Seismic anisotropy and its precursory change before eruptions at Piton de la Fournaise volcano, La Réunion

Abstract

The Piton de la Fournaise volcano exhibits frequent eruptions preceded by seismic swarms and is a good target to test hypotheses about magmatically induced variations in seismic wave properties. We use a permanent station network and a portable broadband network to compare seismic anisotropy measured via shear wave splitting with geodetic displacements, ratios of compressional to shear velocity (Vp/Vs), earthquake focal mechanisms, and ambient noise correlation analysis of surface wave velocities and to examine velocity and stress changes from 2000 through 2012. Fast directions align radially to the central cone and parallel to surface cracks and fissures, suggesting stress-controlled cracks. High Vp/Vs ratios under the summit compared with low ratios under the flank suggest spatial variations in the proportion of fluid-filled versus gas-filled cracks. Secular variations of fast directions (ϕ) and delay times (dt) between split shear waves are interpreted to sense changing crack densities and pressure. Delay times tend to increase while surface wave velocity decreases before eruptions. Rotations of ϕ may be caused by changes in either stress direction or fluid pressure. These changes usually correlate with GPS baseline changes. Changes in shear wave splitting measurements made on multiplets yield several populations with characteristic delay times, measured incoming polarizations, and fast directions, which change their proportion as a function of time. An eruption sequence on 14 October 2010 yielded over 2000 shear wave splitting measurements in a 14 h period, allowing high time resolution measurements to characterize the sequence. Stress directions from a propagating dike model qualitatively fit the temporal change in splitting.

Some Investigations on a Possible Relationship between Ground Deformation and Seismic Activity at Campi Flegrei and Ischia Volcanic Areas (Southern Italy)

“…These changes can induce rotation of the tilt directions, as has similarly been found elsewhere [68][69][70][71]. On macroscopic spatial scale, the fluid-pore pressure is strictly controlled by the local stress and fluid flow regime [72]. Exogenous factors such as Earth tide cycles, barometric pressure variations and rainfall can modify either the stress field or the ground water content, thus in turn causing, at microscopic spatial scale, the opening/closing of voids in the rocks, and changes in permeability and pore pressure [43,[73][74][75].…”

Shear wave automatic picking and splitting measurements at Ruapehu volcano, New Zealand

“…This demonstrates that these two parameters are strongly dependent, either by construction of the MFAST algorithm or by some unknown physical relationship. Similar behavior was found when examining records from earthquake multiplets at Piton de la Fournaise volcano, and there the numbers of events in each group of polarizations/delay times varied in time [Walsh, 2013;Savage et al, 2015]. Similar jumps in delay times were also observed for an earthquake multiplet at Okmok volcano in Alaska, which did not correspond to obvious cycle skipping in ϕ [Johnson et al, 2010].…”

New evidence of CO2 soil degassing anomalies on Piton de la Fournaise volcano and the link with volcano tectonic structures

“…Recently, Savage et al . [] interpreted the spatial distribution of low Vp/Vs seismic ratios at PdF as an evidence of deep pressurized gas‐filled cracks poorly connected with the surface. On the other hand, the very low level of summit degassing mirrors the low level of volcanic activity after the major 2007 eruption [ Roult et al ., ].…”