The seismic signature of rapid inflation at Askja volcano, Iceland

Askja是一座大型的中央火山，位于冰岛北部火山带（NVZ）内，距离Vatnajökull冰帽不到50公里。它有悠久的岩浆活动历史，其中包括1875年的大型普林尼火山（爆炸性）喷发，伴随着大规模的火山口崩塌。这挖出了一个11平方公里的区域，大约250多米深，现在是火山口湖Öskjuvatn的所在地，并将火山沉积在挪威和瑞典。它的最后一次喷发是在1961年，包括三次喷发序列，都是从Öskjuvatn北部的熔岩喷泉开始的，产生了长达8公里的熔岩流。随后的调平调查记录了一段很长时间的地表下沉，尽管随着时间的推移，下沉的速度略有下降，直到2021年8月，InSAR和GPS测量都检测到地表向隆起的转变。从沉陷到隆升的转换意义不是立即明显的；隆起的峰顶位于湖的西部边缘，距离1961年火山喷发的地点约5公里，在那里对地表位移进行了历史性的测量。因此，尚不清楚这是否代表了火山管道系统的根本变化，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，在转向再膨胀后的18个月里，到目前为止，地表已经上升了50多厘米，这比1961年火山喷发以来的总下沉量还要多。此外，由剑桥火山地震学小组（CVSG）运营的地震台站骨干阵列已经在NVZ运行了15年以上，检测到与地表隆起相关的微地震活动正在增加。2023年2月至3月，Öskjuvatn冻结地表整体融化，可能表明浅层熔融侵入与热液系统存在明显的相互作用。毫无疑问，Askja地下正在发生一场大规模的地下岩浆活动，但未来究竟会如何演变还不得而知。因此，显著提高我们在Askja火山口及其周围的观测能力至关重要。目前的主干阵列只有一个地震台站位于InSAR识别的隆起区域，另外四个位于外火山口边缘。虽然这使我们能够探测到地震活动的增加，但我们无法追踪到熔体是如何在浅层深度迁移和积累以产生地表隆起的，我们也无法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站阵列，为期12个月。考虑到火山活动的不可预测性，以及Askja只有在夏季（7月至9月）才能到达的事实，时机至关重要。由于部署，站点密度将大大增加，从50公里到1-2公里的间距。这一巨大的进步将使我们能够探测到一个数量级以上的地震，并以前所未有的细节对浅层地下区域进行成像。新的数据集将使我们能够解决有关浅层岩浆侵入的关键问题，包括：(1)它的深度和横向范围是什么？（ii）它是否直接从地下注入，而目前的地震成像显示，那里存在更深的融水源？（iii）是否有发育的岩脉的证据，这些岩脉为浅层熔体如何演化提供了重要线索，并可能先于喷发活动？（iv）能否阐明岩浆与热液系统之间的相互作用或接触？如果上升的岩浆物质与Öskjuvatn湖相互作用，可能会发生大规模的火山爆发，那么后一个问题尤为重要。

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.