Haiping Ma

Significant differences exist in the ground response of saturated and unsaturated deposits to earthquakes, but few relevant studies exist on strong ground motion in saturated loess. We have selected three loess sites that are highly affected by earthquake‐induced liquefaction hazards, analyzed the nonlinear characteristics of the ground motion response of saturated loess sites through extensive field tests, laboratory experiments, and analytical ground motion response calculations, and compared them with the unsaturated site conditions to reveal the special strong ground motion response characteristics of saturated loess sites. The results show that the PGA of surface ground motion in the Guyuan and Minxian sites is significantly reduced compared with the PGA of input ground motion, and the PGA of surface ground motion in the Tianshui site is slightly increased compared with the PGA of input ground motion. The frequency corresponding to the peak of the Fourier spectrum of the two sites in Guyuan and Minxian decreased significantly, and the frequency corresponding to the peak of the Fourier spectrum of ground motion in the Tianshui site increased slightly. Compared with unsaturated loess, the dynamic stiffness of saturated loess decreases, and the dynamic shear modulus ratio and damping ratio change significantly under high dynamic shear strain. These changes lead to a weakened ground vibration response at the surface, especially at high PGA inputs, highlighting the key differences in ground motions between saturated and unsaturated loess sites. In addition, the dominant frequency of surface motion is significantly lower at saturated loess sites, increasing the risk of site damage during earthquakes. These results emphasize the complex interplay between loess saturation and the dynamic response to earthquakes and demonstrate the need for targeted geotechnical assessment and design in earthquake‐prone areas of the Loess Plateau.

Loess has poor engineering properties, including wet subsidence and dynamic fragility, and the dynamic stability of the loess subgrades can be improved by compacted modified loess mixing industrial wastes such as fly ash and lignin. However, the performance of the modified loess under complex environmental conditions, including dry and wet cycles, as well as freeze-thaw cycles, remains unclear. In this study, the dynamic and structural characteristics of modified loess mixing fly ash and lignin under the coupling effect of dry-wet/freeze-thaw cycles were investigated through laboratory tests, including dry-wet–freeze/thaw cycle tests, dynamic triaxial tests, and scanning electron microscope tests. The cumulative plastic deformation characteristics of the improved loess under different dry-wet cycles and freeze-thaw cycles were analyzed. Combined with the scanning electron microscope test results, the attenuation mechanism of the strength of the improved loess under dry-wet/freeze-thaw coupling was analyzed. The results show that the dry-wet/freeze-thaw cycles have a significant effect on the dynamic deformation of the improved loess. With the increase in dry-wet/freeze-thaw cycles, the cumulative plastic deformation of the improved loess increases logarithmically with the rise in vibration times. With the increase in the number of dry-wet/freeze-thaw cycles, the improved loess becomes loose. The micro-cracks formed in the modified loess due to the connection and directional arrangement of the pores, and become wider and wider with the increase in dry-wet/freeze-thaw cycles. The apparent porosity, average porous diameter, and pore fractal dimension of the improved loess increase, while the probability entropy decreases. Compared with freeze-thaw cycles, dry-wet cycles had a greater effect on the microstructure of the improved loess, which made the deterioration of the dynamic stability of the improved loess more obvious.

To clarify the relationship between GNSS (Global Navigation Satellite System) crustal deformation and the preparation of strong earthquakes in the western Chinese Mainland, a series of methods including velocity field, strain rate field, and strain parameter time series are adopted based on the GNSS observation data from the CMONOC (Crustal Movement Observation Network of China). The abnormal characteristics of crustal deformation movement before the M ≥ 5 earthquakes in the area since 1999 are studied. The corresponding variety of the crustal movement before earthquakes is discussed in combination with the seismogenic structure of earthquakes. The abnormalities of the GNSS velocity field, strain field, and strain parameter time series before earthquakes with different magnitudes are proposed. Finally, the anomaly criterion before medium-strong-large earthquakes in the western Chinese Mainland is proposed according to the GNSS and its combination with the characteristics of geological structure. The results can provide a basis for the forecasting of the trend and location of medium-strong-large earthquakes in the western Chinese Mainland.

A M_S6.2 earthquake struck Jishishan Country in Gansu Province on 18 December 2023. The earthquake occurred on the eastern branch of the Northern Lajishan fault (NLJSF) which has never experienced such a violent seismic event in recorded history. In this study, multiple periods of GNSS campaign observation data and continuous observation data are collected to reveal the crustal deformation evolution characteristics of the eastern branch of the NLJSF before the earthquake, focusing on the dynamic behavior of the causative fault, regional strain parameter features, and the variation of GNSS baselines across the NLJSF. Velocity field data show that the differential movement on both sides of the fault zone before the earthquake accumulated energy for the occurrence of the earthquake. Cross-fault profile analyses indicate a noticeable weakening of horizontal shortening on both fault flanks, suggesting a nearing fault lockup. The baseline time series and strain parameter time series results both show that the study area is mainly NEE-trending compression deformation. In addition, before the earthquake, multiple baseline results and strain parameter time series deviated from the linear trend and gradually flattened, indicating a strain accumulation in the study area. The overall crustal deformation evolution shows a relatively high earthquake risk before the Lajishan earthquake.

To study the characteristics of the present crustal movement in the Tibetan Plateau and explore its relationship between strong earthquakes with magnitudes of 8 and above, the velocity field size was analyzed based on the global position system (GPS) campaign observations and the time series of site north-ward displacement and long baseline were discussed using the GPS continuous observations. The results show that the velocity field size in the Tibetan Plateau decreases from southwest to north, northeast, and southeast, and the value of the velocity in the west is significantly greater than that in the east in the same dimension. The maximum value is located in the southwest and the minimum value is located in the east. The Wenchuan earthquake is located in the mutation region, where the rate and the direction of the crustal movement are quite different. The crustal deformation extent is large in the region close to the seismic source before the earthquake, reflecting that the regional stress accumulation is fast and its required time is relatively short. However, the crustal deformation extent is relatively small in the region away from the seismic source before the earthquake, reflecting that the regional stress accumulation is slow and its required time is relatively long. The N-ward movement became significantly strong after NepalM_S8.1 earthquake; the occurrence of this earthquake may have caused the unlocking of large-scale faults near the seismic source, which further intensified the NE-ward subduction and collision of the Indian Plate. The compression of the Indian Plate to the Tibetan Plateau slowed down after the 2008 WenchuanM_S8.0 earthquake, and increased significantly after 2015, which boosted strain accumulation in the Tibetan Plateau, and attention needs to be paid continuously to strong earthquake risk in this region.

To discuss the crustal movement characteristics in the northeastern region of the Qinghai-Tibet Plateau, the crustal deformation state and the annual movement rate of the GPS continuous baseline time series between the station in the stable blocks and the stations inter the Qinghai-Tibet Plateau are calculated based on the GPS data observed by the continuous stations in the Qinghai-Tibet Plateau and surrounding areas since 1999. The results demonstrate that the compression and shortening movements between the Qilianshan block, the Qaidam block, the Bayan Har block and the two rigid blocks, Alxa block and the Ordos block, are relatively strong. However, the extension or compression rate between the two rigid blocks and the Qilianshan seismic belt, the Longzhong basin tectonic region and the South China block is quite small. It is indicated that the compressed or extended movement between the suture zone of the two blocks and the northeast edge of the Qinghai-Tibet Plateau is small. It is illustrated that certain strain energy may accumulate in the region and have a relatively high potential for strong earthquakes.

Based on the GPS data in the northeastern edge of Qinghai-Tibet Plateau, the velocity field and the extension change rate of baseline between stations are analyzed. The crustal movement characteristics and their dynamic changes in this region are discussed. The result indicates that the baseline in the regions demonstrates an overall characteristic of shrinking in the NE direction and extending in the NW direction; The extrusion of Bayan Har Block in SE direction since 2009 has amplified obviously, which leads to the enhancement of correlated movement in SE direction in the NW side of Longmenshan Fault. In addition, Tianzhu empty regions and the area near the epicenter of Menyuan Ms6.4 earthquake in 2016 are right in the boundary of this differentiated movement. Recently, a certain amount of strain energy might have been accumulated in the nearby area of the northern margin of Alkin Fault Zone, west boundary of Erdos Block and west to105°N on Northern Xiqinling Fault. The seismic risk in the west and east section of the intersecting area between Bayan Har Block and Qaidam Block is high. The west section of Northern Xiqinling Fault belongs to the inner ring of clockwise vortex and possesses medium-strong seismic risk.