Abstract Gravitational instability of discrete lava lobes in an active dome-building volcano may generate rockfalls that travel up to hundreds of meters from the summit. Understanding the behaviour of rockfalls is important for volcanic hazard assessment as rockfalls can precede large dome collapses that generate pyroclastic flows. Here, we investigate the mechanisms, trajectory, velocity and seismic characteristic of rockfalls at the Merapi volcano through high-resolution video and seismic-spectrum frequency analysis. High-resolution videos of rockfall events were converted to images for every second and were scaled with a georeferenced 3D point cloud to convert pixels into scaled units on the order of a meter. The scaled images were analysed using pixel tracking to investigate the mechanism, trajectory, and velocity of rockfalls. Results illustrate that the rockfall mechanisms include gravitational free fall, rolling and bouncing, airborne rotation and rolling over high-friction basal surfaces. The dynamic mechanisms of gravitational rockfalls at Merapi are strongly controlled by the topography and the condition of the basal surface, which is responsible for the continuous fluctuations in rockfall velocity. Maximum rockfall velocity occurs when rocks become airborne while rotating over the steepest area with velocity up to ~90 m/s. Material rolling over gently sloping, high-friction basal surfaces can significantly reduce the rockfall velocity to 2 m/s, which then ceases the rock movement. Our high-resolution video documents the relatively short durations of gravitational rockfall events at Merapi in the range of 24–61 s and has different time duration with the short-period seismic signals as the movement of small rock fragments over sandy area is not detected by seismic signal. Spectral analysis of seismic frequencies indicates that gravitational rockfalls at Merapi are characterized by high frequencies, with a range of 4–17 Hz. This study provides new insights and improves our understanding of the dynamic mechanism of rockfall events at dome-building volcanoes.

中文翻译：

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Merapi熔岩穹顶重力落石的动力速度和地震特征

摘要 一座活跃的圆顶火山中离散熔岩瓣的重力不稳定性可能会产生从山顶传播数百米的落石。了解落石的行为对于火山灾害评估很重要，因为落石可以在产生火山碎屑流的大圆顶坍塌之前发生。在这里，我们通过高分辨率视频和地震频谱频率分析研究了默拉皮火山落石的机制、轨迹、速度和地震特征。岩石坠落事件的高分辨率视频每秒都被转换为图像，并使用地理参考 3D 点云进行缩放，以将像素转换为米量级的缩放单位。使用像素跟踪分析缩放图像以研究落石的机制、轨迹和速度。结果表明，落石机制包括重力自由落体、滚动和弹跳、空中旋转和在高摩擦基面上滚动。Merapi 重力落石的动力机制受地形和基底表面条件的强烈控制，这是造成落石速度连续波动的原因。当岩石以高达 ~90 m/s 的速度在最陡峭的区域上空旋转时，就会出现最大的落石速度。材料在平缓倾斜、高摩擦的基底表面上滚动，可以将落石速度显着降低至 2 m/s，然后停止岩石运动。我们的高分辨率视频记录了默拉皮重力落石事件的持续时间相对较短，范围为 24-61 秒，并且与短周期地震信号的持续时间不同，因为沙区上的小岩石碎片的运动未被检测到地震信号。地震频率的频谱分析表明，默拉皮的重力落石具有高频特征，范围为 4-17 Hz。这项研究提供了新的见解，并提高了我们对圆顶火山落石事件动态机制的理解。