HOLOCENE PALAEOSEISMOLOGIC RECORD AND RUPTURE BEHAVIOR OF LARGE EARTHQUAKES ON THE XIANSHUIHE FAULT

The Xianshuihe Fault, the boundary of Bayan Har active tectonic block and Sichuan-Yunnan active tectonic block, is one of the most active fault zones in the world. In the past nearly 300 years, 9 historical earthquakes of magnitude ≥ 7 have been recorded. Since 2008, several catastrophic earthquakes, such as Wenchuan M_S8 earthquake, Yushu M_S7.1 earthquake and Lushan M_S7 earthquake, have occurred on the other Bayan Har block boundary fault zones. However, only the Kangding M_S6.3 earthquake in 2014 was documented on the Xianshuihe Fault. Thus, the study of surface deformation and rupture behavior of large earthquakes in the late Quaternary on the Xianshuihe Fault is of fundamental importance for understanding the future seismic risk of this fault, and even the entire western Sichuan region. On the basis of the former work, combined with our detailed geomorphic and geological survey, we excavated a combined trench on the Qianning segment of Xianshuihe fault zone which has a long elapse time. Charcoal and woods in the trench are abundant. 30 samples were dated to constrain the ages of the paleoseismic events. Five events were identified in the past 9  000 years, whose ages are:8070-6395 BC, 5445-5125 BC, 4355-4180 BC, 625-1240 AD and the Qianning earthquake in 1893. The large earthquake recurrence behavior on this segment does not follow the characteristic earthquake recurrence model. The recurrence interval is 1000~2000 years in early period and in turn there is a quiet period of about 5 000 years after 4355-4180 BC event. Then it enters the active period again. Two earthquakes with surface rupture occurred in the past 1000 years and the latest two earthquakes may have lower magnitude. The left-lateral coseismic displacement of the 1893 Qianning earthquake is about 2.9m.     

STUDY ON PALEOEARTHQUAKES ALONG THE JINGHE SECTION OF BOLOKENU-AQIKEKUDUKE FAULT

The Bolokenu-Aqikekuduk fault zone(B-A Fault)is a 1 000km long right-lateral strike-slip active fault in the Tianshan Mountains. Its late Quaternary activity characteristics are helpful to understand the role of active strike-slip faults in regional compressional strain distribution and orogenic processes in the continental compression environment, as well as seismic hazard assessment. In this paper, research on the paleoearthquakes is carried out by remote sensing image interpretation, field investigation, trench excavation and Quaternary dating in the Jinghe section of B-A Fault. In this paper, two trenches were excavated on in the pluvial fans of Fan2b in the bulge and Fan3a in the fault scarp. The markers such as different strata, cracks and colluvial wedges in the trenches are identified and the age of sedimentation is determined by means of OSL dating for different strata. Four most recent paleoearthquakes on the B-A Fault are revealed in trench TC1 and three most recent paleoearthquakes are revealed in trench TC2. Only the latest event was constrained by the OSL age among the three events revealed in the trench TC2. Therefore, when establishing the recurrence of the paleoearthquakes, we mainly rely on the paleoearthquake events in trench TC1, which are labeled E1-E4 from oldest to youngest, and their dates are constrained to the following time ranges: E1(19.4±2.5)~(19.0±2.5)ka BP, E2(18.6±1.4)~(17.3±1.4)ka BP, E3(12.2±1.2)~(6.6±0.8)ka BP, and E4 6.9~6.2ka BP, respectively. The earthquake recurrence intervals are(1.2±0.5)ka, (8.7±3.0)ka and(2.8±3)ka, respectively. According to the sedimentation rate of the stratum, it can be judged that there is a sedimentary discontinuity between the paleoearthquakes E2 and E3, and the paleoearthquake events between E2 and E3 may not be recorded by the stratum. Ignoring the sedimentary discontinuous strata and the earthquakes occurring during the sedimentary discontinuity, the earthquake recurrence interval of the Jinghe section of B-A Fault is ~1~3ka. This is consistent with the earthquake recurrence interval(~2ka)calculated from the slip rate and the minimum displacement. The elapsed time of the latest paleoearthquake recorded in the trench is ~6.9~6.2ka BP. The magnitude of the latest event defined by the single event displacement on the fault is ~M_W7.4, and a longer earthquake elapsed time indicates the higher seismic risk of the B-A Fault.     

大凉山断裂带北段石棉断裂的古地震

大凉山断裂带是大型走滑断裂鲜水河-小江断裂系的重要组成部分,其活动性是认识和探讨青藏高原东南缘现今地震活动和构造变形机制的重要基础资料。相较于中段和南段,关于大凉山断裂带北段活动性的相关研究成果,尤其是古地震资料非常缺乏。文中基于野外地质地貌调查,在石棉断裂联合村处开挖了一组(2个)探槽,揭露出断裂全新世活动的直接证据。通过古地震分析和炭样加速器质谱仪(AMS)测年,共获得了4次古地震事件:事件E1:20925—16850BC;事件E2:15265—1785BC;事件E3:360—1475AD;事件E4:1655—1815AD。其中包括全新世以来的3次事件,最新2次事件的复发间隔骤然缩短,反映断裂活动可能正在加剧。     

STUDY ON PALEOEARTHQUKES ALONG THE FODONGMIAO-HONGYAZI FAULT,GANSU PROVINCE

The Fodongmiao-Hongyazi Fault is a Holocene active thrust fault, belonging to the middle segment of northern Qilianshan overthrust fault zone, located in the northeastern edge of the Tibet plateau. The Hongyapu M7(1/4) earthquake in 1609 AD occurred on it. A few paleo-seismology studies were carried out on this fault zone. It was considered that four paleoearthquakes occurred on the Fodongmiao-Hongyazi Fault between(6.3±0.6) ka BP and(7.4±0.4) ka BP, in(4.3±0.3) ka BP, in(2.1±0.1) ka BP and in 1609 AD. The occurrences of the earthquakes suggested the quasi-periodic characteristic with a quasi-periodic recurrence interval between 1 600~2 500a(Institute of Geology, State Seismological Bureau, Lanzhou Institute of Seismology, State Seismological Bureau. 1993; Liu et al., 2014). There was no direct evidence for the Hongyapu M7(1/4) earthquake in 1609 AD from trench research in the previous studies. Great uncertainty exists because of the small number of the chronology data, as a few TL and OSL measurement data and several¹⁴ C data, and it was insufficient to deduce the exact recurrence interval for the paleoearthquakes. Five trenches were excavated and cleared up respectively in the eastern segment, middle segment and western segment along the Fodongmiao-Hongyazi Fault. After detail study on the trench profiles, the sedimentary characteristics, sequence relationship of the stratigraphical units, and fault-cuts in different stratigraphical units were revealed in these five trenches. Four paleoearthquakes in Holocene were distinguished from the five trenches, and geology evidences of the Hongyapu M7(1/4) earthquake in 1609 AD were also found. More accurate constraint of the occurring time of the paleo-earthquakes since Holocene on the Fodongmiao-Hongyazi Fault is provided by the progressive constraining method(Mao and Zhang, 1995), according to amounts of ¹⁴ C measurement data and OLS measurement data of the chronology samples from different stratigraphical units in the trenches. The first paleoevent, E4 occurred 10.6ka BP. The next event, E3 occurred about 7.1ka BP. The E2 occurred about 3.4ka BP. The last event, E1 is the Hongyapu M7(1/4) earthquake in 1609 AD. Abounds of proofs for the occurrences of the events of E1, E2 and E3 were found in the trench Tc1, trench Tc2, trench Tc4 and trench Tc3, located in the eastern, middle and western segments of the Fodongmiao-Hongyazi Fault accordingly. It's considered that the events E1, E2 and E3 may cause whole segment rupturing according to the proofs for these three events found together in individual trenches. The event E4 was only found in the trench Tc5 profile in the west of the Xiaoquan village in the eastern segment of the Fodongmiao-Hongyazi Fault. The earthquake rupture characteristics of this event can't be revealed before more detailed subsequent research. The time intervals among the four paleoearthquakes are ca 3.5ka, ca 3.7ka, and ca 3.0ka. The four events are characterized by ca 3.4ka quasi-periodic recurrence interval.     

LATE QUATERNARY ACTIVITY AND PALEOSEISMIC RUPTURE BEHAVIOR FOR THE SOUTHEAST SECTION OF THE GANZI-YUSHU FAULT

The Ganzi-Yushu Fault, the boundary of Bayan Har active tectonic block, Qiantang active tectonic block and Sichuan-Yunan active tectonic block, is a sinistral strike-slip fault zone with intensive Holocene activity. Thus, the study of activity characteristics and rupture behavior of paleoearthquakes in the late Quaternary on the Ganzi-Yushu Fault is of fundamental importance for understanding the future seismic risk of this fault. The southeast section of Ganzi-Yushu Fault is made up of three segments of Ganzi, Manigange and Dengke, where a M_S7.3 earthquake in 1866, a M_S7.7 earthquake in 1854 and a M_S7.3 in 1896 occurred, respectively. There is still lack of in-depth study on the active features and the cascading rupture possibility of these segments, which hindered the evaluation of seismic risk for the southeast section of Ganzi-Yushu Fault. By the means of field geological survey and micro topography measurement, this paper studied the geological and geomorphological features of the southeast section of the Ganzi-Yushu Fault. The results show that the Ganzi and Dengke segments show obvious extension movement, in addition to the left-lateral movement. For Manigange segment, the characteristics of the movement are mainly left-lateral strike-slip and thrusting, and the maximum vertical displacement of the Holocene strata is greater than 2m. In part areas, the movement is normal faulting, which perhaps relates to the left stepping zone in the local stress environment. Therefore, combining the research results such as the fracture distribution in different motion characteristics, rupture behavior of paleoearthquakes, and the distribution of historical earthquake surface ruptures, we divide the southeast section of Ganzi Yushu Fault into Ganzi, Manigange and Dengke segment, and consider the Yakou and the Dengke Basin as the stepovers and the segments' boundaries. As the small scale of impermanent barriers including Dengke Basin and the ridge near Yakou, of which the width is about 1~2km, they may be broken through in great earthquake rupture in future. A trench was excavated in Zhuqing township to investigate the paleoearthquakes on the Manigange segment, radiocarbon dating was employed and 3 paleoseismic events were revealed in the Zhuqing trench, which are the seismic events occurring respectively at 3875~3455BC, after 775BC, and the latest one that ruptured the surface. Compared with the previous results of paleoseismology in the southeast section of Ganzi-Yushu Fault, it is found that the paleoseismic events in the Manigange segment are obviously different with that in Ganzi segment and Dengke segment. Due to the lack of sufficient data on the southeast section of the Ganzi-Yushu Fault, it still needs further discussion whether the cascade-rupturing between these segments exists.     

NEW EVIDENCES OF HOLOCENE ACTIVITY IN THE JIANGSU SEGMENT OF ANQIU-JUXIAN FAULT OF THE TANLU FAULT ZONE

Anqiu-Juxian Fault is an important fault in the Tanlu fault zone, with the largest seismic risk, the most recent activity date and the most obvious surface traces. It is also the seismogenic fault of the Tancheng M8 1/2 earthquake in 1668. There are many different views about the southern termination location of surface rupture of the Tancheng earthquake and the Holocene activity in Jiangsu segment of this fault. Research on the latest activity time of the Jiangsu segment of Anqiu-Juxian Fault, particularly the termination location of surface rupture of the Tancheng earthquake, is of great significance to the assessment of its earthquake potential and seismic risk. Based on trench excavation on the Jiangsu segment of Anqiu-Juxian Fault, we discuss the time and characteristics of its latest activity. Multiple geological sections from southern Maling Mountain to Chonggang Mountain indicate that there was an ancient seismic event occurring in Holocene on the Jiangsu segment of Anqiu-Juxian Fault. We suggest the time of the latest seismic event is about(4.853±0.012)~(2.92±0.3)ka BP by dating results. The latest activity is characterized by thrust strike-slip faulting, with the maximum displacement of 1m. Combined with the fault rupture characteristics of each section, it is inferred that only one large-scale paleo-earthquake event occurred on the Jiangsu segment of Anqiu-Juxian Fault since the Holocene. The upper parts of the fault are covered by horizontal sand layers, not only on the trench in the west of Chonggang mountain but also on the trench in Hehuan Road in Suqian city, which indicates that the main part of the Jiangsu segment of Anqiu-Juxian Fault was probably not the surface rupture zone of the 1668 Tancheng M8 1/2 earthquake. In short, the Jiangsu segment of Anqiu-Juxian Fault has experienced many paleo-earthquake events since the late Pleistocene, with obvious activity during the Holocene. The seismic activities of the Jiangsu segment of Anqiu-Juxian Fault have the characteristics of large magnitude and low frequency. The Jiangsu segment of Anqiu-Juxian Fault has the deep tectonic and seismic-geological backgrounds of big earthquakes generation and should be highly valued by scientists.     

PALEOEARTHQUAKES ALONG PUXIONG FAULT OF DALIANGSHAN FAULT ZONE DURING LATE QUATERNARY

Daliangshan fault zone (DFZ) constitutes an indispensable part of Xianshuihe-Xiaojiang fault system which is one of the main large continental strong earthquake faults in China.Puxiong Fault,the east branch of middle segment of DFZ,is the longest secondary fault.Its paleoseismic activity plays an important role in evaluating regional seismic activity level and building countermeasures of preventing and reducing the earthquake damage.The active fault mapping as well as the study of paleoseismological trench in recent years illustrates that Puxiong Fault is a slightly west-dipping high-angle left-lateral strike-slip fault with strong activity since late Pleistocene.Two trenches excavated across this fault reveal 2 and 3 paleoearthquakes that ruptured the fault at 8206 BC-1172 AD,1084-1549 AD,and 17434-7557 BC,1577-959 BC and 927-1360 AD,respectively.The OxCal model combining the results from both trenches and the another one in previous study across the fault with the historical earthquake record yields the elapsed time of~0.7ka of the latest paleoearthquake event,and the interval time is~2.3ka between the last two events.In the model,the penultimate event is considered to be recorded in all trenches.As all the three trenches are located at north part of the Puxiong Fault whose strike is apparently different from the south part,the~57km long north secondary segment is supposed to be the seismogenic structure of the paleoearthquake.According to the empirical scaling laws between magnitude and rupture length,the magnitude of the surface ruptured paleoearthquake is estimated to be more than M7 with the coseismic displacement~3.5m.However,the difference between the time of the paleoearthquake events on the middle and south segments of DFZ illustrates their independence as earthquake fracture units,and furthermore,the lower connectivity and the new generation of DFZ.     

山西交城断裂北端及中段3个大型探槽全新世断错现象分析

前人在山西交城断裂带上开挖过多个探槽,揭露出全新世3次古地震事件,但其研究结果尚不能确定该断裂带全新世活动段的北部边界.近期在该断裂带北端和中段又开挖了3个大型探槽,其中在阳曲县泥屯盆地西界开挖的龙王沟探槽,是一个由多个探槽组合成的大探槽,该探槽揭示的地层断错信息,将交城断裂带全新世活动的范围向北延伸了20km.另外2个大型探槽分别为交城断裂带中段瓦窑沟东侧台地前缘的瓦窑探槽与市儿口沟西侧T1阶地前缘的新民探槽.这3个大探槽均揭示出全新世中期(14C测年值为距今5 ～ 6ka)的垆土和淤泥层,以及多组平行分布的断面,所揭示的全新世3次古地震事件具有断错事件活动的同步性,可与前人探槽揭示的全新世断层活动事件相对比.3次断错活动时间分别距今3.06 ～3.53ka、5.32ka左右或6.14ka左右、8.36ka左右;3次事件的时间间隔分别为2.02 ～ 2.84ka和2.22 ～ 3.04ka.这些断错事件的同震垂直位移为1.5～4.7m,显示了7级以上地表破裂型的强震活动.最后讨论了探槽中14C测年样品的影响因素.     

PRELIMINARY STUDY OF PALEOEARTHQUAKES ON THE MIDDLE-EASTERN SEGMENT OF JINTA NANSHAN FAULT

Hexi Corridor is located at the northeastern margin of the Tibetan plateau. Series of late Quaternary active faults are developed in this area. Numerous strong earthquakes occurred in history and nowadays. Jinta Nanshan fault is one of the boundary faults between the Qinghai-Tibet block and the Alxa block. The fault starts from the northwest of Wutongdun in the west, passes through Changshan, Yuanyangchi reservoir, Dakouzi, and ends in the east of Hongdun. Because the Jinta Nanshan fault is a new active fault in this region, it is important to ascertain its paleoearthquakes since late Pleistocene for the earthquake risk study. Previous studies were carried out on the western part, such as field geomorphic investigation and trench excavation, which shows strong activity in Holocene on the western segment of Jinta Nanshan fault. On the basis of the above research, in this paper, we carried out satellite image interpretation, detailed investigation of faulted landforms and differential GPS survey for the whole fault. Focusing on the middle-eastern part, we studied paleoearthquakes through trench exploration on the Holocene alluvial fan and optical luminescence dating. The main results are as follows:Early Pleistocene to late Pleistocene alluvial strata are widely developed along the fault and Holocene sediment is only about tens of centimeters thick. The Jinta Nanshan fault shows long-lasting activity since late Quaternary and reveals tens of centimeters of the lowest scarp which illustrates new strong activity on the middle-east segment of this fault. Since late Pleistocene, 4 paleoearthquakes happened respectively before(15.16±1.29) ka, before(9.9±0.5) ka, about 6ka and after(3.5±0.4) ka, revealed by 4 trenches, of which 2 are laid on relatively thicker Holocene alluvial fan. Two events occurred since middle Holocene, and both ruptured the whole fault.     

LATE QUATERNARY PALEOEARTHQUAKES ON THE MIDDLE SEGMENT OF THE LIJIANG-XIAOJINHE FAULT,SOUTHEASTERN TIBET

More attention has been paid to the late Quaternary activity of the boundary fault of the Sichuan-Yunnan block in eastern Tibet. The Lijiang-Xiaojinhe Fault (LXF) locates along the boundary of the northwest Sichuan and central Yunnan sub-blocks in the Sichuan-Yunnan block. Clear displaced landforms show that the fault has undergone strong late-Quaternary activity. However there is no surface-rupturing earthquake occurring on the LXF in the historical record. The LXF crosses the city of Lijiang, one of the most important tourist cities in Southwest China. The rupture behavior on this fault remains unclear and it is hard to assess its seismic hazard in the future. In this study, on the base of the interpretation of high-resolution satellite imagery, we chose the middle segment of the LXF and dug three trenches at Muzhuda, Hongxing, and Gantangzi sites to constrain the ages of paleoearthquakes combined with radiocarbon dating and OxCal modeling. The Muzhuda trench shows that at least three events occurred on the middle segment of the LXF at 7 940~6 540a BP, 4 740~4 050a BP and 1 830~420a BP, respectively. The Hongxing trench indicates that the LXF underwent two events at 5 120~3 200a BP and 2 100~1 220a BP. The Gantangzi trench reveals at least three paleoearthquakes at 44 980~17 660a BP, 7 210~3 810a BP and 2 540~1 540a BP, respectively. The events in the Gantangzi trench might be incomplete because of stratigraphic gap. These three trenches indicate that three events occurred on the middle segment of the LXF in the Holocene at 7 940~7 210a BP, 4 740~4 050a BP and 1 830~1 540a BP, respectively. Large earthquakes on the middle segment of the LXF appear to fit the quasi-periodic model with the mean recurrence interval of~3 000a and the estimated magnitude 7.5. Given the strong late-Quaternary activity of the middle segment of the LXF and a long elapsed time, we propose that the middle segment of the LXF might have a high seismic hazard potential in the near future.     

Paleoseismic events and the 1596 Keicho-Fushimi large earthquake produced by a slip on the Gosukebashi fault at the eastern Rokko Mountains,Japan

Field surveys and trench excavation investigations revealed that there were at least four large seismic events produced by slips on the Gosukebashi fault in the Holocene in the southeastern Rokko Mountains of Japan. The characteristics of deformed topographies and three-dimensionally excavated exposures show that this fault is a right-lateral strike–slip fault having an average slip rate of 1.0 mm/year, with a reverse displacement component. The principle indicators of past faulting events are: (i) termination of secondary faults; (ii) sedimentary deposits related to faulting; and (iii) injection veins of fault gouge related to seismic faulting in the fractured zone. Radiocarbon dates indicate that the events occurred pre-1660 BC , 1660 BC –220 AD , from ～ 30–220 to 600 AD and 15th century AD . The youngest event is probably associated with the large 1596 AD Keicho-Fushimi earthquake which occurred in the area around Kyoto and Kobe Cities. The second younger event is probably correlated with the 416 AD Yamato earthquake, which is the oldest historic earthquake in Japanese historic records. The results of trench surveys show that the horizontal displacement produced by an individual event is ～ 1.5 m, and the recurrence of seismic event intervals is ～ 1200 years in the Gosukebashi fault.     

PALEOSEISMOLOGY ON THE YEMAHE SEGMENT OF THE YEMAHE-DAXUESHAN FAULT REVEALED BY TRENCH STUDY

A series of NWW striking faults are obliquely intersected by the NEE striking Altyn Tagh fault zone in the western Qilian Mountains. These faults were mostly active in late Quaternary and play an important role in accommodating regional lateral extrusion by both reverse and sinistral slip. Detailed studies on late Quaternary activity, tectonic transformation, paleoseismology, and strain partitioning not only significantly affect our recognition on seismogenic mechanism and zones of potential large earthquakes, but also provide useful information for exploring tectonic deformation mechanism in the northern Tibetan plateau. The Danghenanshan Fault, Yemahe-Daxueshan fault, and Altyn Tagh Fault form a triplet junction point at southwest of Subei county. The Yemahe-Daxueshan fault is one important branch fault in the western Qilian Mountains that accommodated eastward decreasing slip of the Altyn Tagh Fault, which was active in late Holocene, with a length up to 170km. Based on geometry and late Quaternary activity, the Yemahe-Daxueshan fault was subdivided into 3 segments, i.e. the Subei fault, Yemahe fault and Daxueshan Fault. The Yemahe Fault has the most prominent appearance among them, and is dominated by left-lateral slip with a little normal component. The heights of fresh scarps on this fault are only several tens of centimeters. We dug 2 trenches at the Zhazhihu site, and cleaned and reinterpreted one trench of previous studies. Then we interpreted trench profiles and paleoseismic events, and collected ¹⁴C and Optical Stimulated Luminescence samples to constrain event ages. Finally, we determined 3 events on the Yemahe fault with ages(6 830±30) a BP-(6 280±40) a BP, (5 220±30) a BP, (2 010±30) a BP, respectively. The elapsed time of most recent earthquake is(2 010±30) years before present, which is very close to the recurrence interval, so the possibility of major earthquakes on the Yemahe fault is relatively large.     

LATE QUATERNARY ACTIVITY OF FAULTS IN THE EPICENTER AREA OF JINGGU M6.6 EARTHQUAKE

The 2014 Jinggu M6.6 earthquake attacked the Jinggu area where few historical earthquakes had occurred and little study has been conducted on active tectonics. The lack of detailed field investigation on active faults and seismicity restricts the assessment of seismic risk of this area and leads to divergent view points with respect to the seismotectonics of this earthquake, so relevant research needs to be strengthened urgently. In particular, some studies suggest that this earthquake triggered the activity of the NE-trending faults which have not yet been studied. By the approaches of remote sensing image interpretation, structural geomorphology investigation and trench excavation, we studied the late Quaternary activity of the faults in the epicenter area, which are the eastern margin fault of Yongping Basin and the Yixiang-Zhaojiacun Fault, and drew the conclusions as follows: (1)The eastern margin fault of Yongping Basin originates around the Naguai village in the southeastern margin of Yongping Basin,extending northward across the Qiandong, Tianfang, and ending in the north of Tiantou. The fault is about 43km long, striking near SN. The linear characteristic of the fault is obvious in remote sensing images. Structural geomorphological phenomena, such as fault troughs, linear ridges and gully dislocations, have developed along the faults. There are several dextral-dislocated gullies near Naguai village, with displacements of 300m, 220m, 146m, 120m and 73m, respectively, indicating that the fault is a dextral strike-slip fault with long-term activity. In order to further study the activity of the fault, a trench was excavated in the fault trough, the Naguai trench. The trench reveals many faults, and the youngest strata offseted by the faults are Holocene, with ¹⁴C ages of(1 197±51)a and(1 900±35)a, respectively. All those suggest that it is a Holocene active fault. (2)The Yixiang-Zhaojiacun Fault starts at the southeast of the Jinggu Basin, passes through Xiangyan, Yixiang, Chahe, and terminates at the Zhaojiacun. The total length of the fault is about 60km, and is a large-scale NE-trending fault in the Wuliangshan fault zone. Four gullies are synchronously sinistrally dislocated at Yixiang village, with the displacements of 340m, 260m, 240m and 240m, indicating that the fault is a long-term active sinistral strike-slip fault. A trench was excavated in a fault trough in Yixiang village. The trench reveals a small sag pond and a fault. The fault offsets several strata with clear dislocation and linear characteristic. The thickness of strata between the two walls of fault does not match, and the gravels are oriented along fault plane. The offset strata have the ¹⁴C age of(2 296±56)a, (3 009±51)a, and(4 924±45)a, respectively, and another two strata have the OSL age of(1.8±0.1)ka, (8.6±0.5)ka respectively, by which we constrained the latest paleoearthquake between(1.8±0.1)ka(OSL-Y01)and(378±48)a BP(CY-07). This again provides further evidence that the fault is a Holocene fault with long-term activity. (3)Based on the distribution of aftershocks and the predecessor research results, the 2014 Jinggu M6.6 earthquake and the M5.8, M5.9 strong aftershocks are regarded as being caused by the eastern margin fault of Yongping Basin, which is part of the Wuliangshan fault zone. The seismogenic mechanism is that the stress has been locked, concentrated and accumulated to give rise to the quakes in the wedge-shaped area near the intersection of the SN and NE striking faults, which is similar to the seismogenic mechanism in the southwest of Yunnan Province.     

滇西北通甸-巍山断裂中段的晚第四纪滑动速率

通甸-巍山断裂属于红河断裂带的分支断裂,目前对该断裂中段的晚第四纪活动特征研究较少。野外地质地貌调查和年代学研究结果表明,通甸-巍山断裂中段是以右旋走滑运动为主,兼有张性正断的全新世活动断裂,其最新活动时代距今约2.2ka。晚更新世中晚期以来断裂中段平均水平滑动速率为1.25mm/a,全新世晚期以来垂直运动趋于增强。该研究不仅为该断裂的地震危险性评价工作提供了基础资料,而且有助于理解川滇菱形块体西南边界构造变形的空间分配特点     

THE HOLOCENE ACTIVITY AND STRIKE-SLIP RATE OF THE SOUTHERN SEGMENT OF XIAOJIANG FAULT IN THE SOUTHEASTERN YUNNAN REGION,CHINA

The southern segment of the Xiaojiang Fault (SSXF) is located at the intersection of the Xianshuihe-Xiaojiang Fault and Red River-Ailao Shan fault systems in the southeast margin of the Tibetan plateau. Based on the interpretation of remote sensing image, the SSXF clearly shows the linear feature and continuous distribution as a single, penetrating fault. It has a total length of about 70km, trends generally about 20° to the northeast and protrudes slightly in the middle to the east. A typically geomorphologic phenomenon about the synchronous left-lateral dislocation of ridges and gullies can be found at Liangchahe, Longtan Village along the SSXF. The distribution of faults, the sedimentary features, attitude variance and the primary dating results of the offset strata in the trench section across fault sag ponds reveal three paleoseismic events rupturing obviously the surface, which demonstrates that the SSXF has the ability of recurrence of strong earthquakes. High-precision topographic map about two gullies and the platform between them with synchronous dislocation is acquired by using the Trimble 5800 GPS real-time difference measurement system. The dislocation is (18.3±0.5)m. As the top geomorphologic surface between the above two gullies and their adjacent area, the terrace surface T₂ stopped accepting deposits at ~2606a, based on the linear regression analysis of three dating data. According to the geological method, a sinistral strike-slip rate of (7.02±0.20)mm/a on the SSXF in the Holocene is obtained, which has a good consistency with the results provided by using GPS data. The preliminary results about the Holocene activity and slip rate of the SSXF demonstrate that the southward or south-southeast motion of the Sichuan-Yunnan block in the SE Yunnan region has not been absorbed by the possible shortening deformation and the sinistral strike-slip rate of the SSXF has not been drastically reduced. The SSXF is a Holocene fault with obvious activity. This preliminary understanding provides some basic geological data for the seismic risk evaluation of the SSXF in the future, and for the establishment and inspection of the seismotectonic model about the Sichuan-Yunnan block.     

小江西支断裂的滑动速率与强震重复周期

本文通过对各种断错地质地貌现象及被错体年龄的分析,估算出小江西支断裂在第四纪、晚更新世及全新世以来的滑动速率分别为6.7毫米/年、7.0毫米/年及6.4毫水/年。在此基础上,结合历史地震及古地震活动讨论了强震的重复周期和断裂的活动特征,结果表明小江西支断裂的活动以重复错动发生强烈地震为主要特征,强震在断裂带上同一地段的平均重复周期为900年左右,沿整个断裂带的重复周期大于330年     

内蒙大青山山前活动断裂带的地震破裂分段特征

沿内蒙大青山山前活动断裂进行野外调查及探槽开挖的研究结果表明 ,该断裂西部地段及东部地段的最新活动时期在全新世中期以后及全新世晚期以前 ;中部地段在全新世晚期强烈活动 ,公元 84 9年包头地震的地表破裂沿该段展布。大青山山前台地与断层陡坎分布、洪积扇类型及河流阶地断错等地貌特征、全新世晚期断裂活动范围、沿断裂带探槽开挖获得的古地震事件对比 ,以及现今中小地震震中分布表明 ,全新世晚期大青山山前断裂的活动以土左旗为界 ,该界以西全新世晚期断裂强烈活动 ,该界以东全新世晚期断裂活动不明显。全新世时期大青山山前断裂的活动显示了由东向西的迁移     

PALEOSEIMOLOGY AND LATE QUATERNARY SLIP RATE OF THE YOUSHASHAN FAULT AT SOUTHWESTERN MARGIN OF QAIDAM BASIN

The Youshashan Fault lies in the south flank of Yingxiongling anticline, southwestern margin of Qaidam Basin. The Yingxiongling anticline is one of the most active neotectonics, situated at the front of folds expanding southward in the Qaidam Basin. Research on the paleoseimology and Late Quaternary slip rate of this fault is important for hazard assessment and understanding tectonic deformation in this area. We excavated a 27-m-long trench across the Youshashan fault where a pressure bridge formed on the Holocene alluvial fans, measured a profile of the fold scarp created by the fault west of the Youshashan mountain, and collected several samples of finer sands for luminescence dating. Analysis of these data shows that(1) The Youshashan Fault is a Holocene active feature. The fold scarp in the basin indicates that this fault has been active along a same surface trace since at least mid-late Pleistocene. At least two paleoseismic events are revealed by trenching, both occurred in Holocene. The latest event Ⅱ in the trench happened after 500a. The current information fails to confidently support that it is the 1977 Mangya M6.4 earthquake, but cannot excludes the possibility of it is related to this earthquake. The other event Ⅰ occurred about between 1 000a to 4 000a. Erosion after the event Ⅰ prevents us to constrain the event age and to identify more events further. (2)The vertical slip rate of the Youshashan fault is about(0.38±0.06)mm/a since mid-late Pleistocene. Comparing with relative speeds of GPS sites across the Yingxiongling anticline suggests that the Youshashan fault is an important structure which is accommodating crustal shortening in this region.     

THE ACTIVITY OF WESTERN LISHAN FAULT SINCE THE LATE PLEISTOCENE

Along the northern piedmont of Mt. Lishan, the characteristics and locations of the active normal Lishan fault in west of Huaqing Pool provide important evidences for determining the seismotectonic environment, seismic stability evaluation of engineering in the eastern Weihe Basin. After reviewing the results from high-density resistivity method, seismic profile data, geological drillhole section and trenching in west of the Huaqing Pool, it is found that the strike of western normal Lishan Fault changes from EW direction at the eastern part to the direction of N60°W, and the fault consists of two branches, dipping NE with a high dip angle of~75°. The artificial shallow seismic profile data reveals that the attitude of strata near Lishan Fault mainly dips to south, which is presumed to be related to the southward tilt movement of Mt. Lishan since the Cenozoic. The section of geological drillhole reveals that since the late middle Pleistocene, the displacement of the paleo-soil layer S₂ is about 10m. And the maximum displacement of western Lishan Fault recorded in the paleo-soil layer S₁ reaches 7.8m since the late Pleistocene. In addition, evidences from trench profile show that the western Lishan Fault was active at least 3 times since Malan loess deposition with ¹⁴ C dating age(32 170±530)Cal a BP. The multiple activities of the Lishan Fault result in a total displacement about 3.0m in the Malan loess layer L₁. The latest activity of the western Lishan Fault produced a displacement of about 0.9m in the early Holocene loess layer L₀((8 630±20)Cal a BP)and caused obvious tensile cracks in the Holocene dark leoss layer S₀((4 390±20)Cal a BP). Briefly, we have obtained a vertical movement rate of about 0.11~0.19mm/a since the Holocene((8 630±20)Cal a BP)in the western extension of the Lishan Fault, the recurrence interval of earthquakes on the fault is about(10.7±0.5)ka, and the co-seismic surface rupture in a single event is inferred to be about 0.9m.     

海原断裂高湾子地点三维探槽的开挖与古地震研究

对海原断裂高湾子地点进行大比例尺地质地貌填图和三维探槽开挖，揭示出７次古地震事件，除１９２０年地震外，它们的年龄分别为距今（１０００４±３１９６），（６６８９±１６９），（６１２０±５０５），（４２０８±５７７），（２７６３±３７２）和（１００５±４６５）ａ；重复间隔分别为（３３１５±３２００），（５６１±５３２），（１９２０±７６６），（１４２５±６８６），（１５７８±５９５）和（９８０±４６５）ａ。事件Ⅲ，Ⅳ，Ⅴ，Ⅵ，Ⅶ的水平位移量分别为（５．６±２．３），（１．５±１．１），（１．５±１．１），（２±１）和（７±０．５）ｍ。显示出重复间隔时间的分段性和特征行为的分级性。两次类似１９２０年强度的地震间隔期间，有３次位移量为１～２ｍ的地震发生