On the relationship between strain rate and seismicity in the India-Asia collision zone: implications for probabilistic seismic hazard

The increasing density of geodetic measurements makes it possible to map surface strain rate in many zones of active tectonics with unprecedented spatial resolution. Here we show that the strain tensor rate calculated from GPS in the India-Asia collision zone represents well the strain released in earthquakes. This means that geodetic data in the India-Asia collision zone region can be extrapolated back in time to estimate strain buildup on active faults, or the kinematics of continental deformation. We infer that the geodetic strain rates can be assumed stationary through time on the timescale needed to build up the elastic strain released by larger earthquakes, and that they can be used to estimate the probability of triggering earthquakes. We show that the background seismicity rate correlates with the geodetic strain rate. A good fit is obtained assuming a linear relationship (<(N)over dot> = lambda . <(epsilon)over dot>, where <(N)over dot> is the density of the rate of M-w >= 4 earthquakes, <(epsilon)over dot> is strain rate and lambda = 2.5 +/- 0.1 x 10(-3) m(-2)), as would be expected from a standard Coulomb failure model. However, the fit is significantly better for a non-linear relationship (<(N)over dot> = gamma(1) . <(epsilon)over dot>(gamma 2) with gamma(1) = 2.5 +/- 0.6 m(-2) and gamma(2) = 1.42 +/- 0.15). The b-value of the Gutenberg-Richter law, which characterize the magnitude-frequency distribution, is found to be insensitive to the strain rate. In the case of a linear correlation between seismicity and strain rate, the maximum magnitude earthquake, derived from the moment conservation principle, is expected to be independent of the strain rate. By contrast, the non-linear case implies that the maximum magnitude earthquake would be larger in zones of low strain rate. We show that within areas of constant strain rate, earthquakes above M-w 4 follow a Poisson distribution in time and and are uniformly distributed in space. These findings provide a framework to estimate the probability of occurrence and magnitude of earthquakes as a function of the geodetic strain rate. We describe how the seismicity models derived from this approach can be used as an input for probabilistic seismic hazard analysis. This method is easy to automatically update, and can be applied in a consistent manner to any continental zone of active tectonics with sufficient geodetic coverage.