First Security Corp. reports its earnings declined $441,000 in the third quarter, down to $11.36 million from $11.80 million in the same three months last year. Earnings per share for the third quarter were 79 cents, down from 83 cents earned in the third quarter of 1989.

For the first nine months, net income was $36.6 million, up $3.18 million or 9.5 percent from the same period in 1989. Nine month net income per share was $2.53, a gain of 7.7 percent.Chairman Spencer F. Eccles said non-performing assets increased about $18 million from September 1989 to September 1990 but were down some $2 million since last June.

"We continue to be concerned over the loan deteriorations which increased non-performing assets after they had declined 13 of 15 previous quarters," Eccles said.

During the quarter the company increased its loan loss provision and its loan loss reserve. The latter was $70.82 million on Sept. 30, equal to 1.61 percent of total loans and leases, compared with $58.51 million and 1.42 percent a year ago.

"Over the past few months, the general level of economic risk and uncertainty both domestically and internationally has increased," Eccles said. "While the final outcome of events in the Persian Gulf and the extent of national economic weakness cannot be ascertained, caution in our forward planning is necessary."