Shallow SH-wave seismic investigation of the Mt. Angel Fault, Northwest Oregon, USA

The Mt. Angel Fault is likely one of the most active faults near the Portland metropolitan area, and was probably associated with the 1993 Scotts Mills earthquake. SH-wave seismic techniques used to image the Mt. Angel Fault suggest that the fault offsets late Pleistocene gravel (22 to 34 ka) at several locations. Within the study area, displacement of the late Pleistocene gravel along the strike of the Mt. Angel Fault increases from no obvious displacement on the northwest to approximately 18 m on the southeast. This trend of increasing offset along the strike of the fault is paralleled by topographic and geomorphic trends. A reconnaissance geologic investigation at an anomalous bend in the Pudding River near the projected trace of the Mt. Angel Fault revealed potential tectonic deformation in sediments younger than the late Pleistocene gravel imaged by SH-wave data. The results of this study have contributed to the paleoseismic record of the Mt. Angel Fault, laid the groundwork for future geologic investigations along the Pudding River, and determined potential sites for future paleoseismic trenching investigations.     

Quaternary fault segmentation and interaction in the epicentral area of the 1561 earthquake (Mw = 6.4), Vallo di Diano, southern Apennines, Italy

The main structural characteristics of the Caggiano and Polla faults, exposed in the epicentral area of the 1561 earthquake (Mw = 6.4), southern Italy, have been investigated in detail to assess their spatial and temporal properties, and to evaluate their seismogenic potential. These right stepping normal faults show an overlap of about 7 km and an across strike separation of about 4 km. The geometric relationships between the Caggiano and Polla faults, but also the displacement distribution along each fault, demonstrate that they have been strongly interacting throughout the Pleistocene. Nevertheless, geological evidence of Holocene tectonic activity was mainly recognized along the Caggiano Fault (faulted late glacial deposits) and in the southernmost part of the Polla Fault (faulted deposits of probably Late Pleistocene age). This suggests that the Caggiano Fault can be considered as the most tectonically active fault in the Vallo di Diano Fault System. By calculating Coulomb stress changes, we have constrained modes of mechanical interactions between the two faults in a scenario compatible with the 1561 earthquake. This approach allows us to argue that both the Caggiano and the Polla Faults are probably linked at depth, and part of the same seismogenic structure which may be potentially responsible for composite ruptures with magnitude ≥ 6.5.     

Newly found Tunglo Active Fault System in the fold and thrust belt in northwestern Taiwan deduced from deformed terraces and its tectonic significance

We found active faults in the fold and thrust belt between Tunglo town and the Tachia River in northwestern Taiwan. The surface rupture occurred in 1999 and 1935 nearby the study area, but no historical surface rupture is recorded in this area, suggesting that the seismic energy has been accumulated during the recent time. Deformed fluvial terraces aid in understanding late Quaternary tectonics in this tectonically active area. This area contains newly identified faults that we group as the Tunglo Fault System, which formed after the area's oldest fluvial terrace and appears at least 16 km long in roughly N–S orientation. Its progressive deformations are all recorded in associated terraces developed during the middle to late Quaternary. In the north, the system consists of two subparallel active faults, the Tunglo Fault and Tunglo East Fault, striking N–S and facing each other from opposite sides of the northward flowing Hsihu River, whose course may be controlled by interactions of above-mentioned two active faults. The northern part of the Tunglo Fault, to the west of the river, is a reverse fault with upthrown side on the west; conversely the Tunglo East Fault, to the east, is also a reverse fault, but with upthrown side on the east. Both faults are marked by a flexural scarp or eastward tilting of fluvial terraces. Considering a Quaternary syncline lies subparallel to the east of this fault system, the Tunglo Fault might be originated as a bending moment fault and the Tunglo East Fault as a flexural slip fault. However, they have developed as obvious reverse faults, which have progressive deformation under E–W compressive stress field of Taiwan. Farther south, a west-facing high scarp, the Tunglo South Fault, strikes NNE–SSW, oblique to the region's E–W direction of compression. Probably due to the strain partitioning, the Tunglo South Fault generates en echelon, elongated ridges and swales to accommodate right-lateral strike–slip displacement. Other structures in the area include eastward-striking portion of the Sanyi Fault, which has no evidence for late Quaternary surface rupture on this fault; perhaps slip on this part of Sanyi Fault ceased when the Tunglo Fault System became active.     

Deformation at the Easternmost Altyn Tagh Fault: Constraints on the Growth of the Northern Qinghai–Tibetan Plateau

How the Altyn Tagh fault(ATF) extends eastwards is one of the key questions in the study of the growth of the Qinghai–Tibetan Plateau. Detailed fieldwork at the easternmost part of the ATF shows that the ATF extends eastward and bypasses the Kuantan Mountain; it does not stop at the Kuantan Mountain, but connects with the northern Heishan fault in the east. The ATF does not enter the Alxa Block but extends eastward along the southern Alxa Block to the Jintanan Mountain. The Heishan fault is not a thrust fault but a sinistral strike-slip fault with a component of thrusting and is a part of the ATF. Further to the east, the Heishan fault may connect with the Jintananshan fault. A typical strike-slip duplex develops in the easternmost part of the ATF. The cut and deformed Quaternary sediments and displaced present gullies along the easternmost ATF indicate that it is an active fault. The local highest Mountain(i.e., the Kuantan Mountain) in the region forms in a restraining bend of the ATF due to the thrusting and uplifting. The northward growth of the Qinghai–Tibetan Plateau and the active deformation in South Mongolia are realized by sinistral strike-slipping on a series of NE–SW-trending faults and thrusting in restraining bends along the strike-slip faults with the northeastward motion of blocks between these faults.     

Uplift and subsidence from oblique slip: the Ganos–Marmara bend of the North Anatolian Transform, western Turkey

Bends that locally violate plate-motion-parallel geometry are common structural elements of continental transform faults. We relate the vertical component of crustal motion in the western Marmara Sea region to the NNW-pointing 18° bend on the northern branch of the North Anatolian Fault (NAF-N) between the Ganos segment, which ruptured in 1912, and the central Marmara segment, a seismic gap. Crustal shortening and uplift on the transpressive west side of the bend results in the Ganos Mountain; crustal extension and subsidence on the transtensional east side produce the Tekirdağ Basin. We propose that this vertical component of deformation is controlled by oblique slip on the non-vertical north-dipping Ganos and Tekirdağ segments of the North Anatolian Fault. We compare Holocene with Quaternary structure across the bend using new and recently published data and conclude the following. First, bend-related vertical motion is occurring primarily north of the NAF-N. This suggests that this bend is fixed to the Anatolian side of the fault. Second, current deformation is consistent with an antisymmetric pattern centered at the bend, up on the west and down on the east. Accumulated deformation is shifted to the east along the right-lateral NAF-N, however, leading to locally opposite vertical components of long- and short-term motion. Uplift has started as far west as the landward extension of the Saros trough. Current subsidence is most intense close to the bend and to the Ganos Mountain, while the basin deepens gradually from the bend eastward for 28 km along the fault. The pattern of deformation is time-transgressive if referenced to the material, but is stable if referenced to the bend. The lag between motion and structure implies a 1.1–1.4 Ma age for the basin at current dextral slip rate (2.0–2.5 cm/year). Third, the Tekirdağ is an asymmetric basin progressively tilted down toward the NAF-N, which serves as the border fault. Progressive tilt suggests that the steep northward dip of the fault decreases with depth in a listric geometry at the scale of the upper crust and is consistent with reactivation of Paleogene suture-related thrust faults. Fourth, similar thrust-fault geometry west of the bend can account for the Ganos Mountain anticline/monocline as hanging-wall-block folding and back tilting. Oblique slip on a non-vertical master fault may accommodate transtension and transpression associated with other bends along the NAF and other continental transforms.     

Active normal faulting during the 1997 seismic sequence in Colfiorito,Umbria: Did slip propagate to the surface?

In order to determine whether slip during an earthquake on the 26th September 1997 propagated to the surface, structural data have been collected along a bedrock fault scarp in Umbria, Italy. These collected data are used to investigate the relationship between the throw associated with a debated surface rupture (observed as a pale unweathered stripe at the base of the bedrock fault scarp) and the strike, dip and slip-vector. Previous studies have suggested that the surface rupture was produced either by primary surface slip or secondary compaction of hangingwall sediments. Some authors favour the latter because sparse surface fault dip measurements do not match nodal plane dips at depth. It is demonstrated herein that the strike, dip and height of the surface rupture, represented by a pale unweathered stripe at the base of the bedrock scarp, shows a systematic relationship with respect to the geometry and kinematics of faulting in the bedrock. The strike and dip co-vary and the throw is greatest where the strike is oblique to the slip-vector azimuth where the highest dip values are recorded. This implies that the throw values vary to accommodate spatial variation in the strike and dip of the fault across fault plane corrugations, a feature that is predicted by theory describing conservation of strain along faults, but not by compaction. Furthermore, published earthquake locations and reported fault dips are consistent with the analysed surface scarps when natural variation for surface dips and uncertainty for nodal plane dips at depth are taken into account. This implies that the fresh stripe is indeed a primary coseismic surface rupture whose slip is connected to the seismogenic fault at depth. We discuss how this knowledge of the locations and geometry of the active faults can be used as an input for seismic hazard assessment.     

Evidence of paleoearthquakes from trench investigations across Pinjore Garden fault in Pinjore Dun, NW Himalaya

The Pinjore Garden Fault (PGF) striking NNW-SSE is now considered one of the active faults displacing the younger Quaternary surfaces in the piggyback basin of Pinjore Dun. This has displaced the older Kalka and Pinjore surfaces, along with the other younger surfaces giving rise to WSW and SW-facing fault scarps with heights ranging from 2 to 16 m. The PGF represents a younger branch of the Main Boundary Thrust (MBT) system. An ~ 4m wide trench excavated across the PGF has revealed displacement of younger Quaternary deposits along a low angle thrust fault. Either side of the trench-walls reveals contrasting slip-related deformation of lithounits. The northern wall shows displacement of lithounits along a low-angle thrust fault, while the southern wall shows well-developed fault-related folding of thick sand unit. The sudden change in the deformational features on the southern wall is an evidence of the changing fault geometry within a short distance. Out of five prominent lithounits identified in the trench, the lower four units show displacement along a single fault. The basal unit ‘A’ shows maximum displacement of aboutT _o = 2.85 m, unit B = 1.8 m and unit C = 1.45 m. The displacement measured between the sedimentary units and retro-deformation of trench log suggests that at least two earthquake events have occurred along the PGF. The units A and D mark the event horizons. Considering the average amount of displacement during one single event (2 m) and the minimum length of the fault trace (~ 45 km), the behaviour of PGF seems similar to that of the Himalayan Frontal Fault (HFF) and appears capable of producing large magnitude earthquakes.     

中国东部NWW向活动断裂带构造特征:以张家口-蓬莱断裂带为例

中国东部新构造期活动强烈,前人对该时期NE向构造已有很多研究,但NWW向构造研究程度较低.本文以张家口-蓬莱断裂带为例,从几何学、运动学、动力学及地震活动性四个方面对中国东部的NWW向活动断裂带进行了分析.结果表明,中国东部的NWW向活动断裂带具有左行走滑的运动性质,并控制了第四纪盆地左阶雁列的展布样式;NWW向活动断裂带是孕震断裂,诱发了多次地震活动.在动力学上,这些断裂带是扳缘的不同段落变形在板内不同块体间响应调节的产物,且在周边板块的联合作用下,华北和华南南部NWW向断裂可能印度-欧亚板块碰撞的影响占主导,而东北和华南东部NWW向断裂可能太平洋板块俯冲的影响占主导.     

Paleoseismological investigations along Joggers Park Fault,Port Blair,South Andaman: Implications towards delineation of blind thrusting and related crustal deformation through ground penetrating radar (GPR) and electrical resistivity techniques (ERT and VES)

A comprehensive paleoseismological investigations was carried out along Joggers Park Fault (striking NW-SE) reveals pervasive traces of active compressional tectonics (blind thrusting) and shallow-surface recent tectonic signatures along the fault. The geometry and trends in the tectonic deformation of the blind zone (shallow-subsurface stratigraphy) were examined with GPR and electrical resistivity tomography (ERT) and vertical electrical sounding (VES), strongly demonstrates severe tectonic deformation (persistent tectonic style) from Quaternary to Holocene times in a compressive stress regime. The surface manifestation of deformation (topographic expression) along the fault can be marked as hanging wall uplift and can be visually observed in the exposed sections at nearly half kilometer distance from the table land of Joggers Park in the form of a well-defined disconnected high angle thrust fault scarp (reverse fault). The reverse fault kinematically and intimately linked with underlying fault geometry provides adduced evidence to evaluate strike displacement profiles (displacement length relationship) along the fault. In addition, deformed stacked colluvial wedge below the scarp represents a powerful basis to calibrate recent and long term slip rates of the underlying fault.     

Field evidence for normal fault linkage and relay ramp evolution: the Kırkağaç Fault Zone,western Anatolia (Turkey)

Linking of normal faults forms at all scales as a relay ramp during growth stages and represents the most efficient way for faults to lengthen during their progressive formation. Here, I study the linking of normal faulting along the active K?rka?aç Fault Zone within the west Anatolian extensional system to reconstruct fault interaction in time and space using both field- and computer-based data. I find that (i) connecting of the relay zone/ramp occurred with two breaching faults of different generations and that (ii) the propagation was facilitated by the presence of pre-existing structures, inherited from the ?zmir-Bal?kesir transfer zone. Hence, the linkage cannot be compared directly to a simple fault growth model. Therefore, I propose a combined scenario of both hangingwall and footwall fault propagation mechanisms that explain the present-day geometry of the composite fault line. The computer-based analyses show that the approximate slip rate is 0.38 mm/year during the Quaternary, and a NE–SW-directed extension is mainly responsible for the recent faulting along the K?rka?aç Fault Zone. The proposed structural scenario also highlights the active fault termination and should be considered in future seismic hazard assessments for the region that includes densely populated settlements.     

THE LATE QUATERNARY RIGHT LATERAL STRIKE-SLIPPING OF ZHONGDIAN—DAJU FAULT IN NORTHWEST YUNNAN, CHINA

THE LATE QUATERNARY RIGHT LATERAL STRIKE-SLIPPING OF ZHONGDIAN—DAJU FAULT IN NORTHWEST YUNNAN, CHINAthesubject“TherecentdisplacementandDynamicsoflithosphereintheQinghai XizangPlateau”ofnation alclimbingproject“Therecentdisp     

Displacement variation along thrust faults: implications for the development of large faults

Displacement analyses along thrust faults of different maturity (or size) reveal maxima and minima, often associated with minor folding of the adjacent beds, between the tip points. The results show that these faults are segmented, and that they formed through the linkage of smaller (previously independent) faults, and (or) by propagation of a single fault affected by the existence of barriers. Points of potential linkage (marked by displacement minima) are fault bends or distinct fault breaks. Fault nucleation (marked by displacement maxima) occurs within the planar segments of a fault; only in one of eight examples is the nucleation point seen to occur at a fault bend.Displacement variations along inferred or extrapolated regional-scale thrust faults show a variety of patterns, most of which involve constant displacement or a monotonic increase or decrease away from the basal décollement. These data are not considered to be as reliable as those from observed thrusts due to the necessary subjectivity involved in the extrapolation process.In general, displacement variation appears to be a reflection of the symmetry of the thrust fault system, such that, for example, a flat-ramp geometry ending in a steep tip will show an asymmetrical displacement function skewed toward the surface, with a nucleation point above the basal décollement.     

Elastic modeling of the Alto Tiberina normal fault (central Italy): geometry and lithological stratification influences on the local stress field

Two-dimensional, elastic, plane-strain, finite element models (FEMs) are generated to study the state of stress and failure induced by a low-angle normal fault, the Alto Tiberina Fault of the Northern Apennines (central Italy): it is beyond the scope of the present work to show that slip can occur on such a fault. The numerical study was performed to evaluate the influences on the local stress field of the litho-mechanical stratification of formations surrounding the fault, and those induced by the geometry of the fault. The performed models have shown the important role played by evaporites and basement formations of the Umbria-Marche succession as seismogenetic layers. The model results have also underlined that the flat-ramp geometry of the fault induces high relative concentration of stresses in correspondence with the low-angle, east-dipping, synthetic normal faults observed today in shallow depth near the Alto Tiberina surface trace. The stress regime predicted by the final model, in which the behavior of the Alto Tiberina together with the antithetic normal fault of Gubbio was simulated, reconciles available geological and geophysical observable to a greater extent. The numerical results can assist interpretation of the tectonic evolution of the region.     

Uppermost Tortonian to Quaternary depocentre migration related with segmentation of the strike-slip Palomares Fault Zone,Vera Basin (SE Spain)

The Palomares Fault Zone (PFZ) is one of the main strike-slip brittle shear zones found in the Betics. It is segmented in several faults that have been active between the Upper Tortonian and present day. Data from drill cores in the Palomares area have permitted us to define the geometry and location of sedimentary depocentres related with the PFZ. These data show an eastward displacement between the Upper Tortonian to Messinian and the Pliocene–Quaternary sedimentary depocentres, towards the presently active Arteal fault, which bounds the western mountain front of Sierra Almagrera, showing that deformation along this fault zone has migrated towards the east, from the Palomares segment, with its main activity during the Upper Tortonian and Messinian, towards the Arteal fault, active during the Pliocene and Quaternary. To cite this article: G. Booth-Rea et al., C. R. Geoscience 335 (2003).     

黄骅坳陷横向变换带的构造特征及成因

讨论了黄骅坳陷横向变换带的构造特征和成因。根据利用地震反射剖面对盆地构造几何学的研究，圈定了不同尺度的横向变换带，确定了伸展构造背景下的横向变换带是为了保持区域伸展应变调节构造变形的一种调节体系。这种伸展应变守恒是靠三维空间上断层位移沿走向的变化实现的。横向变换带的构造样式为横向地垒状凸起和鼻状凸起或鼻状背斜，并具伸展性质的正断层。横向凸起与非叠复的和叠复的分段断层系伴生及发育在断层位移最小处等事实表明，横向凸起或横向鼻状背斜是由于分段断层系位移沿走向变化形成的。     

High-resolution seismic survey on the Rhine River in the northern Upper Rhine Graben

In the northern part of the Upper Rhine Graben (URG), a high-resolution seismic reflection survey was carried out on the Rhine River over a length of 80 km, and on its tributary Neckar over a length of 25 km. The seismic investigation provides new results to redefine the base of Quaternary fluvial sediments from Oppenheim upstream to the south of Mannheim. The standard Quaternary thickness map of Bartz (1974) was partially revised and completed. Maximum Pleistocene sediment thickness is documented in the area of Mannheim with approximately 225 m. The top of the Pliocene in this area is sub-horizontal and not faulted, and rises downstream continuously towards the fault block of Worms. Intercalated lacustrine pelitic layers play a main role in defining the litho-stratigraphy in this part of the URG. In the north of Worms, Pleistocene sediments are mainly coarse-grained. In the area of Worms, a Pleistocene tectonic phase along N–S striking normal faults with variable displacement along the strike is obvious.     

山东郯城麦坡中更新世地震事件记录*

山东郯城麦坡被命名为典型地震活动断层遗址,其最醒目的标志是郯庐断裂带主干断层(F₂)东盘的紫灰色下白垩统逆冲到断层西盘的红棕色第四系之上且界线截然。野外调查和试验分析表明,郯城麦坡第四系于泉组中发育液化砂涌管、液化砂脉、震裂缝充填构造和同沉积断层等地震引发的软沉积物变形构造——地震事件记录。根据软沉积物变形构造的砂质黏土光释光测年分析,推断这些软沉积物变形构造所记录的地震事件属郯庐断裂带主干断层F₂在中更新世晚期发生的强构造与地震活动。这些地震事件记录为研究郯庐断裂带新构造运动与地震活动提供了新资料,也丰富了该地震活动断层遗址的内涵。     

东营凹陷永8断块断层调节带及其对油气分布的控制作用

利用高精度的三维地震资料,结合油田地质资料对永8 块断层几何学、运动学和动力学进行了研究。结果表明,该 断块为一典型东西向展布的同向叠覆型断层调节带,西侧主断层（辛120 断层）与调节断层在剖面上呈阶梯状组合样式, 东侧主断层（永105 断层）与调节断层呈地垒状组合;辛120 断层活动性减弱时,其位移量传递给永105 断层和调节断层; 力学分析认为该断层调节带是在左旋张扭构造应力场作用下形成的。通过对永8 块油气分布、油气来源以及断裂活动性与 封闭性研究,建立了永8 断层调节带油气成藏模式：主成藏期东侧主断层作为油源断层沟通了烃源灶和储集层并输导油气; 调节断层停止活动时间早形成断块圈闭,成为遮挡断层。永8 块西侧是有利滚动勘探开发区。     

Structure kinematics of a transtensional basin:An example from the Linnan Subsag,Bohai Bay basin,Eastern China

The Linnan subsag is a petroliferous, secondary tectonic unit of the Huimin sag that is located in the western part of the Jiyang depression in the Bohai Bay basin, eastern China. In this study, the authors calculated basin extensional rate and slipping displacement of boundary faults in the dip and strike directions, using seismic and drilling data. The evolution of the Linnan transtensional basin from the Eogene through the Quaternary is quantitatively described, and a dynamic model is established. The Linshan and Xiakou boundary faults of the Linnan subsag are used as a case study to describe a method to calculate the strike and dip slipping displacements of active faults under oblique extension. The results quantitatively illustrate the behavior of the Linnan subsag boundary faults over time. The Linnan subsag transtensional basin experienced four stages of evolution: weak extension during the Kongdian Formation, rapid extension and fault depression during the fourth member of the Shahejie Formation,intensive transtension and fault depression during the third member of the Shahejie Formation-Dongying Formation, and weakening fault depression during the Guantao Formation-Pingyuan Formation.The results of this study provide further understanding of the processes of petroleum migration and accumulation in the region.     

塔里木盆地阿瓦提凹陷周缘的晚新生代张扭性断层带

塔里木盆地西北部的阿瓦提凹陷周缘发育晚新生代正断层,其中第四纪的正断层活动是塔里木盆地构造地质研究的新发现。这些正断层受先存基底断裂控制,平面上沿沙井子断裂带、阿恰断裂带和吐木休克断裂带右阶式雁列状分布,构成右阶左旋张扭性正断层带。剖面上,向下断达下古生界后不清楚,向上断至第四系上部,构成阶梯状或小型地堑(或负花状构造)构造。生长系数计算结果表明,正断层带形成于新近纪末,第四纪早-中期持续活动,到第四纪晚期停止活动。这些张扭性正断层带的成因是阿瓦提地块相对于周边地质体的顺时针旋转而致,其动力学来源于印度板块与欧亚板块陆-陆碰撞,在晚喜马拉雅山期依然持续作用而导致的远程效应。