In a study recently published in Nature Communications , an international team led by researchers from the Institut de physique du globe de Paris (IPGP) has sought to better understand how the 2011 Tohoku-Oki mega-earthquake in Japan disrupted volcanic regions, by monitoring the seismic anisotropy in these regions before and after the earthquake.
What is seismic anisotropy?
A seismic wave propagates in terrestrial rocks at a speed depending on the properties of these rocks. However, within the same rock, the propagation speed is not always constant and can vary depending on the main orientation of the crystals of the rocks, or on the presence of cracks and fluid inclusions in the same axis of orientation within the rock. This is called seismic anisotropy. This anisotropy can be measured and characterized by an amplitude and a direction, which depends on the predominant tectonic forces in the region.
Changes in these seismic velocities, and therefore in the seismic anisotropy, can highlight changes in conditions and orientations of microfractures in the subsoil and thus allow imaging changes in the Earth’s crust.
The study shows that the Tohoku-Oki mega-earthquake deeply affected the tectonic forces of the entire Japanese archipelago. In particular, small variations in anisotropy were observed after the earthquake in volcanic zones. For instance, the volcanic area of Mount Fuji, distant more than 400 km from the epicenter but located at the intersection of 3 tectonic plates (the Pacific plate to the east, the Philippine plate to the south and the Okhotsk plate to the north), seems to have been particularly sensitive to this earthquake.
Seismic observational evidence of a hydrothermal fluid surge:
The temporal variations of the seismic anisotropy clearly show that after the earthquake, the seismic anisotropy changed in the east of Mount Fuji, rotating in the direction of the earthquake. On the other hand, one month after the earthquake, a very rapid variation of the anisotropy is observed in the hydrothermal volcanic zone of Hakone (east of Mount Fuji), whereas researchers were expecting a slow decrease. To explain this phenomenon, the study proposes that the earthquake excited the hydrothermal system and induced a so-called porosity wave, that’s a surge of hydrothermal fluids, which propagated from approximately 3km deep to the surface at a speed of the order of 1mm/s. This appears to be, to the best of our knowledge, the first seismic observational evidence of a hydrothermal fluid surge.
These results demonstrate that a better understanding of the origin of anisotropy and its temporal changes under volcanoes and in the crust can provide new insights into active processes. Continuous measurement of anisotropy could therefore prove very useful for volcanic monitoring and forecasting.
This post was written by Maria Saade
with revisions from Michaela Wenner and Marina Corradini
 Saadé, M., Araragi, K., Montagner, J.P. et al. Evidence of reactivation of a hydrothermal system from seismic anisotropy changes. Nature Com. 10, 5278 (2019). https://doi.org/10.1038/s41467-019-13156-8