祁连山北缘玉门—北大河断裂东段古地震特征

玉门—北大河断裂是酒西盆地南侧的一条重要的活动断裂, 断裂西起青草湾, 向东经老玉门市、 青头山、 大红泉, 止于北大河以东骨头泉一带, 长约80 km, 走向北西西, 倾向南, 倾角20°~60°。 玉门—北大河断裂为一条全新世活动的逆冲断裂, 断裂东段保留了地震破裂带遗迹, 通过野外断错地貌调查和探槽开挖, 揭示该破裂带形成于距今1.7±0.3 ka, 此前断裂在4.1±0.3~5.4±0.3 ka及8.4±1.0 ka还有过2次古地震事件, 利用经验公式和已有震例估算, 每次地震震级约为M7。     

祁连山北缘玉门断裂晚更新世以来的活动速率及古地震

玉门断裂位于青藏高原东北缘的祁连山造山带西段,与阿尔金断裂相邻,构造位置特殊,是青藏高原北缘向外扩展的最新活动证据。近20a越来越多的研究使得对其认识逐渐从弱活动向强活动转变。因此,玉门断裂作为1条青藏高原北缘祁连山造山带中新生的活动断裂和褶皱带,确定它晚更新世至全新世的活动性和古地震复发周期具有重要的意义。文中通过对玉门断裂山前冲积扇面和北大河阶地的影像解译与断层陡坎的测量,以及对2条不同断层陡坎的探槽开挖工作,获得了以下几点认识:1)玉门断裂全新世以来的垂直活动速率为0.41~0.48mm/a,晚更新世晚期以来的垂直活动速率为0.24~0.30mm/a。2)玉门断裂全新世以来共发生4次古地震事件,这4次古地震分别发生在6.12~10.53kaBP、3.6~5.38kaBP、1.64~1.93kaBP和0.63~1.64kaBP。总体上表现出复发间隔逐渐缩短,活动性增强的趋势,并且每次古地震都可能造成多支断层同时破裂,形成陡坎。     

阳关断裂带变形样式及其对阿尔金断裂扩展的指示意义

阳关断裂位于青藏高原北部阿尔金断裂系向北扩展的前缘位置,对其几何学和运动学的深入研究,有助于理解青藏高原向大陆内部扩展的机制。文章通过卫星影像解译、探槽开挖、差分GPS及无人机测量等对阳关断裂开展了详细研究。结果显示：阳关断裂东段发育多条正反向断层陡坎,断层陡坎高度在0.4～8 m之间,平均约2.2 m,探槽揭示断裂倾角约60°,形成高角度逆断层,局部发育正断层;西段断裂向北西前缘扩展,形成一组弧形分布的断层陡坎,陡坎高度多在0.9～2.4 m,平均约1.9 m。同时自南向北,逆冲断层陡坎形态由多级陡坎转为单一陡坎。对探槽剖面分析,显示断裂断错晚更新世冲洪积砾石层,发育的断层倾角较缓,以低角度逆冲为主要特征,约26°,有的甚至沿地层向前推覆。结合前人的研究成果,阳关断裂可能为本区阿尔金向北扩展的北边界,与三危山断裂共同协调吸收了阿尔金断裂东段的部分应变量。     

新疆北天山乌苏南断层晚第四纪活动特征及其构造意义

乌苏南断裂组发育于北天山山前的独山子逆断裂—背斜带(第3排)和西湖隆起(第4排)之间,在卫星影像显示为4～5排近平行断层陡坎组成,断层晚第四纪以来断层活动明显,沿断层断错山前冲洪积扇、冲沟和水系,形成断续分布的线性陡坎地貌,开挖地质探槽显示乌苏南断层组为高倾角逆走滑型断裂。依据断层断错地貌面年代,估算断层晚更新世以来的垂直活动速率约0.125 mm/a,右旋走滑活动速率为1.25±0.02 mm/a。根据乌苏南断层带所处的位置及构造特性,推断该断裂很可能是天山山前第3排背斜带与第4排背斜带之间的过度转换断层。     

天山南部北轮台断裂带东段晚第四纪活动特征

利用卫星影像资料解译,通过野外实地考察并结合气象、地貌、时间的分析,对长逾300 km的北轮台断裂带的晚第四纪活动速率进行定量研究。北轮台断裂带西部为库车坳陷的北部控制性断裂带,东部为发育于山前冲洪积扇面上的断层陡坎,在阿克艾肯段断裂形成了高达38 m的断层陡坎,为多次7~7.5级地震活动的产物,其逆冲抬升速率可达到1.52 mm/a;而在吉格代布拉克段发育有高10~20 m的断层陡坎,此段断裂带的活动速率为0.1~0.9 mm/a。北轮台断裂东段自晚更新世以来活动明显,切割多级阶地面,形成不同高度的古地震形变带,估算最近一次古地震的离逝时间为1.7 ka。     

新疆天山南部北轮台断裂带晚第四纪活动速率

北轮台断裂是1条全新世活动断裂,为南天山与塔里木盆地的分界断裂。晚第四纪以来,北轮台断裂的持续活动使得多期洪积地貌面发生了断错变形与褶皱隆升。利用高精度差分GPS,对北轮台断裂阿克艾肯段和砖厂段内的多期地貌面的断层陡坎形态进行了测量。通过大比例尺活动断层填图发现,阿克艾肯段以逆冲作用为主,而砖厂段则是以褶皱隆升为主。利用光释光测年方法,分别得到了不同期次地貌面(Fan4,Fan3b,Fan3c和Fan2)的年龄,发现自Fan4地貌面形成以来,阿克艾肯段的地壳缩短速率(约2.4mm/a)基本保持恒定;同时,晚第四纪以来砖厂段的SN向地壳缩短速率为1.43~1.81mm/a,较阿克艾肯段有明显下降,推测北轮台断裂带的SN向地壳缩短速率由西向东递减。综合对比南天山山前的逆断裂-褶皱带体系,同样反映出地壳缩短速率由西向东递减的特征。     

阿拉善地块南缘北大山断裂的晚第四纪构造活动证据

阿拉善地块南缘地处青藏高原东北缘地壳扩展前锋带的北侧,对该地区活动断裂晚第四纪的运动性质、滑动速率等开展研究,有助于理解阿拉善地块的晚第四纪构造变形特征及其对青藏高原向N扩展的响应。文中结合遥感影像解译与野外地质地貌考察,对阿拉善地块南缘的北大山断裂进行了分段和活动性研究。结果表明,北大山断裂左旋走滑断错晚第四纪洪积扇和阶地等地貌,形成显著的位错阶地坎、冲沟以及断层陡坎。通过对断错地貌线等标志的测量、复原、统计分析等,发现断裂的地貌位移值分布于3～20m,发育新鲜断层自由面的断层陡坎和左旋错动的纹沟指示了断层的最新一次活动。基于同期洪积扇年龄估算得到北大山断裂晚更新世以来的左旋滑动速率为0.3～0.6mm/a。北大山断裂的运动学特征与区域NE向应力场一致,可能受到了青藏高原NE向扩展的影响。     

帕米尔北缘木什背斜第四纪滑脱褶皱作用与北翼逆断裂的生长

发育在帕米尔弧形推覆构造带最前缘的木什活动背斜是一南缓北陡的第四纪滑脱褶皱,背斜的最小地壳缩短量为0.7km,构造隆升幅度可达1.5km.木什背斜北翼逆断层由一系列坡向北的反向断层陡坎组成,不同断坎间垂直位移分布呈现此消彼长的特征,不论是整个北翼逆断层西段还是单条断坎,其垂直位移均呈东高西低的不对称分布,位移梯度东高西...     

新疆哈密地区东盐池、七角井、托莱泉活动断裂微地貌及新活动特征

大河沿—洛包泉活动断裂带由 7条活断裂呈左阶雁列式排列 ,其中东盐池、七角井、托莱泉活断裂在平面上也呈左阶排列。沿断裂分布有古地震断层陡坎。东盐池断裂具有左旋逆冲性质。断层陡坎高 0 .6— 4 .5 m,断坎坡角为 2 6°— 2 8°,断裂垂直断距 1m,冲沟左错 10— 11.5 m,其垂直平均活动速率为 0 .18mm / a,水平平均活动速率为 1.31— 2 .0 2 mm/ a,为全新世中、晚期活动断裂。七角井断裂具有左旋逆冲或左旋逆走滑性质 ,断层陡坎高为 0 .75— 8.3m ,断坎坡角为 14°— 2 1°,左错为2 .5— 6 .5 m,垂直平均活动速率为 0 .15— 0 .17mm/ a,水平平均活动速率为 0 .35— 0 .4 9mm/ a。该断裂为全新世中、晚期活动断裂。托莱泉断裂具有逆冲性质。断层陡坎西段高为 0 .75— 4 m,坡角为19°— 2 0 .5°;东段高为 3— 12 m,坡角为 10°— 2 9°,垂直平均活动速率为 0 .2 5 mm/ a。该断裂为晚更新世晚期活动断裂     

天山南缘北轮台断裂东段构造变形样式

北轮台断裂是一条全新世活动断裂,全长约130 km,构成天山南麓与山前冲洪积扇的界线。北轮台断裂东段(砖厂段)全长8 km,是一条低角度逆冲断层,晚第四纪以来有强烈的活动。通过对变形阶地进行微地貌测量,并选取2个典型变形位置开挖大型跨断层探槽,分析该段断裂的构造样式和演化模式。结果显示,北轮台断裂砖厂段的活动构造样式不是单一的以逆断层作为表现,而是伴随着褶皱挤压隆起,山体向S推挤形成逆冲断层断错地表,在受到来自盆地方向的阻挡,产生反向的挤压应力,沿断层一线构造挤压应力集中,但是在断层的上盘形成拉张区,应力释放的过程中形成了大量小规模的拉张型正断层。     

盲断裂、褶皱地震与新疆1906年玛纳斯地震

１９０６年玛纳斯７．７级地震时沿准噶尔南缘断裂产生的地表破坏是由非构造成因的振动和重力效应而形成的。天山山前第二排逆断裂和褶皱带是这次地震的发震构造，沿带已发现了长约１３０ｋｍ的断续的地表破裂和最新隆起带。所以１９０６年玛纳斯地震是沿北天山主逆断裂带发生在深部的一次盲断裂地震。地表变形主要以褶皱隆起为主，是一次典型的“褶皱地震     

Coseismic vertical deformation of the M_S8.0 Wenchuan earthquake from repeated levelings and its constraint on listric fault geometry

The devastating MS8.0 Wenchuan earthquake ruptured two large parallel thrust faults along the middle segment of the Longmenshan thrust belt.Preseismic and postseismic leveling data indicated the hanging wall of the YingxiuBeichuan-Nanba thrust fault mainly presented coseismic uplift with respect to the reference point at Pingwu county town, and the observed maximum uplift of 4.7 m is located at Beichuan county(Qushan town)which is about 100 m west of the fault scarp.The foot wall of the Yingxiu-Beichuan-Nan...     

GEOLOGICAL AND GEOMORPHIC EXPRESSIONS OF LATE QUATERNARY STRIKE-SLIP ACTIVITY ON JINTA NANSHAN FAULT IN NORTHERN EDGE OF QING-ZANG BLOCK

Jinta Nanshan Fault is an important fault in northeast front of Qing-Zang Plateau, and it is crucial for determining the eastern end of Altyn Tagh Fault. However, there is still debate on its significant strike-slip movement. In this paper, we study the Late Quaternary activity of Jinta Nanshan Fault and its geological and geomorphic expressions by interpreting aerial photographs and high-resolution remote sensing images, surveying and mapping of geological and geomorphic appearances, digging and clarifying fault profiles and mapping deformation characteristics of micro-topographies, then we analyze whether strike-slip activity exists on Jinta Nanshan Fault. We get a more complete fault geometry than previous studies from most recent remote sensing images. Active fault traces of Jinta Nanshan mainly include 2 nearly parallel, striking 100°~90° fault scarps, and can be divided into 3 segments. West segment and middle segment form a left stepover with 2~2.5km width, and another stepover with 1.2km width separates the middle and east segment. We summarize geomorphic and geologic evidence relating to strike slip activity of Jinta Nanshan Fault. Geomorphic expressions are as follows:First, fault scarps with alternating facing directions; second, sinistral offset of stream channels and micro-topographies; third, pull-apart basins and compressive-ridges at discontinuous part of Jinta Nanshan Fault. Geologic expressions are as follows:First, fault plane characteristics, including extremely high fault plane angle, unstable dip directions and coexistence of normal fault and reverse fault; second, flower structures. Strike-slip rate was estimated by using geomorphic surface age of Zheng et al.(2013)and left-lateral offset with differential GPS measurements of the same geomorphic surface at field site in Fig. 4e. We calculated a strike-slip rate of (0.19±0.05)mm/a, which is slightly larger than or almost the same with vertical slip rate of (0.11±0.03)mm/a from Zheng et al.(2013). When we confirm the strike-slip activity of Jinta Nanshan, we discuss its potential dynamic sources:First, eastern extension of Altyn Tagh Fault and second, strain partitioning of northeastward extension of Qilian Shan thrust belt. The first one is explainable when it came to geometric pattern of several E-W striking fault and eastward decreasing strike slip rate, but the former cannot explain why the Heishan Fault, which locates between the the Altyn Tagh Fault and Jinta Nanshan Fault, is a pure high angle reverse fault. The latter seems more explainable, because oblique vectors may indeed partition onto a fault and manifest strike-slip activity.     

PRESENT KINEMATICS CHARACTERISTICS OF THE NORTHERN YUMUSHAN ACTIVE FAULT AND ITS RESPONSE TO THE NORTHEASTWARD GROWTH OF THE TIBETAN PLATEAU

Qilian Shan and Hexi Corridor, located in the north of Tibetan plateau, are the margin of Tibetan plateau's tectonic deformation and pushing. Its internal deformations and activities can greatly conserve the extension process and characteristics of the Plateau. The research of Qilian Shan and Hexi Corridor consequentially plays a significant role in understanding tectonic deformation mechanism of Tibetan plateau. The northern Yumushan Fault, located in the middle of the northern Qilian Shan thrust belt, is a significant component of Qilian Shan thrust belt which divides Yumushan and intramontane basins in Hexi Corridor. Carrying out the research of Yumushan Fault will help explain the kinematics characteristics of the northern Yumushan active fault and its response to the northeastward growth of the Tibetan plateau.Because of limited technology conditions of the time, different research emphases and some other reasons, previous research results differ dramatically. This paper summarizes the last 20 years researches from the perspectives of fault slip rates, paleao-earthquake characteristics and tectonic deformation. Using aerial-photo morphological analysis, field investigation, optical simulated luminescence(OSL)dating of alluvial surfaces and topographic profiles, we calculate the vertical slip rate and strike-slip rate at the typical site in the northern Yumushan Fault, which is(0.55±0.15)mm/a and(0.95±0.11), respectively. On the controversial problems, namely "the Luotuo(Camel)city scarp" and the 180 A.D. Biaoshi earthquake, we use aerial-photo analysis, particular field investigation and typical profile dating. We concluded that "Luotuo city scarp" is the ruin of ancient diversion works rather than the fault scarp of the 180 A.D. Biaoshi earthquake. Combining the topographic profiles of the mountain range with fault characteristics, we believe Yumu Shan is a part of Qilian Shan. The uplift of Yumu Shan is the result of Qilian Shan and Yumu Shan itself pushing northwards. Topographic profile along the crest of the Yumu Shan illustrates the decrease from its center to the tips, which is similar to the vertical slip rates and the height of fault scarp. These show that Yumu Shan is controlled by fault extension and grows laterally and vertically. At present, fault activities are still concentrated near the north foot of Yumu Shan, and the mountain ranges continue to rise since late Cenozoic.     

卡兹克阿尔特断裂带活动特征

卡兹克阿尔特断裂带是帕米尔和天山新生代造山带间一个重要的活动构造边界,通过对其活动构造特征的详细地质调查和大比例尺填图,可将卡兹克阿尔特断裂带进一步划分为吉勒格由特断裂带、乌恰地震断裂带和木什断裂带3段.吉勒格由特断裂带的地表破裂为一系列的断层陡坎和偏转的冲积扇,经过别尔托阔依河出山口处时,切割了T1至T3堆积阶地.断裂带在T1、T2和T3阶地的断层陡坎高度分别为0.67m、3.90m和36.50m.对采自T2阶地顶部和底部的粉砂样品进行光释光测年,测定的初步结果分别为8900aBP和10500aBP,因此对T3、T2阶地以来的滑动速率估计分别约为3.5mm/a、0.8mm/a.断裂的前缘开挖的探槽揭示出全新世以来有4次古地震活动.乌恰地震带主要切割克兹勒苏河的T3阶地后缘,沿断裂带分布有大小不等的断塞塘和断层陡坎.1985年8月23日在乌恰地震带上发生Ms7.4地震,地震最大位错为1.5m.根据断层陡坎计算出断裂的滑动速率约为0.54mm/a.卡帕河的东岸探槽同样揭示出有4次古地震活动.在乌恰地震带的东端,木什断裂带地表长度约6km,由数十条左阶排列的反向断层陡坎(坡向北)组成,沿这些断坎多处可见冲沟被断错,横跨断层陡坎的探槽揭示出3次古地震活动.     

Coseismic vertical deformation of the MS8.0 Wenchuan earthquake from repeated levelings and its constraint on listric fault geometry

The devastating M_S8.0 Wenchuan earthquake ruptured two large parallel thrust faults along the middle segment of the Longmenshan thrust belt. Preseismic and postseismic leveling data indicated the hanging wall of the YingxiuBeichuan-Nanba thrust fault mainly presented coseismic uplift with respect to the reference point at Pingwu county town, and the observed maximum uplift of 4.7 m is located at Beichuan county (Qushan town) which is about 100 m west of the fault scarp. The foot wall of the Yingxiu-Beichuan-Nanba thrust fault mainly showed subsidence with maximum subsidence of 0.6 m near the rupture. By employing a listric dislocation model, we found that the fault geometry model of exponential dip angle δ= 88°?×1-exp(-9/h) with depth of 18 km and uniform thrust-slip of 5.6 m could fit the observed coseismic vertical deformation very well, which verifies the listric thrust model of the Longmenshan orogenic zone.

     

昌马盆地内发现活动正断层

昌马盆地为祁连山西端的山间盆地,前人一直关注其周边断裂(如昌马断裂)的构造变形,盆地内部变形则鲜有研究。基于遥感解译和野外考察、探槽开挖、差分GPS和放射性碳(¹⁴C)测年等方法,发现昌马盆地西北部的一条活动断层。断层长约4 km,总体走向NEE,倾向SE,倾角陡立,断层地貌表现为陡坎、复陡坎、断层沟槽等,陡坎高度0～5.6 m,由WS向NE逐渐增大。断层运动性质以正断为主,最新活动时代为全新世,并识别出2期古地震事件：6 670～6 885 a B.P.和26 330～26 915 a B.P.。研究结果表明,在青藏高原东北缘向NE方向挤压扩展的背景下,祁连山造山带发生NW-SE向伸展,导致其西端受到SE向拉张作用而形成正断层。     

NEW ACTIVITY CHARACTERISTICS AND SLIP RATE OF THE EBOMIAO FAULT IN THE SOUTHERN MARGIN OF BEISHAN,GANSU PROVINCE

The Ebomiao Fault is a newly discovered active fault near the block boundary between the Tibetan plateau and the Alashan Block. This fault locates in the southern margin of the Beishan Mountain, which is generally considered to be a tectonically inactive zone, and active fault and earthquake are never expected to emerge, so the discovery of this active fault challenges the traditional thoughts. As a result, studying the new activity of this fault would shed new light on the neotectonic evolution of the Beishan Mountain and tectonic interaction effects between the Tibetan plateau and the Alashan Block. Based on some mature and traditional research methods of active tectonics such as satellite image interpretation, trenches excavation, differential GPS measurement, Unmanned Aircraft Vehicle Photogrammetry(UAVP), and Optical Stimulated Luminescence(OSL)dating, we quantitatively study the new activity features of the Ebomiao Fault.
Through this study, we complete the fault geometry of the Ebomiao Fault and extend the fault eastward by 25km on the basis of the 20km-fault trace identified previously, the total length of the fault is extened to 45km, which is capable of generating magnitude 7 earthquake calculated from the empirical relationships between earthquake magnitude and fault length. The Ebomiao Fault is manifested as several segments of linear scarps on the land surface, the scarps are characterized by poor continuity because of seasonal flood erosion. Linear scarps are either north- or south-facing scarps that emerge intermittently. Fourteen differential GPS profiles show that the height of the north-facing scarps ranges from (0.22±0.02)m to (1.32±0.1)m, and seven differential GPS profiles show the height of south-facing scarps ranging from (0.33±0.1)m to (0.64±0.1)m. To clarify the causes of the linear scarps with opposite-facing directions, we dug seven trenches across these scarps, the trench profiles show that the south-dipping reverse faults dominate the north-facing scarps, the dipping angles range from 23° to 86°. However, the south-facing scarps are controlled by south-dipping normal faults with dipping angles spanning from 60° to 81°.
The Ebomiao Fault is dominated by left-lateral strike-slip activity, with a small amount of vertical-slip component. From the submeter-resolution digital elevation models(DEM)constructed by UAVP, the measured left-lateral displacement of 19 gullies in the western segment of the Ebomiao Fault are(3.8±0.5)~(105±25)m, while the height of the north-facing scarps on this segment are(0.22±0.02)~(1.32±0.10)m(L₃-L₇), the left-lateral displacement is much larger than the scarp height. In this segment, there are three gullies preserving typical left-lateral offsets, one gully among them preserves two levels of alluvial terraces, the terrace riser between the upper terrace and the lower terrace is clear and shows horizontal offset. Based on high-resolution DEM interpretation and displacement restoration by LaDiCaoz software, the left-lateral displacement of the terrace riser is measured to be(16.7±0.5)m. The formation time of the terrace riser is approximated by the OSL age of the upper terrace, which is (11.2±1.5)ka BP at (0.68±0.03)m beneath the surface, and(11.4±0.6)ka at (0.89±0.03)m beneath the surface, the OSL age (11.2±1.5)ka BP at (0.68±0.03)m beneath the surface is more close to the formation time of the upper terrace because of a nearer distance to sediment contact between alluvial fan and eolian sand silt. Taking the (16.7±0.5)m left-lateral displacement of the terrace riser and the upper terrace age (11.2±1.5)ka, we calculate a left-lateral strike-slip rate of(1.52±0.25)mm/a for the Ebomiao Fault. The main source for the slip rate error is that the terrace risers on both walls of the fault are not definitely corresponded. The north wall of the fault is covered by eolian sand, we can only presume the location of terrace riser by geomorphic analysis. In addition, the samples used to calculate slip rate before were collected from the aeolian sand deposits on the north side of the fault, they are not sediments of the fan terraces, so they could not accurately define the formation age of the upper terrace. This study dates the upper terrace directly on the south wall of the fault.
Since the late Cenozoic, the new activity of the Ebomiao Fault may have responded to the shear component of the relative movement between the Tibetan plateau and the Alashan Block under the macroscopic geological background of the northeastern-expanding of the Tibetan plateau. The north-facing fault scarps are dominated by south-dipping low-angle reverse faults, the emergence of this kind of faults(faults overthrusting from the Jinta Basin to the Beishan Mountain)suggests the far-field effect of block convergence between Tibetan plateau and Alashan Block, which results in the relative compression and crustal shortening. As for whether the Ebomiao Fault and Qilianshan thrust system are connected in the deep, more work is needed.     

汶川地震震中映秀地区地表破裂特征

汶川8级大震的震中位于映秀镇,地震在映秀地区造成了多处地表破裂,如公路拱曲、地震陡坎,坡中槽新变形等,长度达300余米.经实地全站仪和GPS测量,定量分析了地表破裂的垂直分量与水平分量以及两者之间的比值,以此揭示了映秀-北川断裂的运动性质为逆冲兼右行走滑,在映秀地区逆冲分量大于走滑分量.将本次地震造成的位错数据与震前资料对比,发现汶川地震产生的地表破裂位置与地质历史上映秀-北川断裂造成的断层位错位置是相当吻合的,说明映秀地区Ⅳ级阶地上40余米的的断层陡坎可能是地质历史时期若干次大地震的结果.     

Monitoring degradation of the 1995 Nojima earthquake fault scarps at Awaji Island, southwestern Japan

The Nojima fault on the northwestern coast of Awaji Island, south of Kobe, was reactivated during the January 17, 1995 Hyogoken-nanbu earthquake. This fault rupture was dominated by right-lateral offset (max. 1.7 m) along a high-angle reverse fault which has a maximum vertical displacement of 1.3 m on the southeastern side. We repeatedly measured seven profiles across the fault scarp in two areas (Hirabayashi to the northeast and Ogura to the southwest) for approximately 1 year following the earthquake. The original profile of the fault scarp was an overhanging scarp at Hirabayashi and Ogura, corresponding to the 70–80 ° dip of the fault plane. The fault scarp at Hirabayashi displaces Plio-Pleistocene siltstones of the Osaka Group and is overlain by a thin bed of unconsolidated gravel. The Ogura area is entirely underlain by the Osaka Group. Scarp degradation at Hirabayashi occurred by collapse of the gravel bed and proceeded more quickly than at Ogura, where fault scarp degradation proceeded mainly by exfoliation of the Osaka Group siltstones. The degradation occurred at a very fast rate until March at Hirabayashi, and until June or July at Ogura. Since then, the degradation has been very slow. Our data strongly indicate that the scarp profile was initially controlled mainly by the dip of the fault plane, and scarp degradation has been primarily controlled by lithological factors. The degradation of the Nojima earthquake fault scarp proceeded much more quickly than that of normal fault scarps in the western U.S.A., where many observations of the initial stages of scarp degradation have been carried out. The extremely rapid degradation of the Nojima fault scarp in weak late Neogene siltstones might, in combination with rapid cultural modification of the landscape, explain the paucity of geomorphic scarps along the numerous active faults in Japan. This observation may also have implications for tectonic geomorphology and paleoseismicity studies in other countries characterised by weak bedrock and moderate to high rainfall regimes.