# Seismic Ray

## seismic ray

[′sīz·mik ′rā]## Seismic Ray

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, V_{r} 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, e_{0} is the angle of emergence of the seismic ray on the earth’s surface, and V_{0} is the velocity at the earth’s surface.

Each ray has a deepest point with radius r_{p}. At this point, *e* = 0, cos *e(r) =* 1, and p = r_{p}*/* V_{rp}.

I. V. GORBUNOVA