= Solution
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> fits
$$
t_i^{\rm obs}=t_0+T_i(\mathbf x)+\epsilon_i
$$
for 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>.
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