The seismic signature of rapid inflation at Askja volcano, Iceland

Askja是位于冰岛北部火山带(NVZ)内的一座大型中部火山，距离Vatnajökull冰盖不到50公里。它有很长的岩浆活动历史，其中包括1875年的一次大规模的普林尼安(爆炸性)喷发，并伴随着大规模的火山口坍塌。这一发现挖掘了一个面积为11平方公里的区域，深度约为250+米，这里现在是奥斯克朱瓦顿火山口湖的所在地，并沉积了远至挪威和瑞典的火山灰。它上一次喷发是在1961年，涉及三次喷发序列，都是从奥斯朱瓦顿以北的熔岩喷泉开始的，产生了长达8公里的熔岩流。随后的水准测量记录了一段漫长的地面沉降期，尽管持续的速度随着时间的推移略有下降，直到2021年8月，InSAR和GPS测量都探测到了从沉陷到隆起的转换。从沉陷转换到隆起的意义并不是立即明显的；峰值隆起位于湖的西部边缘，距离1961年喷发的地点约5+公里，那里已经进行了历史上的地表位移测量。因此，目前尚不清楚这是火山管道系统的根本性变化，可能导致重大喷发，还是局部入侵，不会导致任何进一步的活动。然而，自转向再膨胀以来的18个月里，到目前为止，地表已经上升了50厘米以上，这比1961年火山喷发以来经历的总下沉还要多。此外，由剑桥火山地震学小组(CVSG)运营的地震台站主干阵列在过去15年多的时间里一直在NVZ运行，它检测到与地表隆起有关的微震活动正在增加。2023年2月和3月，Öskjuvatn的冰冻表面全部融化，这可能表明浅层熔体侵入与热液系统之间存在重大相互作用。毫无疑问，一次重大的地下岩浆事件目前正在阿斯卡地下进行，但它未来将如何演变还是个未知数。因此，重要的是要显著提高我们在Askja火山口及其周围的观测能力。目前的主干台阵只有一个地震台位于InSAR确定的隆起区，另外四个地震台位于火山口外边缘。虽然这使我们能够探测到地震活动的增加，但我们无法跟踪熔体如何在浅层迁移和积累以产生地表隆起，也没有分辨率使用地震层析成像方法对浅层熔体侵入进行成像。为了利用上述机会，该项目的目标是从2023年7月开始，在Askja火山口内部署密集的地震台站阵列，持续约12个月。考虑到火山过程的不可预测性，以及Askja只有在夏季(7-9月)才能进入的事实，这一时机是至关重要的。部署的结果是，站点密度将大大增加，间距从&gt；5公里增加到1-2公里。这种戏剧性的改进将使我们能够探测到更多数量级的地震，并以前所未有的细节成像浅层地下区域。新的数据集将使我们能够解决与浅层岩浆侵入有关的关键问题，包括(I)它的深度和横向范围有多大？(Ii)它是否直接从下面馈送，目前的地震成像表明那里存在更深的熔融源？(Iii)是否有任何证据表明正在发育的岩墙，这提供了关于浅层熔体如何演化的重要线索，并可能先于喷发活动？(Iv)我们能否阐明岩浆和热液系统之间的任何相互作用或接触？考虑到如果上升的岩浆物质与奥斯朱瓦顿湖相互作用，可能会发生大规模火山喷发，后一个问题尤其重要。

Askja is a large central volcano that sits within the Northern Volcanic Zone (NVZ) of Iceland, less than 50 km from the Vatnajökull ice cap. It has a long history of magmatic activity, which includes a large Plinian (explosive) eruption in 1875 that was accompanied by a massive caldera collapse. This excavated an 11 square km area some 250+ m deep that now hosts the crater lake Öskjuvatn, and deposited tephra as far away as Norway and Sweden. Its last eruption in 1961 involved three eruptive sequences that all initiated from lava fountains to the north of Öskjuvatn and produced lava flows up to 8 km in length. Subsequent levelling surveys have documented a lengthy period of surface subsidence that has continued, albeit at a slightly decreasing rate over time, until August 2021, when a switch to uplift was detected by both InSAR and GPS measurements.The significance of the switch from subsidence to uplift was not immediately obvious; the peak uplift is located on the western edge of the lake, some 5+ km from the locus of the 1961 eruption where historic measurements of surface displacement have been made. It was therefore unclear whether it represented a fundamental change in the volcanic plumbing system that may lead to a significant eruption, or a localised intrusion that would not result in any further activity. However, in the >18 months since the switch to reinflation, the surface has uplifted by over 50 cm to date, which is more than the total subsidence experienced since the 1961 eruption. Furthermore, a backbone array of seismic stations operated by the Cambridge Volcano Seismology Group (CVSG), which has been in place for the last 15+ years in the NVZ, detected an ongoing increase in microseismicity associated with the surface uplift. In February and March 2023, the frozen surface of Öskjuvatn melted in its entirety, potentially indicating significant interaction between melt intrusion at shallow depths and the hydrothermal system. There appears to be little doubt that a major subsurface magmatic event is currently underway beneath Askja, but exactly how it will evolve in future is unknown. It is therefore critical to significantly improve our observational capabilities in and around the Askja caldera. The current backbone array only has one seismic station that sits on the region of uplift identified by InSAR, and another four within the outer caldera edge. While this has allowed us to detect the increase in earthquake activity, we have not been able to track how the melt has migrated and accumulated at shallow depth to produce the surface uplift, nor do we have the resolution to image the shallow melt intrusion using seismic tomography methods.To exploit the opportunity described above, the goal of this project is to deploy a dense array of seismic stations within the Askja caldera for ~12 months, starting in July 2023. The timing is essential, given the unpredictability of volcanic processes, and the fact that Askja is only accessible in summer months (July-September). As a result of the deployment, station density will be greatly increased, from >5 km to 1-2 km spacing. This dramatic improvement will allow us to detect an order of magnitude more earthquakes, and image the shallow subsurface region in unprecedented detail. The new dataset will allow us to address key questions regarding the the shallow magmatic intrusion, including (i) what is its depth and lateral extent?; (ii) is it being fed from directly below, where current seismic imaging suggests there exists a deeper source of melt?; (iii) is there any evidence of developing dykes, which provide important clues about how the shallow melt body is evolving, and may precede eruptive activity?; (iv) can we illuminate any interaction or contact between the magmatic and hydrothermal system? The latter question is particularly significant in light of a potentially large volcanic eruption if rising magmatic material interacts with Lake Öskjuvatn.