First Security Corp. earned $8.85 million in fourth quarter 1988, up 46.8 percent from the $6.03 million earned in the same period a year ago, and $33.04 million for the year, up 22.2 percent over the $27.03 million earned in 1987 - the bank holding company's strongest performance in six years.Common stock earnings for the quarter were 69 cents per share, up from 48 cents for the fourth quarter of 1987. For 1988, earnings per share were $2.58, up from $2.13 the previous year.

Based on the gains, First Security directors voted last October to raise the company's quarterly dividend 9 percent to 30 cents per common share.

First Security Chairman and Chief Executive Spencer F. Eccles credited "careful planning and a willingness to make changes," for the increased earnings.

"For several tough years, we wrestled with the challenges of a declining and fluctuating economy, deregulation and shifts in customer needs. We now are beginning to reap the rewards of the difficult decisions made during that period, as demonstrated by the fact that 1988 was First Security's strongest earnings year since 1982."

Earnings assets rose in 1988, said Eccles, because of increased loans. FSC banks made 29,297 loans totaling $858.6 million in the fourth quarter, up from 26,111 totaling $668.3 million in the same period a year earlier. For the year, 115,137 loans worth $3.8 billion were made, an increase of $1 billion over the $2.8 billion in loans made during 1987.

First Security also boosted deposits in 1988 to $3.9 billion, an increase of $39.9 million from year-end 1987.

Eccles said First Security entered the asset-backed securities market in fourth quarter 1988 when First Security Banks in Utah and Idaho sold a total of $102.3 million worth of consumer loans nationally through the First Boston Corp.

First Security's non-performing assets had been reduced to $94.7 million at the end of 1988, an 18 percent drop of $20.8 million from the figure a year ago. Non-performing assets now represent 2.54 percent of total loans, leases and other real estate owned (OREO) and 1.84 percent of total assets, their lowest level in five years.