Urban haze detectable in seismography (4)

I believe the answer is:

smog

I'm a little stuck... Click here to teach me more about this clue!

'urban haze' is the definition.
(I've seen this in another clue)

'detectable in seismography' is the wordplay.
'SMOG' is hidden within the letters.
But, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for smog that I've seen before include "Thick polluted mist" , "Pollutant haze" , "Pollutant fog" , "Pollution haze/fog" , "Industrially polluted air" .)

I've seen this clue in The Sun.

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