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1.
江娃利  谢新生 《地震地质》2002,24(2):177-187
当探槽开挖长度未跨过断层变形带时，得到的断层垂直位移将偏离断层活动的真实情况，在缺少依据帮助确定断层陡坎原始下坡角的具体位置时，通过断层陡坎高度获得的断层垂直位移也将与实际情况有较大的偏离，文中对此进行了讨论。并讨论了应用断层两侧近水平地层累积变位量的分解确定古地震事件期次的方法，以及探槽剖面中断层两侧同层地层厚度差异是断层活动事件的反映等问题。引用了内蒙古大青山山前断裂和狼山山前断裂、北京平原夏垫断裂和南口－孙河断裂及日本丹那断层探槽开挖的实例。  相似文献   

2.
The weathering characteristics of bedrock fault scarps provide relative age constraints that can be used to determine fault displacements. Here, we report Schmidt hammer rebound values (R‐values) for a limestone fault scarp that was last exposed in the 1959 Mw 7.3 Hebgen Lake, Montana earthquake. Results show that some R‐value indices, related to the difference between minimum and maximum R‐values in repeated impacts at a point, increase upward along the scarp, which we propose is due to progressive exposure of the scarp in earthquakes. An objective method is developed for fitting slip histories to the Schmidt hammer data and produces the best model fit (using the Bayesian Information Criterion) of three earthquakes with single event displacements of ≥ 1.20 m, 3.75 m, and c. 4.80 m. The same fitting method is also applied to new terrestrial LiDAR data of the scarp, though the LiDAR results may be more influenced by macro‐scale structure of the outcrop than by differential weathering. We suggest the use of this fitting procedure to define single event displacements on other bedrock fault scarps using other dating techniques. Our preliminary findings demonstrate that the Schmidt hammer, combined with other methods, may provide useful constraints on single event displacements on exposed bedrock fault scarps. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

3.
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.  相似文献   

4.
We present high-resolution reflection seismic data from four lines (total 1.9 km) that cross a quick-clay landslide scar located close to the shore of the Göta River in southwest Sweden, and compare the results with geotechnical data from boreholes. The seismic data allow the imaging of bedrock topography and normally to weakly consolidated sediments to a subsurface depth of about 100 m. Different types of seismic sources, including sledgehammer, accelerated weight-drop and dynamite were utilized and compared with each other. Analysis of their power spectra suggests that weight-drop and dynamite have higher frequency content and energy than the sledgehammer, which makes these two sources suitable also for waveform tomography and surface-wave data analysis. The shallowest non-bedrock reflector is observed at about 10–20 m below the surface, it overlays the bedrock, and is interpreted to originate from the contact between clay formations above and a coarse-grained layer below. The coarse-grained layer appears to be spatially linked to the presence of quick-clays. It is a regional scale formation, laterally heterogeneous, which deepens to the west of the study area and correlates well with the available geotechnical data. Continuity of the coarse-grained layer becomes obscured by the landslide scar. There may be a link between the coarse-grained layer and landslides in the study area, although this possibility requires further hydrogeological and geotechnical investigations. Reflectors from the top of the bedrock suggest a depression zone with its deepest point below the landslide scar and a bowl-shaped structure in the northern portion of one of the seismic lines.  相似文献   

5.
花海断裂是位于河西走廊西端阿尔金断裂系北侧花海盆地内的一条活动断裂,对该断裂活动性的认识不仅有助于评估该区的地震危险性,而且对深入理解青藏高原向北扩展过程中块体相互作用具有重要的科学意义。遥感解译与地震地质调查表明,花海断裂仅局限于花海盆地内,长度约25 km。断裂走向NNW,南端起自花海镇以南,向北经小泉、大泉、双泉子后穿过山水河,向北逐渐消失在北山山前大型冲积扇前。地貌上,花海断裂南部表现为线性延伸的断层陡坎,北段构成了风成砂丘与冲洪积扇的界线。在断裂北段跨断层陡坎进行了探槽开挖,探槽揭露和光释光年代学测试结果表明,该断裂最新一次古地震事件的时间距今约5万年,全新世以来没有明显的活动迹象,为晚更新世活动断裂。结合陡坎位错分析,花海断裂晚第四纪以来垂直滑动速率小于0.03 mm/a。区域大地构造动力学背景分析表明,花海断裂是在青藏高原向北扩展作用下盆地内形成的次一级活动断裂,是高原外围块体对青藏高原向外扩展的响应。  相似文献   

6.
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.  相似文献   

7.
The Dalrymple Trough marks part of the transform plate boundary between India and Arabia in the northern Arabian Sea. Oblique extension is presently active across this portion of the boundary at a rate of a few millimetres per year, and seismic reflection profiles across the trough confirm that it is an extensional structure. We present new swath bathymetric and wide-angle seismic data from the trough. The bathymetric data show that the trough is bounded by a single, steep, 3-km-high scarp to the southeast and a series of smaller, en-echelon scarps to the northwest. Wide-angle seismic data show that a typical oceanic crustal velocity structure is present to the northwest, with a crustal thickness of ~ 6 km. There is an abrupt change in crustal thickness and velocity structure at the northwestern edge of the trough, and the trough itself is underlain by 12-km-thick crust interpreted as thinned continental crust. Therefore we infer that Dalrymple Trough is an unusual obliquely extending plate boundary at which continental crust and oceanic crust are juxtaposed. The extensional deformation is focused on a single major fault in the continental lithosphere, but distributed over a region ~ 60 km wide in the oceanic lithosphere.  相似文献   

8.
Landslides contribute to dismantle active mountain ranges and faults control the location of landslides. Yet, evidence of the long‐term, regional dependency of landslides on active faults is limited. Previous studies focused on the transient effects of earthquakes on slope stability in compressive and transcurrent regimes. Here we show that in the Peloritani range, NE Sicily, Italy, one of the fastest uplifting areas in the Mediterranean, a clear geographical association exists between large bedrock landslides and active normal faults of the Messina Straits graben. By interpreting aerial photographs, we mapped 1590 landslides and sackungs and 626 fault elements and their facets in a 300 km2 area in the eastern part of the range. We used the new landslide and fault information, in combination with prior geological and seismic information, to investigate the association between bedrock landslides and faults. We find that the distribution and abundance of landslides is related to the presence of large active normal faults, and matches the pattern of the local historical seismicity. Landslide material is more abundant along the East Peloritani Fault System where the long‐term activity of the faults, measured by the average yearly geological moment rate, is larger than in the West Peloritani Fault System where landslides are less abundant. Along the fault systems landslide material concentrates where the cumulated fault throws are largest. We conclude that large landslides and their cumulated volume are sensitive to local rates of tectonic deformation, and discriminate the deformation of the single fault segments that dissect the Peloritani range. Our findings are a direct test of landscape evolution models that predict higher rates of landslide activity near active faults. Our work opens up the possibility of exploiting accurate landslide and fault maps, in combination with geological and seismic information, to characterize the long‐term seismic history of poorly instrumented active regions. © 2015 The Authors Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd  相似文献   

9.
The morphotectonic framework of the Central Apennines is given by faulted blocks bounded by normal faults, mostly trending NW–SE, NNW–SSE and NE–SW, which cut previous compressive structures. Such a structural setting is consistent with the focal mechanisms of the earthquakes which often occur in this area. In this paper, three lithologically different normal fault-generated mountain fronts are analysed in order to assess the relations between their geomorphic features and active tectonics. They border the Norcia depression (Sibillini Mts, Umbria), the Amatrice–Campotosto plateau (Laga Mts, Lazio) and the Fucino basin (Marsica Mts, Abruzzi). The Norcia depression is bounded by a N20°W trending normal fault to the east and by a parallel antithetic fault to the west. The main fault has a 1000 m throw and gives rise to a wide fault escarpment, characterized by: (1) sharp slope breaks due to low angle gravity faults; (2) important paleolandslides; and (3) several fault scarplets on the piedmont belt affecting Quaternary deposits. The Amatrice–Campotosto plateau is delimited by the western slope of Mt Gorzano which runs along a N20°W trending normal fault having a 1500m throw. Minor parallel faults dislocate Quaternary landforms. Large-scale massmovements also occur here. The Fucino basin was struck by the 1915 Avezzano earthquake (I=XI MCS) which produced extensive surface faulting along two parallel NW trending normal fault escarpments on the eastern border of the basin. There is paleoseismic evidence including buried gravity graben in Late Glacial gravels and tectonic dip-slip striations on Holocene calcitic crusts covering bedrock normal fault planes. These data suggest that active extensional tectonics plays a major role in the slope morphogenesis of the Central Apennines and they indicate the importance of geomorphic analysis in seismic zonation of this area.  相似文献   

10.
Changes in surface roughness on carbonate fault scarps often reflect varying durations of subaerial weathering. On the Pleasant Valley fault in central Nevada, the documentation of a surface rupture in 1915, a long recurrence interval of faulting, slow weathering rate, and a relatively high (2–3 m) single-event displacement make the discrimination of the historical and penultimate slip patches unambiguous. Following from a 2018 study, we used a Schmidt hammer and terrestrial laser scanning (TLS) to further test whether these weathering patterns delineate exposed slip patches on a fault scarp. Results show that Schmidt hammer rebound value ranges (termed ΔR – the difference between minimum and maximum R-values in repeat impacts at a point), increase by ~8–10 points across the historical–penultimate event transition zone in two separate scarp transects. TLS-derived surface roughness also indicates a clear difference between the most recent and penultimate events. The average single-event displacement (SED) estimated using the Schmidt hammer and TLS is 2.85 m at two transect sites and is roughly equivalent to the visually estimated 3 m. While this fault is an ideal case where we know some of the slip history, the results demonstrate that these techniques show promise for discriminating slip patches on larger carbonate fault scarps with longer paleoearthquake histories, and could be used alongside 36Cl cosmogenic exposure-age dating to improve paleoseismic records on normal faults. © 2019 John Wiley & Sons, Ltd.  相似文献   

11.
Geomorphic and trench investigations are used toanalyze the seismic potential of the Aremogna-CinqueMiglia fault, an active N- to NW-trending, W-facingnormal fault located in Central Apennines. Wereconstructed a complex 16 km-long, as much as 6m-high, fault scarp that displaces late Holocenesediments in the Aremogna and Cinque Miglia basins.The complex surface expression of the fault, withdouble sub-parallel scarp sections, a change in strikeof about 40° and local complexity showingimportant horizontal component, appears to becontrolled by the presence of older tectoniclineaments. We opened two trenches across the faultscarp, used a quarry exposure, and reinterpreted atrench opened by Frezzotti and Giraudi (1989), to findthe geological evidence for three Holocene surfacefaulting earthquakes on the Aremogna-Cinque Migliafault. Based on radiocarbon dating and stratigraphicand climatic considerations timing of the events isconstrained between 800 B.C. and 1030 A.D., between3735 and 2940 B.C., and between 3540 and 5000 B.C.. The most recent event is not reported in the twomillennia-long Italian Catalogues of HistoricalSeismicity. We suggest that the most recent eventcould be one of the Middle Age earthquakes of unknownorigin for which several felt reports exist in Rome.Moreover, we also consider the hypothesis that one ofthe shocks of the ambiguous September 1349 earthquakesequence could be the Aremogna-Cinque Miglia mostrecent event. Anyway, based on historicalconsideration we indicate A.D. 1349 as the youngestpossible age for this event. Finally, we suggest theAremogna-Cinque Miglia fault is part of the easternsecondary Apennines seismogenic belt. The faultparameters we obtain for this fault (i.e., recurrence interval longer than 2000 yr, verticallong-term slip rate of 0.3–0.5 mm/yr and m 6.5–6.8 forthe event) can be used as a first hand reference tocharacterize the seismic behavior of other faultsalong this section of the Apennines.  相似文献   

12.
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.  相似文献   

13.
The 20 May 2016 MW 6.1 Petermann earthquake in central Australia generated a 21 km surface rupture with 0.1 to 1 m vertical displacements across a low-relief landscape. No paleo-scarps or potentially analogous topographic features are evident in pre-earthquake Worldview-1 and Worldview-2 satellite data. Two excavations across the surface rupture expose near-surface fault geometry and mixed aeolian-sheetwash sediment faulted only in the 2016 earthquake. A 10.6 ± 0.4 ka optically stimulated luminescence (OSL) age of sheetwash sediment provides a minimum estimate for the period of quiescence prior to 2016 rupture. Seven cosmogenic beryllium-10 (10Be) bedrock erosion rates are derived for samples < 5 km distance from the surface rupture on the hanging-wall and foot-wall, and three from samples 19 to 50 km from the surface rupture. No distinction is found between fault proximal rates (1.3 ± 0.1 to 2.6 ± 0.2 m Myr−1) and distal samples (1.4 ± 0.1 to 2.3 ± 0.2 m Myr−1). The thickness of rock fragments (2–5 cm) coseismically displaced in the Petermann earthquake perturbs the steady-state bedrock erosion rate by only 1 to 3%, less than the erosion rate uncertainty estimated for each sample (7–12%). Using 10Be erosion rates and scarp height measurements we estimate approximately 0.5 to 1 Myr of differential erosion is required to return to pre-earthquake topography. By inference any pre-2016 fault-related topography likely required a similar time for removal. We conclude that the Petermann earthquake was the first on this fault in the last ca. 0.5–1 Myr. Extrapolating single nuclide erosion rates across this timescale introduces large uncertainties, and we cannot resolve whether 2016 represents the first ever surface rupture on this fault, or a > 1 Myr interseismic period. Either option reinforces the importance of including distributed earthquake sources in fault displacement and seismic hazard analyses.  相似文献   

14.
宁夏卫宁北山位于青藏高原、阿拉善和鄂尔多斯3大活动地块的交汇地带,查清该地区的活断层对厘清活动地块边界和规划中卫工业园区布局均具有重要意义。采用浅层地震勘探、钻孔联合剖面探测及槽探等手段,针对卫宁北山南缘推测断层开展综合探测,以便查清该断层展布情况和地层断错特征,并结合区域地层时代对比,初步鉴定已探明断层的活动性。结果表明：前人推测的地貌陡坎之下的卫宁北山南缘正断层并不存在,而在卫宁北山南缘地貌陡坎偏北基岩内发育一条逆冲断层,浅层地震勘探剖面揭示该断层上断点埋深35～40 m,钻孔联合剖面探测和槽探发现断层错断了石炭系和新近系,但未错断上覆的上更新统,表明该断层不是活动断层;另外,浅层地震剖面揭示在中卫盆地内部还发育一条北西西向的隐伏正断层,上断点埋深约60 m,但也未错动晚更新世以来地层,因而推测该断层不存在晚第四纪活动性。  相似文献   

15.
Since 1996 paleoseismological investigations have been used to develop the surface- rupturing history of the Bree fault scarp, the morphologically best-defined segment of the southwestern border fault of the Roer Valley graben in northeastern Belgium. The first studies determined that the escarpment is associated with a surface fault, and they exposed evidence for three surface displacements since about 40 ka BP. The most recent eventprobably occurred between 1000 and 1350 yr cal BP. Geophysical and trenching studies at a new site near the southeastern end of the fault scarp reconfirmed the coincidence of the frontal escarpment with a shallow normal fault, which displaces the Middle Pleistocene `Main Terrace' of the Maas River, as well as overlying coversands of Saalian to late Weichselian age. Different amounts of displacement shown by the two youngest coversand units indicate two discrete faulting events, but primary evidence for the coseismic nature of these events is sparse. Radiocarbon and optically stimulated luminescence dating constrainthe age of these events to the Holocene and between 14.0 ± 2.3 ka BP and 15.8 ± 2.9 ka BP, respectively. In addition, four older surface-rupturing events are inferred from the presence of four wedge-shaped units of reworked Main Terrace deposits that are interbedded with coversand units in the hanging wall of the trench and in shallow boreholes. These wedges are interpreted as colluvial wedges, produced by accelerated slope processes in response torejuvenation of the fault scarp, most probably in a periglacial environment. Luminescence dating indicates that five out of a total of six identified faulting events are younger than 136.6 ± 17.6 ka. The antepenultimate event was the largest faulting event, associated with a total fault displacement in excess of 1 m. Thus, the newly investigated trench site represents the longest and most complete record of surface rupturing recovered so far along the Bree fault scarp. This study also demonstrates the viability of the paleoseismological approach to identify past large earthquakes in areas of present-day moderate to low seismic activity.  相似文献   

16.
On 15 July 1995, the Egion earthquake (Ms = 6.2) occurred in the vicinity of Egion, west-central Greece. Macroseismic observations along the 12 km long E-W trending Egion fault represent short-term or earthquake-related deformation characterized by fairly straight E-W trending surface ruptures with small displacements that mimic the Egion fault geologic offsets and segmentation. Hanging wall converging slip vectors along the Egion fault are clearly related to fault motions at depth. Furthermore, peak accelerations of the built-up area of Egion amount to 0.54 g, that is double the estimated peak acceleration of the Egion coastal area, showing thus close relation between fault trace and attenuation of the ground motion.The Egion fault, with a total geological throw of 200 m and dips to the north at about 55 °, accommodating active tectonic deformation of the Egion area. Its morphotectonic expression reflects long-term deformation in competition with the 1995 earthquake related deformation. The Egion fault is controlling the geomorphic evolution of the Egion area as follows: 1) The fault is defining the evolution of fan-deltas (offshore) and the Meganitas river alluvial plain (onshore). 2) The hanging-wall's greatest subsidence is observed, at the Egion bay, at the central portion of the fault. The Egion bay is located at the central part of the fault showing a strong relationship between the long term slip-rate ratio and the recent coastal morphology. The subsidence gradient or the tectonic activity along the fault is defined by the valley-floor width to valley height index (Vf) of small rivers draining the fault scarp. The Meganitas river course is tilted, when crosses the Egion fault trace, towards the area with the highest subsidence along the fault. 3) Stream incision is more important than slope recession at areas close to the fault trace.All these observations suggest that the Egion fault, which probably hosted the last earthquake, are geomorphically controlling the evolution of a 15 km-long by 5 km-wide zone, fairly similar in dimensions to the surface length of the fault.  相似文献   

17.
Two felt moderate-sized earthquakes with local magnitudes of 4.9 on October 11, 1999 and 4.3 on November 08, 2006 occurred southeast of Beni Suef and Cairo cities. Being well recorded by the digital Egyptian National Seismic Network (ENSN) and some regional broadband stations, they provided us with a unique opportunity to study the tectonic process and present-day stress field acting on the northern part of the Eastern Desert of Egypt. In this study, we analyze the main shocks of these earthquakes and present 15 well recorded aftershocks (0.9 ≤ ML ≤ 3.3) which have small errors on both horizontal and vertical axes. The relocation analysis using the double difference algorithm clearly reveals a NW trending fault for the 1999 earthquake. The spatial distribution of its aftershocks indicates a propagation of rupture from the SW towards the NW along a fault length ~5 km dipping nearly ~40°SW. We also determined the focal mechanisms of the two main shocks by two methods (polarities and amplitudes ratios of P, SV and SH and regional waveform inversion). Our results indicate a normal faulting mechanism with a slight shear component for the first event, while pure normal faulting for the second one. The spatial distribution of the 1999 aftershocks sequence along with the retrieved focal mechanism confirmed the NW plane as the true fault plane. While for the 2006 event, the few aftershocks do not reveal any fault geometry; its focal mechanism indicated a pure normal fault nearly trending WNW-ESE that corresponds more likely to the extension of the 1999 earthquake fault. The seismicity distribution between the two earthquake sequences reveals a noticeable gap that may be a site of a future event. The NNE-SSW extensional stress indicated by the mechanisms of these events is in agreement with the regional stress field and the rifting of the northern Red Sea in its northern branches (Gulf of Suez and Gulf of Aqaba). The source parameters (seismic moment, moment magnitude, fault radius, stress drop and displacement across the fault) were also estimated and compared based on both the regional waveform inversion and the displacement spectra and interpreted in the context of the tectonic setting. The obtained results imply a reactivation of the pre-exiting NW-SE faults as a result of extensional deformation from the northern Red Sea-Gulf of Suez rifts.  相似文献   

18.
A remarkable earthquake struck Yutian, China on June 26th, 2020. Here, we use Sentinel-1 images to investigate the deformation induced by this event. We invert the InSAR observations using a two-step approach: a nonlinear inversion to constrain fault geometries with uniform slip based on the rectangular plane dislocation in an elastic half-space, followed by a linear inversion to retrieve the slip distribution on the fault plane. The results show that the maximum LOS displacement is 22.6 ​cm, and the fault accessed to the ruptured characteristics of normal faults with the minor left-lateral strike-slip component. The fault model indicates a 210° strike. The main rupture zone concentrates in the depth of 5–15 ​km, and the fault slip peaks at 0.89 ​m at the depth of 9 ​km. Then, we calculate the variation of the static Coulomb stress based on the optimal fault model, the results suggest that the Coulomb stress of the Altyn Tagh fault and other neighboring faults has increased and more attention should be paid to possible seismic risks.  相似文献   

19.
The 2008 M w 7.9 Wenchuan produced a ～285–300-km-long coseismic surface rupture zone, including a 60-km-long segment along the Qingchuan fault, the northeastern segment of the Longmen Shan Thrust Belt (LSTB), Sichuan Basin, central China. Field investigations, trench excavations, and radiocarbon dating results reveal that (i) the Qingchuan fault is currently active as a seismogenic fault, along which four morphogenic earthquakes including the 2008 Wenchuan earthquake occurred in the past ca. 3500 years, suggesting an average millennium recurrence interval of morphogenic earthquakes in the late Holocene; (ii) the most recent event prior to the 2008 Wenchuan earthquake took place in the period between AD 1400 and AD 1100; (iii) the penultimate paleoseismic event occurred in the period around 2000 years BP in the Han Dynasty (206 BC–AD 220); (iv) the third paleoseismic event occurred in the period between 900 and 1800 BC; and (v) at least three seismic faulting events occurred in the early Holocene. The present results are comparable with those inferred in the central and southwestern segments of the LSTB within which the Wenchuan magnitude earthquakes occurred in a millennium recurrence interval, that are in contrast with previous estimates of 2000–10,000 years for the recurrence interval of morphogenic earthquakes within the LSTB and thereby necessitating substantial modifications to existing seismic hazard models for the densely populated region at the Sichuan region.  相似文献   

20.
The Al Hoceima Mw 6.4 earthquake of 24 February 2004 that occurred in the eastern Rif region of Morocco already hit by a large event in May 1994 (Mw 5.9) has been followed by numerous aftershocks in the months following the event. The aftershock sequence has been monitored by a temporary network of 17 autonomous seismic stations during 15 days (28 March–10 April) in addition to 5 permanent stations of the Moroccan seismic network (CNRST, SPG, Rabat). This network allowed locating accurately about 650 aftershocks that are aligned in two directions, about N10-20E and N110-120E, in rough agreement with the two nodal planes of the focal mechanism (Harvard). The aftershock alignments are long enough, about 20 km or more, to correspond both to the main rupture plane. To further constrain the source of the earthquake main shock and aftershocks (mb > 3.5) have been relocated thanks to regional seismic data from Morocco and Spain. While the main shock is located at the intersection of the aftershock clouds, most of the aftershocks are aligned along the N10-20E direction. This direction together with normal sinistral slip implied by the focal mechanism is similar with the direction and mechanisms of active faults in the region, particularly the N10E Trougout oblique normal fault. Indeed, the Al Hoceima region is dominated by an approximate ENE-SSW direction of extension, with oblique normal faults. Three major 10–30 km-long faults, oriented NNE-SSW to NW-SE are particularly clear in the morphology, the Ajdir and Trougout faults, west and east of the Al Hoceima basin, respectively, and the NS Rouadi fault 20 km to the west. These faults show clear evidence of recent vertical displacements during the late Quaternary such as uplifted alluvial terraces along Oued Rihs, offset fan surfaces by the Rouadi fault and also uplifted and tilted abandoned marine terraces on both sides of the Al Hoceima bay.However, the N20E direction is in contrast with seismic sources identified from geodetic inversions, which favour but not exclusively the N110-120E rupture directions, suggesting that the 1994 and 2004 events occurred on conjugate faults. In any event, the recent seismicity is thus concentrated on sinistral N10-20E or N110-120E dextral strike-slip faults, which surface expressions remain hidden below the 3–5 km-thick Rif nappes, as shown by the tomographic images build from the aftershock sequence and the concentration of the seismicity below 3 km. These observations may suggest that strain decoupling between the thrusted cover and the underlying bedrock and highlights the difficulty to determine the source properties of moderate events with blind faults even in the case of good quality recorded data.  相似文献   

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