ALBERT

Description: We describe a complex geologic model as an aggregate of arbitrarily shaped blocks separated by cubic splines in 2D and triangulated interfaces in 3D. Recently we have introduced a segmentally iterative ray-tracing (SIRT) method based on Fermat's principle of stationary travel time, which has been documented to be robust and fast for a complex block model with a constant velocity defined in each block. In this work, we extend the constant velocity to a generally continuous distribution with an analytical expression of travel time, and develop SIRT in the redefined velocity distribution. As a three-point perturbation scheme, SIRT requires an explicit analytical travel time between two intersection points expressed as a function of coordinates of the two points. In these situations, we derive a general midpoint perturbation formula, and further a detailed perturbation formula for familiar media with a constant velocity gradient. SIRT is a scheme in which we perturb the intersection points of an initial-guess ray path in sequence by the first-order explicit formulas instead of using traditional iterative methods. A key consideration is the fact that the number of intersection points may be variable during the iteration process. Numerical tests demonstrate that SIRT is effective in implementing kinematic two-point ray tracing in complex 2D and 3D heterogeneous media.

Description: Worldwide occurrence and documentation of reverse-type ruptures are sparse. Near Hongkou, the Wenchuan rupture passes through the broad Baisha River valley and provides excellent opportunities to trace the surface faulting in fine details for 13 km distance, one of the longest continuous sections along the entire rupture. In this paper, we present the results of our mapping of the surface rupture in this reach. Based on the discontinuities in slip and geometry, the rupture was divided into four sections for convenience, from west to east: the Shenxi Gou, the Miaoba, the Gaoyuan, and the Bajiao Miao sections, respectively. The vertical offset is large in the Shenxi Gou and the Bajiao Miao sections, locally reaching 5-6 m in maxima, and generally low in the Miaoba section (1-2 m or less in most places). The slip gradient for vertical offset is generally 10 (super -3) , locally up to 10 (super -1) , similar to that in well-documented strike-slip ruptures. Near Gaoyuan village, the surface rupture consists of two subparallel branches, with the northern one exhibiting right-lateral slip with minor southeast-side-up thrusting, while the southern one is almost pure southeast-side-up thrusting. This pattern mimics the incomplete slip-partitioning of oblique thrusting on parallel faults but at a local scale. In addition, the sense of vertical throw on these two strands is opposite to the general northwest-side-up thrusting of the Wenchuan rupture. We propose that it is likely due to the inheritance at shallow depth from the southeast-dipping geological faults, and that old fault zone structures can have a strong effect on the dynamic rupture by guiding the rupture propagation onto paths of preexisting, though locally unfavorable, dipping fault planes. We also discuss the cross-cutting slickenside striations observed near Bajiao Miao, which indicated temporal rake rotation during dynamic rupture, and their geological and seismological implications.