Faulting on the Anninghe fault zone, Southwest China in Late Quaternary and its movement model

The Anninghe fault is one of the significant earthquake-generating fault zones in the Southwest China. Local his-torical record shows that a M≥7 strong earthquake occurred in the year of 1536. On the basis of the detailed air-photographic interpretation and field investigation, we have acquired the following knowledge: 1 The average sinistral strike-slip rate since the Late Pleistocene is about 3~7 mm/a; 2 There is important reverse faulting along the fault zone besides the main left-lateral strike-slip motion, and the shortening rate across the Anninghe fault zone due to the reverse faulting is about 1.7~4.0 mm/a. If the Xianshuihe fault zone is simply partitioned into the Anninghe and Daliangshan faults, we can also get a slip rate of 3~7 mm/a along the Daliangshan fault zone, which is the same as that on the Anninghe fault zone. Moreover, on the basis of our field investigation and the latest knowledge concerning the active tectonics of Tibetan crust, we create a dynamic model for the Anninghe fault zone.     

Inversion of coseismic stress-triggered fault slips using borehole strainmeter observations

Coseismic stress-triggering is becoming a new hot spot of research. Coseismic strain steps recorded by borehole strainmeters are particularly valuable in studying coseismic stress-triggered fault slips. Based on the theory of dis location, one can invert the triggered fault slips with such data if he/she has a well understanding about the local faults. Genetic algorithm can be applied to significantly raise the efficiency of searching a best solution among all possibilities in this kind of inversion. A testifying check of the program and analyses of each parameter＇s influence may further enhance the reliability of inversion results. Taking complexity of geological structure into account, the inversion results should be regarded as the predominant property or a comprehensive effect of triggered local faults＇ activities. As an attempt, we inverted the assumingly active faults＇ slips triggered by the Ms=8.1 Kunlun Mountain earthquake over Beijing area.     

Late Pleistocene and Holocene slip rate of the Northern Wadi Araba fault, Dead Sea Transform, Jordan

The Wadi Araba Valley is a morphotectonic depression along part of theDead Sea Transform (DST) plate boundary that separates the Arabian plateon the east from the Sinai subplate on the west. The Wadi Araba fault(WAF) is the main strike-slip faults one of between the Gulf of Aqaba and the E-Wtrending Khunayzira (Amatzayahu) fault that bounds the southern end ofthe Dead Sea. Just south of the Dead Sea, the WAF cuts across severalgenerations of alluvial fans that formed on tributaries to the Wadi Dahalafter the regression of Late Pleistocene Lake Lisan ca. 15 ka. Geomorphicand stratigraphic evidence of active faulting, including left-laterally offsetstream channels and alluvial-fan surfaces, yielded fault slip-rate data for thenorthern segment of WAF. Typical cumulative displacements of 54 m,39 m, and 22.5 m of stream channels and alluvial-fan surfaces acrossthe fault were measured from detailed geologic and topographic mapping.The 54 m offset of the oldest alluvial-fan surface (Q _f1 ) occurredafter the final lowering of Lake Lisan (16–15 ka) and before 11 ka yieldinga slip-rate range of 3.4 mm/yr to 4.9 mm/yr. Based on radiocarbonages of charcoal and landsnail shell samples from the buried Q _f2 alluvial-fan deposits exposed in trenches excavated across the fault, the39 m and 22.5 m offsets occurred after 9 ka and 5.8 ka, respectively. These data yield a slip-rate range between 3.9 mm/yr and 6.0 mm/yr.The small variability in these slip-rate estimates for different time periodssuggests that the northern Wadi Araba fault has maintained a relativelyconstant slip rate in the past 15 ka. We calculate an average slip rate of 4.7± 1.3 mm/yr since 15 ka based on the three separate displacementsand age estimates. Five separate offsets of 3 m were measured from gullybends and the offset of small fault-scarp alluvial fans. These displacementdata suggest a coseismic slip of 3 m in the last earthquake, or acumulative slip of 3 m in the past few earthquakes. A maximum slip of3 m correspond to a Mw 7 earthquake that ruptures about 49 km offault length. Using an average slip rate of 4.7 ± 1.3 mm/yr togetherwith a 3-m slip-per-event suggests a maximum earthquake recurrence intervalof this fault segment of 500 to 885 years.     

Paleoseismic activity on Wujiahe segment of Serteng piedmont fault,inner Mongolia

Geomorphic study on Wjiahe segment of Serteng piedmont fault,Inner Mongolia is made.Throuth analysis of the available data in combination with the results of predecessors‘studies it can be obtained that average vertical displacement rate is 0.48-0.75mm/a along the Wujiahe segment since the late Pleistocene(14.450-22.340ka BP)and 0.56-0.88mm/s since the early-middle Holocene(5.570-8.830ka BP).Analyzing paleoseismic phenomena revealed in the excavated 5trenches in combination with the results of predecessors‘studies of paleoearthquakes on the fault,we determine five paleoseismic events on the Wujiahe segment of Serteng piedmont fault since 27.0ka BP and the recurrence interval to be about 4.300-4.400ka,A cluster of paleoearthquakes occurred probably during 8.000-9.000ka BP and two paleoeismic events in 10.000-20.000ka BP may be missed.A comparison between height of fault scarps and sum of displacement caused by paleoseismic events revealed in trenches,and recurrence interval of paleoseismic events obtained from average displacement rate along the fault and the disloca-tion by one event suggest that three paleoseismic events are absent in Alagaitu trench.Two paleoseismic events may be absent on the whole active fault segment.     

Shifts of the head of Zhengguo canal dug in Qin dynasty and recent activity of the fault on north margin of the Weihe downfaulted basin

Vertical differential movement rates of the fault on north margin of the Weihe downfaulted basin in recent 2000 years are analyzed by using the data of shifts of the head of Zhengguo canal, dug in Qin dynasty and the data of burial depths of historical relics and the data of levelling. The results show that in Ming dynasty this fault experienced the highest differential up-and-down movement, about 6.6 mm/a, while the rate became lower since Qing dynasty, about 2.6 mm/a. The high rate activity coincided with the period of high seismic activity in the Weihe downfaulted basin in the 15 – 16th centuries. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, 195–200, 1992. This project is sponsored by the Chinese Joint Seismological Science Foundation.     

Seismotectonic environment of occurring the February 3, 1996 Lijiang M=7.0 earthquake, Yunnan Province

Lijiang-Daju fault, the seismogenic fault of the 1996 Lijiang M=7.0 earthquake, can be divided into Lijiang-Yuhu segment in the south and Yuhu-Daju segment in the north. The two segments show clear difference in geological tectonics, but have the similar dynamic features. Both normal dip-slip and sinistral strike-slip coexist on the fault plane. This kind of movement started at the beginning of the Quaternary (2.4～2.5 Ma B.P.). As to the tectonic types, the detachment fault with low angle was developed in the Early Pleistocene and the normal fault with high angle only after the Mid-Pleistocene (0.8 Ma B.P.). Based on the horizontal displacements of gullies and the vertical variance of planation surfaces cross the Lijiang-Daju fault at east piedmont of Yulong-Haba range, the average horizontal and vertical slip rates are calculated. They are 0.84 mm/a and 0.70 mm/a since the Quaternary and 1.56 mm/a and 1.69 mm/a since the Mid-Pleistocene. The movements of the nearly N-S-trending Lijiang-Daju fault are controlled not only by the regional stress field, but also by the variant movement between the Yulong-Haba range and Lijiang basin. The two kinds of dynamic processes form the characteristics of seismotectonic environment of occurring the 1996 Lijiang earthquake.     

海原断裂带老虎山段晚第四纪滑动速率精确厘定与讨论

如何准确测定断裂滑动速率是近年来活动构造研究的前沿与热点.随着高精度地形数据获取手段与第四纪测年方法的不断进步,位错量和地貌面年龄的精度均得到大大提高.在进行滑动速率计算时还要考虑地质过程是否合理,蒙特卡洛方法为获取更加符合地质过程的滑动速率提供了重要工具.本文以滑动速率研究程度较低的海原断裂带老虎山段为例,基于LiDAR高精度地形数据,测得T1—T4阶地面年龄分别为1~3 ka,9~11 ka,15~17 ka,40~45 ka,陡坎前缘的位错分别为7~14 m,28~36 m,59~66 m,180~190 m.综合多地点的左旋走滑位错量及不同时代的地貌面年龄数据,并考虑滑动历史,利用蒙特卡洛模拟方法,将位错-时间两个参数的不确定性定量化,限定老虎山断裂45 ka以来平均滑动速率为4.3±0.16 mm·a^-1,17ka以来的平均滑动速率为4.0±0.15 mm·a^-1,与前人研究得到的狭义海原断裂滑动速率4.5±1.0 mm·a^-1基本一致.综合整个海原断裂带滑动速率,本文结果更支持低滑动速率变化趋势,即海原断裂带整体滑动速率趋于稳定,向东至六盘山断裂,滑动速率开始降低,推测海原断裂带的左旋走滑在尾端主要为马东山—六盘山隆起所吸收.结合老虎山断裂历史地震资料和深部锁闭浅部蠕滑的动力学特征,推测老虎山断裂具备与相邻断裂一起触发强震的能力.     

藏南宁金抗沙西麓断裂晚第四纪活动特征

宁金抗沙西麓断裂位于亚东-谷露裂谷系南部.本文利用15 m分辨率的ETM 遥感影像和20 m分辨率的数字高程模型,结合野外地质考察,重点研究了该断裂的详细结构和晚第四纪以来的活动习性.依据与热隆盆地的关系,该断裂主要分为三段,热隆盆地以北为北段,热隆盆地边界断裂为中段,热隆盆地以南为南段.该断裂在强烈正断的同时兼具有左旋走滑运动,自中更新世以来,主要有三期活动,活动开始时间分别为384 ka1、08 ka和34 ka,累积水平活动速率分别为4.8 mm/a5、.5 mm/a和6.4 mm/a.断裂自中更新中期以来的垂直断错速率为1 mm/a.     

GPS monitoring of temporal deformation of the Xianshuihe fault

Highly precise (σ ~1 mm) temporal deformation measurements are taken across the Xianshuihe fault from two pairs of continuous GPS stations straddling the fault. Baseline vector changes of the two pairs of stations show clearly the difference in deformation behavior between the Qianning and Daofu segments of the fault: the former deforms steadily, and the latter deforms with a strong transient component. The transient deformation across the Daofu segment is possibly related to its irregular geometry, where the fault splits into two branches, that is, the east and west branches. An attempt is made to interpret the baseline vector changes using a kinematic fault model composed of a brittle layer in the upper crust, a ductile layer in the lower crust, and a transition zone in between. The slip in the transition zone of the south segment of the Xianshuihe fault is steady. The slips in the transition zones of the north and Daofu segments of the Xianshuihe fault, however, are not steady, and the average slip rates there are higher than that of the south segment. The difference in deformation behavior is probably associated with the rheological properties of the fault interface, suggesting that the overall fault strength of the south segment is greater than those of the north and Daofu segments, corresponding to longer earthquake recurrence time.     

Fractal research of fault gouge

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Spatial and temporal characteristics of acoustic emission activity during deformation of rock samples with inhomogeneous fault under biaxial compression

The spatial and temporal distributions of acoustic emission （AE） during the deformation of samples containing an inhomogeneous fault have been studied under biaxial compression. The results show that the fault strength and the duration from loading to failure increase and the failure mode changes from abrupt instability to gradual failure with increase of lateral stress σ2. The pre-setting fault and its heterogeneity play an important role in controlling AE spatial distribution during the deformation. The basic pattern of AE spatial distribution is controlled by the pre-setting fault, especially the parts with inhomogeneous strength and the strong segment, and the localization of fracturing starts from the positions with inhomogeneous strength, With increase of σ5, the dense AE distribution area spreads from the positions with inhomogeneous strength to the whole strong segment gradually. AE temporal sequence is significantly affected by σ2. The fault shows abrupt instability in final failure, which occurs on a background of ＂enhancement-quiescence＂ in microfracturing activity at lower σ2. At higher σ5, the fault shows gradual failure, and AE occurrence rate increases continuously and AER increase exponentially before and after the failure, The effect of σ2 on b-value is also remarkable, b-value shows precursory decrease in the weakening stage when fault behavior is abrupt instability, but it shows balanced change when fault behavior is gradual failure,     

Rates of crustal deformation in the North Aegean trough-North Anatolian fault deduced from seismicity

The rates and configuration of seismic deformation in the North Aegean trough-North Anatolian fault are determined from the moment tensor mechanisms of the earthquakes that occurred within this region. The analysis is based onKostrov's (1974) formulation. The fault plane solutions of the earthquakes of the period 1913–1983 withM _s 6.0 are used. The focal mechanism of some of the past events (before 1960) is assumed, based on the present knowledge of the seismotectonics as well as on the macroseismic records of the area studied. The analysis showed that the deformation of the northern Aegean is dominated by EW contraction (at a rate of about 15 mm/yr) which is relieved by NS extension (at a rate of about 9 mm/yr). It was also shown that the northern part of North Anatolia (north of 39.7°N parallel) undergoes contraction in the EW direction (at a rate of about 9 mm/yr) and NS extension as the dominant mode of deformation (at a rate of about 5 mm/yr). It may be stated therefore, that the pattern of deformation of the northern Aegean and the northern part of North Anatolian fault is controlled by the NS extension the Aegean is undergoing as a whole, and the dextral strike-slip motion of the North Anatolian fault. The southern part of North Anatolia is undergoing crustal thinning at a rate of 2.3 mm/yr, NS extension (at a rate of 5 mm/yr) as well as EW extension (at a rate of 4 mm/yr), which are consistent with the occurrence of major normal faulting and justify the separation of North Anatolia into two separate subareas.     

Seismo-tectonic divisions of strong earthquakes with M S≥7.0 and their tectonic geomorphology along Xianshuihe-Xiaojiang fault zone

Seismo-tectonic areas of historical strong earthquakes with M _S≥7 along Xianshuihe-Xiaojiang fault zone are divided, and their individual fault-pattern and tectonic geomorphology are analyzed. Those strong-earthquake areas are located in some special parts of the fault zone, where the major branch-faults of the fault zone form left stepping, parallel, and fork-like patterns. In the strong-earthquake areas structurally complicated basins are developed, such as pull-apart basins in fork-like area, in double stepping area, and in stepping and fork-like areas. Foundation item: Chinese Joint Seismological Science Foundation (9507424). Contribution No. 2001A003, Institute of Crustal Dynamics, China Seismological Bureau.     

Earthquake probabilities and magnitude distribution ( M ≥6.7) along the Haiyuan fault, northwestern China

In recent years, some researchers have studied the paleoearthquake along the Haiyuan fault and revealed a lot of paleoearthquake events. All available information allows more reliable analysis of earthquake recurrence interval and earthquake rupture patterns along the Haiyuan fault. Based on this paleoseismological information, the recurrence probability and magnitude distribution for M≥6.7 earthquakes in future 100 years along the Haiyuan fault can be obtained through weighted computation by using Poisson and Brownian passage time models and considering different rupture patterns. The result shows that the recurrence probability of M _S≥6.7 earthquakes is about 0.035 in future 100 years along the Haiyuan fault. Foundation item: Joint Seismological Science Foundation of China (103034) and Major Research “Research on Assessment of Seismic Safety” from China Earthquake Administration during the tenth Five-year Plan.     

Holocene activities of the Taigu fault zone, Shanxi Province, and their relations with the 1303 Hongdong M =8 earthquake

The Taigu fault zone is one of the major 12 active boundary faults of the Shanxi fault-depression system, located on the eastern boundary of the Jinzhong basin. As the latest investigation indicated, the fault zone had dislocated gully terrace of the first order, forming fault-scarp in front of the loess mesa. It has been discovered in many places in ground surface and trenches that Holocene deposits were dislocated. The latest activity was the 1303 Hongdong earthquake M=8, the fault appeared as right-lateral strike-slip with normal faulting. During that earthquake, the Taigu fault together with the Mianshan western-side fault on the Lingshi upheaval and the Huoshan pediment fault on the eastern boundary of the Linfen basin was being active, forming a surface rupture belt of 160 km in length. Moreover, the Taigu fault were active in the mid-stage of Holocene and near 7 700 aB.P. From these we learnt that, in Shanxi fault-depression system, the run-through activity of two boundary faults of depression-basins might generate great earthquake with M=8. Foundation item: Chinese Joint Seismological Science Foundation (201017). Contribution No. 2003A004, Institute of Crust Dynamics, China Earthquake Administration.     

An equivalent-boundary method for the shell analysis of buried pipelines under fault movement

A new shell finite element method (FEM) model with an equivalent boundary is presented for estimating the response of a buried pipeline under large fault movement. The length of affected pipeline under fault movement is usually too long for a shell-mode calculation because of the limitation of memory and time of computers. In this study, only the pipeline segment near fault is modeled with plastic shell elements to study the local buckling and the large section deformation in pipe. The material property of pipe segment far away from the fault is considered as elastic, and nonlinear spring elements at equivalent boundaries are obtained and applied to two ends of shell model. Compared with the fixed-boundary shell model, the shell model with an equivalent boundary proposed by the study can remarkably reduce the needed memory and calculating time. Foundation item: National Natural Sciences Foundation of China (50078049). Contribution No. 04FE1017, Institute of Geophysics, China Earthquake Administration.     

A 48-kyr-long slip rate history for the Jordan Valley segment of the Dead Sea Fault

We investigate the late Quaternary active deformation along the Jordan Valley segment of the left-lateral Dead Sea Fault and provide new insights on the behaviour of major continental faults. The 110-km-long fault segment shows systematic offsets of drainage systems surveyed at three sites along its southern section. The isotopic dating of six paleoclimatic events yields a precise chronology for the onset of six generations of gully incisions at 47.5 ka BP, 37.5 ka BP, 13 ka BP, 9 ka BP, 7 ka BP, and 5 ka BP. Additionally, detailed mapping and reconstructions provide cumulative displacements for 20 dated incisions along the fault trace. The individual amounts of cumulative slip consistently fall into six distinct classes. This yields: i) an average constant slip rate of 4.7 to 5.1 mm/yr for the last 47.5 kyr and ii) a variable slip rate ranging from 3.5 mm/yr to 11 mm/yr over 2-kyr- to 24-kyr-long intervals. Taking into account that the last large earthquake occurred in AD 1033, we infer 3.5 to 5 m of present-day slip deficit which corresponds to a Mw  7.4 earthquake along the Jordan Valley fault segment. The timing of cumulative offsets reveals slip rate variations critical to our understanding of the slip deficit and seismic cycle along major continental faults.     

Retardations in fault creep rates before local moderate earthquakes along the San Andreas fault system,central California

Records of shallow aseismic slip (fault creep) obtained along parts of the San Andreas and Calaveras faults in central California demonstrate that significant changes in creep rates often have been associated with local moderate earthquakes. An immediate postearthquake increase followed by gradual, long-term decay back to a previous background rate is generally the most obvious earthquake effect on fault creep. This phenomenon, identified as aseismic afterslip, usually is characterized by above-average creep rates for several months to a few years. In several cases, minor step-like movements, called coseismic slip events, have occurred at or near the times of mainshocks. One extreme case of coseismic slip, recorded at Cienega Winery on the San Andreas fault 17.5 km southeast of San Juan Bautista, consisted of 11 mm of sudden displacement coincident with earthquakes ofM _L =5.3 andM _L =5.2 that occurred 2.5 minutes apart on 9 April 1961. At least one of these shocks originated on the main fault beneath the winery. Creep activity subsequently stopped at the winery for 19 months, then gradually returned to a nearly steady rate slightly below the previous long-term average.The phenomena mentioned above can be explained in terms of simple models consisting of relatively weak material along shallow reaches of the fault responding to changes in load imposed by sudden slip within the underlying seismogenic zone. In addition to coseismic slip and afterslip phenomena, however, pre-earthquakeretardations in creep rates also have been observed. Onsets of significant, persistent decreases in creep rates have occurred at several sites 12 months or more before the times of moderate earthquakes. A 44-month retardation before the 1979M _L =5.9 Coyote Lake earthquake on the Calaveras fault was recorded at the Shore Road creepmeter site 10 km northwest of Hollister. Creep retardation on the San Andreas fault near San Juan Bautista has been evident in records from one creepmeter site for the past 5 years. Retardations with durations of 21 and 19 months also occurred at Shore Road before the 1974 and 1984 earthquakes ofM _L =5.2 andM _L =6.2, respectively.Although creep retardation remains poorly understood, several possible explanations have been discussed previously. (1) Certain onsets of apparent creep retardation may be explained as abrupt terminations of afterslip generated from previous moderate-mainshock sequences. (2) Retardations may be related to significant decreases in the rate of seismic and/or aseismic slip occurring within or beneath the underlying seismogenic zone. Such decreases may be caused by changes in local conditions related to growth of asperities, strain hardening, or dilatancy, or perhaps by passage of stress-waves or other fluctuations in driving stresses. (3) Finally, creep rates may be lowered (or increased) by stresses imposed on the fault by seismic or aseismic slip on neighboring faults. In addition to causing creep-rate increases or retardations, such fault interactions occasionally may trigger earthquakes.Regardless of the actual mechanisms involved and the current lack of understanding of creep retardation, it appears that shallow fault creep is sensitive to local and regional effects that promote or accompany intermediate-term preparation stages leading to moderate earthquakes. A strategy for more complete monitoring of fault creep, wherever it is known to occur, therefore should be assigned a higher priority in our continuing efforts to test various hypotheses concerning the mechanical relations between seismic and aseismic slip.     

甘加盆地西缘断裂带发现新活动证据

甘加盆地位于西秦岭北缘断裂带西端,盆地西缘发育了三条近南北向的断层（东支、中支、西支）,表现为西高东低的弧形地貌阶梯带。基于高分辨率卫星遥感影像解译、地质地貌调查、UAV航拍测量、剖面清理与¹⁴C测年等工作,对甘加盆地西缘断裂带进行综合研究,发现该断裂带第四纪晚期有新活动。断层新活动在地貌上表现为不同级别的断层陡坎、阶地与洪积台地断错以及冲沟与阶地右旋等特征。断层剖面与擦痕揭示,该断裂带具右旋逆冲性质,断层断错最新地层为全新统。根据测量与测年数据,初步估算甘加盆地西缘断裂带西支全新世中期以来的平均水平右旋滑动速率为0.89 mm/a,平均垂直滑动速率为0.30 mm/a。     

Slip Rate on the Altyn Tagh Fault on the West of the Cherchen River (Between 85°～85°45′E) Since Late Quaternary

Because of the significance to the formation and evolution of the Tibetan plateau, the displacement and slip rate of the Altyn Tagh fault have been topics full of disputation. Scientists who hold different opinions on the evolution of Tibet insist on different slip rates and displacements of the fault zone. In the article, study is focused on the late Quaternary slip rate of the Altyn Tagh fault west of the Cherchen River (between 85°E and 85°45'E). On the basis of high resolution SPOT images of the region, three sites, namely Koramlik, Aqqan pasture and Dalakuansay, were chosen for field investigation. To calculate the slip rate of the fault, displacement of terraces was measured on SPOT satellite images or in situ during fieldwork and thermo-luminescence (TL) dating method was used. To get the ages of terraces, samples of sand were collected from the uppermost sand beds that lie just under loess. The method for calculating slip rate of fault is to divide the displacement of terrace risers by the age of its neighboring lower terrace. The displacement of rivers is not considered in this article because of its uncertainties. At Koramlik, the slip rate of the Altyn Tagh fault is 11.6±2.6mm/a since 6.02±0.47ka B.P and 9.6±2.6mm/a since 15.76±1.19ka B.P. At Aqqan pasture, about 30km west of Koramlik, the slip rate is 12.1±1.9mm/a since 2.06±0.16 ka B.P. At Dalakuansayi, the slip rate of the fault is 12.2±3.0mm/a since 4.91±0.39ka B.P. Hence, we get the average slip rate of 11.4±2.5mm/a for the western part of the Altyn Tagh Fault since Holocene. This result is close to the latest results from GPS research.