Institute of Surface-Earth System Science

Nitrogen and phosphorus are key elements in controlling eutrophication in the aquatic system. Water and sediment samples were collected from Hongfeng Lake, a seasonally stratified reservoir in southwest China, in winter and summer. Diffusion fluxes of NH4+, NO3−, and labile P in summer using diffusive gradients in thin films technology were 3.4, −37.2, and 0.9 mg m−2 day−1, respectively, based on Fick’s first law. The diffusion flux of labile P was 2.05 mg m−2 day−1 in winter. The contributions fraction of the labile P diffusion flux from sediment to the overlying water were higher in winter than those in summer, because of the relatively lower external input, concentrations and higher diffusion fluxes in winter. After the external input decreased, all of the three diffusion fluxes were lower than the previous record. To understand the influence effect of hydrodynamics, environmental fluid dynamics code modeling was used to simulate the flow and temperature field in winter and summe...

Stable isotopes in precipitation can effectively reveal the process of atmospheric water circulation, serving as an effective tool for hydrological and water resources research, climate change, and ecosystem studies. The scarcity of stable isotope data in precipitation has hindered comprehension of regional hydrology, climate, and ecology due to discontinuities on a temporal scale and unevenness on a spatial scale. To this end, we collated stable hydrogen and oxygen isotope data in precipitation from 1930 stations in Eurasia from 1961 to 2022, totalling 51,752 data records. Stable isotopes in precipitation across various regions of Eurasia, as a whole, decrease with increasing latitude and distance from the coast. In the summer, stable isotopes in precipitation are relatively enriched, while in winter, they are relatively depleted. In recent decades, the stable isotope values of Eurasian precipitation show an overall trend of increasing variation with the advancement of years, which is associated with global warming. Geographical location, underlying surface conditions, seasons, and atmospheric circulation are all factors that determine the characteristics of stable isotopes in precipitation. The dataset of stable isotopes in Eurasian precipitation provides a powerful tool for understanding changes in regional atmospheric water circulation and assists in conducting hydrological, meteorological, and ecological studies in related regions.

1] We use river sediment load data to map the pattern of modern denudation across the Longmen Shan margin of the Tibetan Plateau. Suspended sediment load, with corrections of bed load and solute load contributions, is used to calculate watershed-averaged denudation rates. Decadal erosion is spatially heterogeneous, and seasonally modulated by monsoon flows, which account for 80-90% of the sediment load. Enhanced denudation occurs in a ∼50 km wide band on the hanging wall of the Longmen Shan and Huya fault zones, reaching 0.5-0.8 mm/yr. These rates are similar to kyr-scale rates deduced from cosmogenic 10 Be and to Myr-scale rates from low-temperature thermochronology. The sediment fluxderived erosion rates decrease with increasing distance plateauward, to less than 0.05 mm/yr at a distance ∼200 km northwest of the foot of the Longmen Shan. The gradient in precipitation across this margin alone cannot explain the one order of magnitude spatial difference in erosion. Rather, the river sediment load data delineates a zone of relatively rapid denudation around active faults that carry the Longmen Shan in their hanging wall. From the similarity of denudation rates measured over Myr, kyr, and decadal time scales, we propose that erosion of the Longmen Shan margin has approached a flux steady state. The erosional efflux is balanced by advection of rock toward the Longmen margin above the ∼20°NW dipping ramp of the marginbounding fault. Our results suggest that high amounts of landslide material mobilized by earthquakes such as the Mw 7.9 2008 Wenchuan event are gradually removed by rivers, smoothing sediment flux over time. Our results also suggest that caution should be exercised when interpreting young cooling ages as evidence of the initiation of plateau uplift. Advection of an already high plateau into the belt of higher erosion rate at the Longmen Shan could also give rise to an abrupt cooling history.

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.

Worldwide occurrence and documentation of reverse-type ruptures are sparse. Near Hongkou, the Wenchuan rupture passes through the broad Baisha River valley and provides excellent opportunities to trace the surface faulting in fine details for 13 km distance, one of the longest continuous sections along the entire rupture. In this paper, we present the results of our mapping of the surface rupture in this reach. Based on the discontinuities in slip and geometry, the rupture was divided into four sections for convenience, from west to east: the Shenxi Gou, the Miaoba, the Gaoyuan, and the Bajiao Miao sections, respectively. The vertical offset is large in the Shenxi Gou and the Bajiao Miao sections, locally reaching 5-6 m in maxima, and generally low in the Miaoba section (1-2 m or less in most places). The slip gradient for vertical offset is generally 10 3 , locally up to 10 1 , similar to that in well-documented strike-slip ruptures. Near Gaoyuan village, the surface rupture consists of two subparallel branches, with the northern one exhibiting right-lateral slip with minor southeast-side-up thrusting, while the southern one is almost pure southeast-side-up thrusting. This pattern mimics the incomplete slip-partitioning of oblique thrusting on parallel faults but at a local scale. In addition, the sense of vertical throw on these two strands is opposite to the general northwest-side-up thrusting of the Wenchuan rupture. We propose that it is likely due to the inheritance at shallow depth from the southeast-dipping geological faults, and that old fault zone structures can have a strong effect on the dynamic rupture by guiding the rupture propagation onto paths of preexisting, though locally unfavorable, dipping fault planes. We also discuss the cross-cutting slickenside striations observed near Bajiao Miao, which indicated temporal rake rotation during dynamic rupture, and their geological and seismological implications.

The Haiyuan fault is a major active left-lateral fault along the northeast edge of the Tibet-Qinghai Plateau. Studying this fault is important in understanding current deformation of the plateau and the mechanics of continental deformation in general. Previous studies have mostly focused on the slip rate of the fault. Paleoseismic investigations on the fault are sparse, and have been targeted mostly at the stretch of the fault that ruptured in the 1920 M ϳ8.6 earthquake in Ningxia Province.

1] It is poorly known if fault slip repeats regularly through many earthquake cycles. Well-documented measurements of successive slips rarely span more than three earthquake cycles. In this paper, we present evidence of six sequential offsets across the San Andreas fault at a site in the Carrizo Plain, using stream channels as piercing lines. We opened a latticework of trenches across the offset channels on both sides of the fault to expose their subsurface stratigraphy. We can correlate the channels across the fault on the basis of their elevations, shapes, stratigraphy, and ages. The three-dimensional excavations allow us to locate accurately the offset channel pairs and to determine the amounts of motion for each pair. We find that the dextral slips associated with the six events in the last millennium are, from oldest to youngest, !5.4 ± 0.6, 8.0 ± 0.5, 1.4 ± 0.5, 5.2 ± 0.6, 7.6 ± 0.4 and 7.9 ± 0.1 m. In this series, three and possibly four of the six offset values are between 7 and 8 m. The common occurrence of 7-8 m offsets suggests remarkably regular, but not strictly uniform, slip behavior. Age constraints for these events at our site, combined with previous paleoseismic investigations within a few kilometers, allow a construction of offset history and a preliminary evaluation of slip-and timepredictable models. The average slip rate over the span of the past five events (between A.D. 1210 and A.D. 1857.) has been 34 mm/yr, not resolvably different from the previously determined late Holocene slip rate and the modern geodetic strain accumulation rate. We find that the slip-predictable model is a better fit than the timepredictable model. In general, earthquake slip is positively correlated with the time interval preceding the event. Smaller offsets coincide with shorter prior intervals and larger offset with longer prior intervals.

There has been debate on whether average slip D in long ruptures should scale with rupture length L, or with rupture width W . This scaling discussion is equivalent to asking whether average stress drop σ , which is sometimes considered an intrinsic frictional property of a fault, is approximately constant over a wide range of earthquake sizes. In this paper, we examine slip-length scaling relations using a simplified 1-D model of spatially heterogeneous slip. The spatially heterogeneous slip is characterized by a stochastic function with a Fourier spectrum that decays as k −α , where k is the wavenumber and α is a parameter that describes the spatial smoothness of slip. We adopt the simple rule that an individual earthquake rupture consists of only one spatially continuous segment of slip (i.e. earthquakes are not generally separable into multiple disconnected segments of slip). In this model, the slip-length scaling relation is intimately related to the spatial heterogeneity of the slip; linear scaling of average slip with rupture length only occurs when α is about 1.5, which is a relatively smooth spatial distribution of slip. We investigate suites of simulated ruptures with different smoothness, and we show that faults with large slip heterogeneity tend to have higher D/L ratios than those with spatially smooth slip. The model also predicts that rougher faults tend to generate larger numbers of small earthquakes, whereas smooth faults may have a uniform size distribution of earthquakes. This simple 1-D fault model suggests that some aspects of stress drop scaling are a consequence of whatever is responsible for the spatial heterogeneity of slip in earthquakes.

We document the precise sizes, but not the dates, of the six latest offsets across the San Andreas fault at Wallace Creek, California. Three and perhaps four of these, including the latest in 1857, show dextral offset of 7.5-8 m. The third and fourth offsets, however, are just 1.4 and 5.2 m. The predominance of similar offsets for the latest six events suggests that the fundamental properties of the fault system that control slip size do not vary greatly from event to event. The large offsets imply that ruptures involving this site are typically more than 200 km long.

We analyze the spatial relationship of relocated aftershocks to the principal rupture planes of the M w 7.3 1992 Landers mainshock from a structural point of view. We find that the aftershocks constitute primarily a several-kilometer-wide damage zone centered on the mainshock rupture planes. The intensity of damage decreases away from the principal faults. Less than half of the aftershocks occurred within 1 km of the mainshock planes, and perhaps only 5% of the aftershocks are candidates for rerupture of the mainshock faults. Moreover, it seems that aftershocks along the Landers rupture have b-values that correlate well with the complexity of the mainshock rupture. Low b-values occur along segments that are simple, whereas higher b-values correlate with sections that are more complex. Thus, structural complexity appears to correlate with a greater relative abundance of small earthquakes. These observations imply that aftershock populations reflect fault populations in the medium surrounding the principal faults rather than the behavior of the mainshock planes themselves.

Within the Himalayan collisional belt, granotoids occur along two sub-parallel belts, the North Himalayan Gneiss Dome (NHGD) and the High Himalayan Crystalline Belt (HHC). In the Yardoi area of NHGD, two-mica granite, a new type granite occurs in the core of the Yardoi gneiss dome (YGD), Dala and Quedang from north to south, and extends at least 50 km long. These granites have similar mineral composition, elemental and radiogenic isotope geochemistry, and age of formation. SHRIMP zircon U/Pb dating indicates that the Yardoi and the Quedang two-mica granites formed at 42.6±1.1 Ma and 42.8±0.6 Ma, respectively, similar to the Dala pluton. These two-mica granites have (1) high SiO2 (>68 wt%), Al2O3 (>15 wt%), and A/CNK(>1.0); (2) relatively high Sr and LREE, but low Y(<10 ppm) and Yb (<1 ppm); (3) high Sr/Y (>40 and up to 250) and La/Yb (>30); (4) very weak or no Eu anomalies; and (5) as compared with those in the Himalayan leucogranites, low initial Sr (87Sr/86Sr(i)<0.7120) and similarly unradiogenic Nd (εNd(i)= -8.9~-15.0) isotopic compositions. These granites have initial Sr and Nd isotope compositions similar to those in the amphibolites but significantly different from those in the metapelite and granitic gneiss. Two-mica granites from the Yardoi area are of Na-rich peraluminous granite and have an diluted adakite-like geochemistry. Such features are distinct from those in the younger leucogranites along the HHC as well as in the NHGD, and require melting of source consisting dominantly of amphibolite at overthickened crustal conditions. This is also supported by the presence of amphibolites with similar Sr and Nd isotope compositions, and similar ages of metamorphism. Two-mica granites of similar age also occur in the other NHGD gneiss domes and along the HHC belt, implying that Mid-Eocene melting of overthickened crustal materials was widespread and might be a primary factor that led to the formation of high density materials (e.g. eclogitic rocks) beneath the Tethyan Himalaya.

1] We document geodetic strain across the Nepal Himalaya using GPS times series from 30 stations in Nepal and southern Tibet, in addition to previously published campaign GPS points and leveling data and determine the pattern of interseismic coupling on the Main Himalayan Thrust fault (MHT). The noise on the daily GPS positions is modeled as a combination of white and colored noise, in order to infer secular velocities at the stations with consistent uncertainties. We then locate the pole of rotation of the Indian plate in the ITRF 2005 reference frame at longitude = À 1.34 AE 3.31 , latitude = 51.4 AE 0.3 with an angular velocity of W = 0.5029 AE 0.0072 /Myr. The pattern of coupling on the MHT is computed on a fault dipping 10 to the north and whose strike roughly follows the arcuate shape of the Himalaya. The model indicates that the MHT is locked from the surface to a distance of approximately 100 km down dip, corresponding to a depth of 15 to 20 km. In map view, the transition zone between the locked portion of the MHT and the portion which is creeping at the long term slip rate seems to be at the most a few tens of kilometers wide and coincides with the belt of midcrustal microseismicity underneath the Himalaya. According to a previous study based on thermokinematic modeling of thermochronological and thermobarometric data, this transition seems to happen in a zone where the temperature reaches 350 C. The convergence between India and South Tibet proceeds at a rate of 17.8 AE 0.5 mm/yr in central and eastern Nepal and 20.5 AE 1 mm/yr in western Nepal. The moment deficit due to locking of the MHT in the interseismic period accrues at a rate of 6.6 AE 0.4 Â 10 19 Nm/yr on the MHT underneath Nepal. For comparison, the moment released by the seismicity over the past 500 years, including 14 M W ≥ 7 earthquakes with moment magnitudes up to 8.5, amounts to only 0.9 Â 10 19 Nm/yr, indicating a large deficit of seismic slip over that period or very infrequent large slow slip events. No large slow slip event has been observed however over the 20 years covered by geodetic measurements in the Nepal Himalaya. We discuss the magnitude and return period of M > 8 earthquakes required to balance the long term slip budget on the MHT. Citation: Ader, T., et al. (2012), Convergence rate across the Nepal Himalaya and interseismic coupling on the Main Himalayan Thrust: Implications for seismic hazard,

This contribution provides new constraints on the timing of Tibetan glacial recessions 30 recorded by the abandonment of moraines. We present cosmogenic radionuclide 10 Be inventories at 17 sites in southern and western Tibet (32 crests, 249 samples) and infer 32 the range of permissible emplacement ages based on these analyses. Individual large embedded rock and boulder samples were collected from the crests of moraine surfaces 34 and analyzed for 10 Be abundance. We consider two scenarios to interpret the age of glacial recession leading to the moraine surface formation from these sample exposure 36 ages: 1) Erosion of the moraine surface is insignificant and so the emplacement age of the moraines is reflected by the mean sample age; and 2) Erosion progressively exposes large 38 boulders with little prior exposure, and so the oldest sample age records the minimum moraine emplacement age. We found that depending on the scenario chosen, the moraine 40 emplacement age can vary by > 50% for ~100 ka-old samples. We consider two scaling models for estimating the production rates of 10 Be in Tibet, which has an important, 42 although lesser, effect on inferred moraine ages. While the data presented herein effectively increase the database of sample exposure ages from Tibet by ~20%, we find 44 that uncertainties related to the interpretation of the 10 Be abundance within individual samples in terms of moraine emplacement ages are sufficient to accommodate either a 46 view in which glacial advances are associated with temperature minima or precipitation maxima that are recorded by independent paleoclimate proxies. A reanalysis of published 48 data from moraines throughout Tibet shows that the variation we observe is not unique to our dataset but rather is a robust feature of the Tibetan moraine age database. Thus, when 50 viewed in a similar way with other samples collected from this area, uncertainties within 3 moraine exposure ages obscure attribution of Tibetan glacial advances to temperature 52 minima or precipitation maxima. Our work suggests that more reliable chronologies of Tibetan glaciations will come from improvements in production rate models for this 54 portion of the world, as well as a better understanding of the processes that form and modify these geomorphic surfaces. 56 6 Indeed, the  18 O of ice varies with temperature; therefore it may be used to assess the correlation between temperature minima and glacial expansion using this geochemical 122 proxy . A precipitation-related glacial advance is more difficult to constrain, but we know that an increase of precipitation (during both 124 warm and cold climate) brings snowfall at high elevation and allows glaciers to advance.

Unlike other large rivers flowing out of Tibet, the Yellow River escapes from the plateau towards the NE crossing no less than five NW-SE striking, actively growing ranges and intervening basins. Thick Plio-Quaternary deposits and fluvial terraces testify to a phase of aggradation and sediment infill up to the average surface elevation (3200-3250 m a.s.l.) of the Gonghe, Guide and Qinghai Lake basins. A set of seven main terraces across the Gonghe Basin suggests progressive down-cutting of the Yellow River carving the 500 m deep Longyang gorge at the basin exit. 10 Be and 26 Al concentrations in quartz of surface and sub-surface samples of four terraces constrain the timing of incision by determining the burial age of the deposit and the exposure age of its surface. Modelling the depth dependence of the 10 Be concentration and the 26 Al/ 10 Be ratio allows us to constrain the onset of the ongoing phase of incision to 120-250 ka. These ages suggest long-term incision rates between 2-6 mm a 21 . Together with the present morphology of the Yellow River terraces across the Gonghe basin and the Longyang gorge, our results imply rapid river catchment evolution and interaction between river dynamics, tectonic and climate in northeastern Tibet.

The timing of uplift of the Tibetan Plateau remains controversial, with estimates varying from Eocene to more recent than Pliocene. In particular, the paleoaltimetry of the Qiangtang terrane, on the central Tibetan Plateau, is completely unknown. Here, we present new stable isotope results of fluvial/ lacustrine carbonate cement, pedogenic carbonate and marl from the Kangtuo and Suonahu formations deposited between $ 51 and $ 28 Ma in the Heihuling-Bamaoqiongzong area of the northern Qiangtang terrane. The lithofacies associations indicate that the Kangtuo formation was deposited in alluvial fan and fluvial floodplain environments, and the Suonahu formation was deposited in near shore lacustrine, playa-lake and channelized fluvial environments. Carbon and oxygen isotope values, coupled with the sedimentary facies interpretations, point to evaporation and low respiration rates in the Eocene-Oligocene paleosols, suggesting an arid climate in the high Qiangtang area at the time.

Paleoseismology provides fundamental data for generalizing earthquake recurrence behavior, by revealing past surface-rupturing events. Determining the size of paleoseismic events is notoriously more challenging than their timing. Paleoearthquakes exposed in trenches are vaguely defined as large enough to break to the surface and often assumed to be similar in size. Here we show an example where the paleoseismic record includes events of both moderate and large magnitudes. At the Salt Lake site on the active left-lateral Haiyuan Fault, northern Tibetan Plateau, a high-resolution stratigraphic sequence recorded three and possibly four events since A.D. 1500, constrained by accelerator mass spectrometry 14 C dating. Historical accounts of earthquake damage in the study region suggest that several earthquakes exposed in the trenches markedly differ in magnitude. With the exception of the most recent M~8 earthquake that occurred in A.D. 1920, two earlier events, which occurred in A.D. 1760 (or 1709) and 1638, respectively, are considerably smaller, with magnitude M < 7 and more likely M~6 or less. Thus, this section of the Haiyuan Fault that broke during moderate-magnitude events failed again after a short interval during a large M w 7.8–8.3 earthquake, as part of a larger multisegment rupture. Our study shows that moderate-magnitude events can be preserved in the stratigraphy and exposed by paleoseismic trenching under ideal conditions, for instance, if sedimentation is fast enough and there is no hiatus in deposition. Eventually, the data presented add to the growing body of paleoseismic records containing events of different magnitudes with a large variability in rupture length and coseismic slip.