The construction of the Xiong’an New Area is a national strategy and a long-term plan outlined by the Chinese government. To support the urban planning and development of this area, many scholars have conducted a series of geophysical surveys aimed at understanding the detailed subsurface structure. The Horizontal-to-Vertical Spectral Ratio(HVSR)method, first introduced by Nakamura, has recently gained widespread use for investigating shallow subsurface structures, site response, and microzonation.

In this study, we utilized a large seismic array with an interstation distance ranging from 500 to 1000 meters, deployed across the Xiong’an New Area. The array consisted of over 900 short-period seismographs, covering most of the area. Using ambient-noise recordings, we removed nonrandom transient signals from the waveform data with a short-term-average over long-term-average detector automatic picking algorithm, and applied the Konno-Ohmachi algorithm to smooth the HVSR curves. For each site, we analyzed the amplitude of the peak value of the HVSR curve(A)and the corresponding frequency(f₀). Both parameters were further elaborated through the creation of contour maps using the Kriging interpolation method. Additionally, the peak frequencies from the HVSR curves were used to calculate the sedimentary thickness, based on an average shear-wave velocity and the frequency-depth formula.

The frequency map shows that the peak frequencies range between 0.6 and 1.1Hz, with an overall peak frequency of about 0.7 to 1.0Hz. The lowest frequencies were found predominantly in the vast eastern area of the study region, corresponding to geological features such as the Niubei Slope, Niutuozhen High, and Baxian Sag. According to the frequency-depth formula, a lower peak frequency indicates greater sediment depth. The variation in peak frequencies across stations highlights changes in the bedrock interface, which correspond to fault structures depicted on the geological map. Furthermore, high-amplitude areas were mainly located between the Rongxi fault and Rongdong fault, suggesting an impedance contrast between shallow and deeper layers. Stratigraphic profiles reveal that Quaternary and Tertiary sedimentary layers directly overlie the crystalline basement composed of Proterozoic metamorphic rocks. Combined analysis of peak frequency and amplitude aligns well with the available geological data. Our analysis produced 3D depth images of the Quaternary sedimentary layer interface across the study area, clearly imaging a significant seismic impedance interface at depths of 100-220m. This shallow interface corresponds to the contrast between the Tertiary rocks and the overlying Quaternary sedimentary layers. The sediment thickness progressively increases from east to west across the study area. Interfaces derived from the HVSR profiles display similar characteristics to those on the geological map and are consistent with borehole data and results from the high-density resistivity method. Moreover, we established a power-law relationship correlating the fundamental site resonance frequencies with sedimentary cover thickness obtained from borehole data in the Xiong’an New Area. The undulating characteristics of the sedimentary layers correspond closely to fault locations and geological tectonic units, confirming that faults such as the Rongxi, Rongdong, Niuxi, Niudong, and Xushui-Dacheng faults serve as boundaries for secondary geological tectonic units, influencing the structure of the near-surface sedimentary layers.

We developed a 3D shallow subsurface sedimentary model for the Xiong’an New Area and created contour maps of amplitude(A)and peak frequency(f₀). The results both support and extend previous understandings of the region’s structure. This study demonstrates that the HVSR method, in conjunction with a large seismic array, is a rapid and effective technique for investigating shallow subsurface structures and seismic site responses. The exploration of sedimentary structures and seismic site response characteristics, which are closely related to earthquake hazards, provides a critical foundation for seismic fortification and urban planning in the Xiong’an New Area.

On May 22, 2021, an M_S7.4 earthquake occurred in the Madoi area of Banyan Har block, with a focal depth of about 8km. The seismogenic fault is deduced as the Jiangcuo Fault, a branch of the east Kunlun strike-slip fault. Different with previous strong earthquakes which located at the boundary faults around the Bayan Har block, the Madoi M_S7.4 earthquake occurred inside the block and about 70km away from the boundary fault. Furthermore, there is a contradiction between the small strike-slip component of the seismogenic fault and the large earthquake magnitude. The above phenomena indicate that the Madoi earthquake may have special seismotectonic background and seismogenesis. Strong earthquakes in Tibetan plateau are always closely related to the deep crustal structure and dynamic process. Therefore, it is of great significance to study the crustal structure and the distribution of deep faults in the Madoi area in order to reveal the deep tectonic background and genesis of the Madoi M_S7.4 earthquake. To research the deep seismotectonic environments of the M_S7.4 Madoi earthquake, we reinterpret the deep seismic sounding(DSS)results in Madoi area. The DSS profile reveals fine crustal structure beneath the Madoi area, and divides the crust into 3 crustal layers. From the crustal velocity structure of the Madoi and adjacent area, we found the generation of the Madoi earthquake is closely connected with the deep structure and crustal medium. Through analysis on the velocity structures, we get the following understanding: 1)There is an interface in the upper crust of the Madoi area, which represents the velocity changing from 5.8km/s to 5.6km/s and divides the upper crust into two layers. The upper layer is composed of high velocity structure, indicating a brittle medium environment, while the lower layer consists of low velocity zone and provides the strain accumulation condition for the Madoi earthquake. In addition, the transition between local high velocity zone(HVZ)and the normal crust in the focal area provides an ideal medium environment for earthquake preparation. 2)A wedge-shaped low velocity zone(LVZ)exists in the lower crust south of Madoi, which provides an environment for the movement of weak materials from the SW to NE direction. However, the high-velocity lower crust beneath Madoi area resists the crustal flow and thus transforms the horizontal movement to vertical upwelling, resulting in the stress concentration of the upper crust beneath Madoi area, which may provide dynamic for the preparation of the Madoi M_S7.4 earthquake. 3)The Jiangcuo Fault merges into the East Kunlun Fault in the deep crust, forming a reverse thrust fault structural style dominated by the East Kunlun strike-slip fault. As a branch of the East Kunlun Fault, the strike slip of the Jiangcuo Fault is the adjustment results of strain and movement of the East Kunlun Fault. Moreover, the Jiangcuo Fault and adjacent faults constitute the horsetail-shaped fault zone, combined with the imbricated thrust fault zone profile, reflecting the compressive stress of Modoi area that facilitates the strain concentration. Therefore the occurrence of the Madoi earthquake is related to the left-lateral strike-slip movement of the East Kunlun Fault and the special imbricated thrust fault assemblages. On the other hand, the upwelling of the lower crustal flow and the corresponding sliding of the upper crust may be related with the occurrence of the Madoi earthquake. In conclusion, the Madoi M_S7.4 earthquake is closely related to the ideal medium environment of the upper crust, the lower crustal flow and vertical upwelling beneath Madoi area, as well as the left-lateral strike-slip of the East Kunlun Fault.

In this paper the crustal velocity structure is obtained along Yushu earthquake zone, using wide angle reflection and refraction data. The results reveal strong vertical and lateral heterogeneities in the crust, as well as the basic characteristics of the crustal velocity structure and the tectonics along the seismic sounding profile. Results show that crustal velocity structure is featured with significant regional heterogeneity both in the longitudinal and lateral directions. The crust is of layered structure, and the crystalline basement interface undulates greatly beneath the study area, which is about 8 km in thickness beneath Yushu and gradually thins northwards to 2.5 km beneath the Stake 400.0 km at Wenquan. There are good correlations of the depressions and uplifts on basement interface with different tectonic units. The crust gradually thins towards both south and north direction to a thickness of 62 km from 72 km beneath Nangqian and Yushu. There exist big undulations in 2D velocity contours and the interfaces in the crust between Stakes 200.0～400.0km, and there is presence of an arc-shaped depression in the Moho beneath the Yushu area.

DSS data of Bohai Bay profile was processed in August 2011 and the result obtained in this paper and the results of other profiles,which cross this profile,were interpreted comprehensively in this paper. The DSS data were calculated and interpreted synthetically using 1-D and 2-D processing techniques in order to find out the basic features of 2-D velocity structures,spatial distribution of faults,geological structure of shallow and deep crust in the southwest margin of Bohai Bay and adjacent areas. The result shows that obvious layered structure appears along the profile,and the crustal velocity structures in different regions have obvious heterogeneity in the lateral and vertical directions. The crystalline basement near the Bohai Bay is gradually thinning southwestwards,and beneath the 220km Stake,the depth of G interface is 7.4km. The thickness of the middle layer varies greatly,with the change range up to 4.0km. The crustal depth varies relatively moderately,with a change range of about 2.0km. The Moho deepens gradually from coastal area to the inland along the southwest direction.