Intrinsic and scattering attenuations of the Sichuan-Yunnan region in China from S coda waves

Seismic attenuation is a fundamental property of the Earth's media.Attenuation structure for the complicated geological structures with strong seismicity in the Sichuan-Yunnan region is poorly studied.In this study,we collected 108,399 waveforms of 11,517 local small earthquakes with magnitudes between 1.5 and 3.5 from January 2014 to September 2021 in the Sichuan-Yunnan region and its adjacent areas.We employed an envelope inversion technique for separating the intrinsic and scattering attenuations of the S coda wave,and obtained the intrinsic and scattering attenuation structures for frequencies between 0.25 and 8.00 Hz.The attenuation structures correlate well with the geological units,and some major faults mark the attenuation variations where historic large earthquakes have occurred.The regional average attenuation shows a negative frequency dependence.The average scattering attenuation has a faster descending rate than the average intrinsic attenuation,and is dominant at low frequencies,while at high frequencies the average intrinsic attenuation is stronger.The lateral variation in the intrinsic attenuation is consistent with the variation in heat flow,the scattering attenuation may be related to the scatter distribution and size.The total attenuation is consistent with the previous studies in this region,and the separate intrinsic and scattering attenuation may be useful in understanding regional tectonics and important in earthquake prevention and disaster reduction.

Study on Integrated Recurrence Behaviors of Strong Earthquakes Along Entire Active Fault Zones in the Sichuan-Yunnan Region, China

Based on historical earthquake data, we use statistical methods to study integrated recurrence behaviors of strong earthquakes along 7 selected active fault zones in the Sichuan-Yunnan region. The results show that recurrences of strong earthquakes in the 7 fault zones display near-random, random and clustering behaviors. The recurrence processes are never quasi-periodic, and are neither strength-time nor time-strength dependent. The more independent segments for strong earthquake rupturing a fault zone has, the more complicated the corresponding recurrence process is. And relatively active periods and quiescent periods for earthquake activity occur alternatively. Within the active periods, the distribution of recurrence time intervals between earthquakes has relatively large discretion, and can be modelled well by a Weibull distribution. The time distribution of the quiescent periods has relatively small discretion, and can be approximately described by some distributions as the normal. Both the durations of the active periods and the numbers of strong earthquakes within the active periods vary obviously cycle by cycle, leading to the relatively active periods having never repeated quasi-periodically. Therefore, the probabilistic assessment for middle- and long-term seismic hazard for entireties of active fault zones based on data of historical strong earthquakes on the fault zones still faces difficulty.

The Evolutionary Characteristics of Seismic Activity in the Sichuan-Yunnan Region,China

Based on the earthquake activity characteristics of the diamond block in the Sichuan- Yunnan region and by using the method of the meso-scope damage dynamics and damage evolution, we studied the damage evolution process for moderately strong earthquakes along two seismic belts. The original combination patterns of all the units which illuminate the changes from stable state to destroyed state are given. All these patterns can direct the earthquake prediction practice in this region.

The Two-dimensional Time Coordinate System and Time Prediction Research of M≥6.7 Strong Earthquakes in the Sichuan-Yunnan Region

Since the 20 thcentury,the time intervals of M ≥6.7 strong earthquakes in the SichuanYunnan region show obvious regularity.Using the years of the strong events,a twodimensional time coordinate system is generated,based on which,the time prediction model is constructed for strong earthquakes in the Sichuan-Yunnan region.Prediction analysis shows that there is risk of generating four earthquakes with M ≥ 6.7 in the Sichuan-Yunnan region in the future 16 years,and there are strong signals for M ≥6.7earthquakes for periods 2012-2021 and 2025-2029.The strong earthquakes may occur around 2014-2015,2019 and 2027.

Determination of the degree 120 time-variable gravity field in the Sichuan-Yunnan region using Slepian functions and terrestrial measurements

The terrestrial time-variable gravity measurements are characterized by a high signal-to-noise ratio and sensitivity to the sources of mass change in the Earth's crust.These gravity data have many applications,such as surface deformation,groundwater storage changes,and mass migration before and after earthquakes.Based on repeated terrestrial gravity measurements at 198 gravity stations in the Sichuan-Yunnan region(SYR)from 2015 to 2017,we determine a time series of degree 120 gravity fields using the localized spherical harmonic(Slepian)basis functions.Our results show that adopting the first 6 Slepian basis functions is sufficient for effective localized Slepian modeling in the SYR.The differences between two gravity campaigns at the same time of year show an obvious correlation with tectonic features.The degree 120 timevariable gravity models presented in this paper will benefit the study of the regional mass migration inside the crust of the SYR and supplement the existing geophysical models for the China Seismic Experimental Site.

Analysis of Ground Motion Attenuation Characterization for Moderate Earthquakes in the Sichuan-Yunnan Region

The ground motion attenuation models for PGA,PGV and response spectrum at rock sites and soils sites are derived separately from the digital strong motion records of moderate earthquakes in the Sichuan-Yunnan region after 2007. By comparison with Yu Yanxiangs attenuation model for rock sites in western China,reliability of the model is verified for moderate earthquake. According to the distribution of strong motion data against magnitude and epicentral distance,the applicability and reliability of the results in this paper are discussed.

Numerical simulation of the influence of lower-crustal flow on the deformation of the Sichuan-Yunnan Region

On the basis of distribution of active fault and regional rheological structure, a three-dimensional finite element model of Sichuan-Yunnan region, China, is constructed to simulate contemporary crustal motion and stress distribution and discuss the dynamic mechanism of crustal motion and deformation in the Sichuan-Yunnan region. Linear Maxwell visco-elastic model is applied, which includes the active fault zones, the elastic upper crust and viscous lower crust and upper mantle. Four different models with different boundary conditions and deep structure are calculated. Some conclusions are drawn through comparison. Firstly, the crustal rotation about the eastern syntaxis of the Himalaya in the Sicuan-Yunnan region may be controlled by the special dynamic boundary condition. The drag force of the lower-crust on the upper crust is not negligible. At the same time, the main active fault zones play an important role in the contemporary crustal motion and deformation in Sichuan-Yunnan region.

Study on Postseismic Impact of Wenchuan Earthquake on the Sichuan-Yunnan Region Based on Three-Dimensional Viscoelastic Finite Element Method

Based on the lateral segmentation and vertical stratification characteristics of the crustal medium in Sichuan-Yunnan region,and the asymmetry of the static dislocations on the coseismic fault plane of the Wenchuan M_S8.0 earthquake,we built a three-dimensional viscoelastic finite element model of the crust in the Sichuan-Yunnan region.The postseismic impact of the Wenchuan M_S8.0 earthquake on the Sichuan-Yunnan region was studied.The results show that:①The far-field horizontal deformation caused by the viscoelastic relaxation of the medium in the 10 years after the earthquake is about 0-20 mm within the Sichuan-Yunnan diamond-shaped block,which has a greater influence on north side and smaller on south side.②In the 10 years after the earthquake,the far-field vertical deformation caused by the viscoelastic relaxation effect of the medium is small,and it shows an increase of about 0-4 mm in most areas of the Sichuan-Yunnan diamond-shaped block.③The Xianshuihe fault and the eastern segment of the East Kunlun fault,which are close to the seismogenic fault,show a high gradient on deformation fields after the earthquake.④In order to compare with the strong earthquake activity in the SichuanYunnan region after the Wenchuan earthquake,the horizontal stress state and the Coulomb failure stress change of the active block boundary are also calculated.From the spatial distribution of the coseismic and postseismic displacement field,the fault activity characteristics reflected by the stress state and the stress loading of the fault layer reflected by the Coulomb failure stress change,there is a certain correlation with the spatial distribution of strong earthquake activity in this region.

Study on the Relationship Between Recent Crustal Deformation and the Temporal-Spatial Distribution of Strong Earthquakes (M_s≥6.0) in the Sichuan-Yunnan Region

Analysis of deformation data measured across the faults, regional vertical deformation data and GPS measurements in the Sichuan-Yunnan region made since the 1980s permitted us to conclude that the crustal deformation in the region during this period of time was relatively weak and caused the occurrence of earthquakes (M S≥6.0), which were not distributed along the major boundary active faults in the region after the 1981 Dawu M S 6.9 earthquake and that the seismic activity is characterized by quasi-clockwise migration. Thus, it follows that earthquake prediction research should be focused on the central part of the Sichuan-Yunnan region in the coming years. Finally, a concept of temporal division of the region into active blocks is suggested and the preliminary result of the division is given in the paper.

Tidal Response Characteristics of the Well Water Level in the Sichuan-Yunnan Region

In this paper,the long time series data of the well water-level data of 12 wells in the Sichuan and Yunnan area is analyzed by the Baytap-G tidal analysis software,and well water level tidal response characteristic parameters( amplitude ratio and phase change)are extracted. We analyzed the features of the shape and stage change,and characteristic parameters of the tidal response of well water level before and after the earthquakes,which can provide a new method and approach to analyzing the response relationships between well water level and earth tide and barometric pressure. The results show that Luguhu Well and 9 other wells are affected by earth tides,and their well water level amplitude ratios and phases are relatively stable; the Nanxi Well and Dayao Well water level changes are affected by the barometric pressure combined with tide force,and their well water level amplitude ratios and phases are more discrete. The water level amplitude ratios and phases of Jiangyou Well,Luguhu Well and Dongchuan Well are significant to large earthquakes,and the relationship between seismic energy density and water level amplitude ratios and phases of M_2 wave of the three wells are presented.

Present status and prospective on the seismic hazard studies of major seismic gaps in the Sichuan-Yunnan region

Several seismic gaps have been identified along the Xianshuihe-Xiaojiang fault zone and the Longmenshan fault zone in the Sichuan-Yunnan region, including the Daofu-Shimian gap on the Xianshuihe Fault, the Anninghe-Zemuhe gap, parts of the Xiaojiang Fault, and the Dayi gap between the Wenchuan and Lushan earthquakes. The seismic hazards of the Daofu-Shimian and Dayi gaps have aroused great concern and caused considerable debate. This paper reviews the recent geophysical observations and multidisciplinary studies conducted by domestic and international researchers in these areas, focusing on seismogenic structures, seismic activity, fault locking, stress fields, as well as earthquake triggering, and further analyzes the current controversial viewpoints about the seismic hazards in these regions. Based on the comprehensive analysis, we argue that the kinematic status along the Daofu-Shimian segment may vary significantly along the fault strike. Due to limitations in data and models, there is no consensus on the seismogenic potential of this area. Conversely, the Dayi seismic gap presents a high seismic potential. Furthermore, we delve into the scientific problems associated with major seismic gaps in the Sichuan-Yunnan region and make prospects for the strategy of future work. We propose a research framework that combines dense array and distributed acoustic sensing, artificial intelligence and big data technologies, joint inversion of multiple geophysical fields, as well as numerical simulation techniques for comprehensive seismic hazard analysis in seismic gaps.

Magnitude and distance distribution of strong aftershocks in Sichuan-Yunnan region

研究了川滇地区1966年以来12次主震震级Ms≥6.5的地震序列中，主震与强余震（本文定义为所有Ms≥5的余震）震级差分布特征和强余震与主震距离的分布特征。结果表明，强余震与主震震级差服从截断的指数分布，据此推导出了强余震与主震震级差的概率密度函数；强余震距离分布的优势范围是距主震10-39km，且强余震与主震震中的距离服从正态分布。

GPS-constrained inversion of present-day slip rates along major faults of the Sichuan-Yunnan region, China

A linked-fault-element model is employed to invert for contemporary slip rates along major active faults in the Sichuan-Yunnan region (96°-108°E, 21°-35°N) using the least squares method. The model is based on known fault geometry, and constrained by a GPS-derived horizontal velocity field. Our results support a model attributing the eastward extrusion of the Tibetan Plateau driven mainly by the north-northeastward indentation of the Indian plate into Tibet and the gravitational collapse of the plateau. Resisted by a relatively stable south China block, materials of the Sichuan-Yunnan region rotate clockwise around the eastern Himalayan tectonic syntaxis. During the process the Garzê-Yushu, Xianshuihe, Anninghe, Zemuhe, Daliangshan, and Xiaojiang faults, the southwest extension of the Xiaojiang fault, and the Daluo-Jinghong and Mae Chan faults constitute the northeast and east boundaries of the eastward extrusion, with their left slip rates being 0.3-14.7, 8.9-17.1, 5.1 ± 2.5, 2.8 ± 2.3, 7.1 ± 2.1, 9.4 ± 1.2, 10.1 ± 2.0, 7.3 ± 2.6, and 4.9 ± 3.0 mm/a respectively. The southwestern boundary consists of a widely distributed dextral transpressional zone other than a single fault. Right slip rates of 4.2 ± 1.3, 4.3 ± 1.1, and 8.5 ± 1.7 mm/a are detected across the Nanhua-Chuxiong-Jianshui, Wuliangshan, and Longling-Lancang faults. Crustal deformation across the Longmenshan fault is weak, with short-ening rates of 1.4 ± 1.0 and 1.6 ± 1.3 mm/a across the Baoxing-Beichuan and Beichuan-Qingchuan segments. Northwest of the Longmenshan fault lies an active deformation zone (the Longriba fault) with 5.1±1.2 mm/a right slip across. Relatively large slip rates are detected across a few faults within the Sichuan-Yunnan block: 4.4±1.3 mm/a left slip and 2.7±1.1 mm/a shortening across the Litang fault, and 2.7±2.3 mm/a right-lateral shearing and 6.7±2.3 mm/a shortening across the Yunongxi fault and its surrounding regions. In conclusion, we find that the Sichuan-Yunnan region is divided into more than a dozen active micro-blocks by a large number of faults with relatively slow slip rates. The eastward extrusion of the Tibetan Plateau is absorbed and adjusted in the region mainly by these faults, other than a small number of large strike-slip faults with fast slip rates.

Recent movement changes of main fault zones in the Sichuan-Yunnan region and their relevance to seismic activity

The Wenchuan earthquake occurred near the "triple junction" linking the Bayan Har block, the South China block, and the Sichuan-Yunnan rhombic block, and its influences on the surrounding blocks and the main fault zones in the Sichuan-Yunnan region, i.e., the block boundary zone, cannot be ignored. In this paper, changes of movement and stress of the fault zones before and after a strong earthquake were simulated based on the GPS repetition survey results recently obtained during 1999–2007, 2009–2011, and 2011–2013 with a two-dimensional finite-element contact model and the "block- loading" method. The results show that, before the Wenchuan earthquake, the movement of the Longmenshan fault zone was very slow and its compressive stress accumulated rapidly; after the Wenchuan earthquake, movements toward the E-SSE direction of the Bayan Har, southwestern Yunnan, and rhombic blocks were enhanced, and the dextral and horizontal compressive speeds and annual accumulative compressive stress of the Longmenshan fault zone increased markedly by factors of 4.5, 2.1, and 2.5, respectively. The southern Xianshuihe, Anninghe, Zemuhe, Daliangshan, and Lijiang-Xiaojinhe fault zones accumulated compressive stress rapidly, forming enhanced compressive stress zones along a NE strike crossing the central part of the Sichuan-Yunnan region. The tensional movement of the Xianshuihe fault zone was enhanced and the slip movement in the central part of the zone was reversed in a short time. The changes are tightly related to the medium-intensity earthquakes that occurred during the same period in this region, revealing that the spatial migration of seismic activity is related to changes of movement of the blocks.

Influence of 2008 Wenchuan earthquake on earthquake occurrence trend of active faults in Sichuan-Yunnan region

The Wenchuan earthquake coseismic deformation field is inferred from the coseismic dislocation data based on a 3-D geometric model of the active faults in Sichuan-Yunnan region. Then the potential dislocation displacement is inverted from the deformation field in the 3-D geometric model. While the faults＇ slip velocities are inverted from GPS and leveling data, which can be used as the long-term slip vector. After the potential dislocation displacements are projected to long-term slip direction, we have got the influence of Wenchuan earthquake on active faults in Sichuan-Yunnan region. The results show that the northwestern segment of Longmenshan fault, the southern segments of Xianshuihe fault, Anninghe fault, Zemuhe fault, northern and southern segments of Daliangshan fault, Mabian fault got earthquake risks advanced of 305, 19, 12, 9.1 and 18, 51 years respectively in the eastern part of Sichuan and Yunnan. The Lijiang-Xiaojinhe fault, Nujiang fault, Longling-Lancang fault, Nantinghe fault and Zhongdian fault also got earthquake risks advanced in the western part of Sichuan-Yunnan region. Whereas the northwestern segment of Xianshuihe fault and Xiaojiang fault got earthquake risks reduced after the Wenchuan earthquake.

Wavelet analysis and interpretation of gravity data in Sichuan-Yunnan region, China

The Bouguer gravity anomaly data of Sichuan-Yunnan region and its vicinity were analyzed with wavelet trans- formation method. In the process, complete orthogonal wavelet function system with good symmetry and higher vanishing moment was selected to decompose the gravity anomaly into two parts. With the power spectral analysis on the decomposed anomalies, we interpreted that the two parts of anomalies represent the density variation in upper and middle crust, and in deep crust and uppermost mantle, respectively. The two parts of anomalies indicate the difference between shallow and deep tectonics. The results of shallow-layer apparent density mapping reveal that: a) the crustal density in Sichuan basin is higher than that in Songpan-Garze orogenic zone; b) the density of Kangdian rhombic block is heterogeneous; c) the boundary faults of Kangdian block are of different density fea- tures, suggesting different tectonic signification. The results of deep-layer apparent density mapping show a similar, but not the same, density distribution pattern as the shallow results, and indicate that the tectonics of shallow and deep crust are different, they may be in a status of incomplete coupling. Our results also show that the earthquakes in this area are controlled not only by the fracture zones but also by the deep density distribution.

Strong Earthquake Activity and Its Relation to Regional Neotectonic Movement in Sichuan-Yunnan Region

Based on analyzing space inhomogeneous image of strong earthquake activity, the image of source rupture and the mechanical property of the source fault in Sichuan-Yunnan region, the relations among the strong earthquake activity, active fault, modern movement status of active blocks and structural background of the deep media have been discussed, and the characteristics of strong earthquake activity and possible mechanism have been also discussed.

Recent tectonic stress field zoning in Sichuan-Yunnan region and its dynamic interest

In this paper, we have carefully determined the stress zones in the Sichuan-Yunnan region with reference to the in-situ stress data of hydraulic fracturing and the inverted fault slip data by using the step-by-step convergence method for stress zoning based on focal mechanism solutions. The results indicate that the tectonic stress field in the Sichuan-Yunnan region is divided into 3 stress zones by 2 approximately parallel NNW-trending stress transition belts. The area between the 2 belts is the Sichuan-Yunnan stress zone where the maximum principal stress σ1 is just in the NNW direction. The eastern boundary of Sichuan-Yunnan stress zone （the eastern stress transition belt） is basically consistent with the eastern boundary of Sichuan-Yunnan rhombic block. The western boundary of Sichuan-Yunnan stress zone （the western stress transition belt） is not totally consistent with the western boundary of Sichuan-Yunnan rhombic block. The northern segment of the western stress transition belt extends basically along the Jinshajiang fault and accords with the western boundary of Sichuan-Yunnan rhombic block, while its southern segment does not extend along the southwestern boundary of the rhombic block, i.e., Honghe fault and converge with the eastern stress transition belt, but stretches continuously in the NNW direction and accords with the Yingpanshan fault. We therefore consider that under the combined influence from the northward motion of India Plate, the southeastward shift of east Qinghai-Xizang Plateau and the strong obstruction of South China block, the tectonic stress field in the Sichuan-Yunnan region might not be totally controlled by the previous tectonic frame and new stress transition belt may have possibly formed.

Determinations of directions of the mean stress field in Sichuan-Yunnan region from a number of focal mechanism solutions

Based on the spatial orientation and slip direction of the fault plane solutions, we present the expression of corresponding mechanical axis tensor in geographic coordinate system, and then put forward a method for calculating average mechanical axis tensor and its eigenvalues, which involves solving the corresponding eigenequation. The method for deducing mean stress field from T, B, and P axes parameters of a number of focal mechanism solutions has been verified by inverting data of mean stress fields in Fuyun region and in Tangshan region with fitting method of slip direction, and both results are consistent. To study regional average stress field, we need to choose a population of focal mechanism solutions of earthquakes in the massifs where there are significant tectonic structures. According to the focal mechanism solutions of 256 moderate-strong earthquakes occurred in 13 seismic zones of Sichuan-Yunnan region, the quantitative analysis results of stress tensor in each seismic zone have been given. The algorithm of such method is simple and convenient, which makes the method for analyzing tectonic stress field with large amount of focal mechanism solution data become quantified.

Does digitalization mitigate regional inequalities?Evidence from China

Regional inequality significantly influences sustainable development and human well-being.In China,there exists pronounced regional disparities in economic and digital advancements;however,scant research delves into the interplay between them.By analyzing the economic development and digitalization gaps at regional and city levels in China,extending the original Cobb-Douglas production function,this study aims to evaluate the impact of digitalization on China's regional inequality using seemingly unrelated regression.The results indicate a greater emphasis on digital inequality compared to economic disparity,with variable coefficients of 0.59 for GDP per capita and 0.92 for the digitalization index over the past four years.However,GDP per capita demonstrates higher spatial concentration than digitalization.Notably,both disparities have shown a gradual reduction in recent years.The southeastern region of the Hu Huanyong Line exhibits superior levels and rates of economic and digital advancement in contrast to the northwestern region.While digitalization propels economic growth,it yields a nuanced impact on achieving balanced regional development,encompassing both positive and negative facets.Our study highlights that the marginal utility of advancing digitalization is more pronounced in less developed regions,but only if the government invests in the digital infrastructure and education in these areas.This study's methodology can be utilized for subsequent research,and our findings hold the potential to the government's regional investment and policy-making.