Seismo-acoustic energy partitioning of a powder snow avalanche
Seismo-acoustic energy partitioning of a powder snow avalanche
复制标题
粉雪雪崩的地震声能量分配
DOI：
10.5194/esurf-2019-61
复制
发表时间：
2019
影响因子：
3.4
通讯作者：
G. Barfucci
中科院分区：
文献类型：
--
作者：
E. Marchetti;A. Herwijnen;M. Christen;M. Silengo;G. Barfucci
抽象的。当雪崩向下流动时，它会在地面上辐射地震波，并在大气中辐射次声波。地震能量由在地面上流动的致密基底层辐射，而次声能量可能由粉末前缘辐射。然而，相互的能量划分并没有完全理解。我们呈现了2016年2月5日在瑞士达沃斯上方的Dischma山谷发布的粉末雪崩的次声和地震阵列数据。在距离雪崩路径较近（<1500 m）的地方（<500 m）分别部署了灵敏度在0.1 Hz以上的5单元次声台阵和灵敏度在4.5 Hz以上的7单元地震台阵，利用RAMMS（rapid mass movement simulation）对雪崩动力学进行了模拟，并以阵面速度和流高为特征进行了表征。使用阵列，而不是单一的传感器，使我们能够提高信噪比，并确定事件的后方位角和记录的波场的视速度。从阵列处理中得到的波参数被用来确定雪崩路径，并突出显示发生地震和次声能量辐射的沿着路径的区域。分析表明，地震能量是沿整个雪崩路径沿着辐射的，从开始到沉积区，而次声只从有限的部分辐射，在那里流动加速，形成粉末云。记录的地震信号的特征在于散射的后方位角，这表明地震能量可能是由多个源同时作用辐射的。相反，次声信号的特点是一个明显的变化的后方位角和视速度。这表明次声能量辐射主要由移动点源控制，可能与粉末云一致。由于如此清晰的波参数，次声被发现是特别有效的雪崩检测和路径识别。当次声视速度随着水流的下降而减小时，地震视速度是相当分散的，但在最大次声辐射阶段减小到声速。这表明次声到地震能量转换的有效过程，在我们的情况下，至少在我们的分析频带中，记录的地震振幅增加了20%。当雪崩震级由地震振幅估计时，这种效应就可以考虑进去。所提出的结果清楚地表明，如何由粉末雪崩的地震声能量辐射的过程是非常复杂的，可能控制的粉末云的形成和动力学，因此，该过程的路径几何形状和雪的特性的影响。
Abstract. While flowing downhill, a snow avalanche radiates seismic waves in the ground and infrasonic waves in the atmosphere. Seismic energy is radiated by the dense basal layer flowing above the ground, while infrasound energy is likely radiated by the powder front. However, the mutual energy partitioning is not fully understood. We present infrasonic and seismic array data of a powder snow avalanche, which was released on 5 February 2016, in the Dischma valley above Davos, Switzerland. A five-element infrasound array, sensitive above 0.1 Hz, and a seven-element seismic array, sensitive above 4.5 Hz, were deployed at a short distance (<500 m) from each other and close (<1500 m) to the avalanche path. The avalanche dynamics were modelled by using RAMMS (rapid mass movement simulation) and characterized in terms of front velocity and flow height. The use of arrays rather than single sensors allowed us to increase the signal-to-noise ratio and to identify the event in terms of back-azimuth angle and apparent velocity of the recorded wave fields. Wave parameters, derived from array processing, were used to identify the avalanche path and highlight the areas, along the path, where seismic and infrasound energy radiation occurred. The analysis showed that seismic energy is radiated all along the avalanche path, from the initiation to the deposition area, while infrasound is radiated only from a limited sector, where the flow is accelerated and the powder cloud develops. The recorded seismic signal is characterized by scattered back-azimuth angle, suggesting that seismic energy is likely radiated by multiple sources acting at once. On the contrary, the infrasound signal is characterized by a clear variation of back-azimuth angle and apparent velocity. This indicates that infrasound energy radiation is dominated by a moving point source, likely consistent with the powder cloud. Thanks to such clear wave parameters, infrasound is revealed to be particularly efficient for avalanche detection and path identification. While the infrasound apparent velocity decreases as the flow moves downhill, the seismic apparent velocity is quite scattered but decreases to sound velocity during the phase of maximum infrasound radiation. This indicates an efficient process of infrasound to seismic energy transition, which, in our case, increases the recorded seismic amplitude by ∼20 %, at least in our frequency band of analysis. Such an effect can be accounted for when the avalanche magnitude is estimated from seismic amplitude. Presented results clearly indicate how the process of seismo-acoustic energy radiation by a powder avalanche is very complex and likely controlled by the powder cloud formation and dynamics, and the process is hence affected by the path geometry and snow characteristics.