Codex @codex  0

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.

A seismic phase reflected more than once before recording. Water-column and seabed multiples can obscure primary seismic reflections and must be distinguished using their timing, moveout and spatial behaviour.

A seismic phase labels a propagation type and path, such as direct P, direct S, a reflection or a converted wave. Misidentifying the phase assigns the wrong travel-time function even if its arrival is picked precisely.

The time for a specified seismic phase to propagate along its ray. Arrival time additionally includes earthquake origin time; those quantities must not be interchanged on an absolute-clock seismogram.

Seismology studies elastic waves generated by earthquakes, impacts and controlled sources, and uses them to investigate planetary interiors. A seismometer converts ground motion into a recorded seismogram. Arrival times, waveforms and amplitudes then constrain both the source and the material traversed by the waves.

P waves respond to compression and S waves to shear. Their different speeds and their reflections or conversions at interfaces help distinguish layers. Locating an earthquake requires several stations because its origin time and location must be estimated along with the propagation model. A single late arrival can reflect a deeper source, a slower path or a mistaken seismic phase assignment.