Evidence of reactivation of a hydrothermal system from seismic anisotropy changes

Maria Saade,Philippe Roux,Edouard Kaminski,Jean Paul Montagner,Florent Brenguier,Yosuke Aoki,Kohtaro Araragi

doi:10.1038/s41467-019-13156-8

Abstract

Seismic velocity measurements have revealed that the Tohoku-Oki earthquake affected velocity structures of volcanic zones far from the epicenter. Using a seismological method based on ambient seismic noise interferometry, we monitored the anisotropy in the Mount Fuji area during the year 2011, in which the Tohoku-Oki earthquake occurred (Mw = 9.0). Here we show that even at 400 km from the epicenter, temporal variations of seismic anisotropy were observed. These variations can be explained by changes in the alignment of cracks or fluid inclusions beneath the volcanic area due to stress perturbations and the propagation of a hydrothermal fluid surge beneath the Hakone hydrothermal volcanic area. Our results demonstrate how a better understanding of the origin of anisotropy and its temporal changes beneath volcanoes and in the crust can provide insight into active processes, and can be used as part of a suite of volcanic monitoring and forecasting tools.

Highlights

Seismic velocity measurements have revealed that the Tohoku-Oki earthquake affected velocity structures of volcanic zones far from the epicenter
Temporal changes in seismic anisotropy have been observed in other volcanic areas worldwide[8,9,10], and these have been interpreted as stress changes due to magma intrusion at shallow depths
The horizontal polarization anomaly (HPA) of the surface waves, for the entire period of the study is computed for each pair of receivers, through investigation of the off-diagonal components of the cross-correlation tensor of ambient seismic noise

Summary

Introduction

Seismic velocity measurements have revealed that the Tohoku-Oki earthquake affected velocity structures of volcanic zones far from the epicenter. We show that even at 400 km from the epicenter, temporal variations of seismic anisotropy were observed These variations can be explained by changes in the alignment of cracks or fluid inclusions beneath the volcanic area due to stress perturbations and the propagation of a hydrothermal fluid surge beneath the Hakone hydrothermal volcanic area. As well as crustal deformation, crustal seismic velocity reductions were observed below volcanic regions[7], and in particular for the Mount Fuji area (400 km from the epicenter). We measure here the time evolution of stress-induced anisotropy in the area of Mount Fuji using noisebased seismic interferometry, for the time period covering the 2011 Tohoku-Oki earthquake. Our study aims to identify the transfer and type of fluid that might reactivate the hydrothermal or magmatic system after the mainshock

Methods

Results

Conclusion