earthquake physics directly from near‐field data. Observed near‐field variability may inform on directivity, surface rupture, and slip segmentation. Physics‐based models can identify “what to look for,” for example, in the potentially vast amount of near‐field large array or distributed acoustic sensing data.