Terrestrial, Atmospheric and Oceanic Sciences/地球科學集刊
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中華民國地球科學學會,此刊物暫停授權合作
選擇卷期
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Global warming has resulted in the melting of glaciers in the southeastern region of Tibet. This study used the InSAR time series obtained from ALOS PALSAR data to investigate the melting glacier over Southeast Tibet during 2007 - 2010 with the small baseline subset (SBAS) technique. Additionally, GRACE (Gravity Recovery and Climate Experiment) gravity field model issued by the Center for Space Research (CSR) was used to extract the equivalent water height (EWH) with the deduction of glacial isostatic adjustment (GIA). The results revealed that the monitoring results by InSAR were basically consistent with the EWHs from GRACE. The glacier deformation in the study area presents a downward trend overall. From the InSAR measurements, a 900- km^2 area within the belt subsided -1.6 cm yr^(-1) during 2007 - 2010, and the subsidence exceeded -24 cm yr^(-1) in some regions. On the other hand, as an auxiliary data source, the monitoring result of GRACE is large space scale and comprehensive, the rate is about -0.01 cm yr^(-1) from GRACE. With the variations of glacier shape obtained from MODIS (moderate-resolution imaging spectroradiometer) data, the changing rate of surface temperature was about 0.014°C yr^(-1). The surface temperature change is negatively correlated with the rate of the glacial subsidence with the correlation coefficient of -0.237, which reflects that the melting glacier is influenced by the temperature rising to a certain extent.
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According to Einstein's general relativity theory, a precise (atomic) clock runs faster at a position with higher gravitational potential than a clock at a position with lower potential. Here, we provide a direct clock comparison using a fixed hydrogen clock and a portable hydrogen clock for geopotential determination based on the two-way satellite time and frequency transfer (TWSTFT) technique. After synchronizing the two hydrogen clocks at positions at the same height, the clocks were compared for a height difference of 22 m over a period of 14 days using the TWSTFT technique. Interpretation of the observed time elapse implies a height difference of 175 m +/- 59 m, which matches the frequency stability of our instruments. This study and relevant experiments provide a way of determining the geopotential using ultrahigh precise clocks based on the TWSTFT technique in the near future.
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Lake level change in the Tibetan Plateau is an important indicator for regional and global climate changes. We use altimeter data from Cryosat-2, SARAL, ICESat, and Jason-2 to detect lake level changes at different spatial and temporal resolutions over 2003 - 2017 (Jason-3 data in 2017 for validation). Cryosat-2's SARIn mode provides precise water level time series over 59 lakes. SARAL's waveforms are retracked to generate near monthly, high-quality measurements at 31 lakes. Jason-2 provides a reference for removing inter-altimeter biases, enabling coherent records over lakes with Jason-2 passes. After a decade of rise since the ICESat record of 2003, the lake levels of Nam Co, Selin Co, Ngangzi Co, and Chibuzhang Co became flat in 2014-2016 and started to fluctuate or decline after 2016. Such positive-flat-negative trends are consistent with the trend variations of mass change from Gravity Recovery and Climate Experiment (GRACE). SARAL detected persistent lake level declines over 2013-2016 in southern Tibet that may signify the onset of decadal reduced flows of the Yarlung Tsangpo and Brahmaputra River that could affect the water supply for their downstream regions in India and Bangladesh. Cryosat-2 and Jason-2 detected sudden lake level rises and falls around Zhuonai, Kusai, and Salt Lake associated with a 2011 lake outburst, which is confirmed by lake volume changes from two Landsat-7 images. With a careful processing and calibration, multiple altimeters allow for determining and cross-validating long-term and episodic lake level changes unachievable by a single altimeter.
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We derived a new GPS site velocity field by integrating the data of ten newly built stations across the Liupan Shan Fault, in the northeastern margin of the Tibetan Plateau and those of pre-existing sites in this area for the period of 1999 - 2015. The GPS velocity field in a Eurasian plate-fixed frame shows a clockwise rotation around the Ordos cratonic block. Relative to the Ordos block, GPS sites generally move northeastward. The inferred GPS velocity profile across the Liupan Shan Fault shows that fault-normal velocities decrease from west to east from 5.4 ± 0.9 mm yr^(-1), at the central of the Lanzhou block to 0.1 ± 0.8 mm yr^(-1) at the western Ordos craton. We estimated the locking degree and the slip deficit rate on the Liupan Shan Fault using the tectonic elastic block model. Modeling results show that the Liupan Shan Fault is characteristic of segmentation in terms of locking degree and slip deficit rate. The southern segment has the largest locking degree, with complete locking to the depth of 13 - 15 km; the locking degree in the central segment is smaller than that of the northern and southern segments. On average, the slip deficit rate at the northern segment is larger than that of the Southern and central segments. We estimate the maximum magnitude of an expected earthquake based on the moment budget since the last large earthquake of M 6.7 in 1921. If the accumulated energy was completely released by a single earthquake, the expected maximum magnitude would be M_w 6.7. The model prediction suggests a high potential for seismic hazard in the Liupan Shan area.
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Glaciers in the Himalaya are increasingly retreating and thinning due to climate change. This process is the primary cause of glacial lakes expansion and has increased the possibilities of the glacial lake outburst floods (GLOFs) that have been responsible for heavy loss of life and damage to downstream infrastructures. This study examines the status of the existing potentially dangerous glacial lakes in the Nepalese Himalaya such as Imja Tsho, Tsho Rolpa, Thulagi, Chamlang South, Barun Tsho, and Lumding Tsho; which were more susceptible to GLOF after the devastating earthquake in 2015. We examined the evolution and decadal expansion rate of lakes from 1987 to 2016 using Landsat images. The results show significant expansion of Imja Tsho, Barun Tsho, and Lumding Tsho at the rates of 42.1, 46.8, and 32.9% respectively, during 2006 - 2016; while other glacial lakes (i.e., Chamlang South, Barun Tsho, and Lumding Tsho) are relatively stable. Although the current status of glacial lakes may be stable in term of burst risk, high expanding lakes must be prioritized for detail studies. Continuous model-based monitoring and risk assessment, mitigation measures and disaster management strategies are necessary for reducing the impact of GLOFs.
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We estimate the elastic thickness of a continental lithosphere by using two approaches that combine the Vening Meinesz-Moritz (VMM) regional isostatic principle with isostatic flexure models formulated based on solving flexural differential equations for a thin elastic shell with and without considering a shell curvature. To model the response of the lithosphere on a load more realistically, we also consider lithospheric density heterogeneities. Resulting expressions describe a functional relation between gravity field quantities and mechanical properties of the lithosphere, namely Young's modulus and Poisson's ratio that are computed from seismic velocity models in prior of estimating the lithospheric elastic thickness. Our numerical study in central Eurasia reveals that both results have a similar spatial pattern, despite exhibiting also some large localized differences due to disregarding the shell curvature. Results show that cratonic formations of North China and Tarim Cratons, Turan Platform as well as parts of Siberian Craton are characterized by the maximum lithospheric elastic thickness. Indian Craton, on the other hand, is not clearly manifested. Minima of the elastic thickness typically correspond with locations of active continental tectonic margins, major orogens (Tibet, Himalaya and parts of Central Asian Orogenic Belt) and an extended continental crust. These findings generally support the hypothesis that tectonically active zones and orogens have a relatively small lithospheric strength, resulting in a significant respond of the lithosphere on various tectonic loads, compared to a large lithospheric strength of cratonic formations.
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On 24 June 2017, an enormous landslide struck the village of Xinmo in Mao County, Sichuan Province. Synthetic aperture radar (SAR) images from the Sentinel-1 satellite are chosen to monitor the landslide using the small baseline set (SBAS) technology, following which the deformation time series are obtained for the source area and are found to be consistent with the accelerated creep model. The displacement time series before the landslide clearly show movement processes associated with transient creep, steady-state creep and tertiary creep. The main deformation area is ascertained by calculating the average displacement of 5 representative regions. Three-month time series before the landslide are selected to calculate the failure time of the landslide both separately and together using the inverse-velocity method. The results show that the time series of the main deformation area can fit a linear model of the inverse velocities better than those of the marginal area, and the forecasted time is closer to the actual failure time. The forecasted time calculated using the time series of three regions in main deformation area is June 25, which is only one day apart from the actual failure time.
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A new satellite-only gravity field model entitled HUST-GOGRA2018s is developed by the combination of GRACE and GOCE data in this study. The modified dynamic approach is applied for GRACE data processing, while the space-wise least square method with a cascade filter is utilized for GOCE data processing. The GRACE-only model HUST-Grace2016s and GOCE-only model HUST-GOCE2018s are then computed, respectively. Our new developed GRACE-only model HUSTGrace2016s performs better than AIUB-GRACE03S, GGM05S, Tongji-GRACE01S at higher degrees, and the quality of our GOCE-only model HUST-GOCE2018s is also better than that of GO_CONS_GCF_2_TIM_R2 and GO_CONS_GCF_2_SPW_R2. The combination is subsequently implemented by the superposition of GRACE and GOCE full normal equations. During the combination, the optimal weight is determined by the least-squares combined adjustment method with parametric covariance approach (LS-PCA) and the spectral combination method, respectively. The comparison result demonstrates that LS-PCA is more proper for the combination. As a result, the final HUST-GOGRA2018s model is developed. Validated by external gravity field models, the results demonstrate that the HUST-GOGRA2018s is dominated by GRACE data for the spherical harmonic coefficients lower than degree 60 and GOCE data for the spherical harmonic coefficients higher than degree 150, and its performance is better than that of GOCO01S.
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Bangladesh has experienced multiple freshwater issues including salinization from monsoonal floods and groundwater over-pumping that induces severe land subsidence. Therefore, using satellite observations to virtually build a monitoring network becomes an efficient and innovative means. We focus on the Sylhet Mymensingh haor area that has the highest annual precipitation and the largest inundation area in northeastern Bangladesh. The modified normalized difference water index is first used to extract water area from MODIS and Landsat-5/-7/-8 optical imageries. A weekly flood chance model is then created from a sequence of images to recover water extent from the cloud-covered images. Using MODIS images for water identification achieves an overall accuracy of 84% in rainy season and 41% in dry season as validated with Sentinel-1A radar images. This model can be further used to refine the Shuttle Radar Topography Mission digital elevation model (DEM). As compared with ICESat laser altimetry, the root-mean-square of the height difference is improved from 1.65 m to 1.16 m after DEM modification. By combining the recovered water area and the refined DEM, surface water volume (WV) is quantified. A comparison with the Gravity Recovery And Climate Experiment (GRACE) gravimetry retrieved equivalent water heights (EWHs) in 2002 - 2015 is conducted, where the correlation coefficient and root-mean-square of the EWH difference are 91.7% and 0.09 m, respectively.