Citicorp yesterday reported that earnings before securities transactions fell 40 percent in the second quarter to 84 cents a share from $1.43 a share in the same three-month period last year.

Income before securities transactions in the quarter was $106 million, down from $177 million last year.

For the first half, Citicorp's earnings were down 22 percent to $203 million ($1.61 a share) from $260 million ($1.61) in the first six months of 1980.

Net interest revenue in the first half of 1981 was down 16 percent from 1980 levels, and net interest spreads narrowed 62 basis points to 2.35 percent. 1However, net interest revenue did improve 15 percent from the first quarter, Citicorp said.

Eastern Airlines Inc. said yesterday it lost $4.6 million in the second quarter ended June 30, in part because of high interest expenses, but it was still an improvement from the $15.9 million loss in the same 1980 period.

Eastern, the third-largest domestic airline, showed an operating profit of $2.2 million for the April-to-June period, up sharply from the $12.8 million operating loss in the second quarter of 1980, when a strike halted Eastern's Mexico operations.

The loss came on record revenues of $970.8 million in the second quarter, compared with revenues of $850.8 million a year earlier.

For the six months of 1981, Eastern reported a loss of $5l5,000 on revenues of $1.93 billion, compared with a loss of $12.5 million on revenues of $1.7 billion in the first half of 1980. Operating profits for the first half were $16.1 million compared with $4.5 million a year earlier.

Higher unit volume and prices combined with strong sales in the United States pushed Dow Chemical Co.'s second-quarter profit 37 percent about the level of a year ago, the company reported.

Net income was $181 million (96 cents a share) on sales of $3.07 billion, compared with $171.2 million (94 cents) on sales of $2.5 billion for the second quarter of 1980.

Business conditions improved during the second quarter from earlier in the year, especially in the United States, said Dow financial vice president Robert M. Keil.

So far this year, Dow sales have been $5.96 billion, compared with $5.33 billion in the first six months of 1980. Profits were $310 million ($1.66) compared with $402 million ($2.2l) a year ago.

Santa Fe Industries Inc. says its second-quarter earnings fell 3 percent to $59 million (67 cents per share) from $6l million (70 cents) a year earlier.

Although total revenue increased from $771 million to $1.07 billion during the quarter, Chairman John S. Reed said earnings declined because of a higher income tax rate. Reed said pretax earnings increased $6 million during the quarter.

For the first six months of the year, Santa Fe Industries said it earned $127.1 million ($1.44) up from $113.3 million ($1.31) earned in the first six months of 1980. Six-month revenue rose to $2.13 billion from $1.5 billion.