This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. Relating to or designating a line connecting the points on a map that indicate the places simultaneously affected by an earthquake shock.

n. A coseismal line.

adj. Of or pertaining to a line on a map connecting places simultaneously affected by a seismic event.

The term used by Mallet to designate the curve or line along which a wave of earthquake-shock “simultaneously [synchronously] reaches the earth's surface”; the crest of a wave of shock. See homoseismal, isochrone, isoseismal.

adj. being where earthquake waves arrive simultaneously

"If, however, the number of observations is large for the size of the disturbed area, the construction of coseismal lines may define approximately the position of the epicentre."

unstable   

coseismic