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 (Trimble GX). The acquired sets of DEM data were analyzed using power spectral density and rootmeansquare (RMS) roughness. The fresh rupture surfaces exhibit selfaffine behavior, and the power spectral density and RMS roughness both have power law relationship with the length of profiles. In loglog plot of power spectral density versus spatial frequency, there is an obvious inflexion which divides the spatial frequency into lower frequency domain and higher frequency domain. The wavelength corresponding to the inflexion is called “characteristic wavelength”, and it shows that a single fractal can not describe the roughness scale for the rupture surface. The characteristic wavelength is 7 mm in the direction parallel to the slip for the Bajiaomiao surface (both in Patch Ⅰ and Ⅱ), smaller than that in the direction perpendicular to the slip (10 mm in Patch Ⅰ and 9 mm in PatchⅡ), and 8 mm in the direction parallel to the slip for the Shaba surface, 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 loglog plot is the H exponent, and the H exponent depends on the direction of profile and describes the morphological anisotropy of fault surface. The maximum value and minimum value of H are in the direction perpendicular to the slip and in the direction parallel to the slip, respectively. It is accordant to the slip directions measured in field. A secondary set of Hvalue peaks (85° and 160°) in Shaba rupture surface revealed a set of concealed striations produced by an earthquake prior to the Wenchuan earthquake. But it is not sufficient to determine the time and magnitude of this inferred faulting event. Moreover, through the comparison of the H value between fresh joint surface and fault surface, it shows that whether the H value is larger than 08 probably depends on the type of fault. By linear fitting between the slopes of power spectral density (-α) and the slope of RMS roughness (H) in whole length of profile, a relationship can be obtained: α=122+172H; it does not obey the theoretical relationship α=1+2H strictly. This difference is probably caused by the noise in the data, the multifractal of the rupture surface and the analysis methods of roughness.

Key words:  Wenchuan earthquake, rupture surface, topographic characteristics, roughness