Complex laboratory ruptures and variable ground motions due to heterogeneous normal stress
Sara Cebry
USGS Earthquake Science Center
- Date & Time
- Location
- Hybrid in-person and online seminar via Microsoft Teams
- Host
- Tamara Jeppson
- Summary
Fault heterogeneities such as roughness, stepovers, and other irregularities are known to affect the spectra of radiated waves during an earthquake. To investigate the effect of normal stress heterogeneity on the earthquake spectra, we produced a series of ruptures on a 0.74 m PMMA laboratory fault with a single, localized bump of increased normal stress. By varying the bump prominence (defined here as the increase in the normal stress on the bump divided by the average normal stress across the entire fault), we produced earthquake-like ruptures that ranged from smooth, continuous, sample-spanning ruptures to complex ruptures with variable rupture propagation velocity, slip distribution, and stress drop. Vertical ground motions were measured using an array of eight piezoelectric sensors calibrated using a ball drop technique.
Radiated seismic waves were strongly affected by the bump prominence. 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 high frequency energy showed significant spatial variation in amplitude and timing while continuous ruptures emitted spatially uniform bursts of high frequency energy. Near field peak ground acceleration (PGA) measurements of complex ruptures show nearly an order of magnitude higher PGA near the bump than on the rest of the fault. We expect a similar phenomenon to occur in the earth. We propose that for natural faults, fault geometric heterogeneities, such as roughness or stepovers, may act as earthquake gates and can be a spatially heterogeneous source of enhanced high frequency radiation. This may be a plausible explanation for commonly observed order of magnitude variations in near-fault PGA such as during the 2023 Turkey Mw 7.8 and 7.6 earthquakes.