O.C. quake was small, but it could make history The earthquake may have measured only a 3.9, but it still could make history in Orange County. Monday's temblor, centered in the southern suburb of Laguna Niguel, could be the first measured on a fault discovered only 13 years ago and running along the coast from Newport Beach and Costa Mesa to San Juan Capistrano - close to the San Onofre nuclear power plant. The little-known fault - called the San Joaquin Hills thrust - is similar to the fault that triggered the deadly Northridge quake in the San Fernando Valley 18 years ago. Unlike the famous San Andreas fault, which can be seen on the surface, the fracture in the earth's crust that makes up the San Joaquin Hills thrust fault is entirely underground. Because there is no visible break in the earth's crust at ground level, the fault is perhaps more dangerous because it's unclear exactly where the boundaries are. Scientists weren't aware of the blind thrust faults that triggered the 6.7 Northridge earthquake in 1994, or the 6.0 Whittier Narrows quake in 1987, until after the ground began shaking. Experts say Monday's temblor should serve as a wake-up call, particularly to Orange County residents who mistakenly believe that quakes are more of an L.A. problem. Scientists believe that the San Joaquin Hills thrust fault is capable of generating a magnitude 7 quake or greater. The U.S. Geological Surveyin 2003 conducted a scenario of such a quake and found it could trigger severe shaking on a large swath of southern Orange County, including Costa Mesa, Irvine, Lake Forest, Mission Viejo, Newport Beach, Laguna Beach, Dana Point and San Juan Capistrano. "If this morning's earthquake was on this fault, this is an example of what the fault is capable of doing," said Lisa Grant Ludwig, a UC Irvine associate professor who was the lead author of a paper in the journal Geology in 1999 announcing the discovery of the San Joaquin Hills thrust fault. "I think there's an under-appreciation of the seismic hazard in Orange County," Grant Ludwig said. "There is a general perception in Orange County that we don't have as much earthquake hazard" - in part because the county has not suffered a major, destructive quake since 1933, when the area was sparsely populated. Scientists discovered the San Joaquin Hills thrust fault after noticing evidence of ancient sea life in the hills. The researchers hypothesized that the land was once below sea level, but over hundreds of thousands of years the fault caused the earth to move upward, creating the hilly terrain. In a 2002 follow-up report in the Bulletin of the Seismological Society of America, Grant Ludwig found evidence of marsh deposits about 3 to 12 feet above the current shoreline. That suggested that the fault generated a magnitude 7 temblor sometime between the mid-17th and mid-19th centuries, which "may have generated the largest earthquake in the Los Angeles Basin since Western explorers reached the area." There were no reports of damage in Monday's quake, which hit at 10:37 a.m. Southern California Edison said there was no impact at its San Onofre nuclear plant, which has been shut down since January because of safety concerns. Los Angeles Times staff writers Abby Sewell and Steve Marble contributed to this report.