Focal Mechanism Solutions of the 2008 Wenchuan earthquake sequence from P-wave polarities and SH/P amplitude ratios: new results and implications

The 2008 Wenchuan earthquake, a major intraplate earthquake with M _w 7.9, occurred on the slowly deforming Longmenshan fault. To better understand the causes of this devastating earthquake, we need knowledge of the regional stress field and the underlying geodynamic processes. Here, we determine focal mechanism solutions (FMSs) of the 2008 Wenchuan earthquake sequence (WES) using both P-wave first-motion polarity data and SH/P amplitude ratio (AR) data. As P-wave polarities are more reliable information, they are given priority over SH/P AR, the latter of which are used only when the former has loose constraint on the FMSs. We collect data from three categories: (1) permanent stations deployed by the China Earthquake Administration (CEA); (2) the Western Sichuan Passive Seismic Array (WSPSA) deployed by Institute of Geology, CEA; (3) global stations from Incorporated Research Institutions for Seismology. Finally, 129 events with magnitude over M _s 4.0 in the 2008 WES are identified to have well-constrained FMSs. Among them, 83 are well constrained by P-wave polarities only as shown by Cai et al. (Earthq Sci 24(1):115–125, 2011), and the rest of which are newly constrained by incorporating SH/P AR. Based on the spatial distribution and FMSs of the WES, we draw following conclusions: (1) the principle compressional directions of most FMSs of the WES are subhorizontal, generally in agreement with the conclusion given by Cai et al. (2011) but with a few modifications that the compressional directions are WNW–ESE around Wenchuan and ENE–WSW around Qingchuan, respectively. The subhorizontal compressional direction along the Longmenshan fault from SW to NE seems to have a left-lateral rotation, which agrees well with regional stress field inverted by former researchers (e.g., Xu et al., Acta Seismol Sin 30(5), 1987; Acta Geophys Sin 32(6), 1989; Cui et al., Seismol Geol 27(2):234–242, 2005); (2) the FMSs of the events not only reflected the regional stress state of the Longmenshan region, but also were obviously controlled by the faults to some extent, which was pointed out by Cai et al. (2011) and Yi et al. (Chin J Geophys 55(4):1213–1227, 2012); (3) while the 2008 Wenchuan earthquake and some of its strong aftershocks released most of the elastic energy accumulated on the Longmenshan fault, some other aftershocks seem to occur just for releasing the elastic energy promptly created by the 2008 Wenchuan earthquake and some of its strong aftershocks. (4) Our results further suggest that the Longmenshan fault from Wenchuan to Beichuan was nearly fully destroyed by the 2008 Wenchuan earthquake and accordingly propose that there is less probability for great earthquakes in the middle part of the Longmenshan fault in the near future, although there might be a barrier to the southwest of Wenchuan and it is needed to pay some attention on it in the near future.     

汶川地震前后川滇地区主要断裂活动趋势分析

针对川滇地区地震活动频繁的现状,该文首先对1998—2008年和2008—2015年该地区两期GNSS连续观测数据进行解算(时间节点为汶川地震);然后结合前人对川滇区域棱形块体的划分方案,基于欧拉矢量计算方法求出各块体的欧拉矢量以及旋转极经度、旋转极纬度、旋转角速度,并通过反欧拉矢量方法反演出鲜水河-小江断裂带和龙门山断裂带的错动趋势变化。结果表明:汶川地震造成两条断裂带在错动方向和错动速率方面均有变化,并且主要集中在川滇区域北部和西部的断裂带上。     

川西龙门山前缘雷口坡组油气成藏条件

龙门山前雷口坡组烃源岩为中等烃源岩,干酪根类型以I型干酪根为主,生气丰度30-50×10^8m^3／k^2,生气中心在安县。雷口坡组储集岩为粒屑白云岩,主要发育于雷三段,其次为雷一段,为孔隙性储层。研究表明这些粗结构碳酸盐岩的发育与滩相沉积环境密切相关。中坝雷三气藏成藏主控因素是：早期的构造,有利的储层和良好的保存条件。龙门山前缘雷口坡组天然气勘探前景良好,龙门山前缘中北段双鱼石至金河一带为有利勘探区带。     

龙门山国家地质公园旅游产品设计初探

地质公园是基于自然保护前提下的以地质遗迹景观为主体，集地学、自然、人文景观为一体，以地学科考为主题，具有一定市场功能的综合性多功能的公园旅游区。地质公园内有丰富的旅游资源，适宜开发多种多样的旅游产品，可归为6大类。地质公园旅游产品的成功开发在于重视产品市场定位、旅游形象的系统设计和产品创意与创新。     

汶川地震序列动态跟踪分析及对甘肃震情的判定

在对四川省汶川县地震序列的动态跟踪与对甘肃省震情的动态判定过程中,笔者成功地把握了甘肃震情、一定程度上把握了汶川地震序列的动态发展变化并对5月18日发生在四川省江油市的6.0级地震作出了成功预测。此次成功的地震预测实践为甘肃省的抗震救灾工作提供了正确的科学依据,为减轻地震灾害和稳定被地震扰乱的社会秩序发挥了重要作用,同时也在中国地震预报的历史上写下了新的篇章。从中笔者更加深刻地感悟到,地震预报虽然是一个十分复杂、尚未被攻克的世界性难题,但对于一个特定的地区,如果预报思路和方法得当,要实现一次乃至数次成功的预报是可能的。这对地球科学家们如何认识地震预报问题有一定的意义。     

四川广元清风峡剖面上泥盆统风暴沉积特征及其古环境意义*

泥盆纪吉维特期—弗拉期(G-F)之交的环境突变拉开了晚泥盆世生物大灭绝的序幕。然而,不同学者对于该时期环境变化的认识仍存在争议。为了解泥盆纪G-F之交的环境变化特征,通过对四川广元清风峡剖面泥盆系观雾山组中6套风暴岩的详细沉积学研究,识别出了典型的风暴沉积标志,如底面侵蚀构造、粗粒滞留沉积、粒序层理、丘状交错层理等。根据风暴沉积构造的组合特征,建立了3种近源风暴沉积序列: 序列Ⅰ,底部为具侵蚀构造的砾屑灰岩,顶部为块状泥晶灰岩;序列Ⅱ,底部为具侵蚀构造的砾屑灰岩,顶部为具粒序层理的砂屑灰岩;序列Ⅲ,底部为具侵蚀构造的砾屑灰岩,中部为具粒序层理的介壳灰岩,上部为具丘状交错层理的颗粒泥晶灰岩,顶部为具波状层理和水平层理的泥晶灰岩。牙形石生物组合分析显示,风暴岩形成于晚泥盆世弗拉期初期。基于现代风暴成因分析,认为弗拉期初期全球低纬度地区广泛分布的风暴岩与气候变暖和快速海侵密切相关。研究区风暴岩的发现对认识上扬子地区晚泥盆世沉积环境、古地理、古气候均具有重要意义。     

Metamorphism of Triassic Flysch Along Profile Zoigê-Lushan, Northwest Sichuan, China

We used illite and chlorite crystallinities, index minerals, mineral assemblages, polytype and domain size of white mica, electron microprobe analysis (EMPA), b0 geobarometer and chlorite geothermometer to quantify the diagenetic and metamorphic overprint on the Triassic flysch formations in the Songpan-Garzê orogen along profile Zoigê-Lushan, northwest Sichuan. Two anchizones, two epizones, one diagenetic zone and a transition belt in-between them are defined on the basis of the obtained data. lllite crystallinity correlates strongly with the best mean domain size of mica and moderately with chlorite crystallinity. 2M1 white mica polytype are observed within the epizone whilst 2M1 and 1M polytypes occur in the anchizone and diagenetic zone. Epizonal metamorphism reached maximum temperatures of 370℃±21℃ at low-to intermediate pressure conditions. Clay minerals underwent Ostwaid ripening during metamorphism. Rocks from both sides of the Longmenshan fault reveal contrasting degrees of metamorphic overprint: on the northwest side of the Longmenshan fault, epimetamorphlc conditions contrast with diagenetic rocks on the southeast side.     

青藏高原东缘龙门山晚新生代走滑挤压作用的沉积响应

成都盆地位于青藏高原东缘,夹于龙门山与龙泉山之间,盆地的长轴方向平行于龙门山,呈现为北东—南西向展布的线性盆地。盆地中充填了3.6Ma以来的半固结—松散堆积物,最大厚度为541 m,在垂向上由下部的大邑砾岩、中部的雅安砾石层和上部的上更新统至全新统砾石层组成,其与下覆地层均为不整合接触,显示该盆地是一个单独的成盆期,并非是在中生代前陆盆地基础上形成的继承性盆地。在垂直于龙门山造山带方向上,成都盆地具不对称的楔形结构,沉积基底面整体向西呈阶梯状倾斜,盆地中充填的碎屑物质均来源于盆地西侧的龙门山,具横向水系和单向充填的特征;而且盆地的沉降中心具有逐渐向远离造山带方向迁移的特征,显示盆地的挤压方向垂直于龙门山主断裂,造成了成都盆地在垂直于造山带方向上的构造缩短。在平行于龙门山造山带方向上,成都盆地具有一系列的北东向延伸的次级凸起和凹陷,凹陷和凸起相间分布,且在空间上呈斜列形式展布于盆地的底部,其中次级凹陷（沉降中心）和冲积扇具有向平行龙门山造山带方向迁移的特征,表明成都盆地西缘的龙门山断裂具有右旋走滑的特征。鉴于以上特征,认为成都盆地是在龙门山造山带晚新生代走滑与逆冲的联合作用下形成的走滑挤压盆地。     

四川龙门山区中泥盆统金宝石组核形石岩石学特征及其成因*

四川龙门山区甘溪剖面中泥盆统金宝石组顶部发育一套核形石灰岩。通过野外剖面实测和镜下详细观察,认为该套核形石灰岩垂向上可分为3个沉积旋回,每个旋回均由核形石灰岩和泥晶灰岩组成。葛万藻(Girvanella)丝状体在核形石的核心和包壳中均有发育,对核形石的形成具有重要的作用。结合核形石中的纹层类型、Girvanella丝状体形态以及渗滤砂与核形石纹层的关系,认为核形石是在浅滩和礁后潟湖环境中沉积形成的,核形石及其围岩中赋存的粒内渗滤砂和粒间渗滤砂,形成于核形石沉积之后的大气水淋滤作用。通过对核形石形成环境的分析可知,龙门山区甘溪剖面中泥盆统金宝石组顶部核形石灰岩形成的水动力条件从下到上逐渐减弱,反映了当时相对海平面由低到高的变化特点。     

Tectonic Stress State Changes Before and After the Wenchuan Ms 8.0 Earthquake in the Eastern Margin of the Tibetan Plateau

Crustal tectonic activities are essentially the consequences of the accumulation and release of in situ stress. Therefore, studying the stress state near active faults is important for understanding crustal dynamics and earthquake occurrences. In this paper, using in situ stress measurement results obtained by hydraulic fracturing in the vicinity of the Longmenshan fault zone before and after the Wenchuan Ms 8.0 earthquake and finite element modeling, the variation of stress state before and after the Wenchuan Ms 8.0 earthquake is investigated. The results show that the shear stress, which is proportional to the difference between principal stresses, increases with depth and distance from the active fault in the calm period or after the earthquakes, and tends to approach to the regional stress level outside the zone influenced by the fault. This distribution appears to gradually reverse with time and the change of fault properties such as frictional strength. With an increase in friction coefficient, low stress areas are reduced and areas with increased stress accumulation are more obvious near the fault. In sections of the fault with high frictional strengths, in situ stress clearly increases in the fault. Stress accumulates more rapidly in the fault zone relative to the surrounding areas, eventually leading to a stress field that peaks at the fault zone. Such a reversal in the stress field between the fault zone and surrounding areas in the magnitude of the stress field is a potential indicator for the occurrence of strong earthquakes.