Not all earthquakes are created equal.
Experts say Monday’s 3.9 temblor centered in the area was on a thrust fault under the San Joaquin Hills. Known as the “San Joaquin Thrust,” it was discovered 13 years ago and runs along the coast from Newport Beach and Costa Mesa to San Juan Capistrano.
Monday’s jolt had many residents surprised that it was an actual earthquake. Readers wrote in to Patch that it felt more like a “sonic boom” or thunder rather than the typical shaking and swaying associated with earthquakes.
“Earthquakes feel different depending on your distance from the fault, the size of the earthquake, and the type of material (soil or rock) beneath you," said Lisa Grant Ludwig, associate director of the California Institute for Hazards Research at UC Irvine. "If you are far from an earthquake, it feels more like a rolling or rocking motion. The Baja, California earthquake on Easter 2010 was experienced as rolling or wave-like motions by many residents of O.C,” Grant Ludwig said.
“If an earthquake is close, like the one yesterday, it feels more like a boom or jolt. At my location in Irvine, it was a noisy jolt. Laguna Niguel is in the southern San Joaquin Hills where the ground is mostly rock rather than deep soil. People near the epicenter probably heard things shake and felt a jolt at the same time because the earthquake was right below them.”
"I think there's an under-appreciation of the seismic hazard in Orange County," Grant Ludwig told the Los Angeles Times. "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.
Grant Ludwig said the fault has existed for thousands of years, but it wasn't investigated until after the 1994 Northridge earthquake highlighted the importance of blind thrust faults in urban areas of Southern California. Information about the San Joaquin Hills thrust fault was first published in 1999.
Some reports, including one in the Huffington Post, alluded to Monday’s quake as possibly being a result of a meteor that passed by earlier, but experts at the Jet Propulsion Laboratory in Pasadena don’t buy it.
“It is highly unlikely that the earthquake and the meteor were related," said Andrea Donnellan, principal research scientist at JPL.
Said Kate Hutton, staff seismologist at the California Institute of Technology Seismological Laboratory: “It's not a particularly uncommon event in the area,” adding that about 20 other temblors have been reported within 15 miles of Monday’s quake. She said Cal-Tech has been monitoring these earthquakes since 1933.
Unlike the famous San Andreas fault, which is visible from the ground, 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 of the fault are, Grant Ludwig said.
Scientists discovered the San Joaquin Hills thrust fault after noticing evidence of ancient sea life in what are now the hills, she said. 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.
There were no reports of damage in Monday's quake.