Surface Rupture of the 2008 Wenchuan, China, Earthquake in the Qingping Stepover Determined from Geomorphologic Surveying and Excavation, and Its Tectonic Implications (original) (raw)
Related papers
Bulletin of the Seismological Society of America, 2003
2001
We mapped and analyzed two vertical exposures-exposed on the walls of a 3-to 5-m-deep, 70-m-long excavation and a smaller 3-m-deep, 10-m-long excavation -across the 1999 rupture of the Chelungpu fault. The primary exposure revealed a broad anticlinal fold with a 2.5-m-high west-facing principal thrust scarp contained in fluvial cobbly gravel beds and overlying fine-grained overbank deposits. Sequential restoration of the principal rupture requires initial failure on the basal, east-dipping thrust plane, followed by wedge thrusting and pop-up of an overlying symmetrical anticline between two opposing secondary thrust faults. Net vertical offset is about 2.2 m across the principal fault zone. From line-length changes, we estimate about 3.3 m of horizontal shortening normal to fault strike. The ratio of these values yields a total slip of 4.0 m and an estimate of about 34° for the dip of the fault plane below the excavation. This value is nearly the same as the 35° average dip of the fault plane from the surface to the hypocenter. Restoration of the exposed gravelly strata and adjacent overbank sediments deposited prior to the 1999 event around the principal rupture suggests the possible existence of a prior event. A buried 30-m-wide anticlinal warp beneath the uplifted crest of the 1999 event is associated with three buried reverse faults that we interpret as evidence for an earlier episode of folding and faulting in the site. The prior event is also recorded in the smaller excavation, which is located 40 m south and is oriented parallel to the larger excavation. Radiocarbon dating of samples within the exposed section did not place tight constraints on the date of the previous event. Available data are interpreted as indicating that the previous event occurred before the deposition of the less than 200 14 C yr B.P. overbank sands and after the deposition of the much older fluvial gravels. We interpret the previous event as the penultimate event relative to the 1999 Chi-Chi earthquake. We estimated the long-term slip rate of the Chelungpu fault to be 10-15 mm/yr during the last 1 Ma, based on previously published retrodeformable cross sections. This rate is, however, significantly higher than geodetic rates of shortening across the Chelungpu thrust where two pairs of permanent Global Positioning System stations suggest 7-10 mm/yr of shortening across the fault. Given the 4 meters of average slip, the long-term slip rate yields an interseismic interval of between 267 and 400 yr for the Chelungpu fault.
The~220 km-long rupture of the 2008 Mw 7.9 Wenchuan earthquake breached several km-scale geometric discontinuities along strike, including the previously un-mapped NW-trending Xiaoyudong fault, connecting between the two major, NE-trending rupture planes on the Beichuan and Pengguan Faults. In this paper, we present high-resolution mapping of the 8-km-long surface breaks and sinistral oblique thrusting coseismic slip on the Xiaoyudong fault. Scarp height is the largest at the NW end, reaching 3.5 m, and decreases southward in steps to less than 0.2 m, with an average slip gradient of 6 × 10 −3 at a few tens of meters length scale, but up to 50 × 10 −3 locally. Left-lateral offsets co-vary with the vertical component. The largest sinistral slip vector we observed is 2.2 m. Geological and geophysical evidence suggests that the Xiaoyudong fault is likely a~30°SW-dipping lateral ramp that soles into the Pengguan fault, and at its northwestern end intersects with the Beichuan fault, where the latter has a step in the fault plane. Kinematically, the Xiaoyudong fault functions as a tear and conjugate fault and coincides with significant coseismic slip rake rotations on both the Beichuan and Pengguan Faults. Similar correlation of fault bends with sharp changes in faulting style occurs at other steps along the Wenchuan rupture. The Xiaoyudong fault may have played a positive role in linking coseismic slip partitioning between parallel reverse fault planes, facilitating the growth of a longer and more destructive rupture. This highlights the role of tear faults in bridging ruptures between segments, such that reverse-type ruptures can breach steps wider than anticipated from strike-slip fault examples. Transfer faults are common, and perhaps poorly documented features in reverse fault systems and their roles in ruptures may increase the maximum potential earthquake magnitude for fold-and-thrust belts.
Tectonophysics, 2013
Three faults were involved in the rupture process of the May 12, 2008 Mw7.9 Wenchuan earthquake, representing a rarely documented case of multiple faults rupturing simultaneously. Distinct from the other two major NE-trending surface ruptures, the~7 km long Xiaoyudong surface rupture is oriented NW340°a nd shows thrust-and left-slip faulting. In order to understand what role Xiaoyudong rupture played in the tectonic evolution of Longmenshan thrust system, we conducted a detailed investigation including geomorphic feature observation, coseismic displacement measurement, paleo-rupture event trenching, and fault kinematic estimation of the Xiaoyudong area. Our results not only document an in situ paleoearthquake occurring at 3.2-2.3 ka BP, but also present the Xiaoyudong rupturing event as an oblique thrust with minor components of left lateral slip. We also found vertical and sinistral slip rates estimated as 1.1-1.5 mm/yr and 0.5-0.7 mm/yr, respectively. Finally, we suggest that the Xiaoyudong rupture is not a passive tear fault but an active participator of slip partitioning on multiple faults within the Longmenshan thrust system, and moreover, it gave strong response to both the 2008 and the penultimate earthquake while it kept silence with the antepenultimate event.
Rupture Process of the 2008 Wenchuan, China, Earthquake: A Review
Earthquake and Disaster Risk: Decade Retrospective of the Wenchuan Earthquake, 2019
The May 12, 2008, Wenchuan earthquake (MW 7.9, MS 8.1) is the largest continental intraplate event to strike globally in the last 60 years. It caused great destruction and loss of life along the steep eastern margin of the Tibetan Plateau, adjacent to the Sichuan Basin. The event ruptured multiple faults with a mix of thrust- and right-lateral strike-slip faulting along the northeast-trending Longmen Shan thrust belt, with an overall oblique compressional deformation. Surface displacements of up to ~11 m, the distribution of thousands of aftershocks and landslides, geodetic observations, and seismic wave imaging indicate a total rupture extent of ~280 km, extending unilaterally northeastward from the hypocenter. The primary slip has a patchy distribution along the segmented out-of-sequence Beichuan fault, with large-slip patches in the region from Yingxiu to Xiaoyudong, near Beichuan, and near Nanba. The southwestern segment near Yingxiu, where the hanging wall is comprised of the h...
The morphology of thrust faulting in the 21 September 1999, Chichi, Taiwan earthquake
Journal of Asian Earth Sciences, 2000
The 80-km-long surface rupture of the Chelungpu fault in the 21 September 1999, Chichi, Taiwan Mw=7.6 earthquake resulted in a surface scarp with vertical throws of 2±9 m, and horizontal heaves of 4±8 m. Few major thrust faults have broken the surface in the past century, and the Chelungpu surface rupture is of interest in that it provides a morphological template for the identi®cation of paleo-surface thrusts in similar neotectonic environments such as the Himalaya. The toe of the thrust is found emplaced gently over underlying hanging-wall materials, partly by prograde hanging-wall rotation and partly by simple shear, leaving few overt clues as to the total amount or sense of slip. Despite the large surface slip near the toe of the Chelungpu thrust its emplacement appears to have been relatively slow. MSK (the Medvedev±Sponheuer±Karnik 1981 revision of the Seismic Intensity Scale MSK81 supersedes the Mercalli Intensity scale for the description of acceleration-induced damage to modern buildings) Intensity VIII accelerations were imposed on buildings on the hanging-wall, and Intensity VII on the footwall, decaying in both directions by perhaps one intensity unit a few hundred meters from the rupture. The somewhat moderate amplitude of these accelerations, for a rupture with several meters of slip, is attributed to non-linear dissipative deformation near the toe of the rupture. The partitioning of thrusting into basal slip and hillside steepening in some locations on the Chelungpu fault suggests that the estimation of paleoseismic slip from the oset of piercing points crossing historic thrust faults elsewhere may result in underestimates of fault slip. #
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 3 , locally up to 10 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.
Tectonophysics, 2012
The~220 km-long rupture of the 2008 Mw 7.9 Wenchuan earthquake breached several km-scale geometric discontinuities along strike, including the previously un-mapped NW-trending Xiaoyudong fault, connecting between the two major, NE-trending rupture planes on the Beichuan and Pengguan Faults. In this paper, we present high-resolution mapping of the 8-km-long surface breaks and sinistral oblique thrusting coseismic slip on the Xiaoyudong fault. Scarp height is the largest at the NW end, reaching 3.5 m, and decreases southward in steps to less than 0.2 m, with an average slip gradient of 6 × 10 −3 at a few tens of meters length scale, but up to 50 × 10 −3 locally. Left-lateral offsets co-vary with the vertical component. The largest sinistral slip vector we observed is 2.2 m. Geological and geophysical evidence suggests that the Xiaoyudong fault is likely a~30°SW-dipping lateral ramp that soles into the Pengguan fault, and at its northwestern end intersects with the Beichuan fault, where the latter has a step in the fault plane. Kinematically, the Xiaoyudong fault functions as a tear and conjugate fault and coincides with significant coseismic slip rake rotations on both the Beichuan and Pengguan Faults. Similar correlation of fault bends with sharp changes in faulting style occurs at other steps along the Wenchuan rupture. The Xiaoyudong fault may have played a positive role in linking coseismic slip partitioning between parallel reverse fault planes, facilitating the growth of a longer and more destructive rupture. This highlights the role of tear faults in bridging ruptures between segments, such that reverse-type ruptures can breach steps wider than anticipated from strike-slip fault examples. Transfer faults are common, and perhaps poorly documented features in reverse fault systems and their roles in ruptures may increase the maximum potential earthquake magnitude for fold-and-thrust belts. Tectonophysics j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / t e c t o Please cite this article as: Liu-Zeng, J., et al., Surface ruptures on the transverse Xiaoyudong fault: A significant segment boundary breached during the 2008 Wenchuan earthquake, China, Tectonophysics (2012), http://dx.