Evidence of fluid - filled upper crust from observations of post - seismic deformation due to the 1992 M w 7 . 3 Landers earthquake

I investigate post-seismic deformation due to the 1992 Mw7.3 Landers, southern California, earthquake using the entire catalog of the ERS Synthetic Aperture Radar (SAR) data, and GPS measurements made between 1992 and 1999. The stacked interferometric SAR data spanning the time period of seven years between the Landers and the Hector Mine earthquakes reveal a transient post-seismic deformation with a characteristic decay time of several years. The horizontal displacements measured with GPS exhibit somewhat smaller decay times of 1-2 years. I use a slip model of the Landers earthquake that fits all available geodetic data [Fialko, 2004] to calculate and compare permanent post-seismic displacements due to visco-elastic and poroelastic relaxation. Visco-elastic models assuming weak mantle or lower crust don’t agree with the InSAR data in the limit of complete relaxation, implying either large (> 10 years) relaxation times, or an appreciable yield strength of the lower lithosphere. I demonstrate that a combination of poro-elastic relaxation above the brittle-ductile transition, and localized shear deformation on and below the Landers rupture is able to explain most of the available geodetic data. The InSAR data suggest that pore fluids and interconnected pore space are ubiquitously present throughout the seismogenic layer up to depth of 15 km or greater. The effective hydraulic diffusivity of the upper crust inferred from the kinetics of surface deformation is of the order of 0.1-1 m/s, consistent with the laboratory, field, and deep borehole measurements. The post-Landers geodetic data lend support to a “block tectonics” model of the Eastern California Shear Zone.