Earthquake activity exists in half of built-up area (10)

'earthquake activity' is the definition.
(seismic activity)

'exists in half of built-up area' is the wordplay.
'exists' becomes 'is' ('be' can be a synonym of 'exist').
'in' is an insertion indicator.
'half of' becomes 'semi' (as in a semi-circle).
'built-up area' becomes 'city' ().
'is' inserted into 'semi' is 'seismi'.
'seismi'+'city'='SEISMICITY'