Table of Content

20 October 2023, Volume 45 Issue 5

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Research paper

RELIABILITY EVALUATION OF QUARTZ ESR DATING METHOD ON EARLY PLEISTOCENE SEDIMENT: A CASE STUDY OF JINGYUAN SECTION

WEI Chuan-yi, YIN Gong-ming, WANG Xu-long, WANG Duo, JI Hao, LIU Chun-ru, LI Xin-xiu

2023, 45(5):  1041-1056.  DOI: 10.3969/j.issn.0253-4967.2023.05.001

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As the most recent period of the geological record, the Quaternary climate change, tectonics and river drainage evolution have been well recorded by Quaternary sediment. Establishing the timing of these geological changes, and of their effects on the earth's environment, is a key element in Quaternary research. Because of dating range limit of quartz OSL dating and ¹⁴C dating, lacking of tephra for K-Ar dating, and strict restrictions for ²⁶Al/¹⁰Be cosmogenic nuclide dating, the samples older than 200ka were critical but difficult in Quaternary dating, while electron spin resonance(ESR)dating method could provide absolute age for late Pliocene and Pleistocene samples. Previous studies show that quartz Al center and Ti-Li center are the most suitable signals for sediment ESR dating, and have been successfully applied into middle-late Pleistocene sediment dating. However, the application of those two centers ESR chronology into early Pleistocene or pre-Quaternary sediment remains confusion.

In this study, early Pleistocene Jingyuan gravel layer sediment deposited at Yellow river were collected for ESR dating. The results of comprehensive comparative analysis of high resolution magneto-stratigraphy and deep-sea oxygen isotope curve of loess-paleosol sequences and high credible ²⁶Al/¹⁰Be cosmogenic nuclide dating age make the Jingyuan gravel layer as the ideal material to evaluate the dating range, especially lower dating range, of the quartz Ti-Li center and Al center, respectively. The results show that:

(1)The quartz Ti-Li center and Al center signal intensity of Jingyuan gravel layer was not saturated within 11 000Gy and 130 00Gy additional gamma ray dose, respectively; combined with the long thermal lifetimes of the quartz Ti-Li center(8×10⁶a)and Al center(7.4×10⁹a), guarantee the ESR dating range for million years.

(2)The single saturation exponential function and “EXP+LIN” functions could provide more accuracy fitting result of equivalent dose of quartz Ti-Li center and Al center, respectively, and the fitting goodness is greater than 0.98.

(3)The average ESR dating results of quartz Ti-Li center and Al center of Jingyuan gravel layer is~(1.67±0.15)Ma and~(1.65±0.69)Ma, respectively, which is consistent with the previously well-known age within the error range.

To better understand the lower dating limit of the quartz ESR dating method, based on the previous analysis of the ESR signal thermal stabilities, we discuss the maximum saturation of the ESR signals and ESR signals' sensitivity. Combined with the fitting goodness evaluation of various fitting functions, we propose that the quartz Ti-Li center and Al center ESR dating method could provide reliable chronological constrains on the sand lens of early Pleistocene gravel layer. The results of our study not only provide a solid theorical foundation for the application of quartz ESR dating method for late Pliocene and early Pleistocene fluvial sediments, but also demonstrate a typical practice example of the ESR method on dating late Cenozoic sediments.

FINE CHARACTERISTICS OF EARTHQUAKE SURFACE RUPTURE ZONE BASED ON HIGH-RESOLUTION REMOTE SENSING IMAGE: A CASE STUDY OF LITANG FAULT

YOU Zi-cheng, BI Hai-yun, ZHENG Wen-jun, PENG Hui, LIANG Shu-min, DUAN Lei, QIN Yi-gen

2023, 45(5):  1057-1073.  DOI: 10.3969/j.issn.0253-4967.2023.05.002

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Strong earthquakes(magnitude>6.5)typically cause coseismic surface ruptures of several kilometers or even hundreds of kilometers long on the surface. Coseismic surface rupture is the most intuitive geomorphic representation of an earthquake on the surface, and its geometry and distribution characteristics provide important information about the fault activity. Field investigation is the most basic means for research on coseismic surface fractures, but for areas that are hard to access or have harsh climatic environments, field investigation is often greatly limited. In recent years, the increasing abundance of high-resolution remote sensing images and the rapid development of photogrammetry methods can help us quickly obtain high-resolution topographic and geomorphic data of the study area, to better identify the fine geometry of the earthquake surface rupture zone and measure the offsets of geomorphic markers along the fault. The Litang Fault is a sinistral strike-slip fault located within the Sichuan-Yunnan rhombic block on the eastern edge of the Qinghai-Tibetan plateau. Several historical earthquake events have occurred on this fault, such as the 1890 and 1948 earthquakes, and clear seismic surface ruptures still exist along the fault so far. Previous studies have conducted a series of works on the coseismic surface rupture of this fault, but most of these works were based on field investigations or relatively low-resolution remote sensing images, and there is still a lack of fine research on the coseismic surface rupture of the fault. In this paper, the coseismic surface rupture of the 1890 earthquake which occurred on the Litang Fault was selected as the study object. To obtain high-resolution topographic data of this fault, the WorldView satellite stereo images were used to generate a 0.5-m-resolution orthophoto and a 1-m-resolution Digital Elevation Model(DEM)of the Litang fault based on the photogrammetry method. With the high-resolution topographic data, the fine geometry of the 1890 earthquake surface rupture zone was mapped in detail. The mapping results show that the total length of the surface rupture is about 27km, with an overall strike of N40°W. The rupture is mainly characterized by sinistral strike-slip motion, with a certain degree of dip-slip component in local areas. Except for the interval of approximately 6km with no surface rupture at the Wuliang River floodplain in the Litang Basin, the surface ruptures are relatively continuous at other locations. In addition, various rupture styles have been identified along the fault, including en echelon tension cracks, mole tracks, sag ponds, fault scarps, and displaced gullies. Furthermore, the sinistral offsets of 90 groups of linear geomorphic markers such as gullies and ridges were measured along the fault, which range from 1m to 82.4m. We further estimated the Cumulative Offset Probability Distribution(COPD)of the offsets located on the terrace I of the Wuliang River, which are all in the range of 0-9m. The COPD plot displays four distinct peaks at 1.3m, 2.4m, 4.3m, and6.1m, respectively. Previous studies have reported that the terrace I of Wuliang River formed at about(4 620±40)a BP. Thus, it can be indicated that the Litang fault may have experienced at least four strong earthquake events since(4 620±40)a BP, and the smallest peak of 1.3m may represent the coseismic displacement of the most recent 1890 earthquake. The rupture length of the latest 1890 earthquake was about 27km, and the coseismic sinistral offset was about 1.3m, yielding an estimated moment magnitude of M_W6.8-7.1. The coseismic offset of the other three earthquakes was about 1.8m, 1.9m, and 1.1m from old to new, respectively, yielding a magnitude estimate of M_W7.3, M_W7.3, and M_W7.0, with a size comparable to the 1890 earthquake. The research results fully demonstrate the potential of high-resolution remote sensing images in the study of fine characteristics of earthquake surface rupture.

INTERSEISMIC SLIP RATES AND SHALLOW CREEP ALONG THE NORTHWESTERN SEGMENT OF THE XIANSHUIHE FAULT FROM INSAR DATA

CHEN Yi, ZHAO Bin, XIONG Wei, WANG Wei, YU Peng-fei, YU Jian-sheng, WANG Dong-zhen, CHEN Wei, QIAO Xue-jun

2023, 45(5):  1074-1091.  DOI: 10.3969/j.issn.0253-4967.2023.05.003

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Located in the eastern boundary of the Qinghai-Tibetan plateau, the Xianshuihe fault zone is one of the most active left-lateral strike-slip faults in Chinese mainland. As the southern boundary of the Bayanhar block, the Xianshuihe Fault accommodates the southeastward transport of material toward southeastern Asia. Earthquakes have occurred frequently along this fault, especially in the northwestern segment. More than 20 earthquakes with M_W>6.0 have ruptured since 1700. The most recent M_W>7 earthquake was the Luhuo earthquake in 1973, and the most recent M_W>6 earthquake was the M_W6.6 Luding earthquake in 2022. As one of the most active faults in mainland China, the present slip pattern of the Xianshuihe Fault, especially the shallow creep characteristics along its northwestern segment, has attracted much attention.

The primary goal of determining slip rates of active faults using geodetic data is to quantify the seismic potential of the faults. Illuminating the long-term slip rate and shallow creep distribution of faults is the basis for calculating the seismic moment rate and evaluating the seismic potential. Due to the backwardness of early measurement methods, the seismic deformation along the Xianshuihe Fault was previously based on geologic, cross-fault short baseline and leveling surveys. With the application of GPS in tectonic geodesy, more and more GPS stations are installed near active faults, which provide accurate constraints on the long-term slip rates of the fault. Subsequently, the appearance of InSAR technology has brought a beneficial supplement to GPS, providing high spatial resolution surface velocity maps, which have been widely used to measure deep and shallow creep along active faults. It is the key to accurately characterize the fault slip behavior and evaluate the seismic potential.

In this study, 119 Sentinel-1 satellite descent data from December 2014 to December 2021 were processed to obtain the average line-of-sight(LOS)velocity field of the northwestern segment of the Xianshuihe Fault based on the small baseline InSAR method. Then the elastic screw dislocation model was used to fit the fault normal InSAR LOS velocity profiles to estimate the long-term slip rates and shallow creep rates. Combined with the viscoelastic earthquake cycle model, the effects of the earthquake recurrence period, and rheology of the lower crust and upper mantle on slip rate estimation in Luhuo segment are analyzed. The main results are as follows:

(1)The average InSAR LOS velocity field is in the northwestern segment of the Xianshuihe Fault during 2014—2021 has been obtained with a large range and high spatial resolution. The velocity field results show an obvious velocity gradient across the surface trace of the Xianshuihe Fault, which is consistent with the left-lateral strike-slip characteristics of the Xianshuihe Fault.

(2)To investigate the slip rate variation along the northwestern segment of the Xianshuihe Fault, we used the two-dimensional elastic screw dislocation model to fit the 14 fault-normal velocity profiles selected along the northwestern segment of the Xianshuihe Fault and estimated the long-term slip rates and shallow creep rates using the Markov Chain Monte Carlo(MCMC)method. The results show that the overall slip rates of the NW segment of the Xianshuihe Fault range from 7.2mm/a to 11.0mm/a, and gradually decrease from west to east. The shallow creep rate ranges from 0.3mm/a to 3.1mm/a. The high creep rate appears mainly at Xialatuo and the segment from Daowu to Songlinkou. The shallow creep rates in other places are close to zero, implying that the fault is completely locked.

(3)According to historical earthquake records, the recurrence interval of the Luhuo segment is set to be 150 years, 200 years, and 400 years, and the viscosity of the lower crust and upper mantle is set to be 5.0×10¹⁸Pa·s, 1.0×10¹⁹Pa·s, and 5.0×10¹⁹Pa·s. The slip rate of the Luhuo segment is estimated to be (7.91±0.3)~(9.85±0.4)mm/a using the MCMC method, which is slightly lower than the (10.14±0.5)mm/a obtained by the pure elastic model. In addition, when the earthquake recurrence interval is 150 years and the viscosity of the lower crust and upper mantle is 5.0×10¹⁹Pa·s, we simulate the fault-normal velocity at 5 years, 20 years, 75 years, and 125 years after the 1973 Luhuo earthquake, and find that in any period of the seismic cycle, the estimation of fault slip rate will be biased to some extent if the viscoelastic contribution of the lower crust and upper mantle is ignored.

STUDY ON THE CHARACTERISTICS AND MECHANISM OF LONG-PERIOD GEOELECTRIC FIELD VARIATION: A CASE STUDY FROM DULAN SEISMIC STATION

XI Ji-lou, ZHAO Jia-liu, GAO Shang-hua, WANG Xiao-lei, LI Guo-you, MENG Fan-bo

2023, 45(5):  1092-1111.  DOI: 10.3969/j.issn.0253-4967.2023.05.004

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The geoelectric field is an important geophysical field, and the long-term geoelectric field is also an important component of the Earth's electromagnetic field, which mainly refers to the geoelectric field with a variation period greater than 100,000 seconds. The long-term geoelectric field can dynamically reflect the trend changes of spontaneous electric field, periodic changes of telluric electricity field, and other seasonal disturbances. Furthermore, it can further reveal the basic characteristics and physical mechanisms are related to the sources of the electromagnetic field, through such analysis and research on the changes in the long-term geoelectric field.

In this paper, a systematic study has been conducted on the basic characteristics and variation mechanism of the long-term geoelectric field, which is mainly based on long-term geoelectric field observation data Dulan seismic station in Qinghai Province from 2015 to 2022. Firstly, based on interference preprocessing and reliability analysis, various methods and approaches such as wavelet analysis, median filtering, convolutional filtering, linear fitting are used to filter out high-frequency disturbances in the geoelectric field and to extract and analyze the components of long-term geoelectric field variation. At the same time, based on the analysis results of the abnormal changes in the electric field at the Dawu seismic station before and after the Madoi M_S7.4 earthquake, the possible correlation and main field source mechanism between the abnormal variation of the electric field at the Dulan seismic station and the earthquake has been analyzed and discussed.

The research results show that, from the observation data of geoelectric field at Dulan seismic station: 1)It has relatively strong long-term stability and variation reliability, with the arithmetic mean value of the daily correlation coefficient not less than 0.98, and that of the daily variation difference not more than 0.2mV/km, in the Same measurement direction; 2)The FFT spectrum analysis results include not only short and medium cycle changes such as 24h, 12h, 8h and 6h, but also long cycle changes such as annual cycle, semi-annual cycle and so on; 3)The daily variation range shows seasonal variation with high in summer and low in winter, and the significant variation includes annual cycle, half-year cycle, 27-day cycle, half-month cycle; 4)The trend variation shows a typical periodic annual changing feature as the sine wave change, and the annual extreme value and change amplitude are basically the same; 5)Before and after the Madoi M_S7.4 earthquake, there were obvious abnormal distortions from the annual variation.

Theoretical and mechanism analysis shows that the periodic changes in the daily variation amplitude of the geoelectric field at the Dulan seismic station mainly come from the periodic changes in spatial field sources, and the trend periodic changes of the geoelectric field mainly come from the long-term tidal effect generated by the gravitational action of the sun. During the process of Earth's orbit and rotation around the sun, the periodic changes in direct solar radiation, as well as in the ionospheric plasma concentration and the underground fluid seepage field caused by the gravitational force of the sun, are the important excitation mechanisms for this long-term periodic change of the geoelectric field.

According to the comprehensive analysis, it is believed that the variation of the geoelectric field data observed at the Dulan seismic station has excellent objectivity, reliability, and long-term stability, which can reflect the characteristics of long-term geoelectric field changes and abnormal distortion characteristics of the geoelectric field before and after strong earthquakes. And the change mechanism is in good agreement with the theoretical analysis results and has strong research and application value.

In summary, the research results which carried out on the long-term variation of the geoelectric field, as well as revealed the important mechanisms of the generation and variation of Earth's electromagnetic fields, will have more important enlightening significance and reference value in the multifaceted research work about the geoelectric field observation, such as the site survey, observation system construction, observation data analysis and application et al.

STRESS-INDUCED HEATING HYPOTHESIS BASED ON CORRELATION ANALYSIS OF GRAVITY AND THERMAL FIELDS BEFORE WENCHUAN EARTHQUAKE

SONG Dong-mei, WANG Hui, SHAN Xin-jian, WANG Bin, CUI Jian-yong

2023, 45(5):  1112-1128.  DOI: 10.3969/j.issn.0253-4967.2023.05.005

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As one of the most serious geological disasters, earthquake is of sudden and destructive characteristic. Therefore, it is of great significance to earthquake monitoring and early warning. The phenomenon of surface thermal infrared radiation enhancement is a common precursor of moderate and strong earthquakes and has been used as an important reference information for early warning and short term prediction. A variety of explanations have been given to understand internal mechanism of the above phenomenon, in which the stress-induced heating hypothesis is widely accepted and has been confirmed in the laboratory rock mechanical loading experiments, that is, under ideal conditions in the laboratory, the rock heats up when it is pressed and cools down when it is stretched. Under field conditions in practice, however, weak seismic precursors of thermal anomalies are often interfered by various environmental factors(solar radiation, atmospheric movement and human activities, etc.), and it has not been investigated whether the corresponding relationship between the above crustal compression-extension motions and thermal radiation anomalies can be observed under field conditions. The earth's gravity field, as one of the basic physical fields of the earth, contains the density distribution of crustal structure, which can be served to study the migration of the earth's material, the deformation of the crust and the change of the stress field. In this paper, we use GRACE gravity and MODIS thermal infrared remote sensing data to verify the stress-induced heat hypothesis in the field with Wenchuan earthquake as the time node. Firstly, the crustal mass density obtained by GRACE satellite was compared with thermal infrared radiation. Then, the gravity anomalies extraction method based on maximum shear strain and in-situ temperature method were used to obtain the gravity anomalies and thermal anomalies respectively. Furthermore, the correlation between the two anomalies before the earthquake was detected from the time scale and space scale respectively, and the consistency analysis between the above anomalies and the spatial distribution of the tectonic fault zone was carried out. For this purpose, two important indicators i.e., anomaly intensity and anomaly distribution, were established in time domain and space domain, respectively. The following conclusions could be drawn: 1)The stress-induced heating hypothesis can be verified by remote sensing in field conditions. The warming zone of the crust(positive thermal offset index)corresponds to the compression zone, and the cooling zone(negative thermal offset index)corresponds to the stretching zone. The consistency of positive and negative variation between the crustal mass density and thermal offset index is 88.9%, which provides field observation evidence for the stress-induced heating hypothesis. 2)The spatio-temporal variation of gravity anomalies and thermal anomalies before earthquake has strong correlation. In the time domain, there is a strong correlation between the gravity anomalies and the thermal anomalies, which shows that the intensity of the two anomalies suddenly increases synchronously and reaches the maximum simultaneously three months before the earthquake. In the spatial domain, gravity anomalies mostly occur at the junction of positive and negative values of thermal offset index, which indicates that the spatial distribution of gravity anomalies and thermal anomalies also has a certain correlation. In addition, the two anomalies appear to be distributed along the fault zone for many times, which shows that they are closely related to tectonic activities.

STUDY ON THE REGIONAL GRAVITY GRADIENT FIELD MODELING IN IRREGULAR AREA WITH CONSIDERATION OF SURFACE FLUCTUATION: A CASE STUDY OF YUNNAN PROVINCE

LIU Jin-zhao, LIANG Xing-hui, YE Zhou-run, CHEN Zhao-hui, HU Min-zhang, HAN Yu-fei, WANG Qing-hua, LIU Dong, HAO Hong-tao, ZHANG Shuang-xi, CHEN Ming

2023, 45(5):  1129-1146.  DOI: 10.3969/j.issn.0253-4967.2023.05.006

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The gravity gradient full tensor can sense the slight changes of the gravity vector in different directions, and because of the large number of components(6 components), it can reflect more information on different sides of the same field source than the gravity vector. On the other hand, the ground gravity survey has accumulated a lot of basic gravity vector data, and using the mathematical relationship between the gravity vector and the full tensor of gravity gradient to build a full tensor gravity gradient field model is helpful to the depth mining of existing gravity data information.

Based on the mathematical relation and covariance function relation of disturbance potential, gravity anomaly, gravity disturbance and gravity gradient disturbance, along with the Least Square Configuration(LSC)algorithm, in this paper, the numerical formulas have been derived in detail for modeling the full tensor of regional gravity gradient disturbance field from gravity disturbance or gravity anomaly which distributed on undulating surface with non-grid pattern. The full tensor of gravity gradient derived from the grid distributed gravity anomaly data in West Arnhem Land, Australia by using the spectral domain(two-dimensional fast Fourier transform)method were taken as the “reference value”, Then, the full tensor of gravity gradient, in the same area with irregular regional gravity data distribution, derived form the LSC algorithm based on derived formulas were taken as the “evaluated value”.

By comparing the differences between the “reference value” and the “evaluated value” of the gravity gradient, we found that: 1)The “evaluated value” obtained by the LSC method is consistent in spatial pattern of variation with the “reference value” derived from the spectral domain method. The vertical component of the gravity gradient disturbance describes the boundary characteristics of the field source more precisely than the vertical component of the gravity anomaly, and other gravity gradient disturbance components provide the information representation of the same field source, which provides inspiration for the in-depth interpretation of the field source. 2)Each difference component ΔδΓxxfft-lsc,ΔδΓxyfft-lsc,ΔδΓxzfft-lsc,ΔδΓyyfft-lsc,ΔδΓyzfft-lsc  and ΔΓzzfft-lsc  between the “evaluated value” and the “reference value” of the gravity gradient has systematic deviation, which is 5.54E, 5.30E, 1.85E, 6.55E, 2.09E and 9.67E respectively. It is much lower than the difference between measured gravity gradient and that from constructed model in previous studies. The results show that the accuracy of the two methods is within the allowable range, but the accuracy of the modeling method based on the Least Square Configuration needs to be further verified by the measured data.

Finally, based on the measured surface differential gravity anomaly values in regional area of Yunnan province, the annul gravity gradient field variation model for this region, about 20km in half wavelength, is firstly constructed and presented by using the LSC with the derived formulas in this paper. The different gravity gradient component models show more abundant interannual scale signal variation characteristics, and demonstrate more local signal characteristics from different sensitive directions in Yunnan region during this time period. In addition, in the vicinity of Zhaotong city in northeast Yunnan province, because there is no gravity points available, both the annual difference results of gravity value and the annual change model of gravity gradient show uniform signal characteristics, indicating that the modeling method in this paper does not introduce additional false “anomalies”. In southern Yunnan province, where gravity points are relatively sparse, both the annual difference results of gravity values and the annual change models of gravity gradients are dominated by long-band signals, and no additional signals will be added due to the increase of expansion order of the LSC algorithm, which is also consistent with our intuitive cognition. These provide support for further research on the relationship between crustal material migration, hydrological changes and earthquakes in Yunnan region.

The procedure and method proposed in this paper can improve the efficiency of using measured gravity data(mainly gravity anomaly and gravity disturbance). Moreover, it can provide basic data for better understanding and interpretation of gravity data, gravity gradient data and their relationship with different field sources in geophysics and geology.

Research paper

ANALYSIS FOR SYNTHETIC ADJUSTMENT OF ABSOLUTE AND RELATIVE GRAVITY OF JINZHAI GRAVITY BASELINE IN DABIE MOUNTAIN

XU Ru-gang, ZHANG Xin-lin, LIANG Xiao, SUN Hong-bo, CHU Fei, HUANG Xian-liang, TAN Hong-bo, WANG Jian

2023, 45(5):  1147-1169.  DOI: 10.3969/j.issn.0253-4967.2023.05.007

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In the data process of the regional gravity monitor network, the absolute gravity reference value is the control datum of the gravity monitor network and is usually used for calibrating the scale of the relative gravimeter. Due to the coupling influence of the gravimeter scale and the control datum, it is difficult to quantitatively analyze their influence of on the adjustment processing of the regional gravity monitor network data.

Based on the hybrid gravity observation data of the Jinzhai gravity baseline field in Dabie Mountain, the paper decouples the influence of the relative gravimeter scale and gravimetry control datum. According to analyzing the average point value, the point value, section difference, and the accuracy, we obtain the influence of the gravimeter scale and control datum on data processing of the regional gravity monitor network. The initial gravity value of the Jinzhai gravity baseline corresponds to the optimal combination of the gravimeter scale and control datum and the following conclusions are obtained:

(1)The results of the mutual difference of gravimeters, section difference, and their accuracy calculated by different combinations of gravimeter scale and control datum show that the control datum is of great significance to reduce the inter-difference between gravimeters and obtain reliable measurement results, and the control datum used to the gravimeter scale calibration should form a complete coverage of the reading range of the regional gravity monitor network.

(2)Gravity point values and their accuracy of the Jinzhai gravity baseline calculated by the different combinations of gravimeter scale and control datum show that the point values and accuracy are both affected by the gravimeter scale and control datum. When the solution point is located at the end of the control datum and the solution point is used as the control datum and participates in the solution calculation, the influence of the control datum and gravimeter scale on the solution is the same. When the solution point is not used as the control datum to participate in the solution control, the influence of the gravimeter scale on the solution is stronger than the control datum. When the solution points are between the control datum, the influence of the control datum on the solution is stronger than the gravimeter scale, and the more control points participate, the stronger the influence of the control datum on the solution.

(3)Using the all control data which fully cover the reading range of the measuring network and its corresponding gravimeter scale to resolve the observation data, the point value has the highest accuracy. The solution obtained by a combination of the end control datum and its corresponding gravimeter scale is the second. The accuracies of other references and scale combinations are equivalent and the lowest. The points between the reference are strongly controlled, while the points out of the reference, the farther from the reference, the weaker the control, and the lower the point value accuracy which shows a monotone linear increase.

(4)The data adjustment results of the Jinzhai gravity baseline based on the optimal gravimeter scale and control datum combination show that the accuracies of the section difference are better than 2μGal, and the average accuracy of the point value is better than 2.8μGal. The accuracies of the observation data of the three absolute gravity stations are better than 2.0μGal, and the accuracies about the section difference of the gravity vertical gradient are better than 1.0μGal. The results satisfy the project requirements and can be formally adopted.

(5)The analysis of the difference between scale WYB and the optimal scale 123 shows that the gravimeter scales calibrated in the same reading section can be used directly with each other.

(6)The Jinzhai gravity baseline can meet the requirements of gravimeter calibration for the gravity monitor network of Anhui. The construction and application of the Jinzhai gravity baseline are of great significance to improve monitoring efficiency and save economic cost. The data processing and analysis method of the Jinzhai gravity baseline field can also provide a reference for data processing of other baseline fields and regional gravity monitor networks. The Jinzhai gravity baseline extended to the absolute gravity point with a larger section difference can meet the gravimeter calibration requirements of the gravity monitor network with a larger gravity segment difference.

FOUR-COMPONENT BOREHOLE STRAINMETER TRANSFER FUNCTIONS AND ROCK PROPERTIES ESTIMATION: A CASE OF JIANGNING STATION, JIANGSU, CHINA

YANG Xiao-lin, YANG Jin-ling, SU Li-na, FENG Jing, WANG Jun

2023, 45(5):  1170-1186.  DOI: 10.3969/j.issn.0253-4967.2023.05.008

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To date, more than 60 four-component borehole strainmeters have been deployed in China to provide more data on geodynamics and earthquake precursors. In practice, the strain signals recorded by the four-component borehole strainmeters are greatly disturbed by the effect of barometric pressure at different frequencies. Therefore, precisely deciphering the frequency-dependent strain response to barometric pressure changes in the frequency domain is very significant for continuous high-resolution borehole strainmeter measurements. However, no much related research has been conducted in this day. For these more than 60 four-component borehole strainmeters being operated in Mainland China, the study of properties(e.g., Young's modulus and Poisson's ratio)of borehole surrounding rock are seldom probed via rock mechanical testing has not been given enough attention, compared that with in the USA and Japan, consequently, which makes it difficult in the understanding of the driving mechanism of atmospheric effects.

Beginning at 2006, a YRY-4 type borehole strainmeter was installed approximately at 50m depth in the Jiangning area to precisely monitor the earthquake precursors and tectonic movements in Jiangsu Province. For daily observation, the atmospheric effect is quite obvious in different frequency bands. Therefore, we applied transfer function to uncover features, such as barometric pressure coefficient and phase shift, of frequency dependence observed at Jiangning station in Jiangsu Province in high-(>8cpd), intermediate-(0.5~8cpd), and low-frequency(0.1~0.5cpd)bands. Furthermore, the double bush mechanical model was adopted to estimate the parameters of elastic modulus and Poisson's ratio for the borehole surrounding rock.

The results show that: 1)The values of coherence in the frequency band(0.1~30cpd)are higher than in others. 2)The barometric pressure responses were very stable and valid in the low-frequency band, and remained stable in the high-frequency band(8~30cpd), but significantly fluctuated in the intermediate-frequency band. 3)If the impacts of the diurnal and semidiurnal tidal waves are neglected, the spectra of barometric pressure response coefficients for the four borehole sensors and areal strain were quasi-linear and stable. 4)In the high-frequency band, the spectra of phase responses for borehole strains behaved exponentially, strongly depending on frequency, with the average phase delay of about 24.2°. 5)The estimated elastic modulus and Poisson's ratio were 33.9GPa and 0.27 according to the averaged barometric-pressure response coefficients of areal strain, respectively, in which the estimated parameters showed good agreement with the results obtained via rock mechanical testing.

These above findings will be useful for separating the nonlinear barometric responses from the four-component borehole strainmeter records, as well as for estimating the mechanical parameters of the borehole surrounding rock. In the future, more and more rock cores taken from boreholes may disappear with time. So exploring the potential values of transfer function is an important work in the field of borehole strain study. In the near future, we will reconstruct the rock mechanical parameters of borehole surrounding rock for other unstudied sites in mainland China.

SELF-NOISE MODELS OF SEISMOMETERS BASED ON PROBABILITY DENSITY FUNCTION REPRESENTATION

WANG Kai-ming, YU Da-xin, ZHAO Li-jun, LI Wen-yi, YE Qing-dong

2023, 45(5):  1187-1199.  DOI: 10.3969/j.issn.0253-4967.2023.05.009

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The self-noise levels of different seismometer models directly affect the quality of seismic observations and further limit earth science research based on seismic records. We conducted a test of the self-noise level of seven types of seismometers at the Malingshan seismic station, in which the instrument types included short-period, broadband, very-broadband and ultra-broadband. Three seismometers of each type were set up, and the observation period was from November 22, 2018, to March 26, 2019. In this paper, based on continuous seismic waveforms from seven models of seismometers, the self-noise power spectral density(PSD)of the seismometers was calculated by using the three sensor correlation analysis method, and the probability density distribution of the self-noise PSD of the seven models of seismometers was obtained by using the PDF representation. Based on the mode values of the PDF distribution, the self-noise models of the three channels(UD, EW and NS)of the seven models of seismometers are given respectively.

For the ultra-broadband seismometer CMG-3T-360, in the microseism band(0.1Hz to 1Hz), the self-noise of the horizontal components(EW and NS)is higher than that of the vertical components(UD)and is consistent with the trend of the seismic noise, which may be attributed to misalignment of the horizontal direction between seismometers. In the low frequency band(<0.03Hz), the self-noise level of the horizontal component is higher than that of the vertical component, and small changes in the barometric pressure may lead to higher incoherent noise in the horizontal direction of the sensor at long periods. Compared with the vertical direction, the horizontal direction of the seismometer is more susceptible to air disturbances. At a frequency of 0.005Hz, the instrument self-noise of the horizontal component is close to the seismic background noise, and the instrument self-noise of the horizontal component is the main source of noise recording. Installing a heat and wind shield can effectively reduce the seismometer self-noise in the low frequency band. When using the CMG-3T-360 to observe long-period seismic signals, a shield with both thermal insulation and windproof function is required.

The self-noise level of the short-period seismometer JS-S02 is lower than that of TDV-33S and lower than that of the New Low Noise Model(NLNM)between 0.15Hz and 7Hz. In the UD and EW channels, the self-noise level of TDV-33S is lower than the NLNM model between 0.17Hz and 0.5Hz. The higher instrument self-noise further limits the extraction of long-period seismic signals in the digital recordings of short-period seismometers.

For the broadband seismometer TDV-60B and the very broadband seismometer TDV-120VB, the self-noise levels are basically consistent in the vertical direction and also are higher than those of the broadband seismometer JS-60 and the very broadband seismometer JS-120. In the horizontal direction, the two self-noise levels in the microseism band and the low frequency band show different characteristics, i.e., the self-noise levels of TDV-60B are lower than those of TDV-120VB in the microseism band and higher than those of TDV-120VB in the low frequency band. When the frequency is lower than 0.03Hz, the self-noise levels of TDV-60B and TDV-120VB on the horizontal channels are close to those of JS-60 and JS-120, respectively.

For JS-60 and JS-120, in the vertical direction, the self-noise levels of both are close to CMG-3T-360 in the microseism band. The self-noise level of JS-120 on the vertical channel is lower than 5dB away from CMG-3T-360 in the low frequency band and lies within the 68%confidence interval of the PSD; in the high frequency band(>2Hz), it is higher than CMG-3T-360 confidence interval of the PSD. In the horizontal direction, the self-noise levels of JS-60 and JS-120 are lower than those of CMG-3T-360 between 0.15Hz and 1Hz and in the microseism band, respectively. The self-noise levels of JS-60 on the horizontal channel show good agreement with those of CMG-3T-360 in the high frequency band. The self-noise of JS-120 on NS channel is higher than CMG-3T-360 confidence interval in the low frequency band. When extracting long-period seismic signals, a seismometer with lower noise level in the low frequency band should be selected as much as possible.

Research paper

NUMERICAL SIMULATION OF INFLUENCING FACTORS OF SURFACE RUPTURE IN OVERLYING SOIL LAYER OF THRUST FAULT

GUO Ting-ting, XU Xi-wei, YUAN Ren-mao, YANG Hong-zhi

2023, 45(5):  1200-1218.  DOI: 10.3969/j.issn.0253-4967.2023.05.010

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When the strong earthquakes occur, the deformation and rupture of overlying soil caused by the dislocation of focal faults is one of the important reason for the destruction of ground structures. In the process of a strong earthquake or large earthquake, the deformation reaction and failure of the overlying soil of underground concealed faults are very complicated. To study and analyze the characteristics and influencing factors of surface deformation and fracture of the overlying soil layer, in this paper, the influences of fault dip Angle, fault displacement, and overlying soil thickness on surface deformation and fracture of overlying soil are analyzed by the finite element numerical simulation method comprehensively. The results show that: 1)With the increase of fault vertical dislocation of 1m to 4m, the surface equivalent strain gradually increases, the surface rupture is more likely to occur, and the surface rupture width also wider. With the increase of the thickness of the overlying soil layer from 20m to 60m, and the increase of the fault inclination from 30°, 45°, 70° to nearly 90°, the surface equivalent strain is gradually smaller, the surface rupture is more likely to occur, and the surface fracture width becomes smaller, which means that the amount of dislocation required for the same rupture state needs to increase. 2)When the vertical dislocation of the fault is about 3.3%for the thickness of the overlying soil, the surface rupture occurs only as the fault dip angle is 30°, no surface rupture occurs as the dip angle is 45° and 70°. When the vertical dislocation of the fault is about 5% of the thickness of the overlying soil, the surface rupture occurs only as the fault dip angle is 30° and 45°, no surface rupture occurs as the dip angle is 70° and approaching 90°. When the vertical dislocation of the fault is about 6.6% of the thickness of the overlying soil, the surface rupture occurs as the fault dip angle is 30°、 45° and 70°, and surface rupture is expected to occur as the dip angle is approaching 90°. When the vertical dislocation of the fault is about 10% of the thickness of the overlying soil, the surface rupture occurs as the fault dip angle is 30°, 45°, 60°and approaching 90°. 3)When the amount of vertical dislocation and the thickness of the overlying soil are certain, the ratio of surface rupture width between the hanging wall and footwall which is less affected by fault dip ranges from 3︰1 to 3︰2~1︰1 with the increase of fault dip Angle from 30°, 45° to 70°. When the fault inclination Angle is 30°, with a decrease of vertical dislocation of 4m to 1m, or the increase of overlying soil layer thickness of 20m to 60m, the ratio of surface rupture width between hanging wall and footwall is slightly larger from about 3︰1. When the fault inclination Angle is 45°, with the decrease of vertical dislocation of 4m to 1m, or the increase of overlying soil layer thickness of 20m to 60m, the ratio of surface rupture width between hanging wall and footwall is slightly larger from about 2︰1. When the fault inclination Angle is 75°, with the decrease of vertical dislocation of 4m to 1m, or the increase of overlying soil layer thickness of 20m to 60m, the ratio of surface rupture width between hanging wall and footwall is slightly larger from about 3︰2~1︰1. Under the above fault dip conditions, the ratio of surface rupture width between hanging wall and footwall is less affected by the amount of vertical dislocation and the thickness of overlying soil. As the inclination is approaching 90°, the ratio of surface rupture width between the hanging wall and footwall is about 1︰1, which is not affected by the vertical dislocation and the thickness of the overlying soil layer. 4)The deformation and fracture of the overlying soil layer first began with the soil fracture at the interface of the fault bedrock and soil. With the increase in the amount of dislocation, a fracture point appeared on the surface when the fault dip Angle was 30°, 45°, and 70°. However, when the dip Angle of the fault was close to 90°, there were two initial rupture points on the surface. With the increase of vertical dislocation or the decrease of the overlying soil layer thickness, the overlying soil layer through fracture is finally formed.

RESEARCH ON RAPID ESTIMATION METHOD OF EARTHQUAKE MAGNITUDE BASED ON STRONG-MOTION RECORDS

MENG Rui, ZHANG Yuan-fan, XIE Xiao-feng, NIE Zhi-xi, WANG Zhen-jie, SHAN Xin-jian

2023, 45(5):  1219-1232.  DOI: 10.3969/j.issn.0253-4967.2023.05.011

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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.

DISCUSSION ON SEISMOGENIC STRUCTURE OF THE JUNE 2022 M_S6.1 EARTHQUAKE AND ITS RELATIONSHIP WITH THE APRIL 2013 M_S7.0 EARTHQUAKE IN LUSHAN, SICHUAN PROVINCE

CHEN Han-lin, WANG Qin-cai, ZHANG Jin-chuan, LIU Rui-feng

2023, 45(5):  1233-1246.  DOI: 10.3969/j.issn.0253-4967.2023.05.012

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In this paper, we relocated earthquakes occurred from April 2013 to July 2022 in Lushan seismic zone, inversed focal mechanism solution of the Lushan M_S6.1 earthquake on June 1, 2022 and discussed the seismogenic structure of the Lushan M_S6.1 earthquake and its relationship with the M_S7.0 earthquake in April 2013.

The results of the focal mechanism solution show that the Lushan M_S6.1 earthquake in 2022 is a thrust earthquake. The strike, dip and azimuth of nodal plane Ⅰ are 228°, 46° and 104° and for nodal plane Ⅱ are 28°, 46° and 76° respectively. The results of earthquake relocation show that the focal depth of the Lushan M_S6.1 earthquake sequence is shallow in the north and deep in the south, the fault length is about 10km. The focal depth is mainly concentrated between 10km to 19km. The fault dip is southeast with an angle of 60°. The initial rupture point of the main shock of the Lushan M_S6.1 earthquake is at a depth of 20km, located at the deepest part of the fault. The fault ruptured from deep to shallow. The Lushan M_S7.0 earthquake occurred on April 2013 strikes northeast and dips northwestward, but there exists a reverse fault in the aftershock sequence that has the same direction of strike but the opposite direction of dip. This reverse fault is consistent with the strike and dip of the M_S6.1 earthquake occurred in June 2022. It appears as two parallel faults in the profile. In addition to the reverse fault on the west side, the embryonic of another reverse fault seems to appear on the east side of the middle of earthquake sequence. These faults are about 10km away from the surface. The distribution of earthquakes in two northwest-oriented depth profiles shows that the dip angles of the main shock and the reverse fault of the M_S7.0 earthquake is different at different locations, and these faults are not simple straight planar sections. From one year after occurrence of the M_S7.0 earthquake to occurrence of the M_S6.1 earthquake, the seismic activity on the main fault decreased but the seismic activity on the reverse fault on the west side of the M_S7.0 earthquake sequence was more active during this period, most of the seismic activity occurred near the reverse fault that is parallel to the M_S6.1 earthquake fault.

By analyzing the seismogenic structure and seismic activity characteristics of the Lushan seismic zone, we concluded the Lushan M_S6.1 earthquake on June 1, 2022 is caused by a blind thrust fault with strike towards northeast and dip towards southeast, located 10km away from the surface. It has the opposite directions of strike and dip of the Longmenshan Fault. The epicenters of the Lushan M_S7.0 earthquake in April 2013 and the M_S6.1 earthquake in June 2022 are located near the surface exposure traces of the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault, respectively. However, according to the analysis of the relocation aftershock depth in profile, the aftershock extension to the surface does not coincide with the surface exposure positions of the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault. Therefore, the seismogenic faults of these two earthquakes are not the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault, but two blind reverse faults. The Shuangshi-Dachuan Fault near the M_S6.1 earthquake sequence and the main shock fault of the 2013 M_S7.0 earthquake are thrust faults dipping northwest, while the Lushan M_S6.1 seismogenic fault has opposite direction of dip. The seismogenic fault of the Lushan M_S6.1 earthquake and the main thrust fault of the 2013 M_S7.0 earthquake, which strikes northeast and dips northwest with the reverse thrust fault of the hanging wall, which strikes northeast and dips southeast, together form a double layer Y-shaped structure. These faults are all blind thrust faults and belong to the Qianshan-Shanqian Fault system in the southern segment of the Longmenshan fault zone. The seismogenic structure in the Lushan seismic zone is a complex fault system composed of one main northeast strike fault with dipping northwest, and three faults dipping southeast.

From one year after occurrence of the Lushan M_S7.0 earthquake to the occurrence of the Lushan M_S6.1 earthquake, most of earthquakes in the Lushan seismic zone occurred near a reverse fault which is parallel to the Lushan M_S6.1 earthquake seismogenic fault. These earthquakes are located in the area where the coulomb stress change caused by the M_S7.0 earthquake acts as loading effect. The Lushan M_S6.1 earthquake sequence is mainly distributed in the area where the coulomb stress change plays an unloading role caused by the Lushan M_S7.0 earthquake. The research results showed that the coulomb rupture stress caused by the Lushan M_S7.0 earthquake on the seismic nodal plane of the M_S6.1 earthquake has a restraining effect on the M_S6.1 Lushan earthquake.