The Department of Defense said it may close a military hospital near Riverside and two others in the San Francisco Bay Area because they may be a serious risk in an earthquake. The facilities are the Letterman Army Medical Center at the Presidio, the Oak Knoll Naval Hospital in Oakland and the hospital at March Air Force Base in Riverside. A departmental spokesman said two internal studies have disclosed that Letterman and Oak Knoll would be at "serious risk" if a major earthquake were to occur. The studies indicated that about $140 million in renovations would be needed if the facilities were kept open. An aide to Sen. Pete Wilson (R-Calif.) said the department is forming a task force to study earthquake safety of the three hospitals.