Study on Paleoearthquakes of the Eastern Liupanshan Piedmont Fault Zone

Based on trench excavation and detailed field investigation, the authors have ascertained that 6 paleoearthquake events have occurred in the Haizixia area of the Eastern Liupanshan Piedmont fault zone since 46Ka B.P. The recurrence time of 6 events were about 15000a, 20250a, 14750a, 12150a, 8550a and 4000a B.P., respectively, the recurrence intervals were about 15000a, 5500a, 2600a, 3600a and 4550a, respectively. According to M-D empirical relations, the vertical dislocation of events 1 and 2 corresponded to the dislocation of the earthquake with M8.0; the vertical dislocation of events 3-6, was corresponded to the dislocation of earthquakes with M6.5~7.5.

Using borehole core analysis to reveal Late Quaternary paleoearthquakes along the Nankou-Sunhe Fault,Beijing

The Nankou-Sunhe Fault is a buried active normal fault that traverses the urban area of Beijing.Its seismic risks have caused considerable concerns.This paper studies paleoearthquakes along this fault by analyzing and correlating bore-hole cores obtained from triple-tube coring,incorporating experience acquired from trenching.As a result,a model for identifying earthquake-derived colluvium by sediment-core analysis is proposed.Triple-tube coring technique is useful to collect continuous undis-turbed soil core near the Nankou-Sunhe Fault.By identifying fault-scarp colluviums,determining cumulative displacement,and analysing stratum thickening on the hanging wall,we are able to establish a preliminary paleoearthquake sequence consisting of 13 surface-rupturing events since 60 ka.The seismic history can be divided into three periods based on different recurrence intervals.Between 60 and 40 ka,three earthquakes occurred with recurrence interval of ～10 ka.From 40 to 25 ka,there were six earthquakes with the recurrence interval of about 2.5 ka.In the last 25 ka,four earthquakes have taken place with the recurrence interval varying considerably.The recurrence interval between the last three events is ～5 ka.Smaller recurrence intervals correspond to stages of faster fault slip.The coseismic displacement of a single event is 0.8 to 2.2 m,average 1.4 m,largely equivalent to moment magnitudes 6.7-7.1.This study demonstrates the feasibility of bore-hole drilling in investigating paleoearthquakes along normal faults.It also suggests that closely spaced boreholes with continuous undisturbed cores are essential for reconstructing the complete paleoearthquake sequence.

Colluvial wedges associated with pre-historical reverse faulting paleoearthquakes

Colluvial wedges collapsed from fault scarp can also be used to study reverse faulting paleoearthquakes. Generating processes of reverse faulting colluvial wedges are much more complex than those associated with normal faulting earthquakes. Reverse faulting colluvial wedge is also in triangle shape, and dies away from the fault. Contact between the fault and the colluvial wedge may be a simple straight reverse fault or a combination of an erosive surface in the upper part and a reverse fault in the lower part. Contents and grain sizes increase near the fault and along the base of a colluvial wedge. Based on examples from the piedmont reverse fault and fold along the northern Tainshan, we studied characteristics of reverse faulting colluvial wedges, and discussed the generating processes of reverse faulting colluvial wedges. Reverse faulting generates an unstable scarp hanging in the air immediately after an earthquake. Fallen material deposits along the base of newly formed fault scarp. Erosive

A palaeoearthquake event and its age revealed by the travertine layer along the Litang fault in the southeastern margin of the Qinghai-Tibetan plateau

The Litang fault(LTF),located in the southeast of the Qinghai-Tibetan Plateau,is known for its high level of present-day seismicity,whereas its Pleistocene activity has been scarcely documented.This study focused on a tract of banded travertine deposits precipitated from thermal waters along the NW–SE-trending LTF trace.The role of travertine deposits in recording neotectonic activity has been studied by identifying their internal structure.Typical soft-sediment deformation structures observed within the banded travertines include micro folds,liquefied breccia,and liquefied diapirs.These deformed structures,which are restricted to a single unit separated unconformably by undeformed layers,can be traced for tens of meters,indicating that they were formed by seismic shaking triggered by LTF activity.The deformation of the banded travertine layers is attributed to the combined effects of seismic shaking,liquefaction,and fluidization,and it can be related to a paleo earthquake event with a magnitude of MS>5.The U-series ages obtained from the banded travertine deposits perturbed by the earthquakes are in the range of 130.59–112.94 ka,indicating an important fault-assisted neotectonic activity that occurred during the Middle–Late Pleistocene.Analysis of such structures,in combination with the use of U-series dating methods,can yield a reliable timing of neotectonic activity and provide new evidence for under-standing the seismotectonic setting of the Litang area.

Primary study on regional paleoearthquake recurrence behavior

Based on detailed studies of paleoearthquakes along major active faults in the transition area between the north eastern margin of Tibetan Plateau and the Ordos block, this paper discusses temporal and spatial distribution of paleoearthquakes and their regional recurrent behavior. The regional paleoarthquake recurrence model in the area exhibits features of temporal and spatial clustering, which may be divided into two kinds. One has a time span about 300 years, and the other has about 1 000 years.

Late Quaternary Large Earthquakes on the Western Branch of the Xiaojiang Fault and Their Tectonic Implications

The Xiaojiang fault is a major active left-lateral fault along the southeastern margin of the Tibetan Plateau.The largest historical earthquake in Yunnan Province, with a magnitude 8 and a mean coseismic left-lateral displacement of ~ 6.9 m, occurred on the western branch of the Xiaojiang fault.Studying this fault is important in understanding current deformation and kinematic characteristics of the Tibetan Plateau.Activities and stretches have been well undertaken on the Xiaojiang fault, while paleoseismic research work is always the weak link on this fault.To investigate the paleoseismic history and large earthquake activity of the Xiaojiang fault, we opened a large trench at the northern edge of Caohaizi sag pond on the western branch of the Xiaojiang fault.Six paleoseismic events have been identified, and named E1 through E6 from the oldest to the youngest.Charcoal and woods are abundant, 20 samples were dated to constrain the ages of the paleoseismic events at 40 000–36 300 BC, 35 400–24 800 BC, 9 500 BC–AD 500, AD 390–720, AD 1120–1620 and AD 1750–present.We associate the youngest event E6 with the 1833 M8 earthquake.Events E4, E5 and E6 show a continuous record of the western strand of the Xiaojiang fault in the late Holocene, with a average recurrence interval of 370–480 yr.Large earthquake recurrence in the late Holocene is far less than the recurrence of 2000–4000 yr posed in previous studies.Thus, the seismic hazard on the Xiaojiang fault should be reevaluated.Furthermore, the irregular recurrence of large earthquakes on the Xiaojiang fault and other faults in the Xianshuihe-Xiaojiang system, indicates the uneven southeastward extrusion of the Sichuan-Yunnan block along the southeastern margin of the Tibetan Plateau.

Recurrence Characteristics of Major Earthquakes in the Tangshan area,North China

The Tangshan area lies in the North China plain where an Ms 7.8 earthquake occurred in 1976, which is associated with a hidden active fault. To reveal the recurrence characteristics of major quakes in this area over a relatively long time, we have conducted a comprehensive study using geological investigations, shallow seismic exploration, boreholes, trench observations and geological dating. Five paleoearthquakes were recognized in a 6.4m-deep trench west to the Tangshan Asylum. Among them, the former three events occurred between 56.78 ＋ 4.83ka and 89.39 ~ 7.60 ka, and the fourth event occurred around 6.9 ka, respectively, and then followed by the fifth in 1976. Seven boreholes were deployed crossing the ground fissure formed by the 1976 Tangshan earthquake at the site of No. 10 Middle School, where we have identified 25 liquefaction events in the boreholes TZC6-5 and 6-7. By the comprehensive analysis of the trench, the liquefaction events from the boreholes and the depth-time curves of drill cores, we suggest a new recurrence model of major quakes in this area. It is not a constant recurring cycle since 210 ka, instead consisting of six alternating seismically quiet and active stages. Of them, stage I （〉177 ka） was a quiescent period in seismicity, stage II （from 143 ka to 177 ka） was an active one, stage III （from 102 ka to 143 ka） was quiescent again, stage IV （from 56 ka to 102 ka） had many quakes, stage V （from 6.9 ka to 56 ka） became quiet, and stage VI （from 6.9 ka to now） was the beginning of a new seismically active period.

The Holocene Activity Evidence of the Yema River-Daxue Mountain Fault in Western Qilian Mountain

Altyn Tagh fault controls the deformation characteristics of the northern margin of the Qinghai-Tibet Plateau.The sinistral slip rate of the eastern segment of the fault reduces gradually where the reduction transforms into the deformation within Qilian Mountain,forming a series of thrust faults and strike-slip faults.Among them,the Yema River-Daxue Mountain fault is one of the important structural transform faults in the study area.Based on the differences of the geometrical characteristics and activities,the fault is divided into four segments,the Yema River segment,the Shibandun segment,the Liushapo segment and the Baishitougou segment,among which the former three are Holocene active faults,and the Baishitougou segment belongs to late Pleistocene fault.The excavated trenches imply a total of 6 paleoearthquake events,and at least 4 events have occurred during Holocene,whose occurrence times are 8300±700 yr BP,6605±140 yr BP,4540±350 yr BP,2098±47 yr BP,respectively.The recurrence interval is 2600±600 yr BP that is close to the lapsed time of the last one,2098±47 yr BP,which suggests that the Yema River-Daxue Mountain fault is in a high risk of major earthquakes in the future.The vertical coseismic displacements of the four Holocene paleoearthquake events are 100 cm,42 cm,40 cm and 50 cm,respectively,the horizontal coseismic displacements are 5 m,4.5-5.5 m,5-8 m and 4-5.5 m,separately,and then the reference magnitude of the paleoearthquake events is conjectured to be M7.6±0.1.

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 recur- rence 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 consid- ering different rupture patterns. The result shows that the recurrence probability of MS≥6.7 earthquakes is about 0.035 in future 100 years along the Haiyuan fault.

Recent Activity of the Badu-Longwei Segment,Guguan-Xiangong Fault in Southern Liupanshan,Constrained by Rhythmic Sediment Lithology and Geomorphic Characteristics

This study＇s objective was to investigate the Guguan-Xiangong Fault, which lies in the southern Liupanshan area, through satellite image interpretation and field observations. Guguan- Xiangong Fault is divided into five subsegments; among these, the Badu-Longwei segment has been the most recently active. The geomorphic features of the Badu-Longwei segment are clearly displayed, including multiple high fault scarps with fresh bedrock free faces. There is significant evidence for Holocene activity of the three fault sections, located in Renhuashu, Tianjiagou, and Xinjiecun respectively. The three sections feature distinct episodic deposition and fault scratches. Based on 14C- dating and field observations on the three fault sections, two or more paleoearthquakes across the Badu-Longwei fault segment are ascertained, between 5874±116 and 5430±140 a BP, and after 2037±83 a BP respectively. The Badu-Longwei segment of the Guguan-Xiangong Fault is preliminarily extrapolated as the seismogenic structure of the 600 A.D. Qin-Long earthquake.

Paleoearthquake Investigation along the Chenghai Fault Zone since~500 ka,Southeast Margin of the Tibetan Plateau

The Chenghai fault zone is located in the Sichuan–Yunnan rhombus block,which is surrounded by the Honghe River,Xianshuihe–Xiaojiang,and Jinsha River fault zones.As a mid-continental active fault,it is one of the most important seismogenic fractures in the southeast margin of the Tibetan Plateau.Geological seismic study is an important supplement to the historical seismic record;therefore,identification of paleoearthquake events in this region is of great significance to reveal the pattern and mechanism of earthquake development.In this study,detailed investigation has been carried out on the earthquake traces that formed in the Quaternary sediments in the Jinsha River section of the Chenghai fault zone,and samples for dating chronology testing were also collected.Many paleoearthquake traces were discovered in the field,including earthquake fissions,dammed lake sediments and landslides,earthquake-generated rock falls,seismic faults,and sand liquefaction veins.The collected samples were tested using optically stimulated luminescence,electron-spin resonance,and U-series methods.A total of 68 chronological samples were dated,combined with the results of field investigations,and 10 large paleoearthquake events were discovered in this region since 500 ka,which are at approximately 450,400,345,300,250,190,155,105,75,and 25 ka.Ten tectonic activity periods that produced multiple paleoearthquake events were identified since 500 ka.This study identifies paleoearthquake events in longer scales,larger spaces,and more extensive sediments,which provides new perspectives and new ideas for paleoearthquake research.

A recent paleoearthquake on Qingfengling seismic fault of Tanlu fault zone

Introduction The Tanlu fault zone lies in the eastern China, which is an important huge active fault with a long history. It has experienced a complex generation and evolution process and affects significantly the regional structure, paleogeography, magma activity, minerogenesis and earthquake activity in the area. With a length of 2 400 km, the fault zone consists of 2-4 or more parallel faults of 10-40 km in width, cutting through different geotectonic elements in the eastern China （FANG et al, 1986）. On July 25 in 1668, an extraordinarily large earthquake of M=8.5 occurred on the Changyi-Dadian fault （F1） that is an embranchment of Tanlu fault zone, resulting in a surface rupture with a total length of 130 km （LI et al, 1994; CHAO et al, 1995）. The paleoseismic study reveals that 3 events with a magnitude equal to 8 occurred on the Changyi-Dadian fault. The recent event occurred 3 500 a ago and the reoccurrence interval is about 3 500 a （LIN and GAO, 1987）. During the Tancheng earthquake （on July 25, 1668）, the Anqiu-Juxian fault was not ruptured, which was a Late Pleistocene active fault （ZHENG et al, 1988; GAO et al, 1988; CHAO et al, 1994） and was doubted as the seismogenic fault of the M=7.0 Anqiu earthquake occurred in 70 BC by certain geologists （CHAO et al, 1994）.

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

Geomorphic study on Wujiahe segment of Serteng piedmont fault, Inner Mongolia is made. Through analysis of the available data in combination with the results of predecessors studies it can be obtained that average vertical displacement rate is 0.48~0.75 mm/a along the Wujiahe segment since the late Pleistocene (14.450~22.340 ka BP) and 0.56~0.88 mm/a since the early-middle Holocene (5.570~8.830 ka BP). Analyzing paleoseismic phenomena revealed in the excavated 5 trenches 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 27.0 ka BP and the recurrence interval to be about 4.300~4.400 ka. A cluster of paleoearthquakes occurred probably during 8.000~9.000 ka BP and two paleoseismic events in 10.000~20.000 ka 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.

The Research of the Activity of the Piedmont Fault on the Tangshankou Segment of the Yuguang Basin Southern Marginal Fault

The Yuguang basin is a half-graben basin in the basin-range tectonic zone in northwest Beijing,located at the northern end of the Shanxi graben system,and the Yuguang basin southern marginal fault( YBSMF) controls the formation of this basin. A linear fault escarpment has formed in the proluvial fan on the piedmont fault zone of the Tangshankou segment of YBSMF. A trench across this escarpment reveals three paleo-earthquake events on two active faults. One fault ruptured at about 9 ka for the first time,and then faulted again at about 7. 3 ka,causing the formation and synchronous activity of another fault.Finally,they faulted for the third time,but we cannot determine the faulting time due to the lack of relevant surface deposition. The accumulative vertical displacement of these three events is about 8. 1 m. We estimate that the average recurrence period of the piedmont fault is about 1. 7 ka,and the average slip rate of the piedmont fault is about1. 6 mm/a. We also estimate the reference magnitude of each event according to the empirical formula.

Paleoseismicity and Segmentation Along the Active Fault at the Northern Boundary of Huailai-Zhuolu Basin,Hebei Province

The northern boundary fault of Huailai-Zhuolu basin,Hebei Province,has a total length of 58 km and a general strike of NE.The geometry and feature of activity of the 5 segments of the fault greatly differ from each other.17 paleoseismic events have been recognized within 11 trenches excavated along the different segments of the fault.It is found that each segment is characterized by its distinct recurrence of paleoearthquakes.The recurrence intervals of strong earthquakes for each segment are 3500-7000 a for the shortest and 17,000-20,500 a for the longest.However,the recurrence interval of strong earthquakes for whole fault ranges between 750-8500 a.The boundary of the segments can be recognized as the junction,gap,bending,jog and salient of the fault.The length of the segment ranges between 7.5-14 km.

Preliminary Study on Paleoearthquake Chronology on the Northern Liulengshan Piedmont Fault in Yangyuan, Hebei Province, China

The northern Liulengshan piedmont fault is located along the southern margin of the Yangyuan basin and represents a dip-slip normal fault,generally NEE-trending and NNW-dipping.Landforms offset by the fault are fairly developed and well preserved in the Dushanpu area of Yangyuan County,Hebei Province.Two trenches were excavated in front of a scarp of the northern Liulengshan piedmont fault at Dushanpu and two paleoseismic events were revealed in the trenches,from which several samples were collected for dating.The samples were dated using the IRSL dating technique.The preliminary result shows the possible ages of the two paleoseismic events.

A 26ka Continuous Stratigraphic Record of Surface Rupturing Earthquakes in Drill Logs Across the Xiadian Fault,Hebei Province,China

This paper deals wiah a new method for identification of long-term slip behavior of a conceaiedfault in a plain area from tectono-stratigraphic and chronological data obtained by drilling.Chronological determination and lithological analysis and correlation of sediments in stratigraphic colunms of drills NO.1 and No.2 at Dongliuhe Village on the hanging and foot walls ofXiadian fault allow us to quantitatively reconstruct the differential sedimentary history and vertical dislocation process on the hanging and foot walls of the fault since 26 ka BP and to set upa tectono-stratigraphic indicators for identifying paleoearthquakes.On this basis,taking theaccumulative vertical displacement of both sides of a fault to be a quantitative constraint,thestate before an abrupt dislocation in surface-rupturing earthquake can be reconstructed(de-ducting the later coseismic vertical displacement),then 11 surface-rupturing paleoseismic even-ts can be identified.A close correlation between recurrence behavior of

Characteristics of Large Earthquake Recurrence and Determination of Average Recurrence Interval Value

Paleoearthquakes in the Yanqing-Huailai basin and on the Haiyuan fault are studied in detail.The result indicates that the recurrence behavior of large earthquakes is of a wide variety.Characteristic earthquakes show the behavior characteristics of the activity of most faults,butthey are of different grades,the recurrence interval of large earthquakes is of staged nature,and the interaction between faults has effects on the recurrence sequence of large earthquakes.Thus,when the recurrence behavior of large earthquakes is staged in time or when thegradation of characteristic earthquakes has led to a sharp difference in recurrence intervalbetween paleoearthquakes of different intensities,for estimating the large earthquake risk bythe deterministic method and time-dependent probabilistic method,it is necessary to calculatethe recurrence interval value separately for each specific stage or grade in order that theaverage recurrence interval values of different stages can be determined.