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Quantifying slip rates and earthquake occurrence of active faults on the Shan Plateau, southeast of the easternHimalayan syntaxis, is critical to assessing the seismic hazard and understanding the kinematics and geody-namics of this region. Most previous estimates of slip rates are averaged over either many millions of years usingoffset geological markers or decades using GPS. Well-constrained millennial slip rates of these faults remainsparse and constraints on recurrence rates of damaging earthquakes exist only for a few faults. Here we in-vestigate the millennial slip rate and timing of recent earthquakes on the Jinghong fault, one of the geomor-phically most significant sinistral-slip faults on the central Shan Plateau. We map and reconstruct fault offset(18 ± 5 m) of alluvial fan features at Manpa on the central Jinghong fault, using a 0.1 m-resolution digitalsurface model obtained from an unmanned aerial vehicle survey. We establish a slip rate,≤2.5 ± 0.7 mm/yrover the past ~7000 years, using pit-exposed stratigraphy. This millennial slip rate is consistent with ratesaveraged over both decadal and million-year timescales. Excavations at three sites near the town of Gelanghe onthe northeastern Jinghong fault demonstrate 1) that the last seismic ground-rupture occurred between 482 and889 cal yr BP, most likely in the narrower window 824–767 cal yr BP, if the lack of large earthquakes in thehistorical earthquake record is reliable, and 2) that multiple fault ruptures have occurred since ~3618 cal yr BP.Combining thisfinding with a lack of large earthquakes in the ~800-year-long Chinese historic record in thisregion, we suggest an average recurrence interval of seismic ground-ruptures on the order of ~1000 years. Thisrecurrence interval is consistent with the slip rate of the Jinghong fault and the size and earthquake frequency onother sinistral faults on the Shan Plateau. NRF (Natl Research Foundation, S’pore) Published version

The Kailas conglomerates crop out ubiquitously along the southernmost boundary of the Gangdese batholith. They unconformably overlie the Gangdese batholith and are displaced by the Great Counter thrust (GCT) fault, forming a fault contact with the Xigaze forearc basin, the associated subduction complex and the Tethyan Himalayan sequence. These strata furnish a record of uplift and paleoenvironmental change in the Indus-Yarlung suture zone during the Oligocene-Miocene. Our new and previously published low-temperature thermochronometric data from the Gangdese batholith and the Kailas conglomerates indicate a period of rapid exhumation beginning approximately 17–15 Ma centered on the southern margin of the Gangdese batholith, whereas regional uplift commenced significantly earlier during the deposition of the Kailas conglomerates, based on the presence of an abrupt facies transition from deep-water lacustrine deposits to red alluvial fan or fluvial deposits. The period of rapid exhumation probably lagged behind the initiation of faster uplift, while the related changes in the depositional environment were most likely recorded immediately in the basin stratigraphy. Subsequently, the Kailas conglomerates were buried in association with the development of the north-directed Great Counter thrust, while rapid exhumation was facilitated by efficient incision by the paleo-Yarlung river at approximately 17–15 Ma.

Earthquake gates are fault geometric complexities, common in natural fault systems, that conditionally impede earthquake ruptures. This study centers on modeling of multicycle dynamics of the Pingding Shan earthquake gate along the central Altyn Tagh Fault in northwest China. The earthquake gate includes three geometric complexities: a prominent restraining bend, a 4-km wide stepover to the east, and a releasing bend to the west. We use a 2D finite element method to simulate coseismic spontaneous ruptures with interseismic fault stress evolutions computed by an analytic viscoelastic solution. Paleoseismic records and long-term slip-rates are used to constrain the models. We find that fault-geometry-related heterogenous stresses accumulated over earthquake cycles yield complex rupture patterns and help explain earthquake recurrence intervals revealed by paleoseismic records. The three most important contributions to the heterogeneous stresses come from dynamic ruptures passing fault geometric complexities, fault-strike-dependent tectonic loading and relaxation, and stress history from past earthquakes. In the Pingding Shan earthquake gate, the releasing stepover appears to impede ruptures more effectively than the restraining bend. The combined impact of the restraining bend and the releasing stepover makes the Pingding Shan earthquake gate a very effective barrier to 350-km model-spanning ruptures. The best-fit model yields a low recurrence interval of 4.6 kyrs for 350-km-long ruptures, interspersed with more frequent ruptures limited to individual fault segments. A lower static friction tends to reduce the effectiveness of the earthquake gate to impede ruptures. Local fault geometric complexities together with rapid energy release and restrengthening of friction during dynamic ruptures help explain the 0.66 recurrence interval coefficient of variation (COV) recorded at the Copper Mine paleoseismic trench site on the Xorxoli segment. This study provides a method for applying heterogenous initial stresses for single-event dynamic rupture simulations that consider the effect of past earthquakes and fault geometry.

Competing hypotheses have been proposed to explain the seismic hazard (i.e. whether earthquakes with M ≥ 7 occur) of the southern Longmen Shan (LMS). This region did not rupture during the 2008 Mw 7.9 Wenchuan earthquake, but later generated the 2013 Mw 6.6 Lushan earthquake. Currently, the maximum possible earthquake magnitude, its average recurrence interval, and the seismogenic structure of the southern LMS, remain poorly documented. This study aims to re-evaluate seismogenic structures and seismic hazard along the southern LMS. We first describe the sub-surface structural geometry, as well as the total slip and Quaternary activity of the Range Front blind thrust (RFBT), using high-resolution seismic reflection profiles, borehole data, and intensity-derived macroscopic epicenters. This thrust, which generated the 1970 Ms 6.2 Dayi and 2013 Mw 6.6 Lushan earthquakes, extends for > 250 km along the LMS range front. Integrating new evidence of active faulting and folding and previous quantitative horizontal shortening rate results, we estimate that the Quaternary slip rate of the RFBT is nearly 1 mm/yr, with a minimum total slip of 5 km since 8–5 Ma. Furthermore, we calculate the accumulation rate of seismic moment, 8.04 (± 2.09) × 1017 N·m/yr, for the main active thrusts on the southern LMS, to compare with the moment release rate for earthquakes in the region. When we combine this with the Gutenberg–Richter distribution obtained from historical and instrumental catalogs, we estimate that the potential maximum moment magnitude of an earthquake in the southern LMS is 7.7. Finally, we conclude that the entire southern LMS is capable of generating much larger earthquakes (Mw 7.3–7.7) than seen recently, with an average frequency of once every 1000–1400 years. Therefore, our findings confirm that there is potential for large earthquakes in the southern LMS, especially on the RFBT, which extends beneath the densely populated Chengdu Plain.

We analyze time series from continuously recording GPS stations in Nepal spanning the pre- and post-seismic period associated to the M_w7.8 Gorkha earthquake which ruptured the Main Himalayan Thrust (MHT) fault on April 25th, 2015. The records show strong seasonal variations due to surface hydrology. After corrections for these variations, the time series covering the pre- and post-seismic periods do not show any detectable transient pre-seismic displacement. By contrast, a transient post-seismic signal is clear. The observed signal shows southward displacements consistent with afterslip on the MHT. Using additional data from stations deployed after the mainshock, we invert the time series for the spatio-temporal evolution of slip on the MHT. This modelling indicates afterslip dominantly downdip of the mainshock rupture. Two other regions show significant afterslip: a more minor zone updip of the rupture, and a region between the mainshock and the largest aftershock ruptures. Afterslip in the first ~ 7 months after the mainshock released a moment of [12.8 ± 0.5] × 10^(19) Nm which represents 17.8 ± 0.8% of the co-seismic moment. The moment released by aftershocks over that period of time is estimated to 2.98 × 10^(19) Nm. Geodetically observed post-seismic deformation after co-seismic offset correction was thus 76.7 ± 1.0% aseismic. The logarithmic time evolution of afterslip is consistent with rate-strengthening frictional sliding. According to this theory, and assuming a long-term loading velocity modulated on the basis of the coupling map of the region and the long term slip rate of 20.2 ± 1.1 mm/yr, afterslip should release about 34.0 ± 1.4% of the co-seismic moment after full relaxation of post-seismic deformation. Afterslip contributed to loading the shallower portion of the MHT which did not rupture in 2015 and stayed locked afterwards. The risk for further large earthquakes in Nepal remains high both updip of the rupture area of the Gorkha earthquake and West of Kathmandu where the MHT has remained locked and where no earthquake larger than M_w7.5 has occurred since 1505.

The topography evolution of the southeastern Tibetan Plateau provides a constraint for evaluating various geodynamic models of plateau formation. We reconstruct the Paleocene to Eocene paleoelevation of the Cenozoic Gonjo basin in the Qiangtang terrane, using oxygen and carbon stable isotopic results from pedogenic carbonates of the lower Ranmugou Formation. Lithofacies associations indicate that the lower Ranmugou Formation was deposited in alluvial fan and fluvial floodplain environments. U-Pb dating of volcanics within the middle Ranmugou Formation constrains the deposition of the lower Ranmugou Formation as prior to 43.2 Ma. Paleoelevations are calculated using both a thermodynamic model and an empirical relationship. The empirical relationship of elevation-δ 18 O is determined from a series of modern water samples. Calculated paleoelevations indicate that the Gonjo basin attained a minimum average elevation of 2100–2500 m in the early Eocene. Together with recent paleoaltimetry studies in the region, it can be concluded that the crust of southeastern Tibetan Plateau was already thickened by that time. The calculation is based on the section in northern Gonjo basin, where δ 18 O values of paleosol nodules appear to be unaltered, but we cannot rule out the possibility that δ 18 O values of the pedogenic carbonates had been partially reset. Our preliminary results favor tectonic models compatible with pre-Miocene uplift of the SE-central Tibetan Plateau.

Strong ground shaking during the 2008 Mw 7.9 Wenchuan earthquake, which occurred in the foothills of the Longmen Shan in southwestern China, resulted in widespread coseismic deformation features, such as liquefaction and water ejection. We present a systematic survey of the soil liquefaction and water ejection features caused by this major earthquake. The majority of liquefaction sites occurred along major alluvial fan-building rivers, where the water table was a few meters below the surface. While there is no clear correlation between water fountain height and peak ground acceleration, ~ 58% of liquefaction sites are located 20–35 km from the Beichuan fault. Clusters of sites with anomalously high (> 2 m) water ejections are located near the surface projection of the Range Front blind thrust and its splay faults. The density of anomalously high eruptions within the window surrounding the Range Front blind thrust is 3–6 times greater than outside this region. Our results suggest that geologic structures may play a role in augmenting liquefaction intensity and shaking-related seismic hazards in sedimentary basins. We speculate that the mechanism could be the amplification of shaking by fault zone structures. Alternatively, faults may act as pathways with increased vertical permeability, allowing fluids from deeper confined aquifers to migrate to and enhance liquefaction of the upper soil, as proposed previously by Wang (2007). Liquefaction associated with the Wenchuan earthquake thus demonstrates the importance of considering geologic structures other than the primary earthquake-producing fault in seismic hazard evaluation and earthquake resistance design in areas with similar geologic and hydrological settings.

The ~ 220 km-long rupture of the 2008 Mw 7.9 Wenchuan earthquake breached several km-scale geometric discontinuities along strike, including the previously un-mapped NW-trending Xiaoyudong fault, connecting between the two major, NE-trending rupture planes on the Beichuan and Pengguan Faults. In this paper, we present high-resolution mapping of the 8-km-long surface breaks and sinistral oblique thrusting coseismic slip on the Xiaoyudong fault. Scarp height is the largest at the NW end, reaching 3.5 m, and decreases southward in steps to less than 0.2 m, with an average slip gradient of 6 × 10− 3 at a few tens of meters length scale, but up to 50 × 10− 3 locally. Left-lateral offsets co-vary with the vertical component. The largest sinistral slip vector we observed is 2.2 m. Geological and geophysical evidence suggests that the Xiaoyudong fault is likely a ~ 30°SW-dipping lateral ramp that soles into the Pengguan fault, and at its northwestern end intersects with the Beichuan fault, where the latter has a step in the fault plane. Kinematically, the Xiaoyudong fault functions as a tear and conjugate fault and coincides with significant coseismic slip rake rotations on both the Beichuan and Pengguan Faults. Similar correlation of fault bends with sharp changes in faulting style occurs at other steps along the Wenchuan rupture. The Xiaoyudong fault may have played a positive role in linking coseismic slip partitioning between parallel reverse fault planes, facilitating the growth of a longer and more destructive rupture. This highlights the role of tear faults in bridging ruptures between segments, such that reverse-type ruptures can breach steps wider than anticipated from strike–slip fault examples. Transfer faults are common, and perhaps poorly documented features in reverse fault systems and their roles in ruptures may increase the maximum potential earthquake magnitude for fold-and-thrust belts.

We present three-dimensional (3D) seismic compressional wave velocity ( V p ), shear wave velocity ( V s ) and V p / V s models around the Longmen Shan fault, Sichuan, China region, using aftershocks associated with the 2008 Wenchuan Ms 8.0 earthquake. The velocity and ratio models are obtained using a new version of the double-difference seismic tomography code tomoDDPS (Zhang, 2003) to simultaneously solve for V p , V s , V p / V s and event locations. The data used in inversion include 73,013 P arrival times, 62,287 S arrival times and 61,823 S–P travel times recorded by 63 stations from both permanent and temporary networks in a region 400 km northeast-southwest by 200 km northwest-southeast. The velocity model shows structure heterogeneity both along and across the fault zone. Generally, the velocity model is consistent with the local geology, with older rocks having high velocity and younger rocks having low velocity. The Longmen Shan fault zone is a clear boundary in V p , V s and V p / V s ratio. Down to the depth around 15 km, higher V p and V s and lower V p / V s ratio exist to the west of the fault, corresponding to the Songpan–Ganze Fold System, while lower V p and V s and higher V p / V s ratio exist to the east, corresponding to the Sichuan Basin. Along the fault zone, the velocity structure is generally consistent with various rupture slip models, with two high velocity bodies corresponding well to the two large slip patches. This shows the structure control on the slip distribution along the fault plane.