The Northridge earthquake registered high on the Nielsen scale in the 28 Nielsen-metered markets outside Los Angeles, but the big ratings the networks garnered from their daytime coverage and news specials could end up costing them millions in lost ad revenue.“Emergency news preemptions almost always cost a network a lot of money,” said Paul Schulman, president of media buying agency the Paul Schulman Co. “CBS probably lost $ 500,000-$ 600,000 just for preempting ‘Evening Shade’ with a news special.” CBS and NBC both aired half-hour primetime specials, and all three webs are vulnerable to losses in daytime, where the Big Three provided nonstop coverage from approximately 8 a.m. to 5 p.m. EST. Ad inventory is especially tight in daytime, and having to preempt all afternoon Monday could end up being expensive for the networks, which will have to give “make good” spots to advertisers who were bumped. According to Nielsen overnights, ABC led during daytime with an average 9.4 rating, 21 share, followed by CBS (8.2/19) and NBC (8.1/18). In the evening news race, “ABC World News Tonight” far outdistanced the pack with a 17.4/26 — a big jump compared to its 10.6/20 season-to-date average. “CBS Evening News” and “NBC Nightly News,” which linger about a ratings point behind “World News,” also had substantial increases. “CBS Evening News” recorded a 12.4/18 and “NBC Nightly News” was a tad behind at a 12.2/18. ABC’s newsmag “Day One,” which did a special on the quake, was up three share points from its normal showing. CBS and NBC’s half-hour primetime specials each did a 17 share. A special edition of “Nightline” posted an 8.9/24 to lead latenight coverage, while an NBC News special, which preempted “The Tonight Show With Jay Leno,” logged a respectable 5.0/13, about on par with Leno’s usual performance. But there were signs that the national audience had wearied of quake coverage as the day came to an end: “The Late Show With David Letterman” delivered a 6.9/ 19, roughly 10% higher than its overnight average.