Urgent assessment of the earthquake and tsunami hazard in western Sumatra following the 09/07 Mentawai Islands earthquake sequence.

从2004年12月26日苏门答腊-安达曼大地震M=9.2开始，超过2000公里的巽他巨型逆冲断层，印度洋板块被压在亚洲板块之下，在一系列M>8的大地震中破裂。最近的一次是2007年9月12日发生的8.4级地震。从安达曼群岛到恩甘诺的2500多公里的巨型逆冲断层中，只有300公里在过去三年中没有破裂。西比路岛下的这一部分仍然能够发生8级以上的地震。自1797年以来，它从未失败过，从对该地区地壳变形方式的检查中，我们知道它没有经历过任何缓慢的滑动。利用已知的板块聚合速度进行简单的计算，结果表明1797年释放的所有应变都已经恢复，很明显，另一场地震的时机已经成熟。这随时可能发生。此外，从早期的工作和对最近事件的初步调查中可以清楚地看出，西比路下面的部分现在受到来自北部的2005年事件和来自南部的12/09/07事件的压力。这样的地震很可能会引起海啸。巴塘的最大高度，从侧面到西比路段，只比海平面高出10米，所以海啸是一个真实的威胁。然而，它不一定像2004年海啸那样具有破坏性。事实上，我们最近所做的工作，由NERC在2006年资助，已经表明，群岛离岸巴塘的存在，使大规模海啸的威胁大大减弱。我们对苏门答腊西部巨型逆冲断层地震产生的较小波浪的预测似乎已经得到了最近8.4级地震产生的海啸的初步数据的证实，该地震的高度仅为4米，显然没有造成任何人死亡。最近，我们预测了在苏门答腊海岸观测到的海啸高度与那里经历的垂直运动之间的关系。当其中一次地震发生时，近海岛屿被地震推高，而苏门答腊海岸总是下降/有时下降米。通过一系列详细的地震和海啸耦合模拟，我们已经表明，陆上经历的波高与这种下降的大小成正比。美妙之处在于，垂直运动的大小可以在波浪到达前30分钟测量。最近的地震应该使我们能够检验这种关系。在这个项目中，我们要做三件事。首先，我们计划重新评估西比路岛下的应力占整个地震序列。这些计算，特别是随时间变化的组件，是复杂的，需要复杂的计算，我们将在项目期间完成。其次，我们将测试我们的海啸模拟方法的预测对最近的M8.4事件收集的数据，并调查波高和垂直位移之间的关系。这将包括一个为期2周的实地活动，以收集有关最近遭受海啸袭击的本库鲁市附近的波高信息。海啸留下的伤疤在接下来的几个月里仍然清晰可见，并且可以在现场进行精确测量。使用高分辨率GPS设备，我们将借用NERC地球物理设备设施，我们将准确定位波高估计的位置，并测量海岸的当前高度。这些高度将使我们能够估计地震期间的高度变化。最后，我们将计算另一套海啸情景，考虑到最近的地震和我们的验证工作的结果。苏门答腊西部目前是世界上地震风险最高的地区之一。这个项目不会阻止地震，但它将有助于科学知识的主体，这将有助于人们为它做好准备。

Starting with the great Sumatra-Andaman earthquake M=9.2 of 26 December 2004 over 2000km of the Sunda megathrust, where the Indian ocean plate is being forced under the Asian plate, has been ruptured in a series of great M>8 earthquakes. The most recent, of magnitude 8.4, occurred on 12 September 2007. Of the more than 2500km of the megathrust, from the Andaman Islands to Enganno, only 300km remains un-ruptured in the last three years. This segment, under the Island of Siberut is still capable of an earthquake of greater than magnitude 8. It has not failed since 1797, from examination of the way the earth's crust is being deformed in the area we know that it has not experienced any slow (aseismic) slip. Simple calculations, using the known rate of convergence of the plates here, show that all the strain released in 1797 has been regained and it seems clear that it is ripe for another earthquake. This could happen at any time. In addition, it is clear from earlier work and from preliminary investigation of the recent events, that the segment under Siberut is now being stressed from the north by the 2005 event and from the south by the 12/09/07 event. It is likely that such an earthquake might cause a tsunami. The maximum height of Padang, broadside on to the Siberut segment, is only 10m above sea level so a tsunami is a real threat. However, it does not have to be as destructive as the 2004 tsunami. Indeed recent work we have done, funded by NERC in 2006, has shown that the presence of the Islands offshore Padang make the threat of a massive tsunami recede significantly. Our forecasts of smaller waves generated by western Sumatran megathrust earthquakes would appear to have been borne out by preliminary data from tsunami generated by the recent M8.4 earthquake which only reached 4m in height and apparently killed no one. Recently, we have made predictions relating the height of a tsunami observed on the Sumatran Coast to the vertical movement experienced there. When one of these earthquakes occurs the offshore islands are forced upward by the earthquake while the coast of Sumatra always drops / sometimes by metres. By a detailed series of coupled earthquake and tsunami simulations we have shown that the wave height experienced onshore is directly proportional to the size of this drop. The beauty is that the size of the vertical movement can be measured up to 30 minutes before the wave arrives. The recent earthquake should allow us to test this relationship. In this project we aim to do three things. Firstly, we plan to reassess the stresses under the island of Siberut accounting for the entire earthquake sequence. These calculations, particularly the time varying components, are complex and require sophisticated computations which we will finalise during the project. Secondly, we will test the predictions of our tsunami simulation methods against data collected by the recent M8.4 event and investigate the relationship between wave height and vertical displacement. This will involve a 2 week field campaign to collect information about the wave heights near the city of Bengkulu which was hit by the recent tsunami. Scars left by the tsunami will still be clearly visible for the next couple of months and can be measured accurately in the field. Using high resolution GPS equipment, which we will borrow from the NERC Geophysical Equipment Facility, we will accurately locate the positions of wave height estimates and also measure the present height of the coast. These heights will allow us to estimate the change in height during the earthquake. Finally, we will compute another suite of tsunami scenarios taking into account the recent earthquakes and the results of our validation work. Western Sumatra is currently one of highest earthquake risk areas in the world. This project will not stop the earthquake but it will contribute to a body of scientific knowledge which will help to prepare people for it.