Epicenter

[citation needed] The earliest seismographs were designed to give a sense of the direction of the first motions from an earthquake.

The Chinese frog seismograph[2] would have dropped its ball in the general compass direction of the earthquake, assuming a strong positive pulse.

We now know that first motions can be in almost any direction depending on the type of initiating rupture (focal mechanism).

Instead of merely noting, or recording, the absolute motions of a pendulum, the displacements were plotted on a moving graph, driven by a clock mechanism.

The emphasis is on precision since much can be learned about the fault mechanics and seismic hazard, if the locations can be determined to be within a kilometer or two, for small earthquakes.

[7] Focal depths of earthquakes occurring in continental crust mostly range from 2 to 20 kilometers (1.2 to 12.4 mi).

Once distances from the epicenter have been calculated from at least three seismographic measuring stations, the point can be located, using trilateration.

[12] The word is derived from the Neo-Latin noun epicentrum,[13] the latinisation of the ancient Greek adjective ἐπίκεντρος (epikentros), "occupying a cardinal point, situated on a centre",[14] from ἐπί (epi) "on, upon, at"[15] and κέντρον (kentron) "centre".

[17] It is also used to mean "center of activity", as in "Travel is restricted in the Chinese province thought to be the epicentre of the SARS outbreak.

[20] Garner has speculated that these misuses may just be "metaphorical descriptions of focal points of unstable and potentially destructive environments.