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

位于北大西洋的巨大的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的板块分离（裂谷）造成的空隙。冰岛的大规模火山活动是由于那里的深层地幔温度高于正常水平，因此比正常情况下融化得更多，产生了大量的熔融岩石或岩浆。尽管融冰是在地幔深处40-100公里处形成的，但它的浮力很强，并迅速向地表流动。通常情况下，它会在地表以下5-6公里处的岩浆房中聚集在地壳中，直到足够大的体积积聚起来引起喷发。我们的主要目标是调查发生在冰岛中部Askja活火山系统地表以下15-30公里处的一些异常深度地震。这些地震发生在地壳足够热的深处，以防止脆性破裂产生任何地震。我们在2006年夏天首次发现了这些深层地震，现在将专门针对一系列地震检波器进行进一步调查。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如此深的地震，而且从来没有在裂谷带；这意味着我们可以捕捉到火山通过地壳转移熔融岩石的过程。第二个目标是监测一个新的活动区域，该区域也被认为是由熔体注入引起的，该活动始于2007年2月，位于Askja以东约20公里的裂谷带边缘的一个以前地震平静的区域。到2007年8月中旬，已经发生了3000多个事件，深度从20公里逐渐减少到15公里，后来几乎延伸到地表。这可能代表熔融岩石注入浅岩浆房，这可能导致火山喷发。以往的经验表明，活动可能会持续2年左右，因此我们有机会跟踪熔体注入（可能还有喷发）的几乎整个发展过程。第三，我们将绘制地下储存库（岩浆库）的位置，利用它们上面的地壳寒冷而脆弱的事实，因此，当它破裂时，可以发生许多小地震，而热岩浆库则会变形而不会破裂。因此，通过绘制没有地震的地方，或者地震波通过含有熔体的岩石或高温岩石被吸收的地方，我们可以绘制岩浆房的区域。我们的地震数据还将用于计算该地区的地壳厚度，这是目前知之甚少的。我们的研究结果将与其他研究人员的信息相结合，这些研究人员使用卫星测量来调查该地区的变形，这些卫星测量可以检测到毫米尺度的运动，以及地壳断层的表面测绘，以及岩石的详细岩石学研究的证据，这些研究可以控制熔融岩石在上升到地表时在地壳中形成的深度。Askja地区不适合居住，只有在没有雪的7 - 8月的夏季才容易到达。冬季气温骤降至零下25摄氏度。我们将在2008年夏季安装我们的地震仪阵列，但让它们在冬季远程操作，以获得随时间变化的地震活动的完整图像。地震仪包括一个卫星时钟接收器，可以提供准确的定时。带有大电池的太阳能电池板将为这些仪器在黑暗的冬季提供电力。其中一个站点的数据将通过无线电遥测传输到雷克雅未克，这样剑桥大学就可以通过互联网访问这些站点，而其余站点将在内部记录数据，并将于2009年夏季收集。

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.