Simplify the following text for a Grade 12 native English-speaking person.
Original: However , the figure of 230,000 dead is based on a historical conflation of this earthquake with earthquakes in November 1137 on the Jazira plain and the large seismic event of 30 September 1139 in the Azerbaijani city of Ganja .
Simplified: However , the number of dead comes from a combination of this earthquake with earthquakes in November 1137 on the Jazira plain and 30 September 1139 in Ganja , Azerbaijan in historical records .