Zhuqi Zhang

Clustering of offsets on the Haiyuan fault and their relationship to paleoearthquakes

We used airborne light detection and ranging (LiDAR) data to reevaluate the single-event offsets of the 1920 Haiyuan Ms 8.5 earthquake and the cumulative offsets along the western and middle segments of the coseismic surface rupture zone. Our LiDAR data indicate that the offset observations along both the western and middle segments fall into groups. The group with the minimum slip amount is associated with the 1920 Haiyuan Ms 8.5 earthquake, which ruptured both the western and middle segments. Our research highlights two new interpretations: First, the previously reported maximum displacement of the 1920 earthquake was likely due to at least two earthquakes; second, our results reveal that the cumulative offset probability density (COPD) peaks of the same offset amounts on the western and middle segments do not correspond to one another one-to-one. We suggest that any discussion of the rupture pattern of a certain fault based on the offset data should also consider fault segmentation and paleoseismological data. Therefore, the COPD peaks should be computed and analyzed on fault subsections and not entire fault zones to study the number of paleoearthquakes and their rupture patterns.

Deep crustal deformation of the Longmen Shan, eastern margin of the Tibetan Plateau, from seismic reflection and Finite Element modeling

Rivaling the Himalaya in relief, the Longmen Shan is probably one of the most enigmatic mountain ranges in the world: high mountains reach more than 4000 m relief but without adjacent foreland subsidence and with only slow active convergence. What are geological and geodynamic processes that built the Longmen Shan? Coseismic deformation associated with the 2008 Wenchuan earthquake could hold clues to answer these questions. The primary features associated with the 2008 Wenchuan earthquake rupture have been narrowly distributed coseismic deformation and predominantly vertical displacements that could be interpreted as the result of slips on high-angle listric seismogenic faults. Deep sounding seismic reflection profiling across the seismogenic faults indeed reveals high-angle listric reverse faulting in the brittle upper crust and east-dipping reflectors that we interpret as ductile shearing, in the viscous lower crust. In conjunction with a visco-elastic finite element modeling of coseismic displacements associated with the Wenchuan earthquake, we show that the high-angle listric nature of earthquake faults produces insignificant horizontal shortening across the fault and facilitates upward slips along the fault that both explain the localized coseismic deformation and vertical displacement, as well as the presence of high mountains without adjacent foreland flexure. We suggest that the formation of the Longmen Shan may be better understood in terms of partitioned lithospheric pure-shear thickening in which upward high-angle listric faulting of brittle upper crust is linked to thickening of the more viscous lithospheric mantle through downward ductile shearing of rheologically deformable lower crust.

Morphogenic uncertainties of the 2008 Wenchuan Earthquake: Generating or reducing?

Landscape evolution in active orogen region is inevitably affected by landslides associated with strong earthquakes, rain or storm. However, quantitative studies of the orogenic or eroded volumes are rarely demonstrated. The 2008 Wenchuan Earthquake triggered serious landsliding, consequently, a large amount of landslide material. However, the landslide volume is estimated mainly on the landslides areas interpreted in their semi-automated landslide mapping algorithm. However, the total volume of landslides triggered by the Wenchuan Earthquake amounts to 5–10 times bigger than the expected upper bound according to the empirical correlation between “total landslide volume” and “moment magnitude”. Here we show that the total landslide volume estimation has large uncertainties to be used to determine whether the Wenchuan Earthquake generates or reduces relief. Thus, the widely held view that large dip-slip and oblique-slip earthquakes build mountainous topography may still be applicable to the Wenchuan Earthquake in Longmen Shan area. To challenge this view, simple landslide volume and co-seismic uplift comparison is not enough, i.e., more data are needed.

Erosion Associated with Seismically-Induced Landslides in the Middle Longmen Shan Region, Eastern Tibetan Plateau, China

The 2008 Wenchuan earthquake and associated co-seismic landslide was the most recent expression of the rapid deformation and erosion occurring in the eastern Tibetan Plateau. The erosion associated with co-seismic landslides balances the long-term tectonic uplift in the topographic evolution of the region; however, the quantitative relationship between earthquakes, uplift, and erosion is still unknown. In order to quantitatively distinguish the seismically-induced erosion in the total erosion, here, we quantify the Wenchuan earthquake-induced erosion using the digital elevation model (DEM) differential method and previously-reported landslide volumes. Our results show that the seismically-induced erosion is comparable with the pre-earthquake short-term erosion. The seismically-induced erosion rate contributes ~50% of the total erosion rate, which suggests that the local topographic evolution of the middle Longmen Shan region may be closely related to tectonic events, such as the 2008 Wenchuan earthquake. We propose that seismically-induced erosion is a very important component of the total erosion, particularly in active orogenic regions. Our results demonstrate that the remote sensing technique of differential DEM provides a powerful tool for evaluating the volume of co-seismic landslides produced in intermountain regions by strong earthquakes.

Different earthquake patterns for two neighboring fault segments within the Haiyuan Fault zone

Characteristic slip and characteristic earthquake models have been proposed for several decades. Such models have been supported recently by high‐resolution offset measurements. These models suggest that slip along a fault recurs via similarly sized, large earthquakes. The inter‐event strain accumulation rate (ratio of earthquake slip and preceding interseismic time period) is used here to test the characteristic earthquake model by linking the slip and timing of past earthquakes on the Haiyuan Fault. We address how the inter‐event strain accumulation rate varies over multiple seismic cycles by combining paleoearthquake studies with high‐resolution airborne light detection and ranging (LiDAR) data to document the timing and size of paleoearthquake displacements along the western and middle segments of the Haiyuan Fault. Our observations encompass 5 earthquake cycles. We find significant variations over time and space along the Haiyuan Fault. We observe that on the middle segment of the Haiyuan Fault the rates slow down or increase as an anti‐correlated function of the rates of preceding earthquakes. Here, we propose that the inter‐event strain accumulation rates on the middle segment of the Haiyuan Fault are oscillating both spatially and temporally. However, along the western segment, the inter‐event strain accumulation rate is both spatially and temporally steady, which is in agreement with quasi‐periodic and slip‐predictable models. Finally, we propose that different fault segments within a single fault zone may behave according to different earthquake models.