Oral Presentation

Fault geometric heterogeneities of varying scale, such as roughness, stepovers, or other irregularities influence dynamic rupture and the spectra of radiated seismic waves. To investigate the effect of normal stress heterogeneity on dynamic rupture behavior and radiated spectra, we utilized the compliance and machinability of poly(methyl methacrylate) (PMMA) to create a laboratory fault that is not flat, but instead has a single, localized bump. By varying the size of the bump, we varied the normal stress on the bump relative to the fault-average normal stress (i.e., the bump prominence) and 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 found that a sufficiently prominent bump (local normal stress on the bump was at least six times greater than the sample-average normal stress) acted as a barrier (energy sink) to fault rupture propagation when it was far from critically stressed and as an asperity (energy source) when it was near critically stressed. The bump never acted as a permanent barrier.

The rupture-bump interactions, both when the bump acted as a barrier and an asperity, also enhanced the radiation of high frequency seismic waves. To investigate this more, we used an array of eight piezoelectric sensors to measure vertical ground motions calibrated to determine source spectra and PGA for individual events. Complex events due to high prominence bumps, 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.