Estimation of Terrestrial Water Storage Variations in Sichuan-Yunnan Region from GPS Observations Using Independent Component Analysis

GPS can be used to measure land motions induced by mass loading variations on the Earth’s surface. This paper presents an independent component analysis (ICA)-based inversion method that uses vertical GPS coordinate time series to estimate the change of terrestrial water storage (TWS) in the Sichuan-Yunnan region in China. The ICA method was applied to extract the hydrological deformation signals from the vertical coordinate time series of GPS stations in the Sichuan-Yunnan region from the Crustal Movement Observation Network of China (CMONC). These vertical deformation signals were then inverted to TWS variations. Comparative experiments were conducted based on Gravity Recovery and Climate Experiment (GRACE) data and a hydrological model for validation. The results demonstrate that the TWS changes estimated from GPS(ICA) deformations are highly correlated with the water variations derived from the GRACE data and hydrological model in Sichuan-Yunnan region. The TWS variations are overestimated by the vertical GPS observations the northwestern Sichuan-Yunnan region. The anomalies are likely caused by inaccurate atmospheric loading correction models or residual tropospheric errors in the region with high topographic variability and can be reduced by ICA preprocessing.

Different facets of dry–wet patterns in south-western China over the past 27 000 years

Frequently occurring mega-droughts under current global climate change have attracted broad social attention. A paleoclimatic perspective is needed to increase our understanding of the causes and effects of droughts. South-western (SW) China has been threatened by severe seasonal droughts. Our current knowledge of millennial-scale dry and wet phases in this region is primarily based on the variability of the Indian summer monsoon. However, water availability over land does not always follow patterns of monsoonal precipitation but also depends on water loss from evaporation and transpiration. Here, we reconstructed precipitation intensity, lake hydrological balance and the soil water stress index (SWSI) for the last 27 000 years. Grain size, geochemical and pollen records from Yilong Lake reveal the long-term relationships and inconsistencies of dry–wet patterns in meteorological, hydrological and soil systems in the central Yunnan region, SW China. Our results show that the long-term trends among precipitation, hydrological balance and soil moisture varied through time. The hydrological balance and soil moisture were primarily controlled by temperature-induced evaporation change during periods of low precipitation such as the Last Glacial Maximum and Younger Dryas. During periods of high precipitation (the early to late Holocene), intensified evaporation from the lake surface offset the effects of increased precipitation on the hydrological balance. However, abundant rainfall and the dense vegetation canopy circumvented a soil moisture deficit that might have resulted from rising temperature. In conclusion, the hydrological balance in the central Yunnan region was more sensitive to temperature change while soil moisture could be further regulated by vegetation changes over millennial timescales. Therefore, under future climate warming, the surface water shortage in the central Yunnan region may become even more serious. Our study suggests that reforestation efforts may provide some relief to soil moisture deficits in this region.

Determining Stress State of Source Media with Identified Difference between Groundwater Level during Loading and Unloading Induced by Earth Tides

The groundwater level might be adopted as a useful tool to explore pre-seismic stress change in the earth crust, because it circulates in the deep crust and should be altered by the processes associated with the preparation of earthquakes. This work makes a new attempt that applies the load/unload response ratio (LURR) technique to study the stress state of the source media before the large earthquakes by calculating the ratio between the water levels during the loading and unloading phases. The change of Coulomb failure stress induced by earth tides in the tectonically preferred slip direction on the fault surface of the mainshock is adopted for differentiating the loading and unloading periods. Using this approach, we tested the groundwater level in the wells near the epicenters of some large earthquakes that occurred in the Sichuan-Yunnan region of southwest China. Results show that the LURR time series fluctuated narrowly around 1.0 for many years and reached anomalously high peaks 2~8 months prior to the mainshocks. For the earthquakes with multiple observation wells, the magnitude of the maximum values decreases with the distance from the epicenter. The underlying physics of these changes should be caused by the pre-seismic dilatancy. The corresponding volume variations in the crust could be observed in the geodetic time series in the same neighborhoods and during the same period.

Seismic Reflection Analysis of AETA Electromagnetic Signals

Acoustic and electromagnetics to artificial intelligence (AETA) is a system used to predict seismic events through monitoring of electromagnetic and geoacoustic signals. It is widely deployed in the Sichuan–Yunnan region (22° N–34° N, 98° E–107° E) of China. Generally, the electromagnetic signals of AETA stations near the epicenter have abnormal disturbances before an earthquake. When a significant decrease or increase in the signal is observed, it is difficult to quantify this change using only visual observation and confirm that it is related to an upcoming large earthquake. Considering that the AETA data comprise a typical time series, current work has analyzed the anomalism of AETA electromagnetic signals using the long short-term memory (LSTM) autoencoder method to prove that the electromagnetic anomaly of the AETA station can be regarded as an earthquake precursor. The results show that there are 2–4% anomalous points and some outliers exceeding 0.7 (after normalization) in the AETA stations within 200 km of the epicenter of the Jiuzaigou earthquake (M. 7.0) and the Yibin earthquake (M. 6.0) half a month before the earthquakes. Therefore, the AETA electromagnetic disturbance signal can be used as an earthquake precursor and for further earthquake prediction.

Pure sediment-derived granites in a subduction zone

The Earth is unique in the Solar System due to significant volumes of granite in the lithosphere. However, the origins of granites are still highly debated, especially sediment-derived granites, which are often treated as a geochemical end-member of the continental crust. In the Yunnan region of South China, we identify the occurrence of pure sediment-derived granite in a subduction system. The suite of strongly peraluminous granite reported herein is interpreted to represent pure metasedimentary melts based on their whole-rock elemental and Sr-Nd-B and zircon Hf-O isotopic compositions. These Late Permian−Early Triassic (ca. 254−248 Ma) granites are characterized by radiogenically enriched Sr, Nd, and Hf isotopic signatures. They show δ11B and δ18O signatures akin to those of continental shales. Geochemical modeling indicates no contributions from the mantle that can be detected. Considering the regional tectonic evolution, these granites are suggested to be formed in a subduction zone by decompression melting of rapidly exhumed back-arc sediments. We posit that decompression melting was triggered by widespread extension and thinning of the crust prompted by rollback of the subducting oceanic crust. These granites thus provide evidence that granite formation in subduction zones does not necessarily contribute to crustal growth. These subduction-related pure sediment-derived granites have different elemental ratios and contents (e.g., Al2O3/TiO2 and Yb) from the Himalayan leucogranites. Considering their source compositions (e.g., pelitic rocks), which are similar to those of the Himalayan leucogranites, these differences are likely due to their higher formation temperature and lower pressure despite a great similarity in isotopic compositions. Identification of pure sediment-derived, strongly peraluminous granites (SPGs) in subduction systems provides an important geodynamic mechanism for crustal anatexis, which can both geochemically and tectonically complement their collisional counterparts identified in the Himalayas.

Different facets of dryness/wetness pattern in southwestern China over the past 27,000 years

Frequently happened meta-droughts have arisen broad social attention under current global climate change. A paleoclimatic perspective is expected to gain our understanding on the causes and manifestation more comprehensively. Southwestern China has been threatened by severe seasonal droughts. Our current knowledge of millennial-scale drying/wetting processes in this region is primarily based on the variability of the Indian Summer Monsoon. However, water availability over land does not always follow the monsoonal precipitation but also depends on water loss from evaporation and transpiration. Here, we reconstructed precipitation intensity, lake hydrological balance and soil water stress index (SWSI) covering the last 27,000 yr, based on grain size, geochemical and pollen records from Yilong Lake, to discuss the long-term nexus and discrepancies of dryness/wetness patterns in meteorological, hydrological and soil systems in central Yunnan region, SW China. Our results show that the long-term change trajectories among precipitation, hydrological balance and soil moisture were not completely consistent. During periods of low precipitation, hydrological balance and soil moisture were primarily controlled by temperature-induced evaporation change. This caused opposite status of precipitation with hydrological balance and soil moisture during the Last Glacial Maximum and Younger Dryas. During periods of high precipitation – the early to late Holocene, intensified evaporation from the lake surface offset the effects of increased precipitation on hydrological balance. But meanwhile, abundant rainfall and dense vegetation canopy avoided soil moisture deficit that might result from rising temperature. To sum up, hydrological balance in central Yunnan region was more vulnerable to temperature change while soil moisture could be further regulated by vegetation changes on millennial scale. As such, under future climate warming, surface water shortage in central Yunnan region can be even more serious. But for soil systems, efforts to reforestation may bring some relief to soil moisture deficit in this region.

Determining stress state of source media with identified difference between groundwater level during loading and unloading induced by earth tide

The groundwater might be adopted as a useful tool to explore pre-seismic stress change in the crust, because it circulates in the deep crust and should be altered by the processes associated with the preparation of earthquakes. This work makes a new attempt that applies the load/unload response ratio (LURR) technique to study stress state of source media by calculating the ratio between water level during the loading and unloading phases. The change of Coulomb failure stress induced by earth tide in the tectonically preferred slip direction on the fault surface of the main shock is adopted for differentiating the loading and unloading periods. Using this approach, we test the groundwater level in the wells near the epicenters of some large earthquakes occurred in the Sichuan-Yunnan region of southwest China. Results show that the LURR time series fluctuate narrowly around 1.0 for many years, and climb to the maximum peaks prior to the main shocks. The magnitude of the pre-seismic peaks decreases with the distance from the epicenters. We hypothesized that the underlying physics of these changes might be explained by the pre-seismic dilatancy. The corresponding volume variations could be observed in the geodetic time series in the same neighborhoods.

Probabilistic Seismic Hazard Analysis for Fault Dislocation Magnitude Induced by Strong Earthquakes: A Case Study of the Sichuan-Yunnan Region

Seismic risk in this region is high in Sichuan-Yunnan region of western China, and active faults are well developed in this region. Tunneling in this region would inevitably come across with active faults, and the stability of the tunnel would face serious threats due to the dislocation of an active fault. The magnitude of the fault dislocation is an important design parameter in the design work of tunnels across an active fault. However, traditionally this parameter is estimated as a deterministic value that is often overestimated. In this paper, the probabilistic analysis method was introduced into the estimation of the dislocation magnitude for a tunnel in Sichuan-Yunnan region. To demonstrate the proposed approach, the Xianglushan tunnel in the Central-Yunnan-Water-Transmission Project, which crosses the Longpan–Qiaohou active fault, was taken as an example case. The seismicity parameters in Sichuan-Yunnan region and the dislocation probability caused by earthquakes are considered. The fault dislocation magnitude that may occur during the service life of the tunnel can be estimated as a probability function, and parameter analysis was conducted. Thus, anti-dislocation design work of the tunnel may be conducted based on this reasonable estimation.