Haiyuan Fault Research Articles

Late Pleistocene left-lateral slip rates of the Gulang Fault and its tectonic implications in eastern Qilian Shan (NE Tibetan Plateau), China

The late Pleistocene deformation pattern in the eastern Qilian Shan remains controversial. Study of the geometry and slip rate of the fault holds crucial value for better understanding the regional deformation pattern. The active E-W trending Gulang and Tianjing Shan Fault Zone is located in the southern boundary of the Tengger Desert which plays a significant role in tectonic deformation and slip transfer in the NE Tibetan Plateau. Geomorphic mapping of the active fault traces reveal that the Gulang fault is composed of two segments, separated by the Guanjia Tai pull-apart basin. Offset geomorphic features indicate that the Gulang fault appears to be primarily sinistral strike-slip with no reverse component. Based on offset gullies/terraces and optically stimulated luminescence dating of offset-related sediments, we obtain a consistent late Pleistocene slip rate of ~2.5 mm/a along the western and eastern segments of the Gulang Fault. Meanwhile, the slip vectors indicate the Gulang fault redistributes strain and transforms displacement along the Haiyuan fault, accommodating the westward motion of the Gobi-Alashan block. Moreover, a devastating earthquake of Ms 8.0 struck Gulang on 23rd May 1927. Geomorphically fresh fault scarps and surface rupture zone are observed at Gulang Fault, we modify the Modified Mercalli Intensities (MMI) and revise the isoseismal map for the Gulang earthquake. Combining with the high-resolution deep seismic-reflection data, the Gulang fault may be the source of the earthquake. Regionally, GPS velocities indicate that faulting, crustal shortening, and basin/block rotation accommodate deformation of the plateau's northern margin. In conclusion, the fault geometry, kinematics, and regional GPS velocities reflect that this region experiences eastward movement of the northeastern Tibetan Plateau relative to the Gobi-Alashan.

A Fine Velocity and Strain Rate Field of Present-Day Crustal Motion of the Northeastern Tibetan Plateau Inverted Jointly by InSAR and GPS

Interferometric synthetic aperture radar (InSAR) data from 6 Envisat ASAR descending tracks; spanning the 2003–2010 period; was used to measure interseismic strain accumulation across the Northeastern Tibetan Plateau. Mean line-of-sight (LOS) ratemaps are computed by stacking atmospheric-corrected and orbital-corrected interferograms. The ratemaps from one track with different atmospheric-corrected results or two parallel; partially overlapping tracks; show a consistent pattern of left-lateral motion across the fault; which demonstrates the MERIS and ECMWF atmospheric correction works satisfactorily for small stain measurement of this region; even with a limited number of interferograms. By combining the measurements of InSAR and GPS; a fine crustal deformation velocity and strain rate field was estimated on discrete points with irregular density depending on the fault location; which revealed that the present-day slip rate on the Haiyuan fault system varies little from west to east. A change (2–3 mm/year) in line-of-sight (LOS) deformation rate across the fault is observed from the Jinqianghe segment to its eastern end. Inversion from the cross-fault InSAR profiles gave a shallow locking depth of 3–6 km on the main rupture of the 1920 earthquake. We therefore infer that the middle-lower part of the seismogenic layer on the 1920 rupture is not yet fully locked since the 1920 large earthquake. Benefit from high spatial resolution InSAR data; a low strain accumulation zone with high strain rates on its two ends was detected; which corresponds to the creeping segment; i.e., the Laohushan fault segment. Contrary to the previous knowledge of squeezing structure; an abnormal tension zone is disclosed from the direction map of principal stress; which is consistent with the recent geological study. The distribution of principal stress also showed that the expanding frontier of the northeastern plateau has crossed the Liupan Shan fault zone; even arrived at the northeast area of the Xiaoguan Shan. This result agrees with the deep seismic reflection profile.

Mid-Miocene uplift of the northern Qilian Shan as a result of the northward growth of the northern Tibetan Plateau

The northern Tibetan Plateau, north of the Qaidam Basin and south of the Hexi Corridor (China), consists of a series of WNW- to NW-trending elongated mountain ranges. Deciphering the time-space deformation pattern of these ranges is central to understanding the mechanism of plateau formation and to the controversial issue of whether Tibet has undergone progressive northward growth or synchronous growth since the India-Eurasia collision. Here, we report new constraints on the timing of accelerated uplift of the Tuolai Shan, one of the elongated mountain ranges in the northern Tibetan Plateau. New apatite fission-track data from an elevation transect in the Tuolai Shan provide a definitive tie to rapid cooling that began at 17–15 Ma. We attribute this rapid cooling to accelerated exhumation resulting from thrusting in the hanging wall of the Haiyuan fault in response to progressive northward growth of the plateau. Combining these fission-track data and the published geologic, sedimentological, and thermochronologic data from the northern Qilian Shan and Hexi Corridor, we propose a progressively north-northeastward growth model for the northernmost part of Tibet, suggesting that deformation in the inner Qilian Shan occurred synchronously in the middle Miocene, and subsequently, increasingly further north.

Asymmetrical river valleys and their tectonic significance in the Maxianshan area, NE Tibetan Plateau

The geometry of river networks is particularly sensitive to active tectonic deformation. Morphotectonic analyses conducted using geomorphic indices and numerical models are useful methods with which to identify such deformation. The northeastern (NE) margin of the Tibetan Plateau is characterized by active tectonics, easily erodible bedrock and well-developed valleys, and is thus an ideal region for studying the effects of tectonics on the landscape evolution. Here we firstly identify the systemic asymmetrical valleys in the Maxianshan area, NE Tibetan Plateau. To quantitatively analyze the asymmetrical characteristics of these valleys identified in the southern Maxianshan, 33 parallel streams were investigated by using two drainage basin analytical techniques, namely, the transverse topographic symmetry factor (T) and asymmetry factor (AF). Our results show that the mean T value is 0.14 (magnitude) and 319.08° (direction) and the majority of the AF results are <50, indicating a north-westward migration. In addition, we suggest that this north-westward migration could be caused by the variable rates of uplift experienced across the Maxianshan area, an assertion which is also supported by the inversion results of GPS measurements and earthquake distributions. In contrast to previous studies, we conclude that the variable rates of uplift are probably caused by the distribution of stress within the four quadrants of a strike-slip fault. This mechanism is validated by using the landscape evolution model SIGNUM and studying examples from other strike-slip fault zones, such as the Haiyuan Fault Zone on the NE Tibetan Plateau, Qujiang Fault Zone and Daqing Fault Zone on the southeastern (SE) Tibetan Plateau and Tieluzi Fault Zone in central China. By using quantitative methods and modeling, our study appears to demonstrate that asymmetrical valleys are important geomorphic characteristics of strike-slip faults.

Crustal Deformation on the Northeastern Margin of the Tibetan Plateau from Continuous GPS Observations

We installed 10 continuous Global Positioning System (GPS) stations on the northeast margin of the Tibetan Plateau at the end of 2012, in order to qualitatively investigate strain accumulation across the Liupanshan Fault (LPSF). We integrated our newly built stations with 48 other existing GPS stations to provide new insights into three-dimensional tectonic deformation. We employed white plus flicker noise model as a statistical model to obtain realistic velocities and corresponding uncertainties in the ITRF2014 and Ordos-fixed reference frame. The total velocity decrease from northwest to southeast in the Longxi Block (LXB) was 5.3 mm/yr within the range of 200 km west of the LPSF on the horizontal component. The first-order characteristic of the vertical crustal deformation was uplift for the northeastern margin of the Tibetan Plateau. The uplift rates in the LXB and the Ordos Block (ORB) were 1.0 and 2.0 mm/yr, respectively. We adopted an improved spherical wavelet algorithm to invert for multiscale strain rates and rotation rates. Multiscale strain rates showed a complex crustal deformation pattern. A significant clockwise rotation of about 30 nradians/yr (10−9 radians/year) was identified around the Dingxi. Localized strain accumulation was determined around the intersectional region between the Haiyuan Fault (HYF) and the LPSF. The deformation pattern across the LFPS was similar to that of the Longmengshan Fault (LMSF) before the 2008 Wenchuan MS 8.0 earthquake. Furthermore, according to the distributed second invariant of strain rates at different spatial scale, strain partitioning has already spatially localized along the Xiaokou–Liupanshan–Longxian–Baoji fault belt (XLLBF). The tectonic deformation and localized strain buildup together with seismicity imply a high probability for a potential earthquake in this zone.

Cenozoic cooling history of the North Qilian Shan, northern Tibetan Plateau, and the initiation of the Haiyuan fault: Constraints from apatite- and zircon-fission track thermochronology

The growth of the Tibetan Plateau resulted primarily from India-Asia convergence since initial collision at 65–55 Ma. The Cenozoic Qilian Shan thrust belt and the left-slip strike Haiyuan fault system together define the northeastern margin of the plateau. Although these thrust and strike-slip fault systems play an important role in accommodating continental convergence, our knowledge of the temporal and spatial distribution of upward and outward growth of the northeastern Tibetan Plateau is still lacking. In this study, we integrate new geologic mapping and low-temperature themochronometry (apatite- and zircon-fission-track ages) to provide constraints on the uplift and cooling history of the North Qilian Shan and the initiation of the Haiyuan fault. Cooling ages and thermal history modeling from a traverse across a prominent restraining bend on the Haiyuan fault suggest the North Qilian Shan experienced a three-phase cooling history, including: (1) broad cooling during the Late Triassic to early Cenozoic, (2) long-term tectonic quiescence from late Cretaceous to middle Miocene, and (3) rapid cooling and exhumation from ~15–10 Ma to present. We relate this most recent local cooling (~15–10 Ma) to the initiation of strike-slip faulting along the central Haiyuan fault, which corroborates other recent studies, suggesting a middle Miocene activation of strike-slip deformation along the Haiyuan fault in northern Tibet.

Insight into the 2016 Menyuan Mw 5.9 Earthquake with InSAR: A Blind Reverse Event Promoted by Historical Earthquakes

In 2016, the Mw 5.9 Menyuan earthquake occurred at the western end of the Tianzhu seismic gap in the middle segment of the left-lateral strike-slip Haiyuan fault system. Studying the focal mechanism of the Menyuan earthquake and its correlation with historical seismicity is of great significance in understanding the tectonic style of the Haiyuan fault system and assessing the Tianzhu gap seismic risk. Using Sentinel-1A radar imagery, we obtain the coseismic deformation field of the Menyuan earthquake in the LOS direction and invert the deformation field slip distribution model. Based on the interseismic deformation field reflected by GPS data and the relocated results of aftershocks provided by previous studies, we then analyze the seismogenic nodal plane of the Menyuan earthquake and consider the west-dipping model to be the most likely seismogenic solution. The west-dipping model indicates that the average slip of the Menyuan earthquake is 0.41 m, and the average rake angle is 68°, with a magnitude of Mw 5.905. Considering the historical strong earthquakes and viscoelastic layered crust, the results from Coulomb failure stress modeling indicate that the Coulomb stress loading caused by the historical strong events promoted the occurrence of the 2016 Menyuan earthquake. Both the low seismicity within the Tianzhu gap and the cluster phenomenon of medium earthquakes on the Lao Hu Shan (LHS) and Leng Long Ling (LLL) faults have strong correlations with the stress change distribution resulting from the historical strong earthquakes. A series of medium events occurring on the north side of the LLL fault, represented by the 2016 Menyuan earthquake, would raise the seismic risk on the western end of the Tianzhu gap.

Kinematics of Late Quaternary Slip Along the Qishan‐Mazhao Fault: Implications for Tectonic Deformation on the Southwestern Ordos, China

AbstractThe northwest striking Qishan‐Mazhao fault (QMF) accommodates complex deformation in the Tibet‐Ordos transition zone. We studied the geologic and geomorphic expression of the QMF using interpretations of high‐resolution satellite images and structure‐from‐motion models combined with detailed field investigations. Displaced loess tablelands, stream channels, and fluvial terraces show that the QMF is predominately a left‐lateral strike‐slip fault with a minor normal component. The magnetic susceptibility and optically stimulated luminescence ages of offset fluvial terraces yield left‐lateral slip rates ranging from 0.5 to 1.0 mm/year. Regionally, the QMF and the Haiyuan fault (HYF) form a large right step, in which the Liupanshan Mountains are located. The QMF shares a similar orientation and sense of motion to the HYF, suggesting that the left‐lateral slip of the HYF is not completely absorbed as crustal shortening across the Liupanshan Mountains but is partially transferred to slip along the QMF.

Magnetostratigraphy of the Ganyanchi (Salt Lake) Basin along the Haiyuan fault, northeastern Tibet

Magnetostratigraphy of the Ganyanchi (Salt Lake) Basin along the Haiyuan fault, northeastern Tibet

Distribution and characteristics of loess landslides triggered by the 1920 Haiyuan Earthquake, Northwest of China

On December 16, 1920, an earthquake with a magnitude of 8.5 occurred in Haiyuan County, Ningxia Hui Autonomous Region, Northwest of China. This earthquake triggered several thousand loess landslides which resulted in thousands of deaths and blockages of rivers. The distribution and characteristics of the landslides triggered by the Haiyuan Earthquake in loess areas were studied using satellite images and field investigation. A total of about 3700 landslides with a cumulative area of about 177 km2 were interpreted over an area of 21,000 km2. It was found that landslides triggered by the earthquake were concentrated in a western of 40–55 km from the Haiyuan fault. Landslides were concentrated near ridge crests, with 65.7% of the landslides originating in the upper quadrant of slopes. The aspects of the landslides triggered by the Haiyuan Earthquake were parallel to the faults and there is no back-direction effect in landslides triggered by Haiyuan earthquake. These landslides have long run-out distances(travel distance (L)/height different (H) > 0.6) and deposited materials in river channels, forming 51 dammed lakes that still exist. The relationship between the area (A) and volume (V) of the landslides is V = 4.170 × A1.086. Based on the relationship between number and volume of landslide and the magnitude of earthquake, >100,000 landslides with a cumulative volume of 0.5 × 1010 m3 were triggered by the Haiyuan Earthquake. As loess is sensitive to liquefaction during an earthquake and tends to produce landslides with long travel distances. Hence, the loess depth, slope and geological stresses are the primary factors responsible for the high density of landslides in this region.

Geomorphic offsets along the creeping Laohu Shan section of the Haiyuan fault, northern Tibetan Plateau

Geomorphic offsets along the creeping Laohu Shan section of the Haiyuan fault, northern Tibetan Plateau

Present-day crustal deformation and strain transfer in northeastern Tibetan Plateau

The three-dimensional present-day crustal deformation and strain partitioning in northeastern Tibetan Plateau are analyzed using available GPS and precise leveling data. We used the multi-scale wavelet method to analyze strain rates, and the elastic block model to estimate slip rates on the major faults and internal strain within each block. Our results show that shear strain is strongly localized along major strike-slip faults, as expected in the tectonic extrusion model. However, extrusion ends and transfers to crustal contraction near the eastern margin of the Tibetan Plateau. The strain transfer is abrupt along the Haiyuan Fault and diffusive along the East Kunlun Fault. Crustal contraction is spatially correlated with active uplifting. The present-day strain is concentrated along major fault zones; however, within many terranes bounded by these faults, intra-block strain is detectable. Terranes having high intra-block strain rates also show strong seismicity. On average the Ordos and Sichuan blocks show no intra-block strain, but localized strain on the southwestern corner of the Ordos block indicates tectonic encroachment.

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.

Elastic block and strain modeling of GPS data around the Haiyuan-Liupanshan fault, northeastern Tibetan Plateau

Based on the dense GPS velocity field in the northeastern margin of the Tibetan Plateau from 1999 to 2016, we have produced the deformation and strain characteristics of the Haiyuan fault and the Liupanshan fault. Estimated long-term slip rate along the Haiyuan-Liupanshan fault zones show a gradual decrease from 6.4 ± 1.6 mm/yr at the Tuolaishan fault to 2.9 ± 1.2 mm/yr at the Southern Liupanshan fault. Left-lateral thrusting movement was inverted for the Xiangshan-Tianjingshan fault (XS-TJS), which has an average slip rate of 2.1 ± 3.4 mm/yr during the study period. We also calculated the heterogeneous distribution of interseismic coupling along the fault zones. Our result also shows the locking depth of the Tianzhu seismic gap is ∼22 km. The slip rate deficit, the seismic moment accumulation rate, and the Coulomb stress accumulation rate are high on the fault planes, whereas the second invariant of the strain rate is low at the surface. The Liupanshan fault is locked to a depth of ∼23 km, and the corresponding seismic moment accumulation rate on the fault plane is high, while the strain rate at the surface is low. The accumulated strain along the Tianzhu seismic gap and the Liupanshan fault could be balanced by earthquakes with magnitudes of Mw7.9 and Mw7.4, considering the absence of large earthquakes over the last 1000 years and 1400 years respectively. The Haiyuan segments had ruptured during 1920 Haiyuan earthquake, and the estimated locking depth for period 1999–2016 is 5–10 km. Its seismic moment accumulation rate at depth is low and the strain rate at the surface is high. Our result indicates that 70% of the strike-slip along the Haiyuan segments transforms into thrusting along the Liupanshan fault, while the remaining 30% is related to the orogeny of the Liupanshan. For slip between the Haiyuan fault and the XS-TJS, about 27–34% of the slip is partitioned on the XS-TJS.

Growth of the northeastern margin of the Tibetan Plateau by squeezing up of the crust at the boundaries

In classic orogenic models, the mountain range is underlain by a deep crustal root. Here we present the crustal and upper mantle structures along two receiver function profiles across Qilian, an orogen experiencing recent growth at the northern margin of the Tibetan plateau. Opposite to an expected crustal root beneath the orogen, the Moho beneath Qilian is arch-like, shallower beneath the center and deepens by up to 10 km beneath its southern and northern boundaries. Additional velocity interfaces sub-parallel to the Moho are observed in the lower crust of the basins south of Qilian, which we interpret as the top of a mechanically strong lower crust thrusting several tens of kilometers underneath Qilian. In the north, the small lateral offset between the surface and mantle traces of the thrust system reveals a steep boundary, indicating that the North China cratonic crust acts as a strong resistance to the northward growth of the plateau, forcing the development of the left-lateral strike-slip Haiyuan fault south of the northern Qilian suture. The young Qilian orogen thus has been rising and growing progressively from the boundaries to the center, squeezed up by more rigid tectonic blocks in the north and south.

Partitioning of oblique convergence coupled to the fault locking behavior of fold‐and‐thrust belts: Evidence from the Qilian Shan, northeastern Tibetan Plateau

AbstractOblique plate convergence is common, but it is not clear how the obliquity is achieved by continental fold‐and‐thrust belts. We address this problem in the Qilian Shan, northeastern Tibetan Plateau, using fieldwork observations, geomorphic analysis, and elastic dislocation modeling of published geodetic data. A thrust dips SSW from the northern range front and underlies steeper thrusts in the interior. Cenozoic thrust‐related shortening across the Qilian Shan is ~155–175 km, based on two transects. Elastic dislocation modeling indicates that horizontal strain in the interseismic period is consistent with oblique slip on a single low‐angle detachment thrust below ~26 km depth, dipping SSW at ~17°. We suggest that this detachment is located above North China Block crust, originally underthrust during Paleozoic orogeny. Horizontal shear strain is localized directly above the updip limit of creep on the detachment and is coincident with the left‐lateral Haiyuan Fault. This configuration implies that oblique slip on the detachment below seismogenic depths is partitioned in the shallow crust onto separate strike‐slip and thrust faults. This is consistent with strain partitioning in oceanic subduction zones but has not previously been found by dislocation models of continental interiors. The marginal, strike‐slip, Altyn Tagh Fault influences thrusting within the Qilian Shan for 100–200 km from the fault but does not control the regional structure, where Paleozoic basement faults have been reactivated. The Qilian Shan resembles the main Tibetan Plateau in nascent form: active thrusts are marginal to an interior that is developing plateau characteristics, involving low relief, and low seismicity.

Paleoseismology and slip rate of the western Tianjingshan fault of NE Tibet, China

We present results from a detailed investigation of the horizontal displacement distribution, timing of paleoearthquakes and left-lateral slip rate on the western Tianjingshan fault. Measurements of 240 offset streams and ridges confirm that the fault is left-lateral and record evidence of repeated ∼3–4m coseismic offsets along the 60–km–long fault. This suggests that ∼6 earthquakes may have occurred along the entire western Tianjingshan fault with repeated occurrence of earthquakes of Mw 7.2–7.5. Structural and stratigraphic relationships exposed in our five trenches in combination with previously reported studies further indicate that the fault has ruptured in as many as six paleoearthquakes since the late Quaternary. Paleoseismic data show that the average recurrence interval for Holocene earthquakes is approximately 5,000yr. The most recent earthquake along the western Tianjingshan fault occurred ∼1.2±0.1kyrBP, indicating that this fault segment did not rupture in the M 7.5 historical earthquake of 1709 that ruptured the central Tianjingshan fault. We estimate that the Holocene slip rate of the western Tianjingshan fault is ∼1.1–1.2mm/yr based on measurements of the age and offset of stream channels. Compared with the relatively fast slip rate of the Haiyuan fault (∼4–6mm/yr), we suggest that the Tianjingshan fault acts as an essential active fault accommodating the sinistral displacement and crustal shortening deformation in NE Tibet.

Sentinel-1 observations of the 2016 Menyuan earthquake: A buried reverse event linked to the left-lateral Haiyuan fault

Knowledge on the interaction of active structures is essential to understand mechanics of continental deformation and estimate the earthquake potential in complex tectonic settings. Here we use Sentinel-1A radar imagery to investigate coseismic deformation associated with the 2016 Menyuan (Qinghai) earthquake, which occurred in the vicinity of the left-lateral Haiyuan fault. The ascending and descending interferograms indicate thrust-dominated slip, with the maximum line-of-sight displacements of 58 and 68mm, respectively. The InSAR observations fit well with the uniform-slip dislocation models except for a larger slip-to-width ratio than that predicted by the empirical scaling law. We suggest that geometric complexities near the Leng Long Ling restraining bend confine rupture propagation, resulting in high slip occurred within a small area and much higher stress drop than global estimates. Although InSAR observations cannot distinguish the primary plane, we prefer the west-dipping solution considering aftershocks distribution and the general tectonic context. Both InSAR modelling and aftershock locations indicate that the rupture plane linked to the Haiyuan fault at 10km depth, a typical seismogenic depth in Tibet. We suggest that the earthquake more likely occurred on a secondary branch at a restraining bend of the Haiyuan fault, even though we cannot completely rule out the possibility of it being on a splay of the North Qilian Shan thrusts.

Tectonic geomorphology of the Qilian Shan in the northeastern Tibetan Plateau: Insights into the plateau formation processes

We examined the regional scale topography of the Qilian Shan on the northeastern margin of the Tibetan plateau. Longitudinal profiles and geomorphometric indices, such as slope, local relief and channel steepness reveal that the Qilian Shan was developed as a local plateau with high-relief steep marginal ranges, and interior low-relief topography. Landscape mapping across this local plateau revealed spatially varied origins of the low-relief landscape in the Qilian Shan. North of the Haiyuan fault, universal massive intermontane aggradation was identified. However, the low-relief plateau to the south of the Haiyuan fault was dominated by relict erosional surfaces, even though accompany basin-filling still contributes. The geomorphologic contrasts led us to integrate both the basin-filling processes and antecedent low-relief erosional surfaces into a new insight into the development of the Qilian Shan plateau in the northeastern Tibet. Our present study highlights that explanation for the low-relief plateau formation needs to consider the complex geological evolution of the Qilian Shan, and the Tibetan Plateau itself.

Block-like versus distributed crustal deformation around the northeastern Tibetan plateau

It has been debated for decades whether crustal deformation in and around the Tibetan plateau is distributed or block-like. We model crustal deformation in northeastern Tibet using a deformable-block-motion model, in which kinematic parameters of block motion and internal deformation and the associated boundary slip rates are inverted for using GPS velocity data. The F-test is used to screen out station velocity outliers, justify independence of neighboring blocks, and determine the significance of block internal strains through an iteration process. As a result, fifteen blocks are identified, with their boundary faults slipping at rates of 1–10mm/a. Blocks located east and north of the plateau have large sizes (>104km2 in area) in general, with little internal deformation. Six blocks within the plateau, in contrast, are smaller in sizes, with internal strain rates on the order of 1–10nanostrain/a. Five blocks sitting at the northeast borderland of the plateau have small block sizes but no significant internal deformation. Our results show sinistral slip rates of 4.3±1.6 and 4.6±1.8mm/a across the western and eastern segments of the Haiyuan fault, and 10.8±2.3, 4.6±2.6, and 3.8±2.1mm/a across the western, central, and eastern segments of the East Kunlun fault, respectively. The southwestern, central, and northeastern segments of the Longmenshan fault slip right-laterally at rates of 1.7±1.1, 1.1±0.8, and 1.1±0.8mm/a, with shortening rates of 1.1±1.2, 0.4±0.8, and 0.8±1.1mm/a, respectively. We also develop a scheme to convert geodetic strain rate into seismic moment accumulation rate within blocks and at block boundaries, and estimate the two rates as ∼8.40×1018 and ∼2.06×1019N·m/a, respectively. In comparison, the corresponding seismic moment release rates are estimated as ∼6.06×1018 and ∼2.44×1019N·m/a using an contemporary earthquake catalog of 1920–2015. Such results indicate that the seismic moment accumulation and release rates are comparable for the latest 95years when the earthquake catalog is complete for strong to large events. Both geodetic and seismic estimates suggest that a major portion (∼70–80%) of the total seismic moment is accumulated and released at block boundaries, and a minor but still significant portion (∼20–30%) is accumulated and released within blocks.