Earthquake location from interphase delays ($\Delta t=K\sqrt{(x-x_0{)}^2+z_0^2}$)

In a homogeneous medium, two phases along the same geometric path have delay $\Delta t=KR$, where $K=1/v_B-1/v_A$. The squared delay is a quadratic in station position. Its vertex locates the epicentre, while the ratio of intercept to quadratic coefficient gives squared depth. These spatial parameters do not depend on a particular assigned speed ratio if both speeds are inferred from the same data. Local clock offsets cancel, although uncertainties in the velocity model do not.