四川龙门山断层地质分析

           2008年5月12日，发生在四川汶川地区的 M8.0 大地震，给我国四川等省份带来巨大的人员和经济损失。灾难发生以后，全国人民向灾区伸出了援助之手，上演了一幕幕可歌可泣的感人故事。笔者和广大热心网友一样，密切关注着灾情的最新动向。距灾难发生已经快2周时间了，但在网上好像并未发现中文网站对此次地质灾害从地质构造等方面进行深入分析和讨论。因此笔者特从网上搜索了加利福尼亚理工学院(Caltech Tectonics Observatory)和麻省理工学院(MIT)对此次灾难的分析报告，跟大家分享。希望通过我的贴子。让大家对此次地震从地质学构造有比较全面的了解。另外，我在googel map上建立了四川地质图，主要有2方面内容，实时地震情况、数据为美国国家地震研究所(USGS)5分钟自动更新一次，还有抗震救灾信息的最新动态等.欢迎大家参阅。
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声明：本贴所有信息来自互联网络。本人非地质工作专业人员，有些描述可能词不达意，敬请谅解。来自"加利福尼亚理工学院"(Caltech Tectonics Observatory)的地质学分析报告 

       The Science Behind China‘s Sichuan Earthquake cont...Scientists at the Caltech Tectonics Observatory have started analyzing the devastating earthquake that struck China‘s Sichuan province on May 12. This tragic event is one of a series of earthquakes in the earthquake-prone region and is likely to be repeated. Here we examine the underlying physics.

The earthquake occurred in an area that is deforming as a result of the collision between two tectonics plates, the Indian plate and the Eurasian plate. Figure 1 shows the motion of the Indian plate (large blue arrow pointing northward) relative to that of Eurasia (three smaller blue arrows pointing eastward). The white star indicates the location of the 2008 Sichuan earthquake. The seismicity recorded since 1964 is shown by the circles for earthquakes of magnitude 6.4 and greater.


       中国四川地震背后的科学

       Caltech（加利福尼亚理工学院）构造学观测台的科学家已经开始了对5.12日袭击了中国四川的强破坏性地震的分析。这一悲剧是该地震高发地区一系列地震之一，并很可能再次发生。在这里，我们将分析其潜在的物理学缘由。

   本次地震发生在由于印度板块与欧亚板块相碰撞而形成的地区。Figure 1说明了印度板块的移动（较大的指向北方的蓝色箭头）与欧亚板块移动（类似的三个指向东部的箭头）的关系。白色星号指出了2008年四川地震发生的位置。自1964年以来，所记录的6.4级及以上级别的地震已经用圆圈表示出来。

         

        Figure 1 Map showing the location of the 2008 Sichuan earthquake (white star) as well as the location of all earthquakes occurring between 1964 and 2004 with magnitudes between 4.6 and 9.0 (colored circles). Larger red circles represent the largest magnitude quakes, and smaller yellow circles represent lesser magnitude quakes. Areas of dense circles indicate boundaries between plates. This map was constructed using data from seismometers located at various places over the surface of the Earth. The blue arrows show the northerly motion of the Indian plate and the resulting easterly motion of Tibet. The size of the arrows indicates the relative speed of plate motion.
      This collision, which started 50 million years ago, is the cause of the high mountains and widespread seismicity observed throughout central Asia. The area of dense circles between India and Asia covers a wide region that is undergoing large strain and deformation. It is this strain that led to the Sichuan quake and will lead to others. The Indian plate has been moving northward at a rate of about 4 cm/year (2 inches/yr), pushing into central Asia and thus pushing Tibet eastward, out of its way. The 2008 Sichuan earthquake occurred where the eastern part of Tibet, forced further eastward, overrides the Sichuan basin at a rate of about 4 mm/year (an eighth of an inch/yr). This is the cause of the ongoing rise of the Longmen Shan mountain range that marks the eastern border of Tibet.
    The motion of the land masses is shown in more detail in Figure 2. This velocity data is from GPS stations located in the region. The relatively fast northerly motion of India is evident, as is the somewhat slower easterly motion of Tibet. The blue star indicates the location of the 2008 Sichuan earthquake.

       igure 1 地图显示了2008年四川地震的位置（白色星号）以及在1964-2004年发生的震级在4.6-9.0之间的所有地震（标有颜色的圆圈。较大的红色圆圈代表了震级最大的地震，较小的黄色圆圈代表了震级较小的地震。圆圈密集地区就是板块之间的边界。该地图是根据从位于地球表面各处的地震检波器得到的数据而绘制的。蓝色箭头显示： 印度板块向北移动，从而导致了青藏高原向西移动。箭头的大小代表了相应的板块移动速度。

        这个五亿年前便开始的碰撞，是亚洲中部高山形成和地震广泛分布的原因。印度和亚洲之间圆圈密集的区域覆盖了一个正在承受巨大应力和变形的广阔区域。正是这一应力导致了四川地震，并将引发其他地震。一直以来，印度板块都在以每年4厘米（2英寸/年）的速度向北移动，冲向亚洲中部，并推动青藏高原向东移动，使其离开了原有的位置。2008四川地震之所以发生，是因为青藏高原东部部分被迫继续东移，以大约每年4mm(1/8英寸/年)的速度冲撞四川盆地。这就是标记着青藏高原东部的龙门山山脉正在上升的原因。

        更详细的陆地板块移动情况请查看Figure 2。 这一速度来自于位于该地区的GPS站点。很显然，印度板块在以相对较快的速度向北移动，而青藏高原在以较慢的速度向东移动。蓝色星号代表了2008年四川地震的位置。                         Figure 2 Velocity data from GPS stations located in the region. The blue star indicates the location of the 2008 Sichuan earthquake. Analysis of seismological measurements indicates that the 2008 Sichuan earthquake reached a magnitude of about 7.9, rupturing the front of the Longmen Shan fault which marks the eastern edge of Tibet where the steep front of the Longmen Shan mountain range meets the Sichuan basin.

       A closer look at this region is shown in Figure 3. The rupture of the fault started in the mountains northwest of the city of Chengdu (yellow star in Figure 3) and then, over the next 50 seconds, traveled at least 200 km (100 miles) toward the northeast, along the front of the mountain range.

        Figure 2 上的速度来自于位于该地区的GPS站点。蓝色星号代表了2008年四川地震的位置。地震学测量分析表明，2008年四川地震达到了7.9级，使得龙门山前部断裂。龙门山标记了青藏高原的东部边界；在这里，龙门上的另一头与四川盆地相邻。对该地区更近距离的观察请看Figure 3。该断裂带从位于成都市(Figure 3中的黄色星号)西北部的山脉开始断裂，紧接着，在50秒以后，沿着山脉前面，想东北方向延伸了至少200km(110英里)。    

        Figure 3 Map of fault region showing the epicenter of the first quake (yellow star) as well as the location of aftershocks (yellow circles) occuring within 5 days after the quake. Note that the aftershocks (USGS quick report) occur mainly along and in the vicinity of the fault line ruptured by the original May 12 quake. The white arrows show the horizontal motion of GPS stations located on the ground at those places, indicating the motion of the land. The arrows on the left-hand side are longer than those on the right-hand side, indicating that the land mass on the left is overtaking that on the right. There have been many, daily aftershocks following the initial rupture, as shown by the yellow circles in Figure 3. Note that the aftershocks are concentrated along the same ruptured fault, as well as in the vicinity. Aftershocks are expected to continue and may do so for months or even years afterward, as happened in the recent Sumatra earthquake, though the frequency decreases with time.

       Model results for the earthquake rupture are shown in Figure 4, where the length of the blue rectangle shows the length of the rupture along the surface and the width shows the depth along the plane of the fault. Colors indicate the amount of vertical slippage.  In some places, the vertical slippage along the fault line was as large as 12 meters (39 feet), shown by red-shaded areas in Figure 4. Note that this maximum vertical slippage did not occur at the epicenter but at a location about 60 km northeast of the epicenter.     Figure 3 断裂区图显示了第一次地震的震中位置(黄色星号）以及地震后五天里余震（黄色圆圈）出现的位置。值得注意的是，余震（USGS速报）都是出现在由5月12日主震引起的断裂线上或附近。白色箭头显示了位于这些区域的ＧＰＳ站点的水平位移，同时显示了陆地的移动。左手边的箭头比右手边的长，表示左边的大陆块正在追赶右边的大陆块。正如Figure 3 黄色圆圈所示，首次断裂以后，每天都有多次余震发生。注意：余震集中在相同的断裂带上，以及断裂带附近。余震仍有可能持续，并出现在随后的数月或数年里。正如最近发生在苏门答腊岛的地震一样，尽管它的频率在逐渐变小。    

      Figure 4 Model results of the ruptured fault line (narrow rectangle) superimposed on the actual fault line. The length and depth of the rupture zone are indicated by the sides of the rectangle. Colors within the rectangle indicate the length of vertical slip, i.e., the amount by which the land on the northwest side of the fault moved with respect to the land on the southeast side. Note that the location of the largest slip, indicated by dark red, is northeast of the epicenter (red star). The elevation of the terrain is also indicated by color. Note that the fault lies at the boundary between the high mountains and the low plains. Open circles indicate the aftershocks located by the USGS-NEIC in the 36 hours following the mainshock.
Another view of the amount of slippage as a function of position along the fault (same rectangle as in Figure 4) is shown in Figure 5. Note that at the epicenter (red star), the depth of the rupture was only about 10 km (6 miles) below the surface. The amount of shaking correlates with rupture depth. The closer the rupture is to the surface, the stronger the shaking.       地震断裂的模拟结果显示在Figure 4，蓝色矩形的长度显示了表面断裂的长度，其宽度显示了断裂面的深度。颜色表示其垂直滑动的幅度。在某些地方，沿着断裂线的垂直滑动高达12米，如Figure 4中红色区域所示。值得注意的是，该最大垂直滑动并没有出现在震中位置，而是出现在距离震中东北方向60km处的地方.Figure 4 断裂线（狭窄矩形）断裂的模拟结果在实际的断裂线上成层理式分布。断裂区的长度和深度由矩形的边显示。矩形里的颜色表示了垂直滑动的长度，例如，裂区西北部陆地的移动与西南部的陆地滑动有关。 需要注意的是，暗红色所指示的最大滑动地点位于震中西南部（红色星号）。地形的升高也同样用颜色加以表示。注意：断裂区位于高山与较低平原的边界处。开园表示美国地震局--于主震之后36小时内公布的余震的位置。 

        

      Figure 5 从另一视角分析断裂带（与Figure 4 矩形相同）上的位置与滑动程度的关系。注意，在震中（红色星号）断裂的深度只有大约地表以下10km (6英里)，摇晃程度与断裂深度有关。断裂点离表面越近，摇晃得越严重。

      Figure 5是一个断裂带上的位置和深度与垂直滑动成都的模拟结果图。这个矩形与Figure 4中的相同,尽管它被旋转以使得震中（星号）位于右边。颜色有所不同，最大滑动用橙色显示，而非红色。需要注意的是，在震中（红色星号）断裂在地表以下达到10km(6英里)。沿着断裂带南部边缘的移动是突出的侵入式滑动。(也就是说，断裂带一边陆地的上升与另一边的陆地是有关系的),这导致了山脉的上升与四川盆地的下沉。（详见Figure 6）.

       
       Figure 6 Schematic of a thrust fault (dip slip fault with landmasses coming together). The two landmasses are moving toward each other, as indicated by the red arrows, one landmass sliding under the other.
The effect of the seismic shaking within both the basin and the mountain range has been devastating. Heavy sedimentation in the basin, where close to 100 million people live, amplifies the shaking. As the seismic wave from either the original quake or subsequent aftershocks travels across the basin, the effect of the sediment is two-fold: the waves slow down, so that they spend more time in the area, and their amplitude, or strength, increases. Both effects compound the damage.

      Shaking in the mountain range triggers landslides within the range. Tragically, a number of towns have been reported to be completely buried. Rescue efforts were also hindered by slides in the mountain passes, which blocked several of the main roads into the area. Such landslides also block the flow of water, causing natural dams to form, which later can breach or overflow. This occurred for an earlier earthquake in the same region, the 1933 Diexi earthquake, estimated at magnitude 7.5. That earthquake, located further within the mountain range, caused dramatic landslides and catastrophic draining of lakes behind landslide-dams.

     Can we expect more earthquakes in the future? Historically this has always been an area of frequent earthquakes. Figure 7 shows the numerous faults in the region. Some of these faults now have higher levels of stress and thus will have been brought closer to rupture because of the 2008 Sichuan earthquake.

       Figure 6 插入式断裂带(侵入式滑断层与大陆碰到一起)示意图。两个大陆各自向对方移动，如红色箭头所示，一个大陆滑到另一个的上面。地震摇晃对于盆地和山脉内部来说，都是破坏性的。在这个居住了将近1亿人口的盆地上，严重的下沉更是加剧了其摇晃。来自主震或是后来的余震的地震波通过盆地时，地震便产生了两方面的影响：地震波变慢，这使得其停留在盆地的时间变长；振幅，或者说力度增加。这两种影响都会加重其危害。在山脉里的摇晃出发了该区域的陆地滑动。不幸的是，根据报道，许多城镇已被完全掩埋。救援力量也因为山口的滑坡而受到阻挠，因为通往那个区域的几条主要交通线路被阻挡。这样的山体滑坡也阻止了水流，形成了堰塞湖，随后就可能决堤或者溢出。类似的情况也出现在该地区一个较早的地震，那是1933年的迪西地震，估计其震级为7.5。那场地震，位于更远的山里，导致了山体滑坡，形成了堰塞湖，并造成了可怕的洪灾。

       我们能预测在未来还会发生更多的地震吗？ 从历史角度来讲，这个地区一直都是地震频繁区域。Figure 7 显示了该地区众多的断裂层。一些断裂层目前承受着较大级别的重力，加之2008年的四川地震，它们将会接近断裂的边缘。         

       Figure 7 shows a map of three different types of faults in this region. Red star indicates location of the 2008 Sichuan quake. Note large number of faults that are building up strain. This is an area of high earthquake risk.
Thus, it is likely that more earthquakes will occur in this region. In the absence of prediction and remediation, the best we can do is be prepared for them. More detailed information on the seismological analysis
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      More on the science behind the earthquake: USGS Earthquake Details Interview with Hiroo Kanamori, MIT‘s Technology Review References Avouac, J. P., and P. Tapponnier, Kinematic Model of Active Deformation in Central-Asia, Geophysical Research Letters, 20, 895-898, 1993.
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    Jia, D., G. Q. Wei, Z. X. Chen, B. L. Li, Q. Zen, and G. Yang, Longmen Shan fold-thrust belt and its relation to the western Sichuan Basin in central China: New insights fiom hydrocarbon exploration, Aapg Bulletin, 90, 1425-1447, 2006.
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      Simons, W. J. F., et al., A decade of GPS in Southeast Asia: Resolving Sundaland motion and boundaries, Journal of Geophysical Research-Solid Earth, 112, 2007.
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       © 2004 Tectonics Observatory :: California Institute of Technology :: all rights reserved
 
           Figure 7 显示了该地区三种不同的断裂带。红色星号指示了2008年四川地震的位置。注意，许多断裂层的应力正在增加。这里是一个高地震风险区域。因此，该地区将很有可能发生更多次地震。在缺乏预报和补救措施的情况下，我们所能做的最好的事情就是做好准备。以上是 加利福尼亚理工学院 的分析报告。详请请参 http://www.tectonics.caltech.edu
随后我会继续更新 来自 "麻省理工学院" (MIT )的分析报告 谢谢大家。        armyyn2008-5-26  0:29  [发表于 : 219.145.19.* ]