Co-seismic Surface Rupture Zone Research Articles

Co-seismic Riedel shear structures produced by the 2010 M w 6.9 Yushu earthquake, central Tibetan Plateau, China

Riedel shear structures are often developed within strike-slip fault zones. This study focuses on the Riedel shear structures within the co-seismic strike-slip surface rupture zone produced by the 2010 M w 6.9 Yushu earthquake in the central Tibetan Plateau, China. Field surveys and structural analysis of the surface ruptures reveal that the co-seismic Riedel shear structures are characterized by (i) tension cracks (T fractures); (ii) compression structures, mainly mole tracks (P shears); and (iii) discrete shear faults (R and Y shears) developed in unconsolidated alluvial deposits along the pre-existing left-lateral strike-slip Ganzi–Yushu Fault Zone. The T fractures generally show a right-stepping en echelon pattern. In contrast, the P and R shears indicate a left-stepping en echelon pattern. The Riedel shears are inclined to the general trend of the co-seismic surface rupture zone at mean counterclockwise angles of 41° (T fractures) and 21° (R shears), and a mean clockwise angle of 35° for the P shears. The present results show that (i) the co-seismic Riedel shear structures indicate a left-lateral strike-slip sense for the seismogenic fault, and (ii) the T fractures, and P and R shears are the primary Riedel shear structures formed during the early stages of the evolution of a strike-slip fault constrained by pre-existing geologic structures.

The M S7.1 Yushu earthquake surface rupture and large historical earthquakes on the Garzê-Yushu Fault

As revealed by field investigations, the co-seismic surface rupture zone of the 2010 M S7.1 Yushu earthquake, Qinghai is a characteristic sinistral strike-slip feature consisting of three distinct sinistral primary ruptures, with an overall strike of 310°–320° and a total length of 31 km. In addition, an approximately 2-km-long en-echelon tensile fissure zone was found east of Longbao Town; if this site is taken as the north end of the rupture zone, then the rupture had a total length of ∼51 km. The surface rupture zone is composed of a series of fissures arranged in an en-echelon or alternating relationship between compressive bulges and tensile fissures, with a measured maximum horizontal displacement of 1.8 m. The surface rupture zone extends along the mapped Garze-Yushu Fault, which implicates it as the seismogenic fault for this earthquake. Historically, a few earthquakes with a magnitude of about 7 have occurred along the fault, and additionally traces of paleoearthquakes are evident that characterize the short-period recurrence interval of large earthquakes here. Similar to the seismogenic process of the 2008 Wenchuan earthquake, the Yushu earthquake is also due to the stress accumulation and release on the block boundaries resulting from the eastward expansion of Qinghai-Tibet Plateau. However, in contrast with the Wenchuan earthquake, the Yushu earthquake had a sinistral strike-slip mechanism resulting from the uneven eastward extrusion of the Baryan Har and Sichuan-Yunnan fault blocks.

Selection of emergency shelter sites for seismic disasters in mountainous regions: Lessons from the 2008 Wenchuan Ms 8.0 Earthquake, China

In this paper, we use the 12 May 2008 Wenchuan Earthquake as a background event for analyzing and applying the principles of site selection of emergency shelters for a disastrous earthquake. Based on field investigations and analyses of remote sensing imagery, we identified the distribution of active faults and the locations of co-seismic surface rupture zones—areas in which buildings are at risk of intensive damage. It is important that emergency shelters are located outside of such vulnerable areas. One of the lessons learned from the Wenchuan Earthquake is that high fatality rates occur in areas without life-saving shelters. The principles that underlie the selection of emergency shelter sites are as follows: (1) keep far away from active fault zones, with the distance depending on the characteristics of the fault, including the nature of hangingwall and footwall structures; (2) disaster-mitigation strategies should be developed as a multi-dimensional system for the management of natural hazards, human activities, and urban expansion, involving keeping away from vulnerable slopes and establishing an early-warning system; (3) the accessibility of mountainous regions must be considered, including establishing small emergency shelters that house large numbers of people and covering regions with an uneven distribution of villages; and (4) government and law-making agencies in China must establish new earthquake design codes for buildings, emphasizing the importance of public facilities (including schools, collective welfare institutions, and medical facilities) as emergency shelters during disastrous earthquakes. The site-selection process requires an interdisciplinary approach involving seismologists, engineers, environmental and social scientists, emergency management personnel, and government officials. The parameters upon which the above principles are based can be qualitatively determined, thereby providing a valuable initial database for further quantitative analysis. The preliminary results and knowledge gained in the present paper can be used as a decision-making tool to support the government in earthquake-recovery and reconstruction programs. We also discuss practical examples of site evaluation in regions that suffered heavy damage during the Wenchuan Earthquake.

Co-seismic landslides induced by the 2008 Wenchuan magnitude 8.0 Earthquake, as revealed by ALOS PRISM and AVNIR2 imagery data

The magnitude 8.0 Wenchuan Earthquake occurred on 12 May 2008 in Sichuan Province, China, resulting in numerous landslides along the marginal zone between the Tibetan Plateau and Sichuan Basin. We used Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM) and Advanced Visible and Near Infrared Radiometer type 2 (AVNIR 2) data from the Advanced Land Observing Satellite (ALOS) and Landsat Enhanced Thematic Mapper (ETM) images acquired before and after the earthquake to identify co-seismic landslides. Interpretations of remote sensing images, combined with fieldwork, reveal that co-seismic landslides are mainly restricted to a corridor of <18 km in width about the co-seismic surface rupture zone along a length of >285 km along pre-existing active faults of the Longmenshan Thrust Belt. The landslides mainly occurred upon steep slopes (dips of 30–75°). The distribution and topographic features of co-seismic landslides indicate a close relationship with seismic slip along the co-seismic surface rupture zone. The locations of landslides are controlled by the tectonic topography developed along pre-existing active faults of the Longmenshan Thrust Belt. Our results demonstrate that remote sensing techniques provide a powerful tool for identifying co-seismic landslides produced in intermountain regions by strong earthquakes.

Co-seismic ground-shortening structures produced by the 2008 Mw 7.9 Wenchuan earthquake, China

We observed ground-shortening structures produced by the 2008 M w 7.9 Wenchuan earthquake along the 285-km-long Wenchuan co-seismic surface rupture zone. The structures are characterized by co-seismic thrusting and folding structures that include thrust fault scarps, flexure-slip folds, and mole tracks. Fold structures are generally developed within alluvial deposits, river channel deposits, and sealed road surfaces, mainly restricted to within a < 100-m-wide corridor (generally < 50 m) about the co-seismic thrusting fault scarp along the co-seismic surface rupture zone. The amount of co-seismic horizontal ground shortening accommodated by co-seismic thrusting and folding ranges from 0.1 up to 7.3 m (generally 1–3 m), consistent with GPS observations. Our results, combined with geological and GPS data, confirm that present-day shortening strain upon the eastern marginal zone of the Tibetan Plateau is mainly released by seismic thrusting–folding along active faults within the Longmen Shan Thrust Belt.

Structural model of 2008 Mw 7.9 Wenchuan earthquake in the rejuvenated Longmen Shan thrust belt, China

The 12 May 2008 Wenchuan earthquake ( M w = 7.9) of China occurred in the Longmen Shan thrust belt and is a consequence of the ongoing uplift process of the eastern margin of Tibetan plateau. The coseismic surface ruptures coincide with the pre-existing thrust fault trace. Destruction of buildings and the location of seismically induced landslides are located in the hanging wall of the reactivated thrust faults. In order to detect any indication of coseismic deformation linked to subsurface structures, we interpret the subsurface fault and fold geometries using petroleum seismic reflection profiles, as well as coseismic surface ruptures and seismicity. Our data suggest that the deformation can be divided into two different structural segments related to the regional coseismic deformation during the Wenchuan earthquake. In the southern segment (the Yingxiu–Hongkou–Hanwang segment), two coseismic surface rupture zones coincide with two pre-existing thrust faults (the Yingxiu–Beichuan and Pengguan faults) in the seismic profiles and both coseismic active thrusts become incorporated into the deep main detachment. This through-going thrust fault connecting directly from the hypocenter to the surface break, but cannot be easily extended to the northern segment along the Yingxiu–Beichuan thrust fault zone. In contrast, only one shallow coseismic active thrust fault with oblique-slip occurs as the active passive roof of imbricate thrust sheets in the northern segment (Beichuan–Qingchuan segment). Our results show ramp-flat geometry and the wedge characteristics at the blind front of the rejuvenated thrust belt. We emphasize that there is a potential 15–17 km-deep main detachment associated with this large earthquake in the Longmen Shan belt, and infer that some active displacements along the 6–9 km depth shallow detachment have propagated into the Sichuan basin since late Cenozoic.

Rupture Segmentation and Process of the 2001 Mw 7.8 Central Kunlun, China, Earthquake

Field investigations allow us to constrain the coseismic surface rupture zone of ∼400 km with a strike slip up to 16.3 m associated with the 2001 M w 7.8 Central Kunlun earthquake that occurred along one of the western segments of the Kunlun fault, northern Tibet. The rupture zone may be divided into four segments based on the geological structures, tectonic landform features, spatial displacement distributions obtained from field observations, and analysis of teleseismic waveforms. The deformational characteristics of the surface ruptures and focal mechanism solutions reveal that the earthquake had a nearly pure strike-slip mechanism. The inversion results from seismic data show that the rupture started from the west near the epicentral area bilaterally and rapidly extended to the east in an unilateral manner for 380 km and that the largest slip region was limited in the subsegment 150-280 km east of the epicenter, consistent with field observations. The average stress drop is estimated to be 7 MPa in the area where the largest displacements occurred, a value typical of intraplate earthquakes. The geologic evidence and the inversion results from seismic data clearly show that temporal and spatial displacement distributions and the rupture process are restricted by the pre-existing geological structures of the Kunlun fault.

Co-seismic strike-slip and rupture length produced by the 2001 Ms 8.1 Central Kunlun earthquake.

Field investigations show that the surface wave magnitude (Ms) 8.1 Central Kunlun earthquake (Tibetan plateau) of 14 November 2001 produced a nearly 400-kilometer-long surface rupture zone, with as much as 16.3 meters of left-lateral strike-slip along the active Kunlun fault in northern Tibet. The rupture length and maximum displacement are the largest among the co-seismic surface rupture zones reported on so far. The strike-slip motion and the large rupture length generated by the earthquake indicate that the Kunlun fault partitions its deformation into an eastward extrusion of Tibet to accommodate the continuing penetration of the Indian plate into the Eurasian plate.