No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Remarks on urban active faults exploration and associated activity assessment
Abstract
According to the practice of urban active fault exploration and associated activity assessment conducted in recent years, this paper summarizes the problems encountered in geological, geomorphological, geochemical and geophysical explorations, and proposes the following means and suggestions to solve these problems. To determine the most recent faults or fault zones, emphases should be placed on identifing the youngest active faults and offset geomorphology. To understand the history of faulting and to discover the last offset event, it is suggested to combine geophysical prospecting, drilling and trenching into one profile. Because of significant uncertainties in late Quaternary dating, it is advised to sample systematically and to use multiple dating methods. Shallow seismic reflection has been proven to be the most useful method in urban active fault exploration. However, it is a pressing need to increase the quality of data acquiring and data processing to obtain high resolution images, so that the ability of identifying active faults could be enhanced. Combination of seismic P-wave reflection and S-wave reflection methods is proved to be a powerful means to investigate tectonic environments of deep crust.
... After the active fault survey, reasonable measures should be taken to avoid seismic hazard caused by active fault for general land-use planning and building construction (Xu XW et al., 2016). This preventive measure is significantly important to improve our capability of earthquake disaster reduction (Deng QD et al., 2007). The Chengnanhe fault has no obvious indication in topography and landform, only bedrock scarps and slopes are found locally. ...
For city planning and reducing potential earthquake risk, it’s necessary to detect the information of the buried faults in an urban area especially, including the location and activities. An integrated technique with geophysical and geological methods, including the shallow seismic reflection profile, electrical resistivity measurement, geologic borehole section, and exploration trench, was used to detect the Chengnanhe fault, which is one of the two main faults passing through the Weihai urban area in Shandong province, China. The results show that it is a normal fault striking with E-W direction, and it is relatively inactive and stable. By using the thermoluminescence (TL) dating, we found that the Chengnanhe fault initiated in mid-Pleistocene and there was no offset after late Pleistocene. Such an integrated technique with multiple geological and geophysical methods provides a significant assessment of earthquake risk for city planning in urban areas.
... A buried fault is one that is hidden by loose Quaternary sediments, and is difficult to observe because it has no trace on the surface. Identifying the exact positions of active buried faults and taking corresponding precautionary measures can effectively reduce the damage caused by earthquakes ( Deng et al., 2007;Shao et al., 2007;Zhang et al., 2013); however, questions remain over the best approaches to use to make those identifications. ...
Studying the faults in Wuhai Basin, which is located in the northwestern corner of the Ordos Block in China, can not only guide earthquake-safe construction in the region, but also provide a greater understanding of the structure of the Wuhai Basin and the dynamic environment of Northwestern Ordos Block. In this study, we effectively detected the Yellow River buried Fault (YRF), which we believe currently presents a risk to human life in the area. First, we laid out a deep seismic reflection profile (DSRP) in the north of the Wuhai Basin and found that the YRF is composed of two subparallel faults, the east branch of the Yellow River buried Fault (EYF) and the west branch of the Yellow River buried Fault (WYF), which formed a Y-type graben in the section. We then used 25 shallow seismic exploration profiles to find out how the faults are distributed. Finally, we used drillings and Quaternary dating methods to certify their existence and obtain their activity parameters. We used the sequence stratigraphy method to compare sediment strata and identified paleo-earthquake events by finding the unequal thickness layers (UTLs) on both sides of the fault. The drillings revealed that four earthquakes occurred on the YRF around 25.6 ± 0.11 ka BP, 39.5 ± 0.45–41.7 ± 0.57 ka BP, 58.25 ± 7.13 ka BP, and 111 ± 1.21 ka BP. Since the last activity on the YRF occurred much longer ago than its estimated recurrence cycle, we believe that the fault YRF currently presents a dangerous risk. Our study confirms that the YRF is a normal fault graben and that the Wuhai Bain is an extensional basin formed by the relative movement of the Tibetan Plateau, the Alashan Block, and the Ordos Block.
Due to the difficulty in detecting the upper breakpoint of active buried faults, high-resolution detection of active buried faults in urban areas is very important for urban planning and disaster risk mitigation. The seismic reflection technique is the most common technique used to image faults at depth, but it is difficult to precisely locate the upper breakpoint of active buried faults. Ground-penetrating radar (GPR) provides high-resolution imaging in ultrashallow exploration, but the penetration depth of traditional GPR is shallow. In this work, we conducted six GPR surveys with multifrequency (25 MHz, 50 MHz and 100 MHz) and high-power GPR antennas across the possible range of the Shuiyu fault based on seismic reflection results. We obtained the trace of the Shuiyu fault within 45 m of the surface in Datong city, and the fault was found to dip approximately 70–80° to the SE. According to the GPR results, we selected a location for trenching and excavated one trench across the Shuiyu fault. The trench results verified the accuracy of the fault imaging in the GPR profiles and helped to interpret the stratigraphic information in the GPR profiles. In the accurate detection of active buried faults in other urban areas, the seismic reflection method can be used to determine the main strike of faults at depth, and then GPR can be used to detect the near-surface distribution of the faults in detail and guide trench location selection. The application of high-power GPR antennas with different frequencies in the fast detection of the upper breakpoint of active buried faults in urban areas is of great significance to earthquake hazard analysis and urban planning in Datong city.
In order to redefine the activity of the Benchahe fault, according to target layers with different burial depths, we followed the work rationale of 'from known to unknown, from deep to shallow and step by step'. We adopted not only the multi-level seismic detection method but also the joint exploration of P- and SH-waves. We obtained high-resolution images of the stratum structure in the depth range from 1350 m to near the surface and clear image results of the Benchahe fault. This exploration research accurately locates the Benchahe fault, and further shows that it is not a single fault but rather a fault-terrace belt consisting of two N-dipping faults within the effective detection depth. We clarify that the up-breakpoint of the southern section (F1-1) of the Benchahe fault is shallower and more active. The Benchahe fault is a near EW-trending fault, dipping north with an apparent dip angle of around 60°-75°. Subsequently, through geologic borehole section verification, we show that the results of the multi-level and multi-wave shallow seismic reflection method, determining the relevant parameters and crossing the location of the fractures, are reliable and accurate. We can observe that the reflective layers and fault characteristics revealed by the seismic profiles correspond well with the geologic borehole section. The depth of the upper breakpoint discernible from the borehole section is 93.8 m, which belongs to the top of the middle Pleistocene strata. Therefore, we may safely conclude that the newest active time of the Benchahe fault is late mid-Pleistocene to early late Pleistocene.
Few large earthquakes (Ms ≥ 7) have ever occurred in the historical records in eastern China, especially in southeast continental region of 105° - 120°E, 20° - 35°N. However, many moderate-strong earthquakes with magnitude between 5 and 6 occurred there, such as the 1962 Heyuan Ms 6.4 earthquake and 1969 Yangjiang Ms 6.7 earthquake in Guangdong province, the 1974 Liyang Ms 6.0 earthquake in Jiangsu province, 1979 Guzhen Ms 5.0 earthquake in Anhui province, and the 2005 Jiujiang Ms 5.7 earthquake in Jiangxi province and so on. These earthquakes have no significant earthquake ruptures, and few late Pleistocene active faults were discovered. So, the seismogeological background of the above moderate-strong earthquakes is still unclear up to the present, that is to say, it is very necessary to study the seismogeologic evidence for moderate earthquakes. In the paper, by analysis of the seismogeologic evidence of the 1979 Liyang Ms 6.0 earthquake in Jiangsu province, the 2005 Jiujiang-Ruichang Ms 5.7 earthquake in Jiangxi province, the 1710 Xinhua Ms 5 1/2 earthquake in Hunan province, the 1917 Huoshan Ms 6 1/4 earthquake in Anhui province and the moderate-strong earthquakes around Dongting Lake, some conclusions are obtained as follows: 1) most moderate-strong earthquakes in east China occurred at regions near the early Quaternary active faults, and regions with early-middle Pleistocene active faults; 2)most moderate-strong earthquakes in East China are related to the development and distribution of the Quaternary down-faulted basins. 3) moderate-strong earthquakes in East China may occur at regions with evident active tectonic geomorphological characteristics in Quaternary, such as the areas with linear fault geomorphology or geomorphologic surface; 4 ) moderate-strong earthquakes may occur in the seismic gaps along the seismic zones where earthquakes with magnitudes 4-5 occurred in the history.
Huangzhuang-Gaoliying fault, located in the northwest edge of Beijing fault depression belt, is an important buried fault in the Quaternary tectonic period. To know the geological characteristics of the fault, high-resolution seismic exploration with Mini-sosie vibrator was conducted. The study focuses on the part of the fault through the Olympic park by using Minisosie method and near-surface seismic profile. Three-component wave equation modeling and ray tracing method were implemented to design the geometry and parameters in field data acquisition to meet the requirement for city seismic survey. The decoding principle and the anti-noise capacity of Mini-sosie vibrator were analysed to demonstrate that the Mini-sosie is feasible for the city environment with strong noise. After high-solution data processing, a seismic depth migration section with high-resolution in the strong noise environment in this survey area was obtained. Huangzhuang-Gaoliying fault in the seismic depth migrated section can been seen clearly. The result of geological interpretation clarified the thickness of Cenozoic strata in the two sides of the fault.
In 1585,an earthquake with Ms 5% happened in the south of Chaoxian of Anhui Province,and the parameters of this earthquake listed in earthquake catalogues of varied versions are different. According to the detail textual research of the historical earthquake records, the epicenter location of this earthquake is further confirmed by means of field seismo-geological investigation in the ChaohuTongling region of the western Changjiang valley. Shallow seismic prospecting and drilling methods are applied to study the buried fault, the possibility of existence of seismogenic fault and fault activity in the western Changjiang valley area are analyzed in depth, and the causative tectonic background of the 1585 Ms 5% south Chaoxian earthquake is studied. The result of this study indicates that the Yanjiaqiao-Fengshahu Fault,which was active in early Pleistocene to mid-Pleistocene,is possibly the causative structure of this earthquake. To identify the seismogenic structure of the 1585 south Chaoxian earthquake is helpful to deeply know the tectonic background of the moderate and small earthquake activities in eastern China.
This paper reviews the history and progresses of the research on active tectonics in China and overseas. By giving a brief introduction on histories of active tectonic researches in China and some other countries, the paper sums up the process and development on the quantitative investigation of active tectonics since the 1980's, and puts the focus on the main efforts and progresses that have been made in China on some aspects of the research, such as the basic survey and research and the applied investigation of active tectonics, the study on theories about regional active tectonics and their kinematics and geodynamics, the survey on the coupling relations between deep and shallow structures, the project on active fault survey and prospecting and seismic hazard assessment in urban areas, as well as the efforts on using the Quaternary geochronology. Further, the paper looks back to the Chinese efforts on the quantitative investigation of active tectonics, sums up those cognitions from studies on the determination of several basic and measurable parameters of active tectonics, such as the length of fault and fault-segment, coseismic slip and cumulative slip, fault slip rate, sequence of paleo-earthquake events, and the time elapsed from the latest event. At the same time, efforts and progresses in China on assessing long-term seismic potential for active fault and evaluating potential risk from potential active fault movement have been reviewed by summarizing researches on developing theories, models, methods and on application to the probabilistic assessment of time-dependent seismic potential, magnitude estimation for potential earthquake on active fault, and forecasting the potential risk caused by potential active fault movement. Finally, considering the realities and problems in the research of active tectonics in China, the authors put forward several suggestions for issues worthy of paying more attention to for further investigation in the future.
This paper reviewed the progress of active fault and the related avoidance distance; (1) It summarized the latest on active fault and proposed emphatically the definition of active fault for the project or from the research for setback ; (2) It induced and evaluated the related standard of the world related country and expert ' s relevant research for active fault ; (3) For Setback of Surface-Fault-Rupture, it concluded that the direction of fault precise positioning for construction and setback established for project classification should be researched intentionally. Also the reference and the idea are proposed herein.
Active fault is one of potential geohazards in cities. Locating and dating buried active faults in urban areas have been a difficult issue in active fault exploration. In this paper, we take the detection of the buried active fault performed at Hehuan Road in the north of Suqian city as an example. We preliminarily mapped the fault through field investigation and shallow seismic reflection survey technique. Furthermore, based on the principle of doubling section method, we conducted multiple drilling to constrain the upper faulted point which is located in a range of 5m in horizon and 4.4~6.1 m in depth. Finally, we determined the exact location and latest activity of the fault by trenching. Obviously, good results have been acquired on the accurate location and activity of the Suqian segment of Anqiu-Juxian Fault using multi-level and multi-means detection method. Besides, we observed from the detection at the Hehuan Road site that at least four paleoseismic events occurred during the past 80000 yrs, and the result indicates that the latest faulting event on the fault is younger than (5.9±0.3) ka BP and the buried active fault at the Hehuan Road is a Holocene active fault. The result of buried active fault detection at the Hehuan Road site provides quantitative parameters for evaluation of seismic hazards and planning the width of safety distance in Suqian City.
A numerical experiment using seismic wave equation forward modeling was carried out to test the feasibility of reflection seismic exploration of a thrust underlying an unconformity. The geological targets for the exploration include the fault and a few geological lenses as thin as only a few meters. Using the geological information available, a seismic model was built. Seismic wave equation forward modeling was then performed to mimic the approach of seismic acquisition. In total 38 shot gath-ers of seismic data were synthesized. Then data processing yielded a seismic velocity profiles, a CDP stack profile and a migration profile. The experiment results show that, if high frequency source signals are used, seismic reflection exploration can ef-fectively reveal the thrust fault and the geological lenses. Our experiments show that using source signals with dominant frequency at 150Hz can achieve the best resolution.
Vector resolution, a typical angle-domain de-noising approach, uses the deviation angle between signal and noise to remove noise. We review briefly the vector resolution method and existing improved measures, and propose a new measure which uses curved time-window scheme according to its errors. The key point is to build a curved time-window based on the best time-shift values for maximum correlation coefficients between each trace and the center trace in the time window. Numerical simulation and real data de-noising test results show that the proposed method can better remove noise. We also perform real data tests with the proposed approach and frequency-domain de-noising methods such as pass-band filtering and wavelet threshold de-noising. The proposed approach gets better results event for seismic data in which signal and noise have very close frequency.
Displacements along active faults buried directly beneath major cities create devastating earthquakes that seriously threaten human lives and properties. Exploration and seismic hazard assessment of active faults in urban areas are thus important for disaster mitigation in major cities. It is also a new field for active tectonic studies. The kernel of this work includes determining of the exact location of active faults, dating the ages of last tectonic activity, relating the shallow level faults to structures in the crustal depth, assessing seismic hazard and potential for surface offsets, and formulating countermeasures for disaster mitigations. We use the following simple phrases to express these key scientific problems: Where are the faults? Are they active? How deep are they? Will they create earthquake? Will they form surface offsets? What are the countermeasures? This paper explains these key scientific problems in detail.