Frictional rheology for nonlinear attenuation: Implications for paleoseismology and strong S-waves

Norman H. Sleep

Submitted July 20, 2016, SCEC Contribution #6308, 2016 SCEC Annual Meeting Poster #279

Strong Love waves and near-field velocity pulses impinge on sedimentary basins and hard rocks in California. Rock damage from past shaking provides paleoseismological evidence as well as evidence that frictional failure caused the damage. The dynamic strain eps is the ratio of particle velocity V to phase velocity c. The dynamic stress tau_D is strain times the shear modulus G, tau_D = G*V/c. Frictional failure occurs when tau_F > mu*(P_lith-P_fluid)=~mu*g*z*(rho-rho_f), where mu is the coefficient of friction, P_lith is lithostatic pressure, P_fluid is fluid pressure, g is the accleration of gravity, z is depth, rho is rock density, and rho_f is fluid density. The approximate equality assumes that the water table is at the surface and constant material properties. A testable hypothesis is that the shallow subsurface self-organizes by damage in strong events so the the dynamic stress just reaches the failure stress. Then, G*eps = ~mu*g*z*(rho-rho_f), so that G increases linearly with depth within the damaged zone and G/z is constant. G/z does predictably vary from a constant if mu varies with depth and if the water table is not at the surface. Clay-rich rocks are expected to have lower coefficients of friction than clay poor rocks. Borehole with detailed well logs in sedimentary rocks near Parkfield and accumulating sediments in the Santa Clara Valley show the expected behavior. These results confirm that strong low-frequency shaking sometimes occurs near Parkfield. Numerical models of likely earthquakes are needed to obtain peak ground velocity from strain. Frictional rheology should be used to model high-frequency S-waves. The resolved horizontal acceleration then clips near the effective coefficient of friction and the horiozntal signal is transiently circularly polarized.

Key Words
Parkfield, Santa Clara Valley, paleoseismology, self-organization

Sleep, N. H. (2016, 07). Frictional rheology for nonlinear attenuation: Implications for paleoseismology and strong S-waves. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motion Prediction (GMP)