[S-VC29] Dynamics of volcanic eruptions and their physical and chemical processes

[SVC29-P01] 3D model experiments of explosion craters: preliminary results

*Yoh Tamura¹, Ikuro Sumita² (1.National university corporation Kanazawa university graduate school, 2.Earth and Planetary Science course, Divison of Geosciences and Civil Engineering, Graduate School of Natural Science and Technology, Kanazawa University)

Keywords:Crater

Explosion craters are ubiquitous features at volcanoes, and various laboratory and field experiments have been conducted in order to infer the formation conditions from the crater shapes. Since the shape of explosion craters depends not only on energy but also on explosion depth, scaled depth has been used as the governing parameter. Recently, Liu et al (2020) demonstrated the effectiveness of using the scaled depth by conducting an experiment in which a crater was formed by injecting compressed air at high pressure beneath a layer of glass beads. Liu et al (2020) also performed experiments with different ΔP and τ, but their dependences are not yet fully clear. In this study, we use an experimental setup similar to Liu et al (2020), but using compressed air with a lower pressure (lower energy) and conduct a 3-D explosion cratering experiment in order to clarify the ΔP and τ dependence. In addition to the ΔP, the gas flow rate is measured in the experiments, and these will be used to calculate the kinetic energy of the compressed air and the energy due to its expansion. The cross-sectional profile of the crater is measured using a laser displacement meter, and the formation process is recorded by a high-speed camera. Microphone arrays are positioned near the craters to measure the pressure waves excited by the gas jet. In this presentation, we will present preliminary results obtained so far. Experiments were conducted at gas ejection depths of d = 2.3 and 5.3 cm, ΔP ~5 x 10^4 - 5 x 10^5 Pa and τ= 60ms. The scaled depth for these experimental conditions are 13 - 54 mm/J^1/3, which overlaps the experimental range of Liu et al (2020). For ejection depth of d = 5.3 cm, the power law exponent of the energy scaling was obtained as 0.282 ± 0.03, which is slightly smaller than the commonly known exponent of 1/3. For d = 2.3 cm, the power of the energy scaling law was obtained as 0.05, a very small value. For d = 2.3 cm, the volume of ejected gas is larger than the spherical area (V=πd^3/6) defined by the ejection depth. Therefore, air continues to be ejected even after the crater has formed, after which the crater formation does not proceed much. We thus inferred that a lower percentage of the total energy was used for the crater formation. Furthermore, when the eruption depth is shallow, multiple eruption events occur. As a result, multiple rings form. Analysis of the microphone data indicates that the pressure wave is excited when the granular surface is lifted upwards by the compressed air. The amplitude of the pressure wave is positively correlated with the ΔP and negatively correlated with d, which indicates the possibility of constraining ΔP and d from the pressure wave measurements.