龙门山地震带的地质背景与汶川地震的地表破裂

龙门山位于青藏高原与扬子地台之间,系由一系列大致平行的叠瓦状冲断带构成,自西向东发育汶川—茂汶断裂、映秀—北川断裂和彭县—灌县断裂,并将龙门山划分为3个构造地层带,分别为变形变质构造地层带(主要由志留系—泥盆系浅变质岩和前寒武系杂岩构成)、变形变位构造地层带(主要由上古生界—三叠系沉积岩构成)、变形构造地层带(主要由侏罗系至第三系红层和第四纪松散堆积构成)。龙门山断裂带属地震危险区,3条主干断裂皆具备发生7级左右地震的能力,其中映秀—北川断裂是引发地震的最主要断层,据对彭县—灌县断裂青石坪探槽场地的研究结果表明,在该断裂带上最晚的一次强震发生在930±40a.B.P.左右,据此,可以初步判定,这3条主干断裂的单条断裂上的强震复发间隔至少应在1000a左右,表明龙门山构造带及其内部断裂属于地震活动频度低但具有发生超强地震的潜在危险的特殊断裂,以逆冲-右行走滑为其主要运动方式。汶川地震属于逆冲—走滑型的地震,地表破裂分布于映秀—北川断裂带和彭县—灌县断裂带上。根据近南北向的断裂(小鱼洞断层、擂鼓断层和邓家坝断层)和地表断距可将映秀—北川断层的地表破裂带划分为两个高值区和两个低值区,两个高值区分别位于南段的映秀-虹口一带和位于中北段的擂鼓—北川县城—邓家坝一带;两个低值区分别位于中南段的白水河—茶坪一带和北段的北川黄家坝至平武石坎子一带,两个高值区分别与小鱼洞断层和擂鼓断层相关。根据保存于破裂面上的擦痕,可将该地震破裂过程划分为两个阶段,早期为逆冲作用,晚期为斜向走滑作用,其与地壳增厚构造模式和侧向挤出摸式在青藏高原东缘的推论具有不吻合性。鉴于龙门山的表层运动速率与深部构造运动速率具有不一致性,初步探讨了龙门山地区的地表过程与下地壳流之间的地质动力模型,认为下地壳物质在龙门山近垂向挤出和垂向运动,从而造成导致龙门山向东的逆冲运动、龙门山构造带抬升和汶川特大地震。在此基础上,根据汶川地震所引发的地质灾害,对地震灾后重建提出了的几点建议。

龙门山地震带坡耕地土壤侵蚀对有机碳迁移的影响

坡耕地土壤再分布对土壤有机碳(SOC,soil organic carbon)迁移的作用机制研究已成为土壤侵蚀学研究的热点,然而目前极少有研究关注地震后生态脆弱的龙门山地震带坡耕地土壤侵蚀机理及其导致的土壤有机碳再分布规律。该研究选择龙门山地震带内(都江堰市)一块陡坡耕地和一个梯田系列,采用137Cs法和野外调查,对比分析强震导致田埂垮塌和未受损情况下坡耕地土壤侵蚀空间变化特征和有机碳运移变化机理。结果表明,该区黄棕壤有效137Cs背景值为1 473 Bq/m2;坡度较小的坡式梯田内部上坡表现为侵蚀,下坡表现为沉积,同时,上部梯田的侵蚀速率高于下部梯田,但整个梯田系列净侵蚀量非常小,这表明梯田之间由于缺乏田埂的保护,水力也起着侵蚀、搬运上坡梯田土壤的作用,但是整个坡式梯田系列可以起到较好的保土作用,同时,坡式梯田内部主要以耕作侵蚀为主,是造成梯田上部坡位土壤流失严重的主要原因;陡坡耕地的地形为复合坡,由于田埂垮塌导致其土壤侵蚀速率显著高于坡式梯田系列,在整个坡面上,除了坡顶土壤侵蚀速率高之外,下坡坡度变大(曲率较大)的部位土壤侵蚀速率也非常高,同时,土壤沉积也发生在2个坡位(中下坡坡度较缓的部位和坡脚部位);在梯田系列和陡坡耕地上,SOC与土壤137Cs的空间变化规律较为一致。研究结果表明,在龙门山地震带,质量较好的石埂梯田仍然发挥着较好的土壤保持效果,同时,耕作侵蚀是该区坡耕地上一种重要的土壤侵蚀形式,在制定相应的土壤保持措施时,必须充分考虑耕作侵蚀的作用,才能有效地控制土壤侵蚀,此外,该研究结果还表明采用137Cs核素示踪技术可以比较科学地解释该区域的土壤侵蚀速率和SOC的空间变异规律。

龙门山地震带不同植被类型土壤微生物群落多样性分析

采用BIOLOG-ECO (Ecology Plate,ECO)技术,探讨龙门山地震带——银厂沟地区杂木林、竹林和松树林3种植被类型对土壤微生物群落多样性的影响.结果表明,随着培养时间的延长,土壤微生物群落的平均颜色变化率(Average Well Color Development,AWCD)均呈"S"型曲线增长,但3种植被类型土壤的AWCD值在增长速率和最大值方面存在一定的差异性;碳水化合物类、氨基酸类、醇类和酯类碳源是土壤微生物群落主要利用的碳源类型;当培养0～144 h时,竹林微生物群落的多样性最高,其次是杂木林,松树林最低,当培养144～240 h时,杂木林微生物群落的多样性高于竹林,松树林最低.研究结果说明植被类型可以影响其土壤微生物群落的多样性,同时也为龙门山地震带土壤生态系统的恢复和重建提供了科学依据.

龙门山地震带5.12汶川地震余震空间分布特征

2008年5月12日14时28分至10月12日12时，四川省汶川发生Ms8．0级地震后共记录到余震33308次，Ms≥4．0级余震268次。采用GIS技术，对龙门山地震带上汶川、北川、青川3个县从2008年5月12日～10月12日12时发生的156次Ms≥4．0级余震的空间分布进行了研究。结果显示：在汶川、北川、青川3个县境内：①在东西方向上，若干余震点位的连线与纬线平行，Ms≥4．0级余震呈现纬向地带性特征；②Ms≥4．0级余震亦呈现经向地带性特征；③Ms≥4．0级余震，最位在南北方向与东西方向的连线形成了经纬交织的网格，在空间上呈现等间距网络状分布现象；④Ms≥4．0级的156次余震均发生在经纬交织的网络节点处及其附近，在空间上具有原地复发性的特点。研究结果对于该地震带的趋势分析和震，精监视有一定的意义。

龙门山地震带的大震系列——关于四川汶川8.0级大震的预报探讨

大震系列公式:Ni=(Ni-k/1-b )k在龙门山地震带的地震研究中仍然是适用的。使用该公式处理龙门地震带可知1879年发生MS>7.0级的地震后,约在前三十二年就能预测在茂汶川之间发生MS>7.0级的地震,发震时间是在2008年3月27日(理论值);然后在距约二十四年,龙门山地震带西南端再次再次发生MS>7.0级的地震,结束此次大震系列。

龙门山地震带地震活动特征的研究

本文剖析和研究了龙门山地震带强震活动的时空状态及中等地震活动特征，取得了一些有益的认识。该带强震频度不高，中等地震相对活跃。强震活动的盛衰变化没有明显规律，其主体活动地段为汶川－一茂县一北川段和天全一宝兴段，二者的强震具有交替发生的特点。自然节律显示有规律起伏，可以推测地震的强、弱趋势。本文仔细剖析了中等地震活动情况，提出的若干特征对于监视本带有一定社会影响的４～５级地震有实际意义。

中小地震定量分析在地震区带划分中的应用——以龙门山地震带及邻区为例

采用中小地震能量密度值计算方法,定量分析龙门山地震带及邻区1970年以来现代中小地震空间分布图像特征,探讨其与历史强震震中分布、主要断裂展布和地震带边界的关系,验证了新区划图中龙门山地震带边界修改方案的合理性。同时基于震级-频度分布原理,对所使用的中小地震资料进行完整性分析,分别给出了区域内不同时段的最小完整性震级MC。本研究为此地区地震区、带划分提供了定量依据,可供地震活动性分析和地震危险性评价研究时参考。

浅谈龙门山地震带对汶川县萝卜寨的影响

羌文化是华夏文化的一个重要起源,羌寨的保护和传承也成为一个研究热点。汶川作为羌文化聚集的地区在经历了5.12地震后,许多羌寨都有不同程度的损坏。以研究龙门山地震带为背景对汶川地震的震因以及对在龙门山地区村落选址进行研究,其中以萝卜寨为例重点进行灾后损坏程度分析。

用于龙门山地震带地震预报研究的若干震兆标志

龙门山地震带毗邻平原地区和大都市，多以中等地震活动方式释放应变能。对龙门山地震带地震活动特点研究之后，认真探讨了用于该带地震预报研究的震兆标志。其指标和标志是：1．频度和最大震级分形Rn、Rm≥|±1．5|;2．算法复杂性C(n)≥|±σn-1|；3．η≥|±σn-1|；4．max持续低于长期的max值；5．累积滑动频度∑N持续增高。

龙门山地震带土壤细菌多样性的研究

采用16S r DNA变性梯度凝胶电泳(Denaturing Gradient Gel Electrophoresis,DGGE)技术,结合不同土壤样品的理化性质差异,分析龙门山地震带土壤细菌多样性。结果表明,不同的土壤样品其理化性质存在着明显的差异,三种植被类型的土壤(杂木林、竹林和松树林)中,杂木林的土壤pH值和全氮含量最高,竹林最低;而土壤碱解氮、速效磷、速效钾和有机质含量则是杂木林比竹林高,松树林最低。对DGGE优势条带测序分析表明,回收的11个优势条带(条带A^K)中,条带A、C、D、F、G和H属于变形菌门,条带E、J和K属于酸杆菌门,条带B属于放线菌门,条带I属于装甲菌门,变形菌门占细菌总类群的54.55%,说明龙门山地震带土壤优势菌群为变形菌门细菌。细菌群落多样性与土壤理化性质相关性的统计分析结果显示,影响细菌群落多样性的主要环境因子在杂木林土壤中为速效钾(r=0.80,P>0.05),竹林为有机质(r=0.97,P>0.05),松树林则是pH值(r=0.35,P>0.05),说明龙门山地震带不同植被类型土壤中的细菌群落对土壤理化性质的需求不同。由于地震能够破坏土壤结构,改变土壤理化性质,从而导致土壤细菌群落多样性发生改变,因此,研究龙门山地震带土壤细菌多样性为震后灾区重建工作和维持龙门山地震带生态系统的可持续性提供了科学依据。

汶川8.0级地震的基本特征及其研究进展

2008年5月12日在龙门山发生的汶川8.0级特大地震属于逆冲—走滑型地震。为了尽快和最好地利用汶川地震的资料,推动地震地质科学的研究。在论述了汶川地震科学研究的基本现状和研究进展的基础上,重点讨论了龙门山地震带的地质背景、活动构造、汶川地震的基本特征、地表破裂及组合样式、历史强地震复发周期、观测数据的积累与整合、构造运动学和动力学机制、地质灾害和灾后重建、龙门山地区未来地震的预测和预报等方面的内容。并建议从汶川大地震中汲取教训,整合地质学、地球物理和地球化学等多学科的综合研究,探讨汶川地震的形成过程和机制,以有效地减轻和逐步避免今后可能发生的类似的灾难。

大地震为什么发生在汶川

印度板块每年以50毫米的速度向亚洲板块俯冲,这个地质活动造成青藏高原不断隆升后,在重力的作用下向海拔低的东边滑动,从而与四川盆地挤压,形成包括龙门山断裂带的一系列断裂带。这次汶川地震,就是青藏高原所在的构造,前端逆冲到四川盆地上面,在长期积累后突然滑动造成的。

一个村庄头上的达摩之剑

老蓥华村7队距离蓥华镇两公里,这个美丽的小山村在两座山峰之间的一片谷地上。村前是静静流淌的石亭江。在这个季节里,村民们本应该生活在收获的喜悦里:小麦已经收割,玉米播种下去,长出一尺多高。

练习六 扩展语句与压缩语段

1.用下列四个词语写一段话,所写句子必须包含这四个词语,内容要有联系,意思要完整。

GiT-based structural geologic feature analysis of the southern segment of Longmenshan fault zone for earthquake evidence

The Longmenshan fault is a thrust fault which runs along the base of the Longmen Mountains in Siehuan province, southwestern China. The southern segment of the fault had two distinct responses to the Ms 8 Wenehuan and Ms 7 Lushan earthquakes. This study determines characteristics of the structural geology of the Longmenshan fault to evaluate how it influenced the two aforementioned earthquakes. This research was done within a Geo- information Technologies （GiT） environment based on multi-source remote sensing and crustal movement data extracted from the Global Positioning System （GPS）. The spatial distribution of the southern segment of the Longmenshan fault zone was comprehensively analyzed to study both earthquakes. The study revealed that the Wenehuan and Lushan earthquakes occurred on two relatively independent faults. In addition, there was a nearly constant-velocity crustal movement zone between the two epicenters that probably had a compressive stress with slow motion. Furthermore, the central fault and a mountain back fault gradually merged from north to south. The Lushan earthquake of the Wenchuan earthquake. was not an affershock The research showed that fault zones within 30-50 km of State Highway 318 are intensive and complex. In addition, crustal movement velocity decreased rapidly, with a strong multi-directional shear zone. Thus, activity in that zone was likely stronger than in the northern part over the medium to long term.

The Dynamic Characteristics of Strain Fields and Crustal Movement before the Wenchuan Earthquake (M_S=8.0)

In this paper, we analyze the crustal movements, strain field changes and large scale dynamic characteristics of horizontal deformation before the Wenchuan earthquake （ Ms = 8.0） using GPS data obtained from the Crustal Movement Observation Network of China. The following issues are discussed. First, the strain fields of the Longmeushan fault zone located at the epicenter show slow accumulation, because of the tectonic dynamics process subjected to the eastward movement of the Bayan Har block. Second, the different movements between the Longmenshan fault and South China block are smaller than the errors of GPS observation. Third, the high value of compressive strain （2004 - 2007） is located at the epicenter, which shows that the local squeezing action is stronger than before. Fourth, the data from GPS reference stations in the Chinese Mainland show that crustal shortening is faster than before in the north-eastern direction, which is part of the background of the local tectonic dynamics increase in the Longmenshan fault zone.

Analysis of Dynamic Characteristics of Crustal Horizontal Deformation in the South Segment of Longmenshan Fault Zone Revealed by GPS Data after the Wenchuan Earthquake

In order to obtain deformation parameters in the south segment of Longmenshan fault zone,Euler datum transformation and the least square collocation for data interpolation and smoothing are used to process GPS displacement time series data in the south segment of Longmenshan fault zone,and the rigid and elastic-plastic block motion model is used to calculate the strain parameters in each subarea. Conjoint analysis of displacement,velocity of each station and strain parameters of each subarea reveals that the influence of the Wenchuan earthquake on the south segment of Longmenshan fault zone increases from southeast to northwest,causing a highest deformation rate 6 times the background value and heightening the influence of the hidden faults on the difference of the earth surface along its two sides,which leads to the seismic risk of the southern segment increasing from north to south. The comparison of seismic risk among subareas based on the tectonic and seismicity background indicates that the most dangerous area is on the southeast of Longmenshan faults,and the background strain accumulation and the promoting effect of the Wenchuan earthquake advanced the occurrence of Lushan earthquake and the sinistral strike-slip on the rupture plane. The Wenchuan earthquake also caused a slight two-year long continuous strain release in the south segment of Xianshuihe fault,but the influence is far less than the effect of the compressive strain caused by the Sichuan-Yunnan block.

Variation of the Energy Field of Longmenshan Fault Zone before the Wenchuan M_S 8. 0 Earthquake

During the process of preparation and occurrence of a large earthquake, the stress-strain state along the fault zone has close relation with the weak seismicity around the fault zone. The seismic energy release near the fault zone before an earthquake can better reflect the dynamic process of earthquake preparation. Thus, in this paper, the method of natural orthogonal function expansion has been adopted to discuss the time variation about the energy field of the seismic activity along the Longmenshan fault zone before the Wenchuan MsS. 0 earthquake, 2008. The results show that evident short-term rise changes appeared in the time factors of the typical field corresponding to several key eigenvalues of the energy field along the Longmenshan fault zone before the Wenchuan earthquake, probably being the short-term anomaly message for this earthquake. Through contrastive analysis of earthquake examples such as the 1976 Tangshan earthquake, the authors think that the study of time variation of energy field of seismicity along active fault zone will be helpful for conducting intentional and intensive earthquake monitoring and forecast in active fault regions with high seismic risk based on medium- and long-term earthquake trend judgment.

Tectonic Features of the M_S8.0 Wenchuan Earthquake and Its Aftershocks

The M8.0 Wenchuan earthquake occurred on the Longmenshan fault zone. Based on field investigation of the surface rupture and focal mechanism study of the aftershocks, we discuss the geological relationship of the main, secondary and triggered ruptures. The main rupture is about 200km long and can be divided into the south part and the north part. The south part consists of two parallel fault zones characterized by reverse faulting, with several parallel secondary ruptures on the hanging wall of the main fault, and the north part is a single main fault zone characterized by lateral strike-slip and reverse faulting. Compared to a 300km long aftershock distribution, the surface rupture only occupies 200km, and the remaining lOOkm on the northeast of the main rupture was triggered by aftershocks. Study on the ruptures of this earthquake will be useful for studying the earthquake risk evolution on the Longmenshan fault system.

Microseismic Concentration Zones before and after the February 12,2014 M_S 7.3 Yutian Earthquake and the Possible Indication of an Earthquake Risk Zone

Since 2001, there have occurred in succession the 2001 Kunlun Mountains M S8. 1earthquake,the 2008 Wenchuan M S8. 0 earthquake,the 2010 Yushu M S7. 1 earthquake and the 2012 Lushan M S7. 0 earthquake in the periphery of the Bayan Har block. By comparison of the characteristics of seismic strain release variations before and after the Kunlun Mountains M S8. 1 earthquake in the same time length in the geodynamical related regions,we found that the seismic strain release was obviously enhanced after the earthquake in the Longmenshan area,Batang area,and the NS-trending valleys at the west of the Hot Spring Basin. The Wenchuan earthquake occurred in the first area,and the Yushu earthquake is related to the second area. After the earthquake rupture occurred on the East Kunlun fault zone on the northern boundary of the Bayan Har Block,crustal materials on the south side of the fault zone migrated to the southeast,leading to a concentration of tectonic deformation in the Longmenshan thrust belt, e ventually rupturing on the Longmenshan thrust belt. This earthquake case illustrates that seismicity enhancement zones are possibly prone to long-term destructive earthquakes. After the M S7. 3 earthquake in Yutian,Xinjiang on February 12,2014,earthquake frequency and seismic strain release markedly increased in the junction area between the eastern Qilian Mountain tectonic belt and the Altun Tagh fault zone,where more attention should be paid to the long-term seismic risk.