seismic ray[′sīz·mik ′rā]
a line normal to the front of a seismic wave propagating from the focus of an earthquake. The direction of a ray changes with a change in the velocity of seismic waves along the path of propagation. In a homogeneous, isotropic elastic medium with a constant velocity of wave propagation, a ray is a straight line. In a first approximation for the earth, velocity is taken to be a function of depth; as velocity increases with depth, the paths of the rays become curved, turning convexly downward symmetrically relative to the apexes of the rays. The equation for a ray is
where T is the ray’s travel time, Q is the epicentral distance in radians, R is the radius of the earth, Vr is the velocity of seismic waves along the ray, e(r) is the angle of inclination of the ray to the horizon at a depth corresponding to the radius r, e0 is the angle of emergence of the seismic ray on the earth’s surface, and V0 is the velocity at the earth’s surface.
Each ray has a deepest point with radius rp. At this point, e = 0, cos e(r) = 1, and p = rp/ Vrp.
I. V. GORBUNOVA