By David Shepardson

WASHINGTON (Reuters) - Two U.S. senators on Tuesday sent letters to 13 major airlines expressing concerns that their information technology systems are vulnerable to outages that can strand thousands of passengers similar to recent flight cancellations.

Senators Richard Blumenthal and Edward Markey sent letters after Southwest Airlines Co <LUV.N> and Delta Air Lines Inc <DAL.N> experienced technology issues that resulted in thousands of flight cancellations across the country. They want details on "specific safeguards and backups" to prevent airline IT systems from failing.

Delta was forced to ground about 2,000 flights last week after a small fire resulted in a "massive failure" at the airline's technology center.

Reuters reported last week further outages are likely because major carriers have not invested enough to overhaul reservations systems based on technology dating to the 1960s, citing airline industry and technology experts.

“We are concerned with recent reports indicating that airlines’ IT systems may be susceptible to faltering because of the way they are designed and have been maintained,” the Democratic senators wrote.

"Now that four air carriers control approximately 85 percent of domestic capacity, all it takes is one airline to experience an outage and thousands of passengers could be stranded."

Delta, American and Southwest said they had received the letter. United referred questions to a trade group.

Southwest said it has invested "significant effort and financial investment to ensure the technology that supports our business and our operation has back up/recovery capabilities."

The senators also want airlines to answer if they will rebook passengers on a rival carrier or compensate passengers in the event of delays or cancellations caused by an IT outage. They also want answers about airlines' cybersecurity efforts.

The U.S. Transportation Department said in a statement last week the "responsibility to manage IT systems for airlines’ internal operations - like flight scheduling - falls on the airlines themselves." The department noted that airlines must follow laws "protecting consumers' rights and compensation, and that includes in instances of extensive flight cancellations and delays."

Other recent disruptions include one in July that prompted Southwest to cancel over 2,000 flights and two outages last summer at United Continental Holdings Inc <UAL.N>.

The airlines getting letters included Delta, Southwest, American Airlines Group Inc <AAL.O>, United and JetBlue Airways Corp <JBLU.O>.

The reservations systems of the biggest carriers mostly run on a specialized IBM <IBM.N> operating system known as Transaction Processing Facility, or TPF. It was designed in the 1960s to process large numbers of transactions quickly and is still updated by IBM, which did a major rewrite of the operating system about a decade ago.

(Reporting by David Shepardson; Editing by Cynthia Osterman and Alan Crosby)