Net interest margin (on a fully tax-equivalent basis) for the nine months ended September 30, 2023 was 2.47%, compared to 3.57% for the nine months ended September 30, 2022, a decrease of 110 basis points. Core net interest margin (on a fully tax-equivalent basis), a non-GAAP financial measure which excludes the impact of loan fees and PPP balances, interest, and fees, for the nine months ended September 30, 2023 was 2.39%, a 96 basis point decrease from 3.35% for the nine months ended September 30, 2022.
Average interest earning assets for the nine months ended September 30, 2023 increased $719.0 million, or 19.6%, to $4.38 billion, from $3.66 billion for the nine months ended September 30, 2022. This increase in average interest earning assets was primarily due to strong organic growth in the loan portfolio and continued purchases of investment securities, offset partially by the forgiveness of PPP loans. Average interest bearing liabilities increased $801.8 million, or 33.2%, to $3.22 billion for the nine months ended September 30, 2023, from $2.42 billion for the nine months ended September 30, 2022. The increase in average interest bearing liabilities was primarily due to an increase in interest bearing transaction deposits, brokered deposits, federal funds purchased, and FHLB advances.
Average interest earning assets produced a tax-equivalent yield of 5.04% for the nine months ended September 30, 2023, compared to 4.23% for the nine months ended September 30, 2022. The average rate paid on interest bearing liabilities was 3.49% for the nine months ended September 30, 2023, compared to 1.00% for the nine months ended
September 30, 2022.
Interest Income. Total interest income on a tax-equivalent basis was $165.0 million for the nine months ended September 30, 2023, compared to $115.7 million for the nine months ended September 30, 2022. The $49.3 million increase in total interest income on a tax-equivalent basis was primarily due to strong organic growth in the loan portfolio, continued purchases of investment securities, and higher earning asset yields in the rising interest rate environment.
Interest income on the investment securities portfolio, on a fully-tax equivalent basis, increased $8.1 million during the nine months ended September 30, 2023, compared to the nine months ended September 30, 2022, primarily due to a $106.7 million, or 21.7%, increase in average balances and a 128 basis point increase in yield between the periods.
Interest income on loans, on a fully-tax equivalent basis, for the nine months ended September 30, 2023 was $142.7 million, compared to $104.1 million for the nine months ended September 30, 2022. The $38.5 million increase was primarily due to a 19.3% increase in the average balances of loans outstanding and a 67 basis point increase in yield.
Interest Expense. Interest expense on interest bearing liabilities increased $65.9 million to $83.9 million for the nine months ended September 30, 2023, compared to $18.0 million for the nine months ended September 30, 2022, primarily due to higher rates paid on deposits and increased utilization of federal funds purchased and FHLB advances in the rising interest rate environment.
Interest expense on deposits increased to $66.6 million for the nine months ended September 30, 2023, compared to $12.6 million for the nine months ended September 30, 2022. The $54.0 million increase in interest expense on deposits was primarily due to upward repricing of the deposit portfolio in the higher interest rate environment.
Interest expense on borrowings increased $11.9 million to $17.3 million for the nine months ended September 30, 2023, compared to $5.4 million for the nine months ended September 30, 2022. This increase was primarily due to higher average balances of federal funds purchased and FHLB advances in the higher interest rate environment.
Provision for Credit Losses
The provision for credit losses on loans was $-0- for the third quarter of 2023, compared to $1.5 million for the third quarter of 2022. The provision for credit losses on loans was $2.1 million for the nine months ended September 30, 2023, compared to $6.2 million for the nine months ended September 30, 2022. No provision for credit losses on loans was recorded in the third quarter of 2023 due to a more managed pace of loan growth and continued strong asset quality.