Effect of finite thickness of scattering layer on coda Q of local earthquakes

Abstract

Theories based on the assumption of an infinite medium show that the coda attenuation factor, Q c −1, of local earthquakes should be very close to the intrinsic Q i −1, whereas many observations show that Q c −1 is close to the total Q −( Q t −1). This paper shows by numerical modeling that Q c −1 depends on the thickness of the scattering layer. For source-receiver coincidence, if the thickness is greater than a half of the mean free path, Q c −1 lies between Q i −1 and Q t −1. However, when the thickness is less, Q c −1 may be higher than Q t −1. In addition, Q c −1 depends on the epicentral distance. The thinner the layer, the faster the change with distance in Q c −1. For an unbounded scattering medium Q c −1 is even lower than Q i −1 at long epicentral distances.