Earthquake Early Warning(EEW)is the rapid acquisition of earthquake epicenter, magnitude, and occurrence time after a destructive earthquake has started to issue alerts to the public before the arrival of transverse waves and long-period surface waves. Magnitude estimation plays a significant role in EEW algorithm research, serving as a fundamental component for early warning, post-earthquake disaster assessment, and emergency response. Seismic monitoring methods primarily focus on technologies like High-rate Global Navigation Satellite System (HR-GNSS) and strong-motion instruments. HR-GNSS is capable of capturing high-precision ground deformation signals and offers the advantages of a non-saturation recording range, making it crucial for rapid estimation of earthquake magnitudes during major seismic events. However, due to the low GNSS sampling rate and high instrument noise, observational noise often overshadows the deformation signals obtained during low-magnitude earthquakes. Additionally, the sparse distribution of GNSS stations currently impacts the accuracy and timeliness of magnitude estimation. Strong-motion observation methods, characterized by high sampling rates, low noise, and dense station distribution, are widely applied in magnitude estimation. Prevalent methods for strong-motion magnitude estimation often rely on P-wave arrival time information for timely determination of magnitude, commonly used in earthquake early warning systems. Yet, these methods are susceptible to saturation effects, leading to underestimation of magnitudes for large earthquakes. Moment magnitude estimation methods are closely associated with rupture characteristics of the seismic source and hold clear physical significance. However, determining this magnitude necessitates knowledge of the rupture extent and slip distribution along the fault plane, which are challenging to precisely obtain at the moment of earthquake occurrence. Hence, such methods are generally employed for post-event magnitude calculations.

Addressing these challenges, this paper proposes a novel method for rapidly estimating earthquake magnitudes using Peak Ground Velocity(PGV)derived from strong motion. First, a comprehensive dataset of strong-motion acceleration records is compiled, covering nearly 20 years and including 5 596 records from 23 global seismic events with magnitudes ranging from 6.0 to 9.0. These records encompass epicentral distances from 1km to 1 000km, with source depths within 60km. A uniform processing approach is applied to standardize the records in terms of time domain orientation, measurement units(converted to cm/s²), and file formats. Data from each station is categorized into three directions: East-West(EW), North-South(NS), and Vertical(UD). Subsequently, the data is converted into the Seismic Analysis Code(SAC)file format, which is specialized for digital seismic waveform data exchange. Ensuring accurate PGV measurements from strong-motion data involves meticulous data preprocessing. This includes removing the mean acceleration from the first 5 seconds before the seismic event for simple bias correction, followed by baseline correction using a high-pass filter with a cutoff frequency of 0.02Hz. The preprocessed strong-motion acceleration records are then integrated to obtain velocity, enabling the measurement of PGV. A robust PGV-based magnitude estimation model, suitable for rapid earthquake magnitude estimation, is constructed using the least-squares regression method.

Furthermore, the constructed PGV-based magnitude estimation model undergoes comprehensive experimental analysis. Initially, the residuals between observed PGV values from 5596 strong-motion records and PGV values predicted by the regression model are computed to evaluate the precision of the constructed PGV-based magnitude estimation model. The model is validated using four earthquake events not included in its construction: the 2021 Damasi M_W6.3 earthquake, the 2012 Nicoya M_W7.6 earthquake, the 2008 Wenchuan M_W7.9 earthquake, and the 2014 Iquique M_W8.2 earthquake. This validation process assesses the reliability of the constructed magnitude estimation model. Finally, the paper conducts a study on rapid magnitude estimation to evaluate the timeliness and accuracy of the PGV-based magnitude estimation model within this context.

The experimental results indicate that the predicted values of strong-motion PGV are largely consistent with the observed values for 23 seismic events, with a root mean square error of residuals measuring 0.296. For the four seismic events that were not included in the modeling process, the estimated magnitudes based on strong-motion PGV correspond closely to the moment magnitudes reported by the United States Geological Survey(USGS). The absolute deviations for these events are 0.15, 0.14, 0.05, and 0.13 magnitude units, with an average absolute deviation of 0.12 magnitude units. In the investigation of rapid magnitude estimation, the following outcomes were observed: For the Damasi M_W6.3 earthquake, an initial magnitude of 5.03 was calculated at 13 seconds, approaching the theoretical magnitude at 63 seconds, and reaching a convergent magnitude of 6.09 at 76 seconds. Regarding the Nicoya M_W7.6 earthquake, a preliminary magnitude of 4.57 was computed within 6 seconds, approximating the theoretical magnitude at 30 seconds, and converging to 7.46 at 50 seconds. In the case of the Wenchuan M_W7.9 earthquake, a preliminary magnitude of 4.06 was determined within 19 seconds. At 50 seconds, the calculated magnitude approached the theoretical value, and it converged to 7.81 at 84 seconds. For the Iquique M_W8.2 earthquake, an initial magnitude of 6.45 was estimated within 2 seconds, nearing the theoretical magnitude at 55 seconds, and achieving a convergent magnitude of 8.04 at 70 seconds. The convergence time for rapid magnitude estimation for all four events was consistently under 90 seconds.

This experimental findings underscore the applicability of the constructed PGV-based magnitude estimation model for rapid earthquake magnitude estimation. The model's ability to counter saturation effects and prevent magnitude underestimation reinforces its robustness and offers substantial technical support for earthquake early warning systems and post-earthquake emergency response strategies.

The horizontal movement of the Helan Shan west-piedmont fault is important to determination of the present-day boundary between the Alashan and North China blocks as well as to the exploration of the extent of the northeastward expansion of the Tibetan plateau. Field geological surveys found that this fault cuts the west wing of the Neogene anticline, which right-laterally offset the geological boundary between Ganhegou and Qingshuiying Formations with displacement over 800m. The secondary tensional joints (fissures)intersected with the main faults developed on the Quaternary flood high platform near the fault, of which the acute angles indicate its dextral strike slip. The normal faults developed at the southern end of the Helan Shan west-piedmont fault show that the west wall of this fault moves northward, and the tensional adjustment zone formed at the end of the strike slip fault, which reflects that the horizontal movement of the main fault is dextral strike slip. The dextral dislocation occurred in the gully across the fault during different periods. Therefore, the Helan Shan west-piedmont fault is a dextral strike slip fault rather than a sinistral strike slip fault as previous work suggested. The relationship between the faulting and deformation of Cenozoic strata demonstrates that there were two stages of tectonic deformation near the Helan Shan west-piedmont fault since the late Cenozoic, namely early folding and late faulting. These two tectonic deformations are the result of the northeastward thrust on the Alashan block by the Tibet Plateau. The influence range of Tibetan plateau expansion has arrived in the Helan Shan west-piedmont area in the late Pliocene leading to the dextral strike slip of this fault as well as formation of the current boundary between the Alashan and North China blocks, which is also the youngest front of the Tibetan plateau.

The 1739 M8.0 Pingluo earthquake is the largest destructive earthquake occurring on the Yinchuan plain in history. However, there are different understandings about the seismogenic structure of this earthquake. In this paper, we re-evaluate the seismogenic structure of the 1739 M8.0 Pingluo earthquake after our investigation and detailed measurement of the seismic dislocations on the Great Wall and the surrounding tableland, and also the latest results of trenching, drilling, and shallow seismic exploration are considered as well. The results show that the latest rupture event of the Helanshan piedmont fault occurred after 600~700a BP, the Great Wall built in Ming Dynasty about 500 years ago was faulted by Helanshan piedmont fault. Although the distribution of Yinchuan buried fault coincides much with the distribution of the meizoseismal area, the fault's northward extending stopped at Yaofu town, and its Holocene active segment is less than 36km in length. The latest surface rupture occurred shortly before 3400a BP. The 1739 Pingluo earthquake did not rupture the ground surface along the Yinchuan buried fault. The presence of growth strata and the non-synchronous deformation of strata near the fault demonstrate that Yinchuan buried fault did not rupture at all or there was rupture but absorbed by the loose layers in the 1739 Pingluo earthquake. Therefore, the Helanshan piedmont fault is the seismogenic structure of the 1739 M8 Pingluo earthquake, rather than the Yinchuan buried fault, and there is no synchronous rupture between two faults. The difference of location between the seismogenic structures and the meizoseismal area of the Pingluo M8 earthquake may be caused by the factors, such as fault dip, groundwater depth, basin structure, loose formations, the degree of residents gathering, so on. The phenomenon that the meizoseismal area shifts to the center of the basin of earthquake generated by faulting of a listric fault on the boundary of the basin should be paid more attention to in seismic fortification in similar areas.

Based on the discussions on the basic ideas, methods and procedures for detecting buried faults and taking the example of Luhuatai buried faults in Yinchuan Basin, the paper introduces in detail the multi-means, multi-level detection methods for gradually determining the accurate location of faults. Multi-means refer to the technical methods such as shallow seismic exploration, composite drilling section, trenching, dating of sedimentary strata samples and calculation of upward continuation of fault's upper breakpoints, etc. Multi-levels refer to gradually determining accurate location of fault at different levels with the above means.
Results of shallow seismic exploration reveal that the Luhuatai buried fault has a strike of NNE in general, dip SEE, with the dip angle between 73° to 78°. Geometrically, the fault consists of a main fault and a small north-segment fault in plane. The main fault runs along the NNE direction from Xixia District of Yinchuan City, passing through Jinshan Township to Chonggang Township, and there is a 4km or so intermittent zone between the main fault and the small north-segment fault. The small north-segment fault is 9km long, distributed between the north of Chonggang Township to the south of Shizuishan City. According to dating of sediments sampled from drill holes, the main fault can be further divided into the southern segment and the northern segment. The southern segment of Luhuatai buried fault is active in Pleistocene, while the northern segment is active in Holocene.
Shallow seismic exploration can detect the upper breakpoint of fault deeper than drilling or trenching does. If simply connecting the vertical projections of these breakpoints on the surface, there is a certain bias of fault strike. To this end, we did accurate location for the Holocene active northern segment of Luhuatai buried fault, in which upward continuation calculation is done based on the fault dip to match the upper breakpoint of fault obtained from shallow seismic exploration with the depth of the upper breakpoints obtained from drilling. Through the accurate location of the fault, we get the geometric distribution, occurrence and segmentation of activity of Luhuatai buried fault at the near-surface. Our results provide reliable basis for the safety distance from active faults for engineering construction projects in the Luhuatai buried fault area of Shizuishan City. The methods discussed in this paper for accurate location of buried active faults are of reference value for buried fault exploration in other similar cities or regions.

Yellow River Fault is the longest, deepest fault in the Yinchuan Basin, also is the eastern boundary of the basin. Because its north section is buried, its activity and slip rate remains unknown, which made a negative impact on understanding the evolution and seismic hazard of the Yinchuan Basin. In this study, a composite drilling section with a row of drillholes were laid out along the northern section of the Yellow River Fault based on the results of shallow seismic exploration near the Taole Town, where oil seismic exploration data are available. Fault activity and slip rate are obtained by measuring the age of samples of holes. The results show that the northern section of the Yellow River Fault is a late Pleistocene or Holocene Fault, its accumulative displacement is 0.96m since (28.16±0.12)ka BP, with an average slip rate of 0.04mm/a, which is significantly lower than the southern section. The activity intensity of the northern section of the Yellow River Fault is significantly lower than the southern section since Late Quaternary. In the Yinchuan Basin, the Helanshan eastern piedmont fault is the most active fault since late Quaternary, next is the Yellow River Fault, then, the Yinchuan buried fault and Luhuatai buried fault. Although the Yellow River Fault is the deepest and the longest fault, its maximum potential earthquake is magnitude 7, this seismogenic capability is weaker than the relatively shallower Helanshan eastern piedmont fault, on which occurred the Pingluo M8 earthquake in 1739 AD. Yinchuan Basin is the result of long-term activities of the four major faults, which shaped the special structure of the different parts of Yinchuan Basin. The Yellow River Fault controlled the evolution of the south part of Yinchuan Basin. The two-layer crustal stretching model can help us understand the structural deformation between the upper crust and the lower crust beneath Yinchuan Basin. Deformation of the upper crust is controlled by several brittle normal faults, while the deformation of the lower crust is controlled by two ductile shear zones. The shear sliding on Conrad discontinuity coordinates the extensional deformation of different mechanical properties between the upper and the lower crust. Yellow River Fault might have cut deeply into the Moho in Mesozoic, the tectonic activity in Yinchuan Basin began to migrate and was partitioned into several faults since the beginning of the Cenozoic, mainly in the Helanshan eastern piedmont fault. This may be the reason why the Yellow River Fault has lower seismogenic capability than the shallower Helanshan eastern piedmont fault.

The reconstruction of Green function by cross-correlating long time ambient noise has been extensively used by seismology community and found its applications in many fields such as structural inversion and stress-related velocity monitoring. Analysis on the ambient noise energy, especially its azimuthal distribution and seasonal variation is now becoming more and more important to obtain reliable and precise information from noise cross-correlation function(NCF). In this paper, more than four years vertical records of 33 broadband stations in Ningxia and its adjacent region are cross correlated and stacked monthly to obtain the distribution and variation of noise energy for both 5～10s and 10～20s periods range using normalized background energy flux method. Seasonal variations of strength for both ranges are observed and agree well with the ocean wave activity, which are strong in winter in the northern hemisphere and relatively weak in summer for same hemisphere. But the azimuthal variation are different. For 5～10s noise, the energy mainly comes from the costal line of southeast China. For 10～20s noise the azimuth of the dominant energy has strong seasonal variation. Back projections of the corresponding dominant noise energy azimuth range indicate that the noise field in Ningxia is controlled by several oceans simultaneously but certain ocean may take the main control on the overall noise energy distribution. Due to the none-uniform and none-random properties of noise filed there, we suggest that evaluation of noise field characters should be made before further studies are conducted, especially for time lapse based investigation.

In this paper,an optimized drilling exploration method,the doubling section method,was summarized after many composite drilling section explorations of buried active fault in urban areas.Operation steps of this method are as follows: Firstly,drill a borehole at each of the two ends of the drilling section to make sure that fault is between the two boreholes,then,drill the third borehole at the middle of the two holes; and secondly,confirm again the segment where the fault is and drill the next borehole in the middle of it.By repeating the similar practice,the accurate location of fault can be constrained progressively.Meanwhile,this paper also uses a quantitative indicator,the key horizon gradient between two boreholes,instead of stratigraphic throw,to determine the location of buried fault and puts forward two criterions: 1)the fault is located between two boreholes if the key horizon gradients between these two boreholes are positive and increase with depth; and 2)the fault is located where the key horizon gradients between two boreholes increase obviously relative to the previous values and that of adjacent segments,besides the increase with depth.While in contrast,the key horizon gradient in a normal fault segment decreases obviously.Application cases show that the method can determine precisely the location of buried active fault.

This paper introduces the result of exploration of the Yinchuan buried fault using the composite drilling section method. As one of the main buried faults in Yinchuan plain,the Yinchuan buried fault has restricted seriously the development of Yinchuan City for a long time due to its indistinct location and unclear activity property. So the Yinchuan buried fault was taken as one of main tasks of active fault exploration in Yinchuan City. Most of shallow seismic explorations had been done before the drilling. However,due to the limited precision of shallow seismic exploration,the actual location of the Yinchuan buried fault can't be explored. For obtaining the information about the location and the depth of the upper break point, the active time and slip rate of the Yinchuan buried fault,three composite drilling sections,Xinqushao, Manchun and Banqiao,were laid out along the Yinchuan Fault based on the result of shallow seismic exploration. After comparing with the marker horizons disclosed by drilling,the position,scale and the depth of the upper break point of Yinchuan buried fault were found,and the buried active fault was located precisely. From the exploration result we get the apparent dip of the Yinchuan buried fault as 71 degrees at Xinqushao,71 dgrees at Manchun and 66 degrees at Banqiao,and the depth of the upper break points as 5.18～8.30m,5.01～6.50m and from 10.0～13.59m,respectively. Therefore,the latest active date of the Yinchuan buried fault is determined and the question whether the fault is active or not is answered by dating. The Yinchuan buried fault at Xinqushao and Manchun sections is manifested as a Holocene active fault, and at Banqiao,it is shown as a late Pleistocene active fault. The slip rate of the Yinchuan buried fault since late Pleistocene is 0.14mm/a at Xinqushao,0.05mm/a at Manchun and 0mm/a at Banqiao. Based on the result obtained from seismic exploration and the spatial positions of the three composite drilling sections,we draw the following conclusions:the Yinchuan buried fault can be divided into two segments with Yingu Road as the boundary; the northern segment was active in Holocene and the southern one was active in late Pleistocene; the activity of the northern segment is more recent than that of the southern one.

Yinchuan Basin is a graben-like downfaulted Cenozoic era basin located on the west edge of Ordos Massif.Its activity is violent and deposition is very thick.Yinchuan City is located in the middle of Yinchuan Basin.The seismic petroleum exploration shows that a buried active fault lies in the east of Yinchuan City,named as the Yinchuan buried fault,which strikes NNE and dips west,with a total length of more than 80km.Because the seismic petroleum exploration did not gain any explained signals at the depth ranging from 0 to 400m,so whether the Yinchuan buried fault is active or not in the late Quaternary and its exact surface projective location hasn't been known yet.It has been a “worry” in the urban planning and development of Yinchuan for a long time.Under the financial support of the national and local governments,we launched the project entitled “The prospecting of active fault and earthquake risk assessment in Yinchuan City”.In order to facilitate the exploration,we selected Xinqushao village in the southeast suburb of Yinchuan City to be the site for the integrated test exploration of the Yinchuan buried fault before the exploration,based on the information obtained from the seismic petroleum exploration.Considering that the thick Quaternary sediment in Yinchuan reaches to 1609m,and that the depositional environment is the Yellow River flood plain and the lateral change of lithology is complex,we adopted in the test exploration the train of thoughts of “inferring an unknown fact from a known fact,and from deep to shallow and directly to the top”.The experimentation has been developed step by step according the working order of multilevel seismic exploration→composite geological profile drilling→trenching.Along the same measuring line at Xinqushao,first,we adopted the seismic reflection exploration of primary wave in three levels with the group interval of 10m→5m→1m to catch the master fault of the Yinchuan buried fault,and by tracing upward layer by layer in the order of the three exploration ranges,i.e.1400～400m→600～80m→150～20m,the position of the master fault at ±20m depth under the ground and its offset trace were primarily identified.And then,along the master fault and within the range of 100m at its both sides,9 boreholes of 20.5～100m were arranged for the composite geological profile drilling.The resulting information about the throws of the master fault was obtained,they are 20.34m,9.66m and 2.25m respectively at the depth of 43.75m,20.33m and 13.04m from the ground,and the buried depth of the upper offset point ≤8.34m.At the same time,using the intact core specimen from the fault plane of the borehole No.7,we calculated the dip angle of the fault as 71°at the depth of 55.27m and figured out the exact position of its extension to the earth's surface.Finally,a large-scale trial trench,which is 40 meters long,8～12 meters wide and 6 meters deep,was arranged across the master fault.The trenching revealed that the actual buried depth of the upper offset point of the master fault is 1.5m and there are seismic remains,such as offsets of 5 stages,sand liquefaction and surface rupture,etc.Among the 5 stages offsets,4 events occurred prior to 3170±80 a BP,belonging to the mid to late Holocene paleo-earthquakes.The age of the last event cannot be determined and it is inferred to be the result of the M8.0 Yinchuan-Pingluo earthquake in 1737.In a word,through the comprehensive test exploration,we find that the Yinchuan buried fault is a Holocene active fault,which lays solid base for the next exploration.