REALISTIC GEOLOGIC STRAIN RATES

Pfiffner and Ramsay’s 1982 analysis of geologic strain rates determined that conventional strain rates should range from 10^-13 to 10^-15 s^-1. An update of this summary, including data from GPS geodetic networks in tectonically active areas, reveals that most strain rates fall close to the range defined by Pfiffner and Ramsay. Based on a preliminary compilation of recently calculated active strain rates, extensional rates commonly range from 10^-15 to 10^-17 s^-1, active convergence rates span 10^-12 to 10^-16 s^-1, and many strike-slip strain rates fall between 10^-14 and 10^-15 s^-1.

The uncertainty in strain rate measurement appears to be independent of the calculation technique; geodetic surveys, seismic strain rate tensors, and thermal and tectonic data all yield extensional strain rates that can vary up to one order of magnitude. Contractional strain rates can have uncertainties as high as three orders of magnitude, but are generally closer to one order of magnitude. In strike-slip regimes, uncertainty is also one order of magnitude or less. Detailed analyses demonstrate that strain rates measured a few tens of kilometers apart along strike of a fault zone can vary by ± 10^-1 s^-1.

An ongoing compilation of historical strain rate estimates shows a range of extensionnal rates from 10^-11 to 10^-17 s^-1. Grain size peizometry yields the higher estimates of 10^-11 to 10^-14 s^-1; the lower rates are derived from subsidence histories in sedimentary basins. Estimates for strain rates on Ganymede and Venus range from 10^-14 to 10^-18 s^-1, which are close to the range of geologic strain rates for Earth.