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For surface stations at similar distances, the travel-time derivatives with respect to depth can be small or nearly equal. Their arrival-time shifts are then difficult to distinguish from a common change in earthquake origin time. A deeper source can be paired with an earlier origin. Nearby stations, broad distance coverage, and depth-sensitive phases help separate the two parameters.

An earthquake origin time is the estimated onset time of rupture. Predicted arrivals add this time to a propagation travel time; an uncertain origin time therefore couples to uncertain source depth in some station geometries.

If $r=v_A/v_B$ and both phases share path length, arrival time of A is affine in the interphase delay. With unbiased independent clock errors, the intercept estimates earthquake origin time and slope estimates $1/(r-1)$. A common clock bias is indistinguishable from an origin-time shift; unrestricted station offsets also destroy speed-ratio identifiability. A statistical regression must state its clock-error assumptions.

Identify the event and pick calibrated P wave and, where available, S wave arrivals at stations of known position. Associate each pick with a seismic phase and uncertainty. An earthquake location by arrival times fitsfor the three hypocentre coordinates and earthquake origin time, using travel times from a specified velocity model. A grid search can initialize iterative weighted least squares or a probabilistic inversion. Station coverage must constrain depth as well as horizontal position; merely having four observations does not ensure a well-conditioned solution.

Errors include uncertain or misidentified onsets, clock and station-coordinate errors, wrong velocities or neglected three-dimensional structure, poor azimuthal coverage, inadequate station distances and depth–origin-time tradeoffs. Examine residual patterns and resolution, not only the minimum residual. A small formal covariance within a wrong velocity model can accompany a systematically biased hypocentre.