Imaging volcanic disturbances associated with major earthquakes
IPGP researchers and their colleagues have sought to better understand how the 2011 Tohoku megathrust in Japan disrupted volcanic regions, by tracking variations in seismic anisotropy in these areas before and after the quake.
Lake Hakone and Mount Fuji, 19th century print.
A seismic wave propagates through the Earth’s rocks at a speed that depends on the properties of those rocks. However, within the same rock, the speed of propagation is not always constant and can vary according to the main orientation of the rock crystals, for example, or the presence of fracturing and fluid inclusions in the same axis of orientation within the rock. This is known as seismic anisotropy. This anisotropy can be measured and characterised by an amplitude and direction, which depends on the predominant tectonic forces in the region.
Changes in these seismic velocities, and therefore in seismic anisotropy, within a given area can therefore highlight changes in the conditions and orientations of microfractures in the subsurface and thus image deep-seated changes in the Earth’s crust.
In a study published in Nature Communications at the end of 2019, an international team led by IPGP researchers sought to gain a better understanding of the processes at play in volcanic regions before and after the magnitude Mw=9.0 Tohoku megathrust, which occurred in Japan on March 11th 2011. They therefore studied variations in seismic anisotropy in this region during 2011.
And their results show that this mega-earthquake had a profound effect on the tectonic forces throughout the Japanese archipelago, particularly in volcanic areas, where small variations in anisotropy were observed around volcanoes after the earthquake. The Mount Fuji volcanic zone, more than 400 km from the epicentre but located at the intersection of 3 tectonic plates (Pacific plate to the east, Philippine plate to the south and Okhotsk plate to the north), seems to have been particularly sensitive to this earthquake.
The temporal variations in seismic anisotropy clearly show that after the earthquake, seismic anisotropy changes to the east of Mount Fuji, moving in the direction of the earthquake. On the other hand, the researchers unexpectedly observed a very rapid variation in anisotropy one month after the earthquake in the Hakone hydrothermal volcanic zone (east of Mount Fuji), whereas a slow decrease was expected. To explain this phenomenon, the study proposes that the earthquake excited the hydrothermal system and gave rise to a porosity wave in the hydrothermal fluids that propagated from a depth of around 3km to the surface at a speed of around 1mm/s.
These results show that a better understanding of the origin of anisotropy and its temporal changes beneath volcanoes and in the crust can provide new insights into active processes. Continuous measurement of anisotropy could prove very useful for volcanic monitoring and forecasting.
Ref: Saade, M., Araragi, K., Montagner, J.P. et al. Evidence of reactivation of a hydrothermal system from seismic anisotropy changes. Nat Commun 10, 5278 (2019). https://doi.org/10.1038/s41467-019-13156-8
