Geology topics

Yajing Liu

Publications and source records attributed to Yajing Liu.

13 recordsLinked to original sources

USGS2020

Aseismic transient slip on the Gofar transform fault, East Pacific Rise

Oceanic transform faults display a unique combination of seismic and aseismic slip behavior, including a large globally averaged seismic deficit, and the local occurrence of repeating magnitude (M) ∼ 6 "> ∼ 6 ∼6 earthquakes with abundant foreshocks and seismic swarms, as on the Gofar transform of the East Pacific Rise and the Blanco Ridge in the northeast Pacific Ocean. However, the underlying mechanisms that govern the partitioning between seismic and aseismic slip and their interaction remain unclear. Here we present a numerical modeling study of earthquake sequences and aseismic transient slip on oceanic transform faults. In the model, strong dilatancy strengthening, supported by seismic imaging that indicates enhanced fluid-filled porosity and possible hydrothermal circulation down to the brittle–ductile transition, effectively stabilizes along-strike seismic rupture propagation and results in rupture barriers where aseismic transients arise episodically. The modeled slow slip migrates along the barrier zones at speeds ∼10 to 600 m/h, spatiotemporally correlated with the observed migration of seismic swarms on the Gofar transform. Our model thus suggests the possible prevalence of episodic aseismic transients in M ∼ 6 "> ∼ 6 ∼6 rupture barrier zones that host active swarms on oceanic transform faults and provides candidates for future seafloor geodesy experiments to verify the relation between aseismic fault slip, earthquake swarms, and fault zone hydromechanical properties.

Proceedings of the National Academy of Sciences↗

USGS2020

The community code verification exercise for simulating sequences of earthquakes and aseismic slip (SEAS)

Numerical simulations of sequences of earthquakes and aseismic slip (SEAS) have made great progress over past decades to address important questions in earthquake physics. However, significant challenges in SEAS modeling remain in resolving multiscale interactions between earthquake nucleation, dynamic rupture, and aseismic slip, and understanding physical factors controlling observables such as seismicity and ground deformation. The increasing complexity of SEAS modeling calls for extensive efforts to verify codes and advance these simulations with rigor, reproducibility, and broadened impact. In 2018, we initiated a community code-verification exercise for SEAS simulations, supported by the Southern California Earthquake Center. Here, we report the findings from our first two benchmark problems (BP1 and BP2), designed to verify different computational methods in solving a mathematically well-defined, basic faulting problem. We consider a 2D antiplane problem, with a 1D planar vertical strike-slip fault obeying rate-and-state friction, embedded in a 2D homogeneous, linear elastic halfspace. Sequences of quasi-dynamic earthquakes with periodic occurrences (BP1) or bimodal sizes (BP2) and their interactions with aseismic slip are simulated. The comparison of results from 11 groups using different numerical methods show excellent agreements in long-term and coseismic fault behavior. In BP1, we found that truncated domain boundaries influence interseismic stressing, earthquake recurrence, and coseismic rupture, and that model agreement is only achieved with sufficiently large domain sizes. In BP2, we found that complexity of fault behavior depends on how well physical length scales related to spontaneous nucleation and rupture propagation are resolved. Poor numerical resolution can result in artificial complexity, impacting simulation results that are of potential interest for characterizing seismic hazard such as earthquake size distributions, moment release, and recurrence times. These results inform the development of more advanced SEAS models, contributing to our further understanding of earthquake system dynamics.

Seismological Research Letters↗

USGS2018

Stress rotation across the Cascadia megathrust requires a weak subduction plate boundary at seismogenic depths

The Mendocino Triple Junction region is the most seismically active part of the Cascadia Subduction Zone. The northward moving Pacific plate collides with the subducting Gorda plate causing intense internal deformation within it. Here we show that the stress field rotates rapidly with depth across the thrust interface from a strike-slip regime within the subducting plate, reflecting the Pacific plate collision, to a thrust regime in the overriding plate. We utilize a dense focal mechanism dataset, including observations from the Cascadia Initiative ocean bottom seismograph experiment, to constrain the stress orientations. To quantify the implications of this rotation for the strength of the plate boundary, we designed an inversion that solves for the absolute stress tensors in a three-layer model subject to assumptions about the strength of the subducting mantle. Our results indicate that the shear stress on the plate boundary fault is likely no more than about ∼50 MPa at ∼20 km depth. Regardless of the assumed mantle strength, we infer a relatively weak megathrust fault with an effective friction coefficient of ∼0 to 0.2 at seismogenic depths. Such a low value for the effective friction coefficient requires a combination of high fluid pressures and/or fault-zone minerals with low inherent friction in the region where a great earthquake is expected in Cascadia.

California↗

USGS2015

Isolated cases of remote dynamic triggering in Canada detected using cataloged earthquakes combined with a matched-filter approach

Here we search for dynamically triggered earthquakes in Canada following global main shocks between 2004 and 2014 with M S  > 6, depth < 100 km, and estimated peak ground velocity > 0.2 cm/s. We use the Natural Resources Canada (NRCan) earthquake catalog to calculate β statistical values in 1° × 1° bins in 10 day windows before and after the main shocks. The statistical analysis suggests that triggering may occur near Vancouver Island, along the border of the Yukon and Northwest Territories, in western Alberta, western Ontario, and the Charlevoix seismic zone. We also search for triggering in Alberta where denser seismic station coverage renders regional earthquake catalogs with lower completeness thresholds. We find remote triggering in Alberta associated with three main shocks using a matched-filter approach on continuous waveform data. The increased number of local earthquakes following the passage of main shock surface waves suggests local faults may be in a critically stressed state.

Alberta↗

USGS2010

Depositional environments and cyclo- and chronostratigraphy of uppermost Carboniferous-Lower Triassic -lacustrine deposits, southern Bogda Mountains, NW China - A terrestrfluvialial paleoclimatic record of mid-latitude NE Pangea

Two uppermost Carboniferous–Lower Triassic fluvial–lacustrine sections in the Tarlong–Taodonggou half-graben, southern Bogda Mountains, NW China, comprise a 1834 m-thick, relatively complete sedimentary and paleoclimatic record of the east coast of mid-latitude NE Pangea. Depositional environmental interpretations identified three orders (high, intermediate, and low) of sedimentary cycles. High-order cycles (HCs) have five basic types, including fluvial cycles recording repetitive changes of erosion and deposition and lacustrine cycles recording repetitive environmental changes associated with lake expansion and contraction. HCs are grouped into intermediate-order cycles (ICs) on the basis of systematic changes of thickness, type, and component lithofacies of HCs. Nine low-order cycles (LCs) are demarcated by graben-wide surfaces across which significant long-term environmental changes occurred. A preliminary cyclostratigraphic framework provides a foundation for future studies of terrestrial climate, tectonics, and paleontology in mid-latitude NE Pangea. Climate variabilities at the intra-HC, HC, IC, and LC scales were interpreted from sedimentary and paleosol evidence. Four prominent climatic shifts are present: 1) from the humid–subhumid to highly-variable subhumid–semiarid conditions at the beginning of Sakamarian; 2) from highly-variable subhumid–semiarid to humid–subhumid conditions across the Artinskian-Capitanian unconformity; 3) from humid–subhumid to highly-variable subhumid–semiarid conditions at early Induan; and 4) from the highly-variable subhumid–semiarid to humid–subhumid conditions across the Olenekian-Anisian unconformity. The stable humid–subhumid condition from Lopingian to early Induan implies that paleoclimate change may not have been the cause of the end-Permian terrestrial mass extinction. A close documentation of the pace and timing of the extinction and exploration of other causes are needed. In addition, the semiarid–subhumid conditions from Sakamarian to Artinskian–Kungurian (?) and from middle Induan to end of Olenekian are in conflict with modern mid-latitude east coast meso- and macrothermal humid climate. Extreme continentality, regional orographic effect, and/or abnormal circulation of Paleo-Tethys maybe are possible causes. Our work serves as a rare data point at mid-latitude NE Pangea for climate modeling to seek explanations on the origin(s) of climate variability in NE Pangea from latest Carboniferous to Early Triassic.

Global and Planetary Change↗

USGS2009

A formal framework for scenario development in support of environmental decision-making

Scenarios are possible future states of the world that represent alternative plausible conditions under different assumptions. Often, scenarios are developed in a context relevant to stakeholders involved in their applications since the evaluation of scenario outcomes and implications can enhance decision-making activities. This paper reviews the state-of-the-art of scenario development and proposes a formal approach to scenario development in environmental decision-making. The discussion of current issues in scenario studies includes advantages and obstacles in utilizing a formal scenario development framework, and the different forms of uncertainty inherent in scenario development, as well as how they should be treated. An appendix for common scenario terminology has been attached for clarity. Major recommendations for future research in this area include proper consideration of uncertainty in scenario studies in particular in relation to stakeholder relevant information, construction of scenarios that are more diverse in nature, and sharing of information and resources among the scenario development research community. ?? 2008 Elsevier Ltd.

Environmental Modelling and Software↗

USGS2009

Instability of seawater pH in the South China Sea during the mid-late Holocene: Evidence from boron isotopic composition of corals

We used positive thermal ionization mass spectrometry (PTIMS) to generate high precision ??11B records in Porites corals of the mid-late Holocene from the South China Sea (SCS). The ??11B values of the Holocene corals vary significantly, ranging from 22.2??? to 25.5???. The paleo-pH records of the SCS, reconstructed from the ??11B data, were not stable as previously thought but show a gradual increase from the Holocene thermal optimal and a sharp decrease to modern values. The latter is likely caused by the large amount of anthropogenic CO2 emissions since the Industrial Revolution but variations of atmospheric pCO2 cannot explain the pH change of the SCS before the Industrial Revolution. We suggest that variations of monsoon intensity during the mid-late Holocene may have driven the sea surface pH increase from the mid to late Holocene. Results of this study indicate that the impact of anthropogenic atmospheric CO2 emissions may have reversed the natural pH trend in the SCS since the mid-Holocene. Such ocean pH records in the current interglacial period can help us better understand the physical and biological controls on ocean pH and possibly predict the long-term impact of climate change on future ocean acidification. ?? 2008 Elsevier Ltd. All rights reserved.

Geochimica et Cosmochimica Acta↗

USGS2009

The SCEC/USGS dynamic earthquake rupture code verification exercise

Numerical simulations of earthquake rupture dynamics are now common, yet it has been difficult to test the validity of these simulations because there have been few field observations and no analytic solutions with which to compare the results. This paper describes the Southern California Earthquake Center/U.S. Geological Survey (SCEC/USGS) Dynamic Earthquake Rupture Code Verification Exercise, where codes that simulate spontaneous rupture dynamics in three dimensions are evaluated and the results produced by these codes are compared using Web-based tools. This is the first time that a broad and rigorous examination of numerous spontaneous rupture codes has been performed—a significant advance in this science. The automated process developed to attain this achievement provides for a future where testing of codes is easily accomplished. Scientists who use computer simulations to understand earthquakes utilize a range of techniques. Most of these assume that earthquakes are caused by slip at depth on faults in the Earth, but hereafter the strategies vary. Among the methods used in earthquake mechanics studies are kinematic approaches and dynamic approaches. The kinematic approach uses a computer code that prescribes the spatial and temporal evolution of slip on the causative fault (or faults). These types of simulations are very helpful, especially since they can be used in seismic data inversions to relate the ground motions recorded in the field to slip on the fault(s) at depth. However, these kinematic solutions generally provide no insight into the physics driving the fault slip or information about why the involved fault(s) slipped that much (or that little). In other words, these kinematic solutions may lack information about the physical dynamics of earthquake rupture that will be most helpful in forecasting future events. To help address this issue, some researchers use computer codes to numerically simulate earthquakes and construct dynamic, spontaneous rupture (hereafter called “spontaneous rupture”) solutions. For these types of numerical simulations, rather than prescribing the slip function at each location on the fault(s), just the friction constitutive properties and initial stress conditions are prescribed. The subsequent stresses and fault slip spontaneously evolve over time as part of the elasto-dynamic solution. Therefore, spontaneous rupture computer simulations of earthquakes allow us to include everything that we know, or think that we know, about earthquake dynamics and to test these ideas against earthquake observations.

Seismological Research Letters↗

USGS2008

The decline of winter monsoon velocity in the South China Sea through the 20th century: Evidence from the Sr/Ca records in corals

A modern massive Porites coral was collected from the Longwan Bay (19??20???N, 110??39???E) on the east coast of the Hainan Island, China. The coral was sectioned vertical to the growth axis into discs of double density-bands representing annual growth. The samples were analyzed for the Sr/Ca ratio by inductively coupled plasma atomic emission spectrometry. The history of winter sea-surface temperature (SST) is reconstructed using the Sr/Ca ratio in winter bands of corals. The winter SST at Xisha in the middle of the South China Sea (SCS) is weakly correlated with the instrument-measured winter monsoon velocity (WMV) with a correlation coefficient of 0.19. The winter SST data from corals at Longwan Bay, Hainan, in the northern SCS are moderately correlated with the WMV (r = 0.40). Interestingly we found that the difference of winter SSTs between the two sites (Xisha and Longwan Bay, Hainan) (the X-H index) is significantly negatively correlated with the WMV (r = - 0.73). This negative correlation may be related to the intrusion of the warm Kuroshio Current into the SCS through the Luzon Strait promoted by the strong northeastern monsoon winds in the winter. Using the relationship between our coralline data and observed WMV, the calculated winter monsoon velocity (WMVc) was obtained for 87??years. This data set in combination with the instrument-measured data between 1993 and 1998 generate a record of WMVc for a period of 93??years from 1906 to 1998. The WMVc in the 20th century shows significant interannual and decadal variability with a trend of persistent decline in the whole 20th century at the rate of decrease of - 0.02 (m/s)/a. The lowest wind velocity occurred during the last two decades of the 20th century. The WMVc has decreased significantly by about 30% from the early to the late of 20th century. The 20th century decline of winter monsoon velocity evidenced from the SCS coral records is consistent with the atmosphere-ocean general circulation models (AOGCMs) simulations for monsoon response to increasing temperatures. In addition, an obvious decline shift of WMV around 1976 can be seen in both instrumental and proxy records and it coincides with many other Pacific records. This shift is likely to correspond to a Pacific-wide change in the Pacific Decadal Oscillation occurring at the same time. ?? 2008 Elsevier B.V. All rights reserved.

Global and Planetary Change↗

USGS2006

Belowground carbon balance and carbon accumulation rate in the successional series of monsoon evergreen broad-leaved forest

The balance, accumulation rate and temporal dynamics of belowground carbon in the successional series of monsoon evergreen broadleaved forest are obtained in this paper, based on long-term observations to the soil organic matter, input and standing biomass of litter and coarse woody debris, and dissolved organic carbon carried in the hydrological process of subtropical climax forest ecosystem—monsoon evergreen broad-leaved forest, and its two successional forests of natural restoration—coniferous and broad-leaved mixed forest and Pinus massoniana forest, as well as data of root biomass obtained once every five years and respiration measurement of soil, litter and coarse woody debris respiration for 1 year. The major results include: the belowground carbon pools of monsoon evergreen broad-leaved forest, coniferous and broad-leaved mixed forest, and Pinus massoniana forest are 23191 ± 2538 g · m −2 , 16889 ± 1936 g · m −2 and 12680 ± 1854 g · m −2 , respectively, in 2002. Mean annual carbon accumulation rates of the three forest types during the 24a from 1978 to 2002 are 383 ± 97 g · m −2 · a −1 , 193 ± 85 g · m −2 · a −1 and 213 ± 86 g · m −2 · a −1 , respectively. The belowground carbon pools in the three forest types keep increasing during the observation period, suggesting that belowground carbon pools are carbon sinks to the atmosphere. There are seasonal variations, namely, they are strong carbon sources from April to June, weak carbon sources from July to September; while they are strong carbon sinks from October to November, weak carbon sinks from December to March.

Science in China, Series D: Earth Sciences↗

USGS2006

A ubiquitous thermoacidophilic archaeon from deep-sea hydrothermal vents

Deep-sea hydrothermal vents are important in global biogeochemical cycles, providing biological oases at the sea floor that are supported by the thermal and chemical flux from the Earth's interior. As hot, acidic and reduced hydrothermal fluids mix with cold, alkaline and oxygenated sea water, minerals precipitate to form porous sulphide-sulphate deposits. These structures provide microhabitats for a diversity of prokaryotes that exploit the geochemical and physical gradients in this dynamic ecosystem. It has been proposed that fluid pH in the actively venting sulphide structures is generally low (pH < 4.5), yet no extreme thermoacidophile has been isolated from vent deposits. Culture-independent surveys based on ribosomal RNA genes from deep-sea hydrothermal deposits have identified a widespread euryarchaeotal lineage, DHVE2 (deep-sea hydrothermal vent euryarchaeotic 2). Despite the ubiquity and apparent deep-sea endemism of DHVE2, cultivation of this group has been unsuccessful and thus its metabolism remains a mystery. Here we report the isolation and cultivation of a member of the DHVE2 group, which is an obligate thermoacidophilic sulphur- or iron-reducing heterotroph capable of growing from pH 3.3 to 5.8 and between 55 and 75??C. In addition, we demonstrate that this isolate constitutes up to 15% of the archaeal population, providing evidence that thermoacidophiles may be key players in the sulphur and iron cycling at deep-sea vents. ?? 2006 Nature Publishing Group.

Nature↗

USGS2004

U-Pb zircon constraints on the tectonic evolution of southeastern Tibet, Namche Barwa area

The eastern syntaxis of the Himalayas is expressed in the crust as a pronounced southward bend in the orogen. The change in strike of geologic features coincides with the high topography of the Namche Barwa region, the exposure of granulite-grade metamorphic rocks, and a 180-degree bend in the Yalu Tsangpo. We have conducted a geochronologic and geochemical investigation of several suites of granitoids collected from the Namche Barwa massif and subjacent terranes of southeastern Tibet, ranging from cm-scale dikes and sills to larger, outcrop-scale intrusions. U-Pb SHRIMP-RG zircon ages establish at least five magmatic episodes: ∼400 to 500 Ma, ∼120 Ma, 40 to 70 Ma, 18 to 25 Ma, and 3 to 10 Ma . These episodes broadly correlate to spatial patterns in sample localities, as follows: 400 to 500 Ma ages occur in zircon cores collected from within the massif proper; ∼120 Ma granites, related to early Gangdese arc plutonism, are primarily located northeast of Namche Barwa; later (40-70 Ma) Gangdese activity is expressed in granites west of Namche Barwa. 18 to 25 Ma granites occur both along the suture zone west of Gyala Peri, and directly north of Namche Barwa along the area of the Jiali fault zone, and are attributed both to shearing within the Jiali fault zone and to an early Miocene Gangdese Thrust event. Exceptionally young (<10 Ma) zircon ages are clustered near the core of the massif, along the Yalu Tsangpo gorge. Trace-element geochemical data indicates the presence of both fluid-present and fluid absent melts, with a fluid-absent (decompression) melting regime dominating near the core of Namche Barwa.

Tibet↗

USGS1994

Coal compositional changes along a mire interior to mire margin transect in the Mary Lee coal bed, Warrior Basin, Alabama, USA

Vertical increment samples of the Mary Lee coal bed, collected along an inferred mire interior to mire margin transect, were studied palynologically, petrographically and geochemically to determine if any vertical or lateral changes in coal composition could be detected. Results show the Mary Lee palynoflora to be dominated by Lycospora spp. (arboreous lycophytes), with other lycophyte genera, Crassispora (Sigillaria) and Densosporites (small lycophytes), occurring less frequently. Commonly encountered fern/pteridosperm miospore genera include Leiotriletes, Granulatisporites, Lophotriletes and Schulzospora . Calamospora , representing calamites, and Florinites , representing cordaites, are minor constituents of the overall palynoflora. Petrographically, the Mary Lee coal bed contains high percentages (> 75% mineral matter free, mmf) of vitrinite macerals, with cryptotelinite being by far the most common type. Cryptogelocollinite is the second most abundant vitrinite maceral. Inertinite maceral percentages are variable, ranging from 8% to 30% (mmf) and liptinite macerals are a relatively minor constituent of the Mary Lee coal bed (3–8% mmf). Both vertical and lateral changes in coal composition occur along the transect studied. Lateral changes include an increase in ash yield and sulfur content from the interior to margin; for example, the average ash yield and sulfur content at location 073 (most interior) are 13.7% and 0.55%, respectively, whereas at location 083 (most marginal) the average ash and sulfur values (calculated from coal increments only) are 32.3% and 3.62%, respectively. Palynofloral changes, from the interior to margin, include a general increase in Lycospora pusilla (Lepidodendron) over Lycospora pellucida (Lepidophloios) and an increase in fern and calamite spores and cordaite pollen. An increase in poorly preserved miospore assemblages was also observed in this direction. At locations 016 and 026, coal increments between the lower and middle partings were barren of palynomorphs, except for one increment in column 016. Petrographically, these increments were completely (> 95% mmf) dominated by cryptotelinite; both liptinite and inertinite macerals were conspicuously absent. In contrast, increments above the middle partings at these locations contained elevated percentages of inertinite macerals, relative to more interior locations. Vertical changes were also noted. At mire interior locations, basal coal layers contain either an arboreous lycophyte-dominant assemblage, with abundant Lycospora pusilla , or a lycophyte-fern/pteridosperm co-dominant assemblage. This palynoflora grades upward into a Lycospora pellucida, L. granula -dominant palynoflora in the middle layers and ultimately into a palynoflora containing an abundant fern/pteridosperm element. Petrographically, the coal at these locations is dominated overall by cryptotelinite, with an increase in inertinite occurring in the uppermost increments. The overall dominance of arboreous lycophytes and cryptotelinite in the Mary Lee coal bed suggests that the Mary Lee paleomire developed under predominantly wet conditions. The moderate (at interior locations) to high (at marginal locations) ash yield of the coal further suggests that the mire developed under planar to near-planar conditions.

International Journal of Coal Geology↗