Abstract

It is very important to describe accurately the topography of rupture surfaces for our understanding of seismic faulting because the topographic characteristics of the rupture include much information about the earthquake and fault mechanics. Two fresh rupture surfaces of the Mw 7.9 2008 Wenchuan earthquake, referred to as the Bajiaomiao surface and the Shaba surface, have been measured by scanning with a 3D portable laser scanner. The acquired sets of Digital Elevation Model (DEM) data are analyzed using power spectral and root mean square (rms) roughness. The fresh rupture surfaces exhibit self-affine behavior, and both the power spectral density and rms roughness have a power law relationship with the length of profiles. The roughness of the surface parallel to the slip direction is generally less than the roughness perpendicular to the slip direction. In a log–log plot of power spectral density versus spatial frequency, there is an obvious inflection that divides the spatial frequency into a lower-frequency domain and a higher-frequency domain. The wavelength corresponding to the inflection is 7 mm in the direction parallel to the slip for the Bajiaomiao surface (in both Patch I and Patch II), smaller than that in the direction perpendicular to the slip (10 mm in Patch I and 9 mm in Patch II), and 8 mm in the direction parallel to the slip, but larger than that in the direction perpendicular to the slip (6 mm). The slope of least-squares fitting line to the rms roughness curve in a log–log plot is the H exponent (also called the Hurst exponent), which depends on the direction of the profile and describes the morphological anisotropy of the fault surface. The slip directions indicated by the minimum H values, 85° and 75° on the Bajiaomiao surface and 45° on the Shaba surface, are equal to the slip directions observed in the field. A secondary minimum H value, 160° in the Shaba rupture surface, reveals a set of concealed striations produced by an earthquake prior to the Wenchuan earthquake. Moreover, the H value of the topographic profile perpendicular to the slip on the Bajiaomiao surface is slightly larger than 0.8, whereas it is quite smaller than 0.8 on the Shaba surface, probably indicating reverse faulting on the Bajiaomiao surface and normal faulting on the Shaba surface. Through linear fitting between the slopes of power spectral density (-α) and the slopes of rms roughness (H) in the whole length of profile, a relationship can be gain, α=1.22+1.72×H. It does not adhere to theory relationship strictly, α=1+2×H, probably caused by the noise in the data, the multifractal of the rupture surface, and the analysis methods of roughness.

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