Multiple scattering from icequakes at Erebus volcano, Antarctica, and practical convergence of coda-based correlations

We examine seismic coda from an unusually dense deployment of over 100 short-period and broadband seismographs in the summit region of Mount Erebus volcano on a network with an aperture of approximately 5 km. We investigate the energy-partitioning properties of the seismic wavefield generated by thousands of small icequake sources originating on the upper volcano and use them to estimate Green’s functions via coda cross correlation. Emergent coda seismograms suggest that this locale should be particularly amenable to such methods. Using a small aperture subarray, we find that modal energy partition between S and P wave energy between ∼1 and 4 Hz occurs in just a few seconds after event onset and persists for tens of seconds. Spatially averaged correlograms display clear body and surface waves that span the full aperture of the array. We test for stable bidirectional Green’s function recovery and note that good symmetry can be achieved at this site even with a geographically skewed distribution of sources. We estimate scattering and absorption mean free path lengths and find a power law decrease in mean free path between 1.5 and 3.3 Hz that suggests a quasi-Rayleigh or Rayleigh-Gans scattering situation. Finally, we demonstrate the existence of coherent backscattering (weak localization) for this coda wavefield. The remarkable properties of scattered seismic wavefields in the vicinity of active volcanoes suggests that the abundant small icequake sources may be used for illumination where temporal monitoring of such dynamic structures is concerned.

We present a novel optimization approach to improve the convergence of interstation coda correlation functions towards the medium's empirical Green's function. For two stations recording a series of impulsive events in a multiply scattering medium, we explore the impact of coda window selection through a Markov Chain Monte Carlo scheme, with the aim of generating a gather of correlation functions that is the most coherent and symmetric over events, thus recovering intuitive elements of the interstation Green's function without any nonlinear post-processing techniques. This approach is tested here for a 2-D acoustic finite difference model, where a much improved correlation function is obtained, as well as for a database of small impulsive icequakes recorded on Erebus Volcano, Antarctica, where similar robust results are shown. The average coda solutions, as deduced from the posterior probability distributions of the optimization, are further representative of the scattering strength of the medium, with stronger scattering resulting in a slightly delayed overall coda sampling. The recovery of singly scattered arrivals in the coda of correlation functions are also shown to be possible through this approach, and surface wave reflections from outer craters on Erebus volcano were mapped in this fashion. We also note that, due to the improvement of correlation functions over subsequent events, this approach can further be used to improve the resolution of passive temporal monitoring.

我们针对埃里伯斯火山（Mount Erebus）火山口区域部署了超100台短周期与宽频带地震仪组成的密集台阵，台阵孔径约5 km，以此研究该区域的地震尾波。我们对源自火山上部的数千次小型冰震所激发的地震波场能量分配特性展开分析，并通过尾波互相关法估算格林函数（Green's function）。该区域的初至尾波地震图表明，此类方法在此处尤为适用。利用小孔径子台阵，我们发现在事件触发后数秒内，约1~4 Hz频段内的S波与P波模式能量便完成分配，且该分配状态可持续数十秒。空间平均互相关图呈现出清晰的体波与面波，其覆盖了整个台阵孔径范围。我们针对稳定的双向格林函数恢复开展测试，结果显示即便震源的地理分布存在偏斜，该区域仍可实现良好的对称性恢复。我们估算了散射与吸收平均自由程，发现在1.5~3.3 Hz频段内平均自由程呈幂律衰减，这暗示此处为准瑞利或瑞利-甘斯散射环境。最后，我们证实了该尾波波场存在相干背散射（弱局域化）现象。活动火山周边散射地震波场的独特特性表明，丰富的小型冰震源可被用于对这类动态构造开展被动时序监测。

我们提出一种全新的优化方法，用于提升台站间尾波互相关函数向介质经验格林函数收敛的效果。针对多重散射介质中两台站记录到的一系列脉冲型事件，我们通过马尔可夫链蒙特卡洛（Markov Chain Monte Carlo）方案探索尾波窗口选择的影响，目标是生成一组在事件间具备最高相干性与对称性的互相关函数集合，从而无需任何非线性后处理技术即可还原台站间格林函数的直观特征。我们分别在二维声学有限差分模型中测试了该方法，获得了大幅优化的互相关函数；同时在南极洲埃里伯斯火山的小型脉冲冰震数据库中开展验证，也得到了类似的稳健结果。通过优化的后验概率分布推导得到的平均尾波解，可进一步反映介质的散射强度：散射越强，整体尾波采样的时间延迟略长。该方法还可实现互相关函数尾波中的单次散射至相的还原，并且通过该方式绘制出了埃里伯斯火山外火山口的面波反射图像。此外我们注意到，由于该方法可提升后续事件的互相关函数质量，因此还可进一步用于提升被动时序监测的分辨率。