seismometer n.

before vowels seism-, word-forming element meaning "earthquake," from Greek seismos "a shaking, shock; an earthquake," also "an extortion" (compare colloquial shake (someone) down), from seiein "to shake, agitate, sway; to quake, shiver" from PIE root *twei- "to agitate, shake, toss; excite; sparkle" (source also of Sanskrit tvesati "to excite; to be excited, inflame, sparkle" and words in Avestan for "fears" and "fright, danger").

word-forming element meaning "device or instrument for measuring;" commonly -ometer, occasionally -imeter; from French -mètre, from Greek metron "a measure," from PIE root *me- (2) "to measure."