A method for mapping apparent stress and energy radiation applied to the 1994 Northridge earthquake fault zone

Abstract

Using the Northridge earthquake as an example, we demonstrate a new technique able to resolve apparent stress within subfaults of a larger fault plane. From the model of Wald et al. (1996), we estimated apparent stress for each subfault using τ a = (G/β)<Ḋ>/2 where G is the modulus of rigidity, β is the shear wave speed, and <Ḋ> is the average slip rate. The image of apparent stress mapped over the Northridge fault plane supports the idea that the stresses causing fault slip are inhomogeneous, but limited by the strength of the crust. Indeed, over the depth range 5 to 17 km, maximum values of apparent stress for a given depth interval agree with τ a (max)=0.06S(z), where S is the laboratory estimate of crustal strength as a function of depth z. The seismic energy from each subfault was estimated from the product τ a DA, where A is subfault area and D its slip. Over the fault zone, we found that the radiated energy is quite variable spatially, with more than 50% of the total coming from just 15% of the subfaults.