How Earthquakes Become Data

A signal, not a photograph

An earthquake begins as a rupture below the surface. It releases energy as seismic waves, but the rupture itself is not photographed from above. Networks of seismometers record ground motion as those waves reach different stations. The first usable facts are therefore time series: how the ground moved, when distinct wave phases arrived and how strong the motion was at each instrument.

A single station can show that shaking occurred. Locating the source requires several stations, a model of how waves travel through Earth and an iterative comparison between predicted and observed arrival times.

Location, depth and origin time are solved together

Analysts and automated systems estimate latitude, longitude, depth and origin time by finding the source that best explains the arrival times across a network. The earliest automatic solution can change when more stations report, noisy signals are removed or an analyst reviews the waveforms. A map point is a best estimate with uncertainty, not a direct GPS reading from underground.

Depth matters because it changes the distance waves travel to the surface. Yet depth alone does not determine impact: magnitude, distance, rupture direction, local geology and building vulnerability all contribute to experienced shaking.

Magnitude is calculated after detection

Magnitude is derived from measured wave amplitudes or seismic moment after correcting for distance, instrument response and the scale being used. Different methods can produce slightly different early values. Large events are commonly summarized with moment magnitude because it relates to fault area, slip and rock rigidity and does not saturate as readily as some older scales.

Magnitude and intensity answer different questions. Magnitude describes the estimated size of the source; intensity describes shaking and effects at a particular place. One earthquake may have one preferred magnitude and many intensity values.

Why records are revised

Networks continue receiving waveforms after the first alert. Agencies may add stations, refine velocity models, associate additional phases or replace an automatic magnitude with a reviewed solution. PlanetVexa preserves source time and update time separately so a later revision is visible without implying that the earthquake happened again.