UC Berkeley

Earthquake faults, and all frictional surfaces, establish contact through asperities. A detailed knowledge of how asperities form will enable a better understanding of the manner in which they communicate during foreshock failure sequences that are observed, leading to the larger main shock. We present results of experiments where a pressure sensitive film was used to map, size and measure the magnitudes of the normal stresses at asperities along a seismogenic section of a laboratory simulated fault. We measured seismicity acoustically and foreshocks were found to be the result of localized asperity failure during the nucleation phase of gross fault rupture. Since surface roughness plays an important role in how asperities are formed, two Hurst exponents were measured to characterize a highly worn interface using roughness profiles: (i) long wavelength estimates (H ~ 0.45) and (ii) short wavelength estimates (H ~ 0.8-1.2). The short wavelength roughness estimates were computed at the scale of single asperity junction points. Macroscopically, the number of asperities and real contact area increased with additional application of normal force while the mean normal stress remained constant. The ratio of real to nominal contact area was low - ranging from 0.02 < Ar/A0 < 0.05-predicting that the asperities should be elastically independent of each other. Results from the pressure sensitive film showed that asperities were closely spaced and could not be treated as mechanically independent. Larger asperities carried both higher levels of average normal stress and higher levels of normal stress heterogeneity than smaller ones. Using linear stability theorem, the critical slip distance on foreshocking asperities was estimated to be d0 ~ 0.65-3 μm. The critical slip distance d0 was ~1.8-11.5 per cent of the premonitory slip needed to initiate gross fault rupture of the interface (20-40 μm) and the overall slip necessary to initiate gross fault rupture was on the order of the average asperity diameter (52 μm). Foreshocks may be due to a change in the critical slip distance, at localized sections of the fault, caused by the two distinct roughness profiles measured at short and long length scales.