The ceremony took place on Friday, at the Government House, Birnin Kebbi.
The Governor said Ambursa was sworn-in because the State House of Assembly refused to confirm Karatu as substantive CJ.
Karatu had petitioned the National Judicial Council (NJC,) alleging the Assembly failed to confirm her because of her Christian faith.
Bagudu said he lobbied the lawmakers to ensure Karatu was confirmed but they did not.
He also disclosed that he wrote to NJC in his bid to ensure Karatu was confirmed by the House, stressing that the refusal to confirm her was not based on her faith.
According to the governor, who showed documents to journalists, the former acting chief justice altered her date of birth and primary school certificate, which made her ineligible for confirmation since June 2018, after the governor had sent her name for confirmation for the second time.
The embattled CJ is due to retire on July 5.
Earlier, the Speaker, Alhaji Abdulmumeen Kamba, said: “It is worthy to note that the former Chief Judge of the state, before Karatu, was of Christian faith. The House, not governor, has the power to confirm a chief judge. Kebbi State is a multi-ethnic and multi-religious state and every faith or ethnicity is treated equally.”
He said that Ambursa is the most senior among the judges in the state High Court.
The Guardian visited Karatu’s home in Kebbi yesterday, but was told she had gone to Abuja.