Experimental Evidence of Inhomogeneous P Wave at Very Low Strain

Particle motion of body waves allows to differentiate between dissipative and non dissipative soil type. Particularly relevant is to establish the strain threshold between elastic and non elastic soil behavior and, in case of the presence of anelasticity, to check the reliability of simple models. In the present paper we analyse the records of two superficial open quarry explosions of unexploded 2nd world war bombs dropped by Allies air raid, recently rescued at a few meters depth close to Milano Central station. The seismic stations located at 2.5 km and 550 m epicentral distance, respectively, show a clear separation between body waves and surface waves. The results show: 1) despite the low strain (max strain not exceeding 10-7) elastic approx is not applicable; 2) the data evidence a clear elliptical prograde P wave particle motion with g greater than 0 and less than 30 degrees (g is angle between the P propagation and the A attenuation vector, P precedes A); 3) the Homogeneous Isotropic Linear Viscoelastic model is sufficient to describe conveniently this inhomogeneous P wave motion.