Subduction zone segmentation and controls on earthquake rupture: The 2004 and 2005 Sumatra earthquakes

2004年12月26日的苏门答腊地震是有记录以来的第二大地震。人口越来越集中在大地震易发地区，因此研究控制这些地震的过程是一个紧迫的问题。苏门答腊地震是第一次可以应用现代地球物理工具的地震，因此为这类研究提供了独特的机会。大多数大地震（8级及以上）发生在两个板块交汇的地方；这些地区大部分位于水下，这使得调查困难，这意味着海啸的危险增加了地面震动的危险。在2004年12月26日的地震中，澳大利亚板块和欧亚板块在一条沿地球表面延伸1200公里的断层上相互滑动了25米。一个显而易见的问题是：为什么这次地震这么大？但也许问题应该是：为什么这次地震没有更大？因为在2005年3月的第二次大地震中，板块边界附近400公里的地方发生了滑动。所有板块边界都被划分成不同的断层段，这些断层段彼此不同，要么被间隙隔开，要么有不同的方向。地震段之间的边界提供了屏障，限制了地震的传播范围。一次大地震可能使一整段板块边界破裂，但一次大地震通常使不止一段板块边界破裂。然而，我们不知道是什么决定了地震是停留在板块边界的一个板块内（并保持相对较小），还是跨越板块之间的障碍（成为一场大地震）。苏门答腊地震为解决这个问题提供了一个独特的框架。对苏门答腊地震辐射的地震波的详细分析给出了控制地震大小的障碍的准确位置。2004年12月的地震发生在班达亚齐附近，几乎全部向北蔓延。虽然在断裂的南端附近发生了25米的滑动，但几乎没有滑动向南扩散；显然，这里有一个重要的屏障阻止了地震向下一个断层段蔓延。2005年3月，另一场大地震发生在下一段，并向南蔓延，直到它被南端的屏障所阻挡。我们将对板块边界进行详细的地球物理调查，以确定这两个屏障的性质。在陆海联合实验中，利用气枪爆炸将地震波从板块边界内的结构上反射回来，从而获得大尺度（1-20公里分辨率）的板块边界图像。在一个长期的实验中，地震仪放置在陆地和海床上几个月，将收集到来自遥远地震的地震信号。这些波向上穿过地球到达地震仪阵列，可以用来（以类似于CAT扫描的方式）形成地壳和上地幔深层的三维图像。同时，为了将板块边界的静态结构与地震的动力学联系起来，将开发新的技术，以提供两次地震中更精确的滑动分布图像。海底岩心将显示过去这些断层上发生大地震的时间，以及2004-5年看到的分割是否与过去的事件相似。此外，我们将收集断层滑动影响海床并产生海啸的证据。其结果将对当地和全球产生重大影响。将2004年和2005年地震之间的屏障与2005年地震南部的屏障进行比较是很重要的，因为直接向南的板块边界在1833年滑动，造成了毁灭性的海啸。更一般地说，研究结果将对其他汇聚边界产生影响，比如日本下面的边界，以及与喜马拉雅山脉相关的陆地。

The Sumatran earthquake of December 26th 2004 was the second-largest earthquake on record. The growing concentrations of population in regions prone to great earthquakes makes it a matter of urgency to study the processes that control these earthquakes. The Sumatran earthquake is the first to which modern geophysical tools can be applied, so offers a unique opportunity for such study. Most great earthquakes (magnitude 8 and larger) take place where two plates converge; such regions lie mostly under water, which makes them difficult to investigate, and which means that the hazard of tsunamis is added to the dangers of ground shaking. In the December 26th 2004 earthquake, the Australian and Eurasian plates slipped towards each other by up to 25 metres, on a fault that runs for 1200 km along the earth's surface. An obvious question is: Why was this earthquake so large? But perhaps the question should be: Why wasn't this earthquake larger? because, in March 2005 an adjacent 400 km of the plate boundary slipped in a second huge earthquake. All plate boundaries are divided into segments - pieces of fault that are distinct from one another, either separated by gaps or with different orientations. The boundaries between segments provide barriers that limit how far an earthquake can spread. A large earthquake may rupture a whole segment of plate boundary, but a great earthquake usually ruptures more than one segment at once. However, we do not know what determines whether an earthquake stays within one segment of plate boundary (and remains relatively small), or jumps across barriers between segments (to become a great earthquake). The Sumatran earthquakes give a unique framework to attack this problem. Detailed analyses of the seismic waves radiated by the Sumatran earthquakes give accurate locations for the barriers controlling the sizes of the earthquakes. The December 2004 earthquake started close to Banda Aceh and spread almost entirely northwards. Although 25 metres of slip occurred near the southern end of the rupture, almost no slip spread to the south; clearly, an important barrier here prevented the earthquake spreading to the next fault segment. In March 2005 another great earthquake occurred within this next segment, and spread southwards until it was stopped by a barrier at its southern end. We will conduct detailed geophysical surveys of the plate boundary to determine the nature of these two barriers. Large-scale (1-20km resolution) images of the plate boundary will be obtained in a combined land-sea experiment using air-gun explosions to bounce seismic waves off structures inside the plate boundary. In a longer-term experiment, seismometers left on land and on the sea-bed for several months will pick up the seismic signals from distant earthquakes. These waves, travelling upwards through the earth to the array of seismometers, can be used (in a fashion similar to CAT scanning) to form 3-dimensional images of the deeper parts of the crust and upper mantle. At the same time, new techniques will be developed to give more precise pictures of the distribution of slip in the two earthquakes, in order to link the static structure of the plate boundary to the dynamics of the earthquakes. Cores from the seabed will show when large earthquakes have occurred on these faults in the past, and whether the segmentation seen in 2004-5 was similar in the past events. In addition, we will collect evidence for the ways in which fault slip affects the seabed and generates a tsunami. The results will be significant both locally and globally. It is important to compare the barrier between the 2004 and 2005 earthquakes with the barrier at the south of the 2005 earthquake, because the plate boundary immediately to the south slipped in 1833, causing a devastating tsunami. More generally, the results will have implications for other convergent boundaries, such as those beneath Japan, and on land associated with the Himalayas.