A generic model for the shallow velocity structure of volcanoes

Abstract

The knowledge of the structure of volcanoes and of the physical properties of volcanic rocks is of paramount importance to the understanding of volcanic processes and the interpretation of monitoring observations. However, the determination of these structures by geophysical methods suffers limitations including a lack of resolution and poor precision. Laboratory experiments provide complementary information on the physical properties of volcanic materials and their behavior as a function of several parameters including pressure and temperature. Nevertheless combined studies and comparisons of field-based geophysical and laboratory-based physical approaches remain scant in the literature. Here, we present a meta-analysis which compares 44 seismic velocity models of the shallow structure of eleven volcanoes, laboratory velocity measurements on about one hundred rock samples from five volcanoes, and seismic well-logs from deep boreholes at two volcanoes. The comparison of these measurements confirms the strong variability of P-and S-wave velocities, which reflects the diversity of volcanic materials. The values obtained from laboratory experiments are systematically larger than those provided by seismic models. This discrepancy mainly results from scaling problems due to the difference between the sampled volumes. The averages of the seismic models are characterized by very low velocities at the surface and a strong velocity increase at shallow depth. By adjusting analytical functions to these averages, we define a generic model that can describe the variations in P-and S-wave velocities in the first 500 m of andesitic and basaltic volcanoes. This model can be used for volcanoes where no structural information is available. The model can also account for site time correction in hypocenter determination as well as for site and path effects that are commonly observed in volcanic structures.

Identification of infrasonic and seismic components of tremors in single-station records: application to the 2013 and 2018 events at Ioto Island, Japan

“…Seismic speed of the ground at a shallow depth can be similar or even smaller than α in the case of this study as explained in Appendix A. However, the very low seismic velocity is expected in the depths smaller than tens of meters (Lesage et al 2018). When the seismic velocity propagates from deeper layers,ĉ should be larger regardless of the shallow low-velocity layer, considering the velocity continuity.…”

“…Then, P-wave velocity becomes 78.2 m/s, 629 m/s and 666 m/s, respectively. The latter two values seem reasonable for the shallow P-wave velocity at a volcanic area (Lesage et al 2018).…”

“…Then, the velocities of the S-wave and the surface waves can be smaller than the sound velocity. A generic velocity structure model for volcanic areas (Lesage et al 2018) also supports the existence of quasi-sonic or subsonic low-velocity layers within tens of meters below the surface. Besides, the incident angle of the airplane noise is not horizontal.…”

“…As the shallow ground at volcanic areas generally has very low seismic velocities (Lesage et al 2018), the above approximation is not always valid. We calculate Eq.…”

Absence of Detectable Precursory Deformation and Velocity Variation Before the Large Dome Collapse of July 2015 at Volcán de Colima, Mexico

“…When an increasing effective pressure is applied to volcanic rocks, which are porous and pervasively microcracked, the most complient cracks and pores close yielding the elastic modulus and seismic velocities to increase (Vinciguerra et al, 2005;Stanchits et al, 2006;Nara et al, 2011;Heap et al, 2014). This process is involved in the increase of velocities with depth in volcanic structure (e.g., Lesage et al, 2018 for a review). When deviatoric stresses are applied, cracks normal to the axis of the maximum principal stress close, while those parallel to it remain almost unaffected (Nur, 1971).…”