Collaborative Research: Dynamic Response of the Ross Ice Shelf to Wave-induced Vibrations

Ice shelves span approximately 45% of the Antarctic coastline. Importantly, they create a buttressing/restraining arch that slows the discharge of grounded ice sheets to the sea, thus, mitigating Antarctic contributions to global sea level change. Some ice shelves have partially or totally collapsed in recent decades and, most are presently experiencing thinning and mechanical weakening, driven by ocean warming and associated ocean current changes and/or ocean and atmospheric warming. Ice shelves, and their associated ice sheets, are susceptible to a variety of atmospheric, oceanic, and solid Earth perturbations, including scenarios in which multiple processes may reinforce ice shelf destabilization. Ocean waves impacting ice shelves induce vibrations that can trigger fracturing and the calving of icebergs, and even ice shelf disintegration. Glacial seismology can reveal short-time scale dynamic processes that are not resolvable via remote sensing, geodetic, and other methods.

Understanding and being able to anticipate changes in the glaciological regime of the Ross Ice Shelf (RIS) and West Antarctic Ice Sheet (WAIS) are key to improving sea level rise projections due to ongoing ice mass loss in West Antarctica. The fate of the WAIS is a first-order climate change and global societal issue for this century and beyond that affects coastal communities and coastal infrastructure globally.

Ice shelf--ocean interactions include impacts from tsunami, ocean swell (10-30s period), and very long period ocean waves that impact ice shelves and produce vibrations that induce a variety of seismic signals detected by seismometers buried in the ice shelf surface layer, called firn. To study the wave-induced vibrations in the RIS, an extensive seismic array was deployed from Nov. 2014 to Nov. 2016. This unique seismometer array deployment on an ice shelf made continuous observations of the response of the RIS to ocean wave impacts from ocean swell and very long period waves. An extensive description of the project motivation and background (including photos and videos of the deployment operations), and list of published studies of analyses of the seismic data collected by this project, are available at the project website https://iceshelfvibes.ucsd.edu.

Two types of seismic signals detected by the seismic array are most prevalent: flexural gravity waves (plate waves) and icequakes (signals analogous to those from earthquakes but from fracturing of the ice).
Long period ocean waves flex the ice shelf at the same period as the ocean waves, with wave energy at periods greater than ocean swell more efficient at coupling energy into flexing the ice shelf. Termed flexural gravity waves or plate waves (Chen et al., 2018), their wave-induced vibrations can reach 100’s of km from the ice edge where they are excited, with long period wave energy propagating in the water layer below the shelf coupled with the ice shelf flexure. Flexural gravity waves at very long periods (> 300 s period), such as from tsunami impacts (Bromirski et al., 2017), can readily reach grounding zones and may play a role in long-term grounding zone evolution.
Swell-induced icequake activity was found to be most prevalent at the shelf front during the austral summer (January – March) when seasonal sea ice is absent and the associated damping of swell by sea ice is minimal (Chen et al., 2019).

In addition to the seismic array, a 14 station GPS (global positioning system) array was installed during seismic data retrieval and station servicing operations in October-November 2015. The GPS stations, co-located with seismic stations, extended from the shelf front southward to about 415 km at interior station RS18. Due to logistical constraints associated with battery weight during installation, only one station (at DR10) operated year-round. The GPS data collected give a detailed record of changes in iceflow velocity that are in close agreement with the increasing velocity estimates approaching the shelf front from satellite observations. Importantly, the year-round data at DR10 show an unprecedented seasonal cycle of changes in iceflow velocity, with a speed-up in northward (seaward) ice flow during Jan.-May and then a velocity decrease from June-Sep. (returning to the long-term mean flow velocity). This annual ice flow velocity change cycle has been attributed in part to seasonal changes in ice shelf mass (thinning, reducing buttressing) due to melting at the RIS basal (bottom) surface from intrusion of warmer ocean water (Klein et al., 2020).

冰架（ice shelf）约覆盖南极海岸线的45%。至关重要的是，这类冰架会形成一道支撑约束拱，减缓接地冰盖（grounded ice sheets）向海洋的泄出速率，从而缓解南极冰体对全球海平面变化的贡献。近数十年来，部分冰架已发生部分或完全坍塌，且当前多数冰架正经历变薄与力学弱化过程，这一过程由海洋变暖、相关洋流变化，以及/或者海洋与大气变暖所驱动。冰架及其关联的冰盖易受多种大气、海洋及固体地球扰动影响，其中包括多进程共同加剧冰架失稳的情景。撞击冰架的海浪会引发振动，进而触发冰体断裂与冰山崩解，甚至导致冰架整体解体。冰川地震学（glacial seismology）能够揭示遥感、大地测量及其他方法无法解析的短时尺度动态过程。

理解并能够预测罗斯冰架（Ross Ice Shelf, RIS）与西南极冰盖（West Antarctic Ice Sheet, WAIS）的冰川系统变化，是改善因西南极持续冰量损失导致的海平面上升预测的关键。西南极冰盖的命运是本世纪及未来关乎全球沿海社区与沿海基础设施的首要气候变化与全球社会议题。

冰架-海洋相互作用涵盖海啸、海浪（周期10~30秒）以及超长周期海浪对冰架的影响，这类海浪会引发振动，产生多种地震信号，可被埋设于冰架表层（称为雪冰层firn）中的地震仪检测到。为研究罗斯冰架内由海浪引发的振动，研究团队于2014年11月至2016年11月间部署了一套大型地震台阵。这套独一无二的冰架地震台阵实现了对罗斯冰架响应海浪（包括涌浪与超长周期海浪）冲击的连续观测。该项目的研究动机与背景详情（含部署作业的照片与视频），以及基于本项目采集的地震数据发表的分析研究列表，均可在项目官网https://iceshelfvibes.ucsd.edu查询获取。

该地震台阵检测到的两类最为普遍的地震信号分别为：弯曲重力波（flexural gravity waves，又称板波plate waves）与冰震（icequakes，即类似地震但源于冰体断裂的信号）。长周期海洋海浪会以与海浪相同的周期使冰架发生弯曲，周期大于涌浪的海浪能量更高效地将能量耦合至冰架弯曲过程中。这类被称为弯曲重力波或板波的信号（Chen等，2018），其由海浪引发的振动可在距激发点冰缘数百公里的范围内传播，超长周期海浪能量会与冰架弯曲耦合，在冰架下方的水层中传播。周期超过300秒的超长周期弯曲重力波（例如由海啸引发的振动，Bromirski等，2017）可轻易抵达接地带，并可能在接地带长期演化过程中发挥作用。涌浪引发的冰震活动在南极夏季（1月至3月）于冰架前缘最为活跃，此时季节性海冰已消融，海冰对涌浪的阻尼作用降至最低（Chen等，2019）。

除地震台阵外，研究团队于2015年10月至11月间在地震数据回收与台站维护作业期间，部署了一套包含14个站点的全球定位系统（global positioning system, GPS）台阵。这些GPS站点与地震台站同址布设，布设范围从冰架前缘向南延伸至内陆站点RS18处，距离约415公里。受安装期间电池重量相关的后勤限制，仅有DR10站点实现了全年连续运行。采集的GPS数据详细记录了冰流速度的变化，这与卫星观测得到的冰架前缘区域流速递增的结果高度吻合。尤为重要的是，DR10站点的全年连续数据展现了前所未有的冰流速度季节周期：1月至5月间北向（向海）冰流加速，6月至9月间流速回落至长期平均流速。这种年度冰流速度变化周期，部分归因于暖海水侵入导致罗斯冰架底部基面（basal surface）融化引发的冰架质量季节性变化（变薄、支撑作用减弱）（Klein等，2020）。