T24B-01
Structure of the imbricate thrust system that sourced the 2008 M7.9 Wenchuan earthquake
The 2008 M7.9 Wenchuan earthquake ruptured an imbricate thrust fault system in the frontal part of the Longmen Shan range, which forms the eastern boundary of the Tibetan plateau. We use industry seismic reflection data combined with surface geologic maps and relocated aftershocks to produce a three dimensional model of the thrust faults that ruptured in the earthquake. We identify a segmented pattern to the fault system, with slip in the southern region partitioned between the steeply- dipping Beichuan fault and the shallower Pengguan fault, which likely merge at depth forming an imbricated thrust stack. Based on the mapped surface rupture, the steeply dipping Beichuan thrust exhibits a combination of dip and strike slip displacements, whereas the more shallowly dipping Pengguan fault has primarily dip slip motions (Xu et al., 2008). In contrast, slip is limited to the Beichuan fault in the northern part of the rupture. The transition in rupture patterns occurs in the vicinity of a major lateral geometric segment boundary in the Beichuan fault. Thus, the Wenchuan earthquake clearly demonstrates the ability of thrust fault earthquakes to involve multiple thrust splays and rupture across significant lateral segment boundaries. Prior to the earthquake, this mountain range showed very low geodetic and geologic shortening rates, leading to the suggestion of lower crustal inflation rather than brittle crustal shortening as the main mechanism creating and maintaining its significant topographic relief. In addition to constraining the three dimensional geometries of the faults in this range front, we also show that the relief in the eastern margin of the Longmen Shan can be explained by crustal shortening alone, without calling on lower crustal flow. The Wenchuan earthquake demonstrates the role of active crustal shortening in developing and supporting the range, and illustrates the potential for large and very destructive earthquakes on similar active thrust faults that have been observed in other populated regions of the world.
T24B-02 INVITED
Active faults and surface rupture in the 12 May 2008 Sichuan earthquake
The 12 May 2008 Sichuan, China, earthquake displayed a complex pattern of surface rupture on at least three faults that extend for ~220 km along the Longmen Shan mountains in Sichuan Province. Two of these faults - the Beichuan and Pengguan faults - had been previously recognized as active on the basis of geological and geomorphological relationships. The active traces of the Beichuan and Pengguan faults are northeast-striking, steeply-dipping to vertical shear zones that show a combination of dextral strike-slip and thrust movement, and offset a variety of Quaternary landforms. Evidence for previous earthquakes on both faults is patchy, but available data indicate that surface-rupturing earthquakes have occurred within the last 8-12 kyr on the Beichuan fault, and within the last 1-3 kyr on the Pengguan fault. During the 12 May earthquake, up to 6 m of oblique dextral-thrust slip occurred on strands of the Beichuan fault. Slip during the early stages of the rupture was primarily thrust, and the component of strike-slip deformation became progressively more important as the rupture propagated to the northeast. In places the surface rupture appears somewhat discontinuous, and we speculate that significant near-surface deformation was absorbed by bedding-parallel slip rather than by offset along a discrete fault plane. The Pengguan fault experienced nearly pure thrust deformation along ~100 km of surface rupture, despite geomorphological evidence of Quaternary dextral strike-slip displacement. The third fault that was involved in the 12 May earthquake, informally termed the Xiaoyudong fault, is a northwest-striking, moderately west-dipping fault with approximately equal components of sinistral strike-slip and thrust deformation. The Xiaoyudong fault is largely confined to the floor of the Jian Jiang river valley and was not previously recognized as an active fault. The continuity of geological contacts on either side of the valley appears to rule out long-term displacements of more than a few km on the fault. Observations of apparent compound scarps, however, indicate that the Xiaoyudong fault likely had some surface expression in older fluvial deposits before the 12 May earthquake. Interestingly, despite the extensive surface rupture and widespread evidence for large ground motions, the density of landsliding associated with the earthquake appears to depend much more strongly on bedrock lithology than on proximity to the active fault traces.
T24B-03 INVITED
Geomorphic insights into the growth of eastern Tibet and implications for the recurrence of great earthquakes
The great earthquake of 12 May 2008 in Sichuan Province occurred along one of the more enigmatic mountain fronts in the world. Despite nearly 6km of relief between the Longmen Shan and the Sichuan Basin, geodetic data prior to the earthquake revealed little to no shortening across this margin of the plateau (Burchfiel et al., 2008). How such topography can develop and be maintained in the face of limited shortening of the upper crust remains an outstanding problem. Here, I review the results of geomorphic studies aimed at characterizing the rates and patterns of differential rock uplift across the plateau margin, present new thermochronologic constraints on the rates of exhumation over million-year timescales, and discuss the implications of millennial deformation rates recorded by fluvial terraces. Collectively these data indicate that, prior to the mid-late Miocene, much of the Longmen Shan was characterized by extremely slow exhumation rates (~1-10 m/My). Extrapolation of present-day relationships channel longitudinal profile relief and erosion rate suggests that relief across this margin of the plateau must have been less than ~1 km. The abrupt onset of rapid cooling at ca. 12 Ma inferred from both multi-system thermochronologic and age-elevation relationships is interpreted to mark the onset of Cenozoic deformation and rock uplift. Rocks in the immediate hanging wall of the Yingxiu-Beichuan fault, the primary structure responsible for the 12 May earthquake, have been exhumed from 6 - 8 km depth, at average exhumation rates of 500 - 600 m/My. These data provide a minimum bound on the magnitude and average rate of displacement along the frontal fault system and are consistent with a relatively long recurrence interval (4000 - 8000 yr) for great earthquakes. Geomorphic analyses of the longitudinal profiles of major river systems in the region reveal a locus of active differential rock uplift coincident with the edge of the Tibetan plateau and with the southern portion of the surface rupture. Moreover, this region of high rock uplift rate trends away from the Sichuan Basin, following the Min Shan northward along the topographic margin of the plateau. The striking correspondence between inferred patterns of rock uplift and the along-strike transition from reverse-sense to right-lateral displacement during the rupture suggests that such events may be somewhat characteristic of this fault system. Finally, the distribution of fluvial terraces along two rivers draining the Min Shan reveal a contrast in the style of upper crustal deformation associated with proximity to the fault systems along the margin of the Sichuan Basin. Relatively short wavelength differential incision in the hanging wall of the Yingxiu-Beichuan fault is interpreted to reflect deformation associated with repeated earthquakes along this fault, whereas long-wavelength (>80 km) warping of terrace treads along the Bailong Jiang apparently reflects surface deformation in response to deep-seated flow in the lower crust and/or upper mantle.
T24B-04
Landslide erosion associated with the May 12, 2008 Sichuan Earthquake
The M7.9 earthquake of May 12, 2008 in Sichuan, China triggered a remarkable number of landslides. As time progresses and satellite imagery becomes available, the landslide inventory associated with this event will be a valuable dataset for understanding the dynamics and distribution of landslides triggered by large earthquakes and for constraining models of landscape evolution in steep topography where landslides are the dominant mode of hillslope erosion. The landslides associated with Sichuan earthquake will also have interesting implications for the tectonic and landscape evolution of the Longmen Shan. Preliminary analysis of satellite imagery reveals that the highest density of landslides and therefore the greatest amount of landslide driven erosion was focused along Longmen Shan front associated with steep, high relief topography and is closely tied to the region of greatest ground motion in the hanging wall of the fault rupture. The rocks underlying the highest density of landslides are Precambrian granites and gneisses, and landslide density decreases moving to the northeast in areas underlain by deformed Paleozoic sediments. Short-term erosion rates in the Longmen Shan region measured prior to the earthquake from concentrations of 10Be in quartz extracted in 5 river catchments were 0.22-0.32 mm/yr, reflecting the last 1,500-3,000 years - a time period shorter than the estimated recurrence interval of large earthquakes along the fault that ruptured. Long-term erosion rates measured by low-temperature thermochronology, meanwhile, are higher, 0.5-0.6 mm/yr (Kirby et al., 2002). This suggests that the earthquake and landslide erosion associated with it allowed the landscape to catch up and balance the tectonic growth of Longmen Shan.
T24B-05 INVITED
Lack of Young Subsidence in the East Tibetan Foreland: Implications for Crustal Thickening Processes at Depth
The Wenchuan earthquake of May 12, 2008 occurred on a west-dipping reverse fault (with a pronounced right-slip component) located along the steep eastern edge of the Tibetan plateau. It has been suggested that thrust faulting here may not be indicative of large-scale shortening and thickening of the upper crust, but may rather be an expression of vertical uplift of the upper crust, with minor shortening. This interpretation is compatible with the lack of young flexural subsidence in the Sichuan foreland provided that the flexurally competent layers of the Sichuan foreland lithosphere are loaded from below, or internally, by thickening crustal domains deep within the mid or lower crust of the eastern plateau, rather than from above, by emplacement of thrust sheets at shallow crustal levels onto the flexurally competent layer of the foreland. This interpretation reconciles gravity anomalies across the plateau margin, the young age of the high topography of eastern Tibet, and the old age of the Sichuan basin with the lack of Cenozoic flexural subsidence in the Sichuan foreland. A similar lack of asymmetric foreland subsidence is also present along the northeastern margin of the Tibetan plateau where it abuts the (southeastern) Tarim Basin, suggesting that a similar mechanism may operate here.
T24B-06
Interseismic Activity and Coseismic Activity of the Longmen Shan in the Late Cenozoic: New Insights From the Wenchuan Earthquake
As the eastern margin of the Tibetan plateau, the Longmen Shan in southwestern China is characterized by
steep gradients in topography, crustal thickness, lithosphere thickness and seismic velocities. Although the
extrusion model of Tapponnier et al. (1982) successfully explained the movements of blocks in Asia caused
by the collision of the Indian and Eurasian plates, a low-viscosity zone in the mid- to lower crust has been
proposed to fit low GPS shortening rates, the lack of coeval foreland subsidence and low-angle thrust faults
with young high topography and thickened crust in the Longmen Shan (Burchfiel et al., 1995). However, on
12 May 2008, a magnitude 7.9 earthquake occurred at Wenchuan County along a NE-striking, NW-dipping
thrust fault beneath the Longmen Shan and produced >4 m coseismic slip with comparable magnitude of
reverse and right-slip components, which requires different deformation mechanisms in the Longmen Shan.
A fault-propagation fold at depth of ductile-brittle transition zone with bi-phase activity is provided here to
better understand the late Cenozoic deformation of the Longmen Shan. In the upper crust of the Longmen
Shan, a series of NE-striking, NW-dipping thrust faults cut Precambrian crystalline basement and converge to
a decollement at depths of 10-20 km, which is the flat in a crustal-scale fault-propagation fold and behaves
as the rheological boundary between the strong upper crust and weak mid-lower crust. Due to the uplift and
eastward enlargement of the Tibetan plateau, the mid-lower crust beneath the Longmen Shan flows eastward
and drags the upper crust to move slowly. The eastward weak crustal material was obstructed by the strong
lithosphere of the Sichuan basin, and consequently, accumulated and formed a NW-dipping low-angle ramp
at depth. Therefore the long-term interseismic activity is controlled by the ductile flow of deep crust and
results in crustal thickening, high relief, low GPS shortening rates and strain accumulation. When the growth
of plastic strain excesses a critical point, the strong upper crust decouples from the mid-lower crust, moves
fast along the flat and ramp, and produces instantaneous coseismic activity and breakthrough faults (e.g.,
Beichuan fault, Yingxiu-Beichuan fault, Guanxian-Anxian fault).
The Wenchuan earthquake and aftershocks provide a good example of coseismic activity. The reverse
displacement dominates over right-slip in the initial rapture, but the magnitude of right-slip increases
northeastward with time along 300-km-long ruptures, suggesting a thrust of the hanging wall block with anti-
clockwise rotation around the pole of the fault plane under transpression. The fold geometry depends on the
fault steepness and the amount of imposed shear. In a constant-thickness model of fault-propagation fold,
assuming the ramp angle is 30°, the amount of layer parallel shortening almost equals to that of vertical
uplift. Taking into account of the average relief of 4 km and 5-8 km for denudation, the crustal shortening of
the Longmen Shan is estimated to be <12 km in past 12-15 Ma. But it is more probable that the ramp
angle is < 30° and the crustal thickening will also increase the structural relief, more detailed modeling
is required to constrain the development of Longmen Shan fault-propagation fold. Because the slip of
breakthrough faults only contributes a small amount to the total slip of fault-propagation fold, the late
Cenozoic uplift of the Longmen Shan is largely governed by long-term interseismic activity while the crustal
shortening is more effective in coseismic activity.