Earthquake aftereffects and triggered seismic phenomena.

The physical basis for delayed aftereffects of large crustal earthquakes is analyzed. These effects apparently range in scale from typical aftershocks near the source region of a prior earthquake to the triggering of groundwater and deformation related anomalies, possibly including other large earthquakes, at locations as much as a few hundred kilometers from the source of a great earthquake. Observational reports of such aftereffects are reviewed and possible mechanisms for their occurrence are discussed. It is noted that distant coseismic stress changes associated with representative M=7 to M=8 earthquakes seem far too small to induce aftereffects of the type reported. It is shown, however, that a substantial increase of distant stresses in the crustal seismogenic layer can occur as relaxation processes take place, and approximate stress estimates are presented. These relaxation processes include, on a presumably short time scale possibly on the order of a year or less, deep aseismic viscous slip or concentrated shear flow on downward lithospheric extensions of the rupture zone and, on a longer time scale of possibly tens of years, viscous flow in the asthenosphere. The higher stresses occur in the relaxed condition first because lithospheric stress changes are then carried predominantly by the seismogenic layer, and second because the alleviation of base shear stress causes stress alterations to be channeled in the lithospheric plate, consequently causing less rapid stress attenuation with distance. The conclusion is that a fault segment in the vicinity of a large earthquake experiences a sudden coseismic stress change and, as seems more important at greater distances, an alteration in local tectonic loading rate due to the relaxation processes mentioned. In this sense a large earthquake can trigger delayed seismic or aseismic phenomena elsewhere. Using a specific experimentally motivated rate and evolving state constitutive description for frictional slip, the paper closes with an elementary analysis of the transient slippage response of a single-degree-of-freedom fault model to stress perturbations of this type. Depending on material properties, especially on whether the fault surface exhibits long-term velocity strengthening or weakening, on a normal stress-dependent measure of fault stiffness in interaction with its surrounding, and on the size of the stress perturbations and preperturbation state of the fault surface, the response may involve slip motions that accelerate to a delayed seismic instability or that transiently accelerate and then decelerate in rate, accomplishing stress relaxation in an aseismic manner. The concept of a stability domain is introduced in a phase plane whose points locate all possible conditions of the surface, and the dependence of the boundary of this domain on stiffness and constitutive properties is reported. [ABSTRACT FROM AUTHOR]