Heterogeneous high frequency seismic radiation from complex ruptures

Fault geometric heterogeneities such as roughness, stepovers, or other irregularities are known to affect the spectra of radiated seismic waves that result from dynamic slip on a fault. To investigate the effect of normal stress heterogeneity on radiated spectra, we created a laboratory fault with a single, localized bump by utilizing the compliance and machinability of poly methyl methacrylate (PMMA). By varying the normal stress on the bump and the fault-average normal stress, we produced earthquake-like ruptures that ranged from smooth, continuous ruptures to complex ruptures with variable rupture propagation velocities, slip distributions, and mechanical stress drops. We used an array of eight piezoelectric sensors to measure vertical ground motions calibrated to determine source spectra and PGA for individual events. High prominence bumps produced complex events that radiated more high frequency energy, relative to low frequency energy, than continuous events without a bump. In complex ruptures, the radiated high frequency energy was spatially variable and correlated with local variations in peak slip rate and maximum mechanical stress drop caused by the bump. Continuous ruptures emitted spatially uniform bursts of high frequency energy as the rupture propagated along the fault. Near-field peak ground acceleration (PGA) measurements of complex ruptures show nearly an order-of-magnitude higher PGA near the bump than elsewhere. We propose that for natural faults, geometric heterogeneities may be a plausible explanation for commonly observed order-of-magnitude variations in near-fault PGA.