Pressure dependence of density, porosity, compressional wave velocity of fault rocks from the ruptures of the 2008 Wenchuan earthquake, China

Abstract

The uniaxial compressibility (β), density (ρ), porosity (φ) and P-wave velocity (Vp) of fault rocks from the rupture zone of the 2008 Wenchuan earthquake, China, were systematically measured at pressures ranging from 10 MPa to 600 MPa on oedometer at room temperature. This paper provides a new approach to measure total pores as the function of pressure. The total pores can be divided into two portions, compliant pores and framework ones. The former is fully closed at the pressure Pcp — “closure pressure of compliant pore” and the latter can survive in specimens even when the pressure reaches 600 MPa. The observations demonstrate that ρ and Vp nonlinearly rise with pressurization at low pressure regime followed by a linearly upward tendency at pressure beyond Pcpρ and Pcpv. However, the β-values, pressure derivatives of density (Dρ) and of velocity (Dv) are at least one order of magnitude higher than the counterparts of crystallized rocks, which are attributed to the persistence of framework pores at pressure beyond Pcp. Hence, we conclude that a linear rise in elastic wave velocity with pressurization should not be regarded as the criterion for the judgment of pore-free materials. Furthermore, the experimental results provide a reliable constraint on the porosity of fault rocks as the function of depth.