Tracking melt injection under the Mid-Atlantic Rift near Askja, central Iceland

位于北大西洋的大型海洋岛屿冰岛的地壳几乎完全是由连续的火山喷发形成的，这些火山喷发填补了北美和欧洲所在板块分离(裂解)造成的缺口。冰岛的大规模火山活动是由于那里的深部地幔比正常情况下更热，因此比正常情况下熔化得更多，从而产生了大量的熔岩或岩浆。尽管熔体形成在40-100公里深的地幔深处，但它非常浮力，并迅速向上向地表渗出。通常情况下，它会聚集在地表下约5-6公里处的岩浆室中，直到形成足够大的体积，导致火山喷发。我们的主要目标是调查发生在冰岛中部活跃的Askja火山系统下地表下15-30公里处的一些特别深的地震。这些地震发生在地壳应该足够热的深处，以防止脆性破坏产生任何地震。我们在2006年夏天首次发现了这些深部地震，现在将专门针对一系列地震仪对其进行进一步调查。我们的假设是，它们是由地壳深处的熔体运动引起的。这是一个令人兴奋的观察结果，因为以前很少观察到这样的深部地震，而且从未在裂谷地带观察到；这意味着我们可以在通过地壳转移熔岩的过程中捕捉到火山。第二个目标是监测一个新的活动区，也被认为是由熔融注入引起的，该活动区始于2007年2月，位于阿斯卡以东约20公里处裂谷带边缘的一个以前地震平静的地区下面。截至2007年8月中旬，发生了3000多次地震，深度从20公里逐渐减少到15公里，最近几乎延伸到地表。这可能是将熔岩注入浅层岩浆室，从而可能导致火山喷发。以前的经验表明，这种活动可能会持续大约两年，因此我们有机会跟踪熔融注入(可能是喷发)事件的几乎整个发展过程。第三，我们将绘制地下储藏库(岩浆室)的位置图，利用它们上方的地壳冷而脆的事实，从而在破裂时发生多次小地震，而热岩浆室则在没有破裂的情况下延展性变形。因此，通过绘制没有地震的地方，或者地震波通过含有一些熔体的岩石吸收的地方，或者高温下的岩石，我们可以绘制岩浆室的区域。我们的地震数据也将被用来计算这一地区的地壳厚度，这一点目前知之甚少。我们的研究结果将与其他研究人员的信息结合在一起，这些研究人员使用卫星测量来调查该地区的变形，这些测量可以探测到毫米级的运动，以及地壳中断层的表面测绘，以及来自岩石详细岩石学研究的证据，这些岩石研究提供了对熔岩在上升到地表期间在地壳中积聚的深度的控制。Askja地区不适宜居住，只有在7-8月没有雪的夏季月份才能很容易到达。冬季气温骤降至零下25摄氏度。我们将在2008年夏天安装我们的地震仪阵列，但让它们在整个冬天远程运行，以获得随时间推移的地震活动的完整画面。地震仪包括一个卫星时钟接收器，可以提供准确的计时。带有大型电池的太阳能电池板将在黑暗的冬季为仪器供电。其中一个空间站的数据将通过无线电遥测到雷克雅未克，因此可以从剑桥通过互联网访问，而其余的空间站将在内部记录数据，将于2009年夏天收集。

The crust of the large oceanic island of Iceland in the North Atlantic is formed almost entirely from successive volcanic eruptions which fill the gap caused by the separation (rifting) of the plates on which North America and Europe sit. The massive volcanism in Iceland is caused by the deep mantle there being hotter than normal, and so melting more than normal to produce large amounts of molten rock, or magma. Although the melt is formed in the deep mantle at depths of 40-100 kilometres, it is very buoyant and bleeds rapidly up towards the surface. Normally it ponds in the crust in a magma chamber about 5-6 kilometres beneath the surface until a sufficiently large volume has built up to cause an eruption. Our main objective is to investigate some exceptionally deep earthquakes occurring 15-30 kilometres beneath the surface under the active volcanic system of Askja in central Iceland. These occur at depths where the crust should be hot enough to prevent brittle failure generating any earthquakes. We first discovered these deep earthquakes in summer 2006, and will now target an array of seismometers specifically to investigate them further. Our hypothesis is that they are caused by melt moving in the deep crust. This is an exciting observation, since such deep earthquakes have only rarely been observed before, and never in a rift zone; it means that we can catch the volcano in the act of transferring molten rock through the crust. A second objective is to monitor a new area of activity, again thought to be caused by melt injection, which started in February 2007 beneath a previously seismically quiet area on the edge of the rift zone some 20 km east of Askja. Over 3000 events have occurred up to mid-August 2007, gradually decreasing in depth from 20 to 15 km and latterly extending almost to the surface. This may represent injection of molten rock into a shallow magma chamber which may result in a volcanic eruption. Previous experience suggests that activity is likely to persist for around 2 years, so we have an opportunity to track almost the entire development of a melt injection (and possibly eruption) episode. Thirdly we will map the location of the underground storage reservoirs (magma chambers), by using the fact that the crust above them is cold and brittle, therefore allowing numerous small earthquakes to occur as it fractures, whereas the hot magma chambers deform ductilely without fracturing. So by mapping where the earthquakes are absent, or where seismic waves are absorbed by passing through rock containing some melt, or rock at high temperatures, we can map the region of the magma chamber. Our seismic data will also be used to calculate the crustal thickness in this region, which is presently poorly known. Results form our studies will be integrated with information from other researchers investigating deformation in the region using satellite measurements which can detect motions at a millimetre scale, together with surface mapping of faults in the crust and evidence from detailed petrological studies of the rocks which provide control on the depths at which the molten rock has ponded in the crust during its ascent to the surface. The Askja area is inhospitable, and is easily accessible only in the summer months of July-August when snow is absent. Winter temperatures plummet to -25C. We shall install our seismometer array during summer 2008, but leave them operating remotely through the winter to gain a complete picture of the seismicity through time. The seismometers include a satellite clock receiver which provides accurately timing. Solar panels with large batteries will power the instruments through the dark winter months. Data from one of the stations will be radio-telemetered to Reykjavik and hence accessible via the internet from Cambridge, while the remainder will record data internally, to be collected in summer 2009.