Co-seismic Surface Rupture Zone Research Articles

Slip rate deficit partitioned by fault-fold system on the active Haiyuan fault zone, Northeastern Tibetan Plateau

This study focuses on the complexity of fault-fold system developed in the Hasishan section of the Haiyuan fault zone in the northeastern margin of the Tibetan Plateau, along which a 237-km-long coseismic surface rupture zone formed during the 1920 M 8.5 Haiyuan earthquake. Interpretations of satellite imageries combined with field investigations and structural analyses of fault geometry and kinematics reveal that the fault-fold system developed in the Hasishan section are characterized by four main elements (i) the left-lateral strike-slip Haiyuan primary fault F1; (ii) a discrete branch fault F2 with an identical sense of movement; (iii) a tension structure, characterized by a NE-trending normal fault F3 developed in piedmont terrace deposits; and (iv) a compression structure P, characterized by a NW-trending en echelon folds within the Cretaceous–Cenozoic sedimentary sequences including the most recent late Pleistocene–Holocene unconsolidated and/or weakly-consolidated deposits. Previous geologic and geophysical observations show that the slip rate along the whole Haiyuan fault zone is non-uniform, indicating an eastward decrease with a gradient of ∼2.65 mm/100 km. In contrast, the slip rate for the primary fault F1 in the Hasishan section was reported to be 3.2 ± 0.2 mm/yr, ∼3 mm/yr less than the result of 6.2 mm/yr inferred from the slip rate gradient. The finding in this study demonstrates that the fault-fold system consisting of faults F2, F3, and fold P has partitioned ∼3 mm/yr slip within the Hasishan section of the Haiyuan fault zone. This study documents that the slip rates at different sections along the primary strike-slip faults are partially dependent on local geometric structure and fault complexity, and the recognition and analyses of related fault-fold system within large strike-slip fault zone are important for better characterizing the total slip rate budget across primary strike-slip faults, which will contribute to improve seismic hazard assessments.

Typical Riedel shear structures of the coseismic surface rupture zone produced by the 2021 Mw 7.3 Maduo earthquake, Qinghai, China, and the implications for seismic hazards in the block interior

Large earthquakes in the Tibetan Plateau generally occur on block-boundary faults and produce coseismic surface ruptures that indicate the sense of block movement. However, seismic hazard in the block is less concerned. The Mw 7.3 Maduo earthquake on 22 May 2021 hit the internal Bayan Har block, which provides a rare opportunity to study the deformation features of the intra-block tectonics in central Tibet and related kinematics implications. We developed the high-resolution survey via the DJI drone of Phantom 4 Pro RTK and detailed field observations, to sum up the typical characteristics of the Maduo coseismic surface rupture zone. Then we classified these surface ruptures based on rupture orientation, spatial distribution, and stress state and analyzed their kinematic mechanisms. Our results indicate that (1) the typical surface ruptures of the Maduo earthquake are shear fractures, tensional cracks, and mole tracks, which follow the Riedel shear model; (2) the Riedel shear structures reveal a sinistral strike-slip movement sense for the seismogenic fault, accordant with regional tectonics; (3) seismic potential on minor faults in the interior block should be emphasized highly. This study will not provide insights into the kinematics of surface ruptures on a strike-slip fault but will strengthen the understanding of seismic hazards on an intra-block active fault.

Anomalous co-seismic surface effects produced by the 2014 Mw 6.2 Ludian earthquake, Yunnan, China: An example of complex faulting related to Riedel shear structures

Riedel shear structures are often developed within strike-slip fault zones. This study focuses on the Riedel shear structures within the co-seismic surface rupture zone and the co-seismic landslides produced by the 2014 Mw 6.2 Ludian earthquake. All the geologic, seismic, and geodesic data indicate a WNW–ESE principal compressive stress acting on the Baogunao-Xiaohe fault zone (BXFZ). Based on the idealized Riedel shear model, together with evidences form the particular distribution of aftershocks and co-seismic surface effects, the 2014 Ludian earthquake could be with a complex faulting process related to the Riedel shear structures under this present tectonic stress, i.e., NNW-striking sinistral strike-slip fault plane (Y shear) which trends approximately N40°W, and ENE-striking dextral strike-slip fault plane (R' shear) which is nearly perpendicular to former. Field surveys and structural analysis of the co-seismic surface rupture zone are performed based on Unmanned Aerial Vehicle (UAV), borehole drilling, and adit excavation, to the north of Wangjiapo Village, southeast of the epicenter. The results indicate that these co-seismic surface ruptures were produced following a pre-existing sinistral transtensional duplex structure in plan view, which mainly comprises i) typical Riedel shear structures characterized by en echelon cracks running northwest and ii) tensile cracks subparallel to the slope contours running northeast. And the inward-dipping geometry of faults under the sinistral transtentional duplexes converges downwards into a single shear zone, which is defined as a negative (normal faulted) flower structure in vertical cross-section. In addition, on the basis of the analyses of three large-scale landslides, this study demonstrates that the slopes in a state of marginal static stability just situated inside the region of main energy release, which was predominantly controlled by the Y shear fault, are the reasons why this moderate earthquake induced anomalous co-seismic landslides.

Structural analysis of the 1997 Mw 7.5 Manyi earthquake and the kinematics of the Manyi fault, central Tibetan Plateau

On 8th November 1997, the Mw 7.5 Manyi earthquake ruptured the left-lateral strike-slip Manyi fault in the central Tibetan Plateau. Despite its large magnitude, a comprehensive geological study on the co-seismic surface rupture has not yet been reported so far. Here we reported on our new results of the co-seismic surface ruptures and left-lateral offsets caused by the earthquake based on detailed interpretation of recently acquired high-resolution WorldView and SPOT-5 images. We recognized a primary ∼185-km-long surface rupture that trends N80°E, in consistence with previous results. Along the poorly developed segment at the eastern end of the Manyi fault, a new rupture is also identified, which trends N57°E and extends ∼17 km. The co-seismic surface rupture zone exhibits many distinct features such as stepovers, extensional cracks, ponds, restraining and releasing bends in relation to fault geometrical complexities. By visual analysis of the displaced geomorphic features using satellite imagery, we measured 198 co-seismic offsets along the ∼200 km fault trace. The results show a ∼3.7 m average offset and a maximum offset of up to ∼7.5 m near the epicenter area. The newly found ∼17-km-long rupture is interpreted to be linked to the main fault during the event. If the Manyi fault continues this northeastward propagation for additional 100 km, it will separate the BayanHar Block into two sub-blocks. Such a new hypothetical block model will influence assessment of seismic potential near the block boundaries and estimate of deformation of the BayanHar Block and the central Tibetan Plateau.

Investigating the Milun Fault: The coseismic surface rupture zone of the 2018/02/06 ML 6.2 Hualien earthquake, Taiwan

Investigating the Milun Fault: The coseismic surface rupture zone of the 2018/02/06 ML 6.2 Hualien earthquake, Taiwan

Co-seismic surface rupture of Papatea fault and reactivation mechanism of the Clarence landslide during the 2016 Mw7.8 Kaikoura earthquake, New Zealand

Large earthquake-triggered landslides near a seismogenic fault are usually closely linked to fault slipping. Therefore, it is important to research the relationship between the landslide movement and the deformation pattern of faults to better understand the location and influencing factors related to the large landslides. A large landslide, on the right bank of the Clarence River, New Zealand, was reactivated during the 2016 Mw 7.8 Kaikoura earthquake. The landslide is located in close proximity to the Papatea Fault which ruptured during the earthquake. The special tectonic location of this landslide provided favorable conditions to research the failure mechanism of earthquake-triggered landslides. Field investigation and remote-sensing images revealed that the co-seismic surface rupture zone of the Papatea Fault is relatively complicated and characterized by a wide deformation zone with a width of ~80–170 m. The northern section showed as left-lateral strike-slip and thrust-slip, with a vertical displacement of up to 4–6 m. However, it exhibited thrust motion in the Clarence landslide area. The Clarence landslide was a reactivation of a pre-existing landslide. The thrust faulting of the Papatea Fault produced a wide deformation and fractured zone near the top of the landslide mass, and resulted in over-steepened topography in the landslide area. It should be noted that long-team seismic shaking, especially the main frequency of the seismic acceleration and rapid co-seismic vertical slip of the fault, played the principal role in further accelerating topographic instability during the earthquake. The landslide body thus started to slide along a pre-existing sliding surface under these actions. Our results indicate that the deformation pattern of the seismogenic fault could also play an important role in the occurrence of earthquake-triggered landslides, in addition to geological and geomorphological conditions and local seismic acceleration.

Millennium recurrence interval of morphogenic earthquakes on the Qingchuan fault, northeastern segment of the Longmen Shan Thrust Belt, China

The 2008 M w 7.9 Wenchuan produced a ∼285–300-km-long coseismic surface rupture zone, including a 60-km-long segment along the Qingchuan fault, the northeastern segment of the Longmen Shan Thrust Belt (LSTB), Sichuan Basin, central China. Field investigations, trench excavations, and radiocarbon dating results reveal that (i) the Qingchuan fault is currently active as a seismogenic fault, along which four morphogenic earthquakes including the 2008 Wenchuan earthquake occurred in the past ca. 3500 years, suggesting an average millennium recurrence interval of morphogenic earthquakes in the late Holocene; (ii) the most recent event prior to the 2008 Wenchuan earthquake took place in the period between AD 1400 and AD 1100; (iii) the penultimate paleoseismic event occurred in the period around 2000 years BP in the Han Dynasty (206 BC–AD 220); (iv) the third paleoseismic event occurred in the period between 900 and 1800 BC; and (v) at least three seismic faulting events occurred in the early Holocene. The present results are comparable with those inferred in the central and southwestern segments of the LSTB within which the Wenchuan magnitude earthquakes occurred in a millennium recurrence interval, that are in contrast with previous estimates of 2000–10,000 years for the recurrence interval of morphogenic earthquakes within the LSTB and thereby necessitating substantial modifications to existing seismic hazard models for the densely populated region at the Sichuan region.

Fault zones ruptured during the early 2014 Cephalonia Island (Ionian Sea, Western Greece) earthquakes (January 26 and February 3, Mw 6.0) based on the associated co-seismic surface ruptures

The early 2014 Cephalonia Island (Ionian Sea, Western Greece) earthquake sequence comprised two main shocks with almost the same magnitude (moment magnitude (Mw) 6.0) occurring successively within a short time (January 26 and February 3) and space (Paliki peninsula in Western Cephalonia) interval. Each earthquake was induced by the rupture of a different pre-existing onshore active fault zone and produced different co-seismic surface rupture zones. Co-seismic surface rupture structures were predominantly strike-slip-related structures including V-shaped conjugate surface ruptures, dextral and sinistral strike-slip surface ruptures, restraining and releasing bends, Riedel structures (R, R′, P, T), small-scale bookshelf faulting, and flower structures. An extensional component was present across surface rupture zones resulting in ground openings (sinkholes), small-scale grabens, and co-seismic dip-slip (normal) displacements. A compressional component was also present across surface rupture zones resulting in co-seismic dip-slip (reverse) displacements. From the comparison of our field geological observations with already published surface deformation measurements by DInSAR Interferometry, it is concluded that there is a strong correlation among the surface rupture zones, the ruptured active fault zones, and the detected displacement discontinuities in Paliki peninsula.

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.

Co-seismic surface ruptures produced by the 2014 Mw 6.2 Nagano earthquake, along the Itoigawa–Shizuoka tectonic line, central Japan

Field investigations reveal that the Mj 6.8 (Mw 6.2) Nagano (Japan) earthquake of 22 November 2014 produced a 9.3-km-long co-seismic surface rupture zone. The slip occurred on the pre-existing active Kamishiro Fault, which developed along the Itoigawa–Shizuoka tectonic line, and is inferred as the boundary between the Eurasian and North American plates. The surface-rupturing earthquake produced dominant thrusting and subordinate strike-slip displacement. Structures that developed during the co-seismic surface rupture include thrust faults, fault scarps, en-echelon tension cracks, folding structures such as mole tracks and flexural folds, and sand-boils. The surface displacements measured in the field range from several centimeters to 1.5m in the vertical (typically, 0.4–1m), accompanied by a strike-slip component that reached 0.6m along NNE trending ruptures. These observations indicate a thrust-dominated displacement along the seismogenic fault. Our results show that (i) the pre-existing Kamishiro Fault, which strikes NNE–SSW, controlled the spatial distribution of co-seismic surface ruptures and displacements; and (ii) the style and magnitude of thrust displacements indicate that the present-day shortening strain on the Eurasian–North American plate boundary in the study area is released mainly by seismic thrust displacements along the active Kamishiro Fault.

Microstructural and mineral analysis on the fault gouge in the coseismic shear zone of the 2008 M w 7.9 Wenchuan earthquake

The 2008 Mw 7.9 Wenchuan earthquake formed two coseismic surface rupture zones with the trend of N35°E, known as the Beichuan–Yingxiu rupture and the Pengguan rupture. The Beichuan–Yingxiu rupture is the principle one with abundant fault gouge development along its length. In the exploratory trench at the Saba village along the Beichuan–Yingxiu rupture, the new fault gouge zone is only ~3 mm wide, which suggests that fault slip was constrained in a very narrow zone. In this study, we thus carried out detailed microstructural and mineral component analysis on the oriented fault gouge samples from the Saba exploratory trench to understand their features and geological implication. The results show that different microstructures of localized brittle deformation can be observed in the fault gouges, including Y-shear, R1-shear, R2-shear, P-shear as well as tension fracture, bookshelf glided structure and so on. These microstructures are commonly recognized as the product of seismic fault slipping. Furthermore, within the area between two parallel Y-shears of the fault gouge, a few of microstructures of distributed ductile deformations were developed, such as P-foliation, elongation and asymmetrical trailing structure of detrital particles. The microstructure features of fault gouges implicate the thrust movement of the fault during the Wenchuan earthquake. In addition, the fault gouge has less quartz and feldspar and more clay than the surrounding rocks, which indicates that some quartz and feldspar in the surrounding rocks were transformed into clay, whereas the fault gouge has more illite and less illite/montmorillonite mixed layers than the surrounding rocks, which shows that the illite/montmorillonite mixed layer was partly converted into illite due to temperature increasing induced by coseismic fault slipping friction (also being affected partly by the chemical action of solutions). Such microstructures features and mineral component changes recorded the information of fault slip and provide criterions for discussing the genesis of fault gouge and recognition of the direction of fault movement.

Timing and Spouting Height of Sand Boils Caused by Liquefaction during the 2010 Mw 6.9 Yushu Earthquake, Tibetan Plateau, China

The 2010 Mw 6.9 Yushu earthquake produced a ~33-km-long co-seismic surface rupture zone along the pre-existing active Yushu Fault on China’s central Tibetan Plateau. Sand boils occurred along the tension cracks of the co-seismic surface rupture zone, and locally spouted up above the ground to coat the top of limestone blocks that had slid down from an adjacent ~300-m-high mountain slope. Based on our observations, the relations between the arrival times of P- and S-waves at the sand-boil location and the seismic rupture velocity, we conclude that 1) the sand boils occurred at least 18.24 s after the main shock; 2) it took at least 4.09 - 9.79 s after the formation of co-seismic surface rupture to generate liquefaction at the sand-boil location; 3) the spouting height of sand boils was at least 65 cm. Our findings help to clarify the relationships between the timing of lique-faction and the spouting height of sand boils during a large-magnitude earthquake.

Features and genesis of micro-nanometer-sized grains on shear slip surface of the 2008 Wenchuan earthquake

In coseismic surface rupture zones caused by the 2008 Mw 7.9 Wenchuan earthquake, some thin-layered fault gouges with strong deformation were observed in different locations. In this paper, fault gouge samples were taken as research objects from the Bajiaomiao village in the south-west segment of the principal rupture and the Heshangping village and the Shaba village in the north-east segment of the principal rupture where larger displacements were measured. Fabric characteristics of the fault gouge samples and the morphologies and structures of micro-nanometer grains on Y-shear surfaces were then analyzed by using a stereoscope and SEM. Observation results showed that obvious Y- and R-shears and obvious scratches were well developed in coseismic gouges caused by the 2008 Wenchuan earthquake. Micro-nanometer grains in the fault gouge of the Wenhcuan earthquake were formed mainly due to breaking, grinding, and powdering of fault slipping friction surface. Heat caused by fault slipping (maybe also including heat caused by thermal decomposition) played an important role in producing micro-nanometer sized grains. Existence occurrence state of micro-nanometer sized grains on fault slip surface includes singled grains and their complexes with shapes of ball, silkworm, pancake and mass. The structures mainly include dispersed and close-packed structures besides a few of striped and layered structures. All these structures were formed at the extreme unbalance conditions caused by rapid deforming during an earthquake. There always exist some voids between structures due to loosely contact. Only alienated grains are included in the stripped structure. But there are some singled grains with no deformation in dispersed and close-packed structures besides complexes of grains with morphologies of ball, silkworm, pancake and mass. The striped and close-packed structures are the results of plastic deformation, and the dispersed and layered structures are the results of brittle deformation whereas loose contact of different structures was caused mainly by discontinuous dynamic friction (fault stick-slipping). The structures of the micro-nanometer sized grains in coseismic fault gouge caused by the Wenchuan earthquake are the geological records of seismic fault slipping (it is not pseudotachylite), which could be used as an index of paleo-seismic events.

Soil gas distribution in the main coseismic surface rupture zone of the 1980,Ms = 6.9, Irpinia earthquake (southern Italy)

AbstractSoil gas measurements of different gas species with different geochemical behaviors were performed in the area of the Pecore Plain, a 200 m × 300 m sized, fault‐bounded extensional basin located in the northern Mount Marzano massif, in the axial belt of the southern Apennine chain. The Pecore Plain area was affected by coseismic surface faulting during theMs = 6.9, 1980 Irpinia earthquake, the strongest and most destructive seismic event of the last 30 years in southern Italy. The collected data and their geostatistical analysis provide new insights into the control exerted by active fault segments on deep‐seated gas migration toward the surface. The results define anomalies that are aligned with the NW‐SE trending coseismic rupture of the 1980 earthquake along the western border of the plain, as well as along the southern border of the plain where a hidden, E‐W striking fault is inferred. Geospatial analysis highlights an anisotropic spatial behavior of222Rn along the main NW‐SE trend and of CO2along the E‐W trend. This feature suggests a correlation between the shape and orientation of the anomalies and the barrier/conduit behavior of fault zones in the area. Furthermore, our results show that gas migration through brittle deformation zones occurs by advective processes, as suggested by the relatively high migration rate needed to obtain anomalies of short‐lived222Rn in the soil pores.

Temporal slip change based on curved slickenlines on fault scarps along Itozawa fault caused by 2011 Iwaki earthquake, northeast Japan

We observed co-seismic slickenlines caused by the 2011 Mw 6.6 Iwaki earthquake along the Itozawa fault co-seismic surface rupture zone. The structures are characterized by co-seismic curved slickenlines and slickenlines with cross-cutting relationships on the fault scarps. The curved slickenlines indicate that the direction of fault motion during the rupture of the 2011 Iwaki earthquake shifted from normal faulting with a left-lateral component to that with a right-lateral component. The angular misfits between the slip direction predicted from the NW–SE trending extensional stress before the 2011 Iwaki earthquake and that predicted from each component of the curved slickenlines on the fault scarps are ~ 33° to 65° and ~ 2° to 17°, respectively. The misfit changes show that the co-seismic slip direction, discordant with the extensional stress, shifted to normal faulting and is explained by the stress during the faulting. These results suggest that co-seismic rupture processes near the surface are a key in revealing the detailed dynamic processes of a seismic event. • 2011 Mw6.6 Iwaki earthquake was a triggered event after 2011 Mw9.0 Tohoku earthquake. • We observed curved slickenlines along the Itozawa fault co-seismic surface ruptures. • Slickenlines indicate a change in the direction of fault motion during rupture. • Fault motion shifted from that with a left- to a right-lateral component. • Fault motion shift is explained by the local stress perturbation during faulting.

Rupture process of the Wenchuan earthquake (Mw 7.9) from surface ruptures and fault striations characteristics

On 12 May 2008, the Wenchuan earthquake (Mw 7.9) produced complicated thrust-type co-seismic surface rupture zones, which encompass the dextral-slip thrust of the Yingxiu–Beichuan fault, the approximately pure thrust of the Guanxian–Anxian fault, and the sinistral-slip thrust of the Xiaoyudong rupture zone located between the former two. In order to understand the faulting mechanism, we discuss the rupture process by examining the segmentation and kinematics of the surface rupture zones, together with the co-seismic fault striations at various sites. Based on the two along-strike main displacement peaks (6–6.5 m and 11–12 m) and on the different geometric and kinematic patterns for the southern and northern segments of the surface rupture zones, we find that the Wenchuan earthquake might have consisted of two rupture stages, which is in agreement with seismic wave inversion results. By comparing the kinematics of fault striations occurring in the Bajiaomiao and Beichuan areas, it suggests that during the first stage, thrusting along both the Yingxiu–Beichuan fault and Guanxian–Anxian fault produced the ~ 80–100 km-long Yingxiu–Qingping surface rupture segment and the ~ 80 km-long Guanxian–Anxian surface rupture zone, respectively. Then, faulting was triggered along the Yingxiu–Beichuan fault by the first rupture process, yielding the second rupture stage, which was characterized by dextral strike-slip (or dextral oblique thrusting). Due to the overlap between the two rupture stages, the southern segment (Yingxiu–Qingping) of the Yingxiu–Beichuan rupture zone comprises two different processes while the northern segment (Gaochuan–Beichuan–Shikan) only suggests one rupture phase.

Coseismic Deformation of the 2010 Mw 6.9 Yushu Earthquake Derived from GPS Data

On 13 April 2010 UTC 23:49:17 (Local time 7:49:17 on 14 April 2010), a large earthquake ( M w 6.9) struck Yushu county, Qinghai, China, killing 2,700 people, and causing widespread damage in the high mountainous regions of the central Tibetan Plateau. This event occurred on the pre‐existing left‐lateral strike‐slip Ganzi–Yushu fault, which is the western part of the Yushu‐Ganzi‐Xianshuihe fault zone. The Ganzi‐Yushu fault extends for ∼200 km along a northwest–southeast strike (Wang et al. , 2008). The event of 13 April 2010 is the largest one in this region since the last destructive earthquake about 272 years ago (Zhou et al. , 1996; 1997). Previous studies from geodetic surveys inferred an average slip of 14.4 mm/yr along the Ganzi‐Yushu‐Xianshuihe fault (Gan et al. , 2007). The principal horizontal strain rates over the Yushu area are characterized by a combination of north‐northeast–south‐southwest compression and west‐northwest–east‐southeast extension (Calais et al. , 2006), which are significant at the 95% confidence level. Field investigation revealed an ∼51‐km left‐lateral strike‐slip surface rupture (Chen et al. , 2010; J. Zhang et al. , 2010) with a coseismic displacement of 0.3–3.2 m (Lin et al. , 2011). Satellite image analyses show various lengths of surface breakage ranging between 39 and 80 km (Li et al. , 2011; Tobita et al. , 2011; Zha et al. , 2011). However, estimates from initial seismological inversion show the rupture belt is composed of at least 3 segments, with a total length of 50–70 km. Field observation demonstrates that coseismic surface rupture zone produced by the main shock is 31–33 km in length. Teleseismic waveform analysis suggested that the earthquake could be resolved into two subevents of up to 2.0‐m slip on a 119°‐striking fault dipping southwest (Y. Zhang …

Field evidence of rupture of the Qingchuan Fault during the 2008 Mw7.9 Wenchuan earthquake, northeastern segment of the Longmen Shan Thrust Belt, China

Although many studies of the 2008 Mw 7.9 Wenchuan earthquake have described the ground deformation features, rupture mechanism, and structural features of the seismogenic fault zone associated with this event, debate remains concerning the total length of the co-seismic surface rupture zone and whether the earthquake ruptured the Qingchuan Fault in the northeastern segment of the Longmen Shan Thrust Belt (LSTB), China. In this paper, we present new field evidence that the Qingchuan Fault was ruptured by the 2008 Wenchuan earthquake and that the total length of the co-seismic surface rupture zone is up to 285–300km. Field investigations reveal that the earthquake produced a ≥60-km-long surface rupture zone along the pre-existing Qingchuan Fault, with the offset being mainly right-lateral strike–slip and a distinct component of vertical slip. Co-seismic surface ruptures are characterized by faults and extensional cracks. Field measurements indicate co-seismic right-lateral strike–slip displacements along the Qingchuan Fault of 0.3–0.6m and vertical offsets of 0.2–0.5m, which differs to the displacements observed along the central and southwestern segments of the Wenchuan surface rupture zone in the displacement amount and sense. The change in slip sense from thrust-dominated slip in the central and southwestern segments of the LSTB to right-lateral strike–slip-dominated displacement along the Qingchuan Fault (northeastern segment of the LSTB) reflects a change in the orientation of compressive stress along the LSTB, associated with eastward extrusion of the Tibetan Plateau as it accommodates the ongoing penetration of the Indian Plate into the Eurasian Plate.

Riedel shear structures in the co-seismic surface rupture zone produced by the 2001 M w 7.8 Kunlun earthquake, northern Tibetan Plateau

We present a case study of Riedel shear structures related to the co-seismic surface ruptures produced during the 2001 M w 7.8 Kunlun earthquake in northern Tibet, which are related to strike-slip movement along the Kunlun Fault. Field investigations and interpretations of high-resolution remote sensing images show that the 2001 co-seismic surface ruptures, striking WNW–ESE, are mainly characterized by Riedel shear structures, including T fractures, and R, Y, and P shears. A left-lateral shear sense is indicated. To assess the Riedel shear fabrics quantitatively, we measured the azimuth of 19,455 co-seismic surface rupture strands, the width of rupture zones at 474 profiles, and 336 fold axes of mole track structures, using 1-m-resolution IKONOS and 0.61-m-resolution QuickBird images acquired soon after the earthquake. The analytical results show that i) the co-seismic surface ruptures are generally concentrated in up to five subparallel sub-rupture zones, with individual sub-rupture zones varying in width from 3 to 350 m (generally <100 m); ii) the total width across all sub-rupture zones is generally <500 m (though locally >1–2 km); iii) T fractures are mainly developed within the alluvial deposits at counterclockwise angles of 15–40° relative to the general trend of the rupture zone; iv) P shears are also mainly found in the alluvial deposits, but at counterclockwise to clockwise angles of 5–10° relative to the general trend; and v) Y and R shears are developed within both the alluvial deposits and basement rocks, mainly representing the reactivation of pre-existing fault traces. The results demonstrate that the co-seismic Riedel shear structures are primarily controlled by the local geology during surface rupture formation, consistent with the idea that Riedel shear structures are common fault patterns within strike-slip shear zones and that their development is related to the early stages of fault evolution.

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.