Volcanic eruptions, earthquakes, landslides, and other events can trigger seismic waves that travel through Earth. Smaller triggers, such as road traffic or rivers, can produce quieter, ambient waves. No matter the wave’s source, changes in the velocity of seismic waves can reveal key mechanical characteristics of the shallow geological structures they pass through.
New research by Takano et al. explores changes in ambient seismic velocity at a nonerupting volcano. Past research has focused on velocity changes caused by large earthquakes or volcanic eruptions. However, the sparseness of such events makes it difficult to monitor shallow structures continuously. Instead, the new study shows other more reliable sources of seismic velocity change can prove useful.
From 2012 through 2015, the research team monitored ambient seismic velocity changes at Izu Oshima, a volcanic island off the coast of Japan that has not erupted since 1990. They gathered continuous measurements from four sensor stations deployed on the island and mathematically cross correlated the data to determine daily changes in the velocity of ambient seismic waves of different frequencies.
Without any major earthquakes or eruptions during the study period, the scientists were able to investigate how relatively gentle deformation of the ground due to relatively quiet volcanic activity affected seismic wave velocity. Such volcano deformation is known to change seismic velocity by generating internal pressure, or stress, within rock, as well as by changing the rock’s shape (strain).
The researchers found a strong correlation between ambient seismic velocity changes and observed strain caused by volcano deformation. This suggests that volcanic pressure is the main driver of such changes at Izu Oshima, as opposed to pressure caused by heavy seasonal precipitation or ocean tides, which they also analyzed.
Further analysis revealed that most of the observed changes in seismic wave velocity occurred in the upper 1 kilometer of the ground beneath the volcano. By incorporating the results of previous studies, the researchers also showed that seismic wave velocity is more sensitive to stress at shallower depths.
These findings could enable improved understanding of Izu Oshima’s underlying geology, and similar techniques could aid investigation of subsurface geological structures in other locations. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB014340, 2017)
—Sarah Stanley, Freelance Writer