Subparallel Dipping Faults that Ruptured during the 2008 Wenchuan Earthquake (original) (raw)
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Geophysical Journal International, 2013
The Mw 7.9 2008 Wenchuan earthquake ruptured about 280 km of faults in the Longmen Shan of Sichuan province, China, at the eastern edge of the Tibetan Plateau. We use teleseismic waveforms with geodetic data from Global Positioning System, synthetic aperture radar interferometry and image amplitude correlation to produce a source model of this earthquake. The model describes evolution of fault slip during the earthquake. The geodetic data constrains the spatial distribution of fault slip and the seismic waveforms constrain mostly the time evolution of slip. We find that the earthquake started with largely thrust motion on an imbricate system of faults beneath the central Longmen Shan, including the Beichuan Fault and Pengguan Fault, with fault slip at depth extending up to 50 km northwest of the mountain front. The fault ruptures continued northeast along the Beichuan Fault with more oblique slip (right--lateral and thrust) and the proportion of lateral motion increasing in the northern Longmen Shan. The northernmost fault segment has a much steeper dip, consistent with nearly pure strike--slip motion. The kinematic source model shows that the rupture propagated to the northeast at about 2.5--3.0 km/s, producing a cascade of sub--events with a total duration of about 110 s. The complex fault ruptures caused shortening and uplift of the extremely steep central Longmen Shan, which supports models where the steep edge of the plateau is formed by thrusting over the strong crust of the Sichuan Basin.
1] We derived a coseismic slip model for the M w 7.9 2008 Wenchuan earthquake on the basis of radar line-of-sight displacements from ALOS interferograms, GPS vectors, and geological field data. Available interferometric synthetic aperture radar (InSAR) data provided a nearly complete coverage of the surface deformation along both ascending (fine beam mode) and descending orbits (ScanSAR to ScanSAR mode). The earthquake was modeled using four subfaults with variable geometry and dip to capture the simultaneous rupture of both the Beichuan fault and the Pengguan fault. Our model misfits show that the InSAR and GPS data are highly compatible; the combined inversion yields a 93% variance reduction. The best fit model has fault planes that rotate from shallow dip in the south (35°) to nearly vertical dip toward the north (70°). Our rupture model is complex with variations in both depth and rake along two major fault strands. In the southern segment of the Beichuan fault, the slip is mostly thrust (<13 m) and occurred principally in the upper 10 km of the crust; the rupture progressively transformed to right-lateral strike slip as it propagated northeast (with maximum offsets of 7 m). Our model suggests that most of the moment release was limited to the shallow part of the crust (depth less than 10 km). We did not find any "shallow slip deficit" in the slip depth distribution of this mixed mechanism earthquake. Aftershocks were primarily distributed below the section of the fault that ruptured coseismically.
Slip maxima at fault junctions and rupturing of barriers during the 2008 Wenchuan earthquake
Nature Geoscience, 2009
The disastrous 12 May 2008 Wenchuan earthquake in China took the local population as well as scientists by surprise. Although the Longmen Shan fault zone-which includes the fault segments along which this earthquake nucleated-was well known, geologic and geodetic data indicate relatively low (<3 mm yr −1 ) deformation rates. Here we invert Global Positioning System and Interferometric Synthetic Aperture Radar data to infer fault geometry and slip distribution associated with the earthquake. Our analysis shows that the geometry of the fault changes along its length: in the southwest, the fault plane dips moderately to the northwest but becomes nearly vertical in the northeast. Associated with this is a change in the motion along the fault from predominantly thrusting to strike-slip. Peak slip along the fault occurs at the intersections of fault segments located near the towns of Yingxiu, Beichuan and Nanba, where fatalities and damage were concentrated. We suggest that these locations represent barriers that failed in a single event, enabling the rupture to cascade through several fault segments and cause a major moment magnitude (M w ) 7.9 earthquake. Using coseismic slip distribution and geodetic and geological slip rates, we estimate that the failure of barriers and rupture along multiple segments takes place approximately once in 4,000 years.
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.
Fault Geometry and Slip Distribution of the 2010 Yushu Earthquakes Inferred from InSAR Measurement
Bulletin of the Seismological Society of America, 2011
We construct a coseismic deformation interferogram for the April 2010 Yushu earthquakes using ALOS/PALSAR data from the ascending track (path 487). We then infer the trace of the Yushu fault using the coherence image, and we build five fault models for the Yushu fault. To determine the fault geometry parameters that give the best fit to the coseismic interferogram, we apply an elastic dislocation algorithm. Our preferred fault model consists of two faults dipping to the northeast. One strikes ∼N60°W with a dip of 82°; the other strikes ∼N67°W with a dip of 86°. Lastly, we infer the coseismic slip distributions of the Yushu earthquakes by the inversion of the displacement in the line of sight (LOS). The results show that three high-slip concentrations are located at a depth of 5 ∼ 8 km, with a peak slip of 1.32 m at (96.93 E, 33.03 N). The Yushu fault is a left-lateral strike-slip faulting with small northside-up, dip-slip components.
Bulletin of the Seismological Society of America, 2013
An extensive data set of teleseismic and strong-motion waveforms and geodetic offsets is used to study the rupture history of the 2008 Wenchuan, China, earthquake. A linear multiple-time-window approach is used to parameterize the rupture. Because of the complexity of the Wenchuan faulting, three separate planes are used to represent the rupturing surfaces. This earthquake clearly demonstrates the strengths and limitations of geodetic, teleseismic, and strong-motion data sets. Geodetic data (static offsets) are valuable for determining the distribution of shallower slip but are insensitive to deeper faulting and reveal nothing about the timing of slip. Teleseismic data in the distance range 30°-90°generally involve no modeling difficulties because of simple ray paths and can distinguish shallow from deep slip. Teleseismic data, however, cannot distinguish between different slip scenarios when multiple fault planes are involved because steep takeoff angles lead to ambiguity in timing. Local strong-motion data, on the other hand, are ideal for determining the direction of rupture from directivity but can easily be over modeled with inaccurate Green's functions, leading to misinterpretation of the slip distribution. We show that all three data sets are required to give an accurate description of the Wenchuan rupture. The moment is estimated to be approximately 1:0 × 10 21 N · m with the slip characterized by multiple large patches with slips up to 10 m. Rupture initiates on the southern end of the Pengguan fault and proceeds unilaterally to the northeast. Upon reaching the cross-cutting Xiaoyudong fault, rupture of the adjacent Beichuan fault starts at this juncture and proceeds bilaterally to the northeast and southwest.
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.
Geodetic data are increasingly being used to infer coseismic slip distribution due to its advantages of widecoverage and high accuracy. However, it is difficult to obtain a comprehensive rupture pattern at depthwhen a source model is only constrained by geodetic surface deformation. In this study, a joint inver-sion approach incorporating stress changes and GPS surface displacements is explored and applied tocharacterize the fault slip of the 2008 Mw 7.9 Wenchuan earthquake, China. The earthquake data for the20-year period before the main quake, which are collected from the background seismicity catalogues,and one month of aftershock data are statistically analysed to determine the fault stress changes basedon the Dieterich model. The coseismic surface deformation measurements from 158 GPS surveying sitesare jointly used to constrain the solution. Our preferred rupture model reveals four high-slip concentra-tions on the Yingxiu–Beichuan fault and one on the subparallel PengGuan fault. The spatial distributionsuggests that the coseismic slip occurs not only above the hypocentre but also with a significant thrust-ing motion, with a mean slip of 8.5 m and a maximum of 9.7 m at a depth of 10–16 km. A significanthigh-slip concentration is found for the first time in this study. The coseismic faulting extends toward∼16 km southwest of the Yingxiu–Beichuan fault and has a dextral strike-slip with a mean displacementof 4.8 m at a depth of 7–19 km. The joint inversion model misfits (GPS: 1.7 cm, stress change: 0.02 MPa)exhibit a good compatibility between the two types of datasets. The derived slip model, which has animproved resolution at depth, explains 98% of the coseismic surface displacements and 93% of the faultstress changes.