A Model Experiment of Fracture Induced Long-Period Events: Injection of Pressurized Gas Into a Viscoelastic Rock Analog

Low-frequency events observed in volcanic areas preceding phreatic or phreatomagmatic eruptions may be excited by the injection of pressurized gas into damaged/heated rocks or solidifying magmas. To simulate this process, we conducted a series of gas injection experiments in gels as an analog of such soft rocks. When we inject the pressurized air into a quasi-Maxwell fluid with higher and lower shear moduli, cracks or a slug appears, respectively. The crack propagation velocity consists of the shear wave velocity of the gel. At the beginning of the crack generation, multiple cracks emerge, causing pulse-like low-frequency signals. In contrast, the slug grows faster than the shear wave velocity and excites high-frequency oscillation. These results show the possibility that the low-frequency seismicity observed in volcanic regions is the result of not only the resonance of a crack but also the crack propagation in soft rocks.