Olga, I'll reply to your comment here.  

I still think "of" is the better choice.  The reason is that "of" is related not only to "the coastal city", but also to the directional word "southwest".  "of" is the natural preposition used after directional words used to show the location of one place relative to another.  After reading your comment, I googled - earthquake "southwest of the coastal city of" --   and I obtained 3,830,000 links.   I then substituted the word "off", and there were no hits other than your own. 

"off" would work if it appeared in a separate clause not containing a directional word:  "An earthquake hit 106 km. southwest of the city of Whistler, off the coast of Chiapas."

Earthquake of a 5.3 Magnitude - please correct

A 5.3 magnitude earthquake hit (or: struck) the southern state of Chiapas, Mexico, this past Sunday, seismologists reported. The epicenter was [or: was located] 106 km. southwest of the coastal city of Whistler, [1] Itar-Tass reported. No injuries or property damage have [2] yet been reported.

Or: An earthquake at 5.3 magnitude struck (or: hit) the......?

Or: An earthquake of the magnitude of 5.3 stuck (or: hit    or:  shook   or: rocked   or:  was felt in) the . . .

[1]  Or:  The epicenter was [or: was located] in the ocean, 106 km. southwest of the coastal city of Whistler, Chiapas.     This sentence has the advantage of specifying that the epicenter was in the ocean, and not on land.  

[2]  I googled "no injuries or property damage have been reported", and was given about 134,000 links.  Then I substituted "has", and got about 22,100 links.  I personally prefer "have", because the word "injuries" is plural. 

Thank you!