ALBERT

Description: The global spatiotemporal pattern of mortality risk and burden attributable to tropical cyclones is unclear. We aimed to evaluate the global short-term mortality risk and burden associated with tropical cyclones from 1980 to 2019.

Description: Desertification has hampered eco-environment sustainable development in arid, semi-arid, and dry sub-humid areas. However, the effect of the desertification process on vegetation carbon sources and sinks remains unclear in Central Asia. Based on Landsat images and cloud computing, this study applied and evaluated five machine learning methods (i.e., classification and regression tree, random forest, support vector machine, gradient tree boost (GTB), and naive bayes) and desertification difference index method to improve desertification estimation by integrating vegetation, soil, terrain, and climate conditions. According to the optimal method and net ecosystem production (NEP) model, we quantitatively explored vegetation carbon sources and sinks in Central Asia from 1990 to 2020, and then the effect of the desertification process on them was quantified under different aridity stress. The results showed that GTB method performs best on the test set and spatial pattern, which has higher overall accuracy (82.1 %) and Kappa coefficient (0.78) than other five methods. The desertification area has decreased by 8.58 % (341,643 km2) from 1990 to 2020. Among them, the severe and slight desertification areas decreased by 62.42 % and 32.11 %, respectively, while the moderate and high desertification areas increased by 24.6 % and 13.11 %, respectively. In particular, land restoration areas where the desertification restored one or above levels, accounted for 33.91 % of the total area. NEP in Central Asia showed an increasing trend at a rate of 0.54 g C m−2 yr−1 during 1990–2020, and the area passed the t-test (p 〈 0.05) was mainly located in Kazakh Steppe, Kazakh Uplands, and the edge of Tianshan Mountains. In general, restoring the land of degraded ecosystems has stored up 61.08 × 103 t carbon, accounting for 59.61 % of the total net change of NEP, but the fragile ecological environments in the existing desertification areas have been further aggravated.

Description: Postseismic surface deformation associated with great subduction earthquakes is controlled by asthenosphere rheology, frictional properties of the fault, and structural complexity. Here by modeling GPS displacements in the 6 years following the 2010 Mw 8.8 Maule earthquake in Chile, we investigate the impact of heterogeneous viscosity distribution in the South American subcontinental asthenosphere on the 3-D postseismic deformation pattern. The observed postseismic deformation is characterized by flexure of the South America plate with peak uplift in the Andean mountain range and subsidence in the hinterland. We find that, at the time scale of observation, over 2 orders of magnitude gradual increase in asthenosphere viscosity from the arc area toward the cratonic hinterland is needed to jointly explain horizontal and vertical displacements. Our findings present an efficient method to estimate spatial variations of viscosity, which clearly improves the fitting to the vertical signal of deformation. Lateral changes in asthenosphere viscosity can be correlated with the thermomechanical transition from weak subvolcanic arc mantle to strong subcratonic mantle, thus suggesting a stationary heterogeneous viscosity structure. However, we cannot rule out a transient viscosity structure (e.g., power law rheology) with the short time span of observation.

Description: The formation and evolution of permafrost in China during the last 20 ka were reconstructed on the basis of large amount of paleo-permafrost remains and paleo-periglacial evidence, as well as paleo-glacial landforms, paleo-flora and paleofauna records. The results indicate that, during the local Last Glacial Maximum (LLGM) or local Last Permafrost Maximum (LLPMax), the extent of permafrost of China reached 5.3×106−5.4×106 km2, or thrice that of today, but permafrost shrank to only 0.80×106−0.85×106 km2, or 50% that of present, during the local Holocene Megathermal Period (LHMP), or the local Last Permafrost Minimum (LLPMin). On the basis of the dating of periglacial remains and their distributive features, the extent of permafrost in China was delineated for the two periods of LLGM (LLPMax) and LHMP (LLPMin), and the evolution of permafrost in China was divided into seven periods as follows: (1) LLGM in Late Pleistocene (ca. 20000 to 13000−10800 a BP) with extensive evidence for the presence of intensive ice-wedge expansion for outlining its LLPMax extent; (2) A period of dramatically changing climate during the early Holocene (10800 to 8500−7000 a BP) when permafrost remained relatively stable but with a general trend of shrinking areal extent; (3) The LHMP in the Mid-Holocene (8500−7000 to 4000−3000 a BP) when permafrost degraded intensively and extensively, and shrank to the LLPMin; (4) Neoglaciation during the late Holocene (4000−3000 to 1000 a BP, when permafrost again expanded; (5) Medieval Warming Period (MWP) in the late Holocene (1000−500 a BP) when permafrost was in a relative decline; (6) Little Ice Age (LIA) in the late Holocene (500−100 a BP), when permafrost relatively expanded, and; (7) Recent warming (during the 20th century), when permafrost continuously degraded and still is degrading. The paleo-climate, geography and paleopermafrost extents and other features were reconstructed for each of these seven periods.

Description: Modern GPS measurements have provided essential constraints on the kinematics of the continental lithosphere at an unprecedented spatial and temporal resolution and have, in turn, revolutionized our view of crustal deforming processes spanning the earthquake cycle in the subduction zone. These measurements have been particularly useful in constraining viscous deformation of the asthenosphere. The accumulation of geodetic time series in many subduction zones has led to many significant refinements on the concept of subduction earthquake cycle. In this thesis, I present a broad spectrum of interrelated topics about the underlying deformation mechanisms during the subduction zone earthquake cycles. I integrate Finite Element Method (FEM) modeling and geodetic constraints from GPS observations to geomechanically explore the tectonophysical processes at different stages of the earthquake cycle with case studies mainly confined to the Chilean plate boundary margin. For the interseismic period, I investigate the control of viscoelasticity of the asthenosphere on interseismic deformation and its effects on the apparent locking degree determination. Most previous models explain the interseismic deformation with purely elastic solution and neglect the potential viscoelastic effects, hence the associated interpretations are potentially misleading. To highlight the pitfalls of interpreting the geodetic data with purely elastic models for both the forward and inverse problems, I develop a novel FEM-based viscoelastic inversion method and apply it to the Peru-North Chile subduction zone. My results confirm that elastic models are prone to overestimating the interseismic locking depth and indicate that the signals interpreted as back-arc shortening in the elastic model can be alternatively explained by viscoelastic deformation, which, in turn, dramatically refines the interseismic locking pattern in both dip and strike directions. Hence it is necessary to thoroughly reevaluate existing locking models that are based on purely elastic models, some of which attribute viscoelastic deformation to different sources such as microplate sliver motions. For the coseismic period, I investigate the influence that megathrust earthquake slip has on the activation of splay faults, taking into account the effects of gravity and variations in the frictional strength properties of splay faults. My results indicate that the static triggering process is controlled by a critical depth of megathrust slip distribution. Megathrust slip concentrated at depths shallower than the critical depth will favor normal displacement, while slip concentrated at depths deeper than the critical depth is likely to result in reverse motion. This work thus provides a useful tool for predicting the activation of secondary faults and may have direct implications for tsunami hazard research. For the earthquake cycle, especially the postseismic period, I investigate how the effective viscosity varies in asthenosphere. We use a set of 3-D FEM models and continuous GPS observations to constrain the effective viscosities of the asthenosphere and investigate the spatio-temporal variability of the effective viscosity. Our results reveal a sudden decrease in effective viscosities in near field following the earthquake and the slow recovery of these effective viscosities during the postseismic phase. While in far field, there is no sudden effect, rather a gradual viscosity decrease. The variations of the viscosity in these bodies may reflect a dependence of the viscosity on the stress state of the materials, which is suddenly elevated by coseismic-introduced stress perturbation. Therefore, we suggest this geophysical process may explain the first order change in wavelength of surface deformation away from the trench before and after a great earthquake. While the viscosity variation of the asthenosphere is significant enough to be measured by geodetic instruments, significant challenges remain for refining the model of viscoelastic deformation in the subduction earthquake cycle.

Description: Along a subduction zone, great megathrust earthquakes recur either after long seismic gaps lasting several decades to centuries or over much shorter periods lasting hours to a few years when cascading successions of earthquakes rupture nearby segments of the fault. We analyze a decade of continuous Global Positioning System observations along the South American continent to estimate changes in deformation rates between the 2010 Maule (M8.8) and 2015 Illapel (M8.3) Chilean earthquakes. We find that surface velocities increased after the 2010 earthquake, in response to continental-scale viscoelastic mantle relaxation and to regional-scale increased degree of interplate locking. We propose that increased locking occurs transiently during a super-interseismic phase in segments adjacent to a megathrust rupture, responding to bending of both plates caused by coseismic slip and subsequent afterslip. Enhanced strain rates during a super-interseismic phase may therefore bring a megathrust segment closer to failure and possibly triggered the 2015 event.

Description: The time‐variable features of interseismic deformation in subduction zones are poorly understood and commonly ignored. Here, by incorporating century‐long leveling data and contemporary global navigation satellite systems (GNSS) velocities, we investigate the temporal evolution of interseismic deformation in southwest Japan using two‐dimensional viscoelastic earthquake‐cycle models. We find that a steady‐state continental mantle viscosity of 1019 Pa·s is required to explain these two geodetic data sets. Our preferred model predicts significant variations in the surface velocity field throughout the entire interseismic period due to viscous mantle flow driven by ongoing megathrust locking, indicating that the multiyear GNSS‐derived velocity field represents a snapshot of time‐varying interseismic deformation. The locking depth and locking time exert significant influences on the horizontal and vertical deformation patterns. Our findings highlight the importance of the interseismically relaxing mantle and the necessity of considering such an effect in determining the current locking state along the Nankai subduction zone.

Description: These data sets accompany the article "Forming a Mogi Doughnut in the years prior to and immediately before the 2014 M8.1 Iquique, Northern Chile earthquake" (Schurr et al., 2020). The data sets consist of an earthquake catalog (2020-011_schurr-et-al_mogi_eqk_cat.txt) preceding the 2014 M8.1 Iquique, northern Chile earthquake, an inter-seismic locking model derived from GPS data for the northern Chile subduction zone (as plain text table and Generic Mapping Tools [GMT, Wessel et al. 2019] grid file: 2020-011_schurr-et-al_mogi_locking.txt and .grd) and the gravity field corrected for water column and subducted slab of the source region (GMT grid file: 2020-011_schurr-et-al_mogi_gravity). All data files are combined in one zip folder.