Origin of the Temporal Evolution of Elastic Properties During Laboratory Seismic Cycle.

Abstract:

Recent seismological observations highlighted that earthquakes are associated to drops in elastic properties around the fault zone (Brenguier et al., 2008). This drop is often attributed to co-seismic damage produced at the rupture tip, and can mostly be observed at shallow depths. However, it is known that in the upper crust, faults are surrounded by a zone of damage (Caine, Evans, & Forster, 1996). Because of this, the origin of velocity drops associated to earthquakes, as well as its recovery in the months following the rupture remains highly debated.

We conducted stick-slip experiments to explore the evolution of elastic waves velocities during the entire seismic cycle. The tests were run on saw-cut La Peyratte granite samples presenting different initial degrees of damage, obtained through thermal treatment. Three types of samples were studied: not thermally treated, thermally treated at 650 °C and thermally treated at 950 °C. Seismic events were induced in a triaxial configuration apparatus at different confining pressures ranging between 15 MPa and 120 MPa. Active acoustic measurements were carried through the whole duration of the tests and P-wave velocity was measured.

P-wave velocity evolution was in agreement with the evolution of shear stress acting on the fault, showing drops during stick slip events. P-wave velocity drops evolution with increasing confining pressure did not show a unique trend; the largest drops were observed at low confining pressure (15 MPa) and decreasef for intermediate confining pressures (up to 45 MPa), while for higher confining pressures (60 MPa to 120 MPa), velocity drops slightly increased with confining pressure.

We show how P-wave velocity drops associated to stick-slip events can be described by the combination of bulk properties (i.e. initial microcrack density) and applied stress conditions and mainly due to microcracks re-opening during fault' stress release.

These results bring a different interpretation to the seismic velocity drops observed in nature, often attributed to co-seismic damage, being the re-opening of microcracks in the medium surrounding the fault.