ИСТИНА

Deep long-period (DLP) earthquakes are observed in many volcanic regions at depths between 15 and 40 km and are considered as one of the important precursors to volcanic eruptions. Their origin is often attributed to pressure fluctuations that result from unsteady processes in deep parts of the plumbing system of volcanoes. The exact mechanism resulting in strong and rapid pressure perturbations at these depths remains, however, unclear. Here, we present a possible generating mechanism related to the rapid changes of pressure caused by nucleation and growth of gas bubbles in response to the slow decompression of over-saturated magma. The pressure variation in the bubbly magma is simulated using a model that accounts for multiple dissolved volatiles (H2O-CO2) and diffusive gas transfer in magma based on the full solution of advection-diffusion equation. Fast expansion of the bubbly magma deforms the surrounding rocks which respond elastically at the time scales associated with the bubble growth. We show that the time scale of this response is mainly controlled by the gas and bubble content in the magma and under certain conditions can be sufficiently fast to generate seismic waves. In particular, we show that amplitudes and frequency content of DLP earthquakes observed in one of the World’s most active volcanic system, the Klyuchevskoy group of volcanoes in Kamchatka (Russia) can be predicted by our model when considering pressure changes of a few tens of MPa in a volume of ~103-104 m3 and magmas containing ~4 wt% of H20 and ~0.6 wt% of CO2. Our results provide evidence for the role of the deep degassing in the generation of volcanic seismicity and contribute to the better understanding of the link between the deep long-period earthquakes and pre-eruptive activation of the volcanic systems.