Extraction and Discrimination of MBT Anomalies Possibly Associated with the Mw 7.3 Maduo (Qinghai, China) Earthquake on 21 May 2021

Earthquakes are one of the most threatening natural disasters to human beings, and pre- and post-earthquake microwave brightness temperature (MBT) anomalies have attracted increasing attention from geosciences as well as remote sensing communities. However, there is still a lack of systematic description about how to extract and then discriminate the authenticity of seismic MBT anomalies. In this research, the first strong earthquake occurring near the northern edge of eastern Bayan Har block in nearly 20 years, the recent Mw 7.3 Maduo earthquake in Qinghai province, China on 21 May 2021, was selected as a case study. Based on the monthly mean background of MBT, the spatiotemporal features of MBT residuals with 10.65 GHz before and after the earthquake was firstly revealed. Referring to the spatial patterns and abnormal amplitudes of the results, four typical types of evident MBT positive residuals were obtained, and the time series of intensity features of each category was also quantitatively analyzed. Then, as the most influential factor on surface microwave radiation, air temperature, soil moisture and precipitation were analyzed to discriminate their contributions to these residuals. The fourth one, which occurred north to the epicenter after the earthquake, was finally confirmed to be caused by soil moisture reduction and thus ruled out as being related to seismicity. Therefore, the three retained typical MBT residuals with 10.65 GHz could be identified as possible anomalies associated with the Maduo earthquake, and were further analyzed collaboratively with some other reported abnormal phenomena related to the seismogenic process. Furthermore, through time series analysis, the MBT positive residuals inside the Bayan Har block were found to be more significant than that outside, and the abnormal behaviors of MBT residuals in the elevation range of 4000–5000 m reflected the shielding effect on microwave radiation from thawing permafrost on the plateau in March and April, 2021. This research provides a detailed technique to extract and discriminate the seismic MBT anomaly, and the revealed results reflect well the joint effect of seismic activity and regional coversphere environment on satellite-observed MBT.

Velocity Structure of the Northeastern End of the Bayan Har Block, China, and the Seismogenic Environment of the Jiuzhaigou and Songpan-Pingwu Earthquakes: Inferences from Double-Difference Tomography

ABSTRACT The 2017 Mw 6.5 Jiuzhaigou mainshock hit the northeastern end of the Bayan Har block, which has experienced many historical earthquakes, including the 1976 M 7.2 Songpan-Pingwu earthquake swarm. We used the double-difference tomography method to perform a joint inversion of the seismic source and P-wave velocity (VP) structure of the Jiuzhaigou-Songpan-Pingwu region. The results show significant lateral heterogeneity in the VP in the mid-upper crust. The velocity structure in the shallow crust correlates well with the surface geology. The Jiuzhaigou mainshock and Songpan-Pingwu earthquake swarm both occurred at the boundary between high- and low-VP anomalies. The Songpan-Pingwu earthquake swarm may be related to the eastward flow of low-viscosity material in the mid-lower crust of the Tibetan plateau. Low-viscosity material intrudes into the bedrock when it encounters the rigid Motianling massif, resulting in surface uplift and thrust earthquakes. By contrast, the Jiuzhaigou earthquake is associated with strain energy accumulating at the boundary between high- and low-VP anomalies related to the different movement rates of the low-VP material in the mid-lower crust and the high-VP body in the mid-upper crust. In this case, the high-VP body ruptures with a strike-slip sense to the southeast.

Digitalized Earth's most severe sea-level regression and extinction

End-Permian mass extinction is the largest bio-crises in the past 542 million years in Earth's history. Despite half a century of study, what caused the catastrophe remains equivocal. Fossil collections in the study area of Bayan Har, NW China, suggest a continuous Permian sequence, whereas most mid-to-upper Permian strata were missing. By correlating the Permian sequence reconstructed from reworked carbonate clasts with the measured Permian section, we corroborate a sea-level fall of at least 354 m caused by plume-induced uplift, resulted in the erosion of the last 15-Myr Permian carbonate strata, from Uppermost Permian to the fusulinid zone. The marine regression and resultant erosion occurred not only in China but also in Canadian Arctica[1], Oman[2], Canadian Rockies[3], Norway[3], North America[3] all over the world. New sections and digitalized sea-level regression demonstrate that the period of extinction falls within the hiatus, a break in deposition between the uppermost Permian carbonate strata and the clasts reworked from Permian platforms, representing a duration of sea-level drop 354 m. Carbonate clasts, Siberian Traps volcanism, global warming, anoxia, and ocean acidification are all post-extinction geological events. Why did the extinction occur during the falling stage? We will never know because we can't study a hiatus unrepresented by strata unless we associate the extinction with the sea-level drop.

Role of rigid block in tectonic transformation zone at east Tibet

<p>In recent decades, plateau margins have attracted attention because the understanding of their dynamics and history provides insights into the modes of crustal deformation responsible for the plateau structure and morphology and more widely into the deformation of continental lithosphere. The slip transformation and strain partitioning mechanism at the eastern termination of the Kunlun fault system remain unclear. Geophysics investigations revealed the Ruoergai Basin as a rigid block; however, insufficient information is available on the role of this block in tectonic transformation zone at east Tibet. We employed the finite element method in our simulations to delimitate the presence of the Ruoergai block and determine how it affects the surrounding area. We found that the Ruoergai block moves independently to the east or northeast, and its motion differs from that of the Bayan Har block in the eastward escape process of this last-named block. The formation and behavior of Awancang fault and Longriba fault seems to impact by the Ruoergai block. The influence of the Ruoergai block in the east margin should not be ignored. The Awancang fault and Ruoergai block absorbed the north vector velocity of the Bayan Har block, after which the Bayan Har block started moving southeast. The strain partitioning at the eastern margin of the Tibet Plateau is progressively complete[A1]  from the Awancang fault, Ruoergai block, and Longriba fault area to the Longmenshan block. The presence of the Ruoergai block could decrease the strike-slip rate of the Maqin–Maqu section of the Kunlun fault. Given its influence in the region, the Ruoergai block should be incorporated in future studies on regional deformation and in deformation and tectonic transformation models. Then we compared the deformation and tectonic transformation models in the northern margin of the Tibet Plateau. Proposed a rigid block compression pattern unite the tectonic transformation and deformation issue, further explain most of the fault behaviors in the northern margin and eastern margin of Tibet.</p><p> </p>

Crustal Density Structure of the Jiuzhaigou Ms7.0 Earthquake Area Revealed by the Barkam–Jiuzhaigou–Wuqi Gravity Profile

The Barkam–Jiuzhaigou–Wuqi gravity profile extends across the Jiuzhaigou Ms7.0 earthquake (in 2017) zone and passes through several historical big earthquakes’ zones. We have obtained Bouguer gravity anomalies along the profile composed of 365 gravity observation stations with Global Positioning System (GPS) coordinates, analyzed the observed data and inverted subsurface density structure. The results show that the Moho depth has a big lateral variation from southwest to northeast, which shallows from 57 km to 43 km with maximum variation up to 14 km within 800 km. The most acute depth change of the Moho is in the boundary region between the Bayan Har block and West Qinling–Qilian block. According to our analysis, it is related to the eastward movement of the Bayan Har block. There are three main pieces of evidence that support it: (1) Density is higher in the east of the Bayan Har block and smaller in the west, which is the same as seismic activity; (2) Two thin low-density layers exist in the upper and middle crust of the Bayan Har block, which may promote inter-layer slip and the Jiuzhaigou Ms7.0 earthquake occurred in the boundary area of the two low-density layers, where the crustal density and Moho surface fluctuate sharply; (3) the GPS velocity field in the southwestern part gravity profile is significantly larger than that of the northeastern part, which is consistent with the density structure. Our studies also suggest that the large undulation of the Moho prevents the movement of the Bayan Har block, and strain is prone to accumulate here. The dynamic background analysis of the crust in this area indicates that the Moho surface uplifts in the West Qinling–Qilian block, which decelerates the eastern migration of material on the Qinghai–Tibet Plateau, and leads to the weak tectonic activity of the north part of the Bayan Har block.