= Earthquake location from interphase delays
{title2=$\Delta t=K\sqrt{(x-x_0)^2+z_0^2}$}
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.
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