An earthquake of magnitude 4.6 shook residents awake across the Bay Area early Thursday morning.
The earthquake, at 1:41 a.m., was centered near Boulder Creek in the Santa Cruz Mountains. Scientists with the U.S. Geological Survey believe it was triggered by an unmapped fault below the mountains.
Residents of Oakland and San Francisco felt shaking of a few seconds or more, with some reporting a sharp jolt. In the Santa Cruz County communities near the epicenter, books, vases and other belongings were knocked off shelves. No major damage was immediately reported aside from scared pets and, for some, broken dishes, a shattered television and a busted picture frame.
“It’s the type of earthquake that can pop off anywhere in California,” said Susan Hough, a USGS seismologist based in Pasadena. “It doesn’t need a big fault to trigger it.”
People across Santa Cruz County and the Bay Area got earthquake alerts, which go out for earthquakes of magnitude 4.5 and above. The alerts typically will arrive at communities near the epicenter just after the shaking has begun, but before the shaking for areas that are farther away.
The quake was originally reported at 5.1 magnitude but quickly revised downward to 4.9 and subsequently to 4.6.
Hough said the nearby San Andreas Fault likely didn’t play a role. The 6.9 magnitude Loma Prieta earthquake that struck in 1989 was caused by the San Andreas Fault, with its epicenter near Mount Loma Prieta, along the border of Santa Cruz and Santa Clara counties in the Santa Cruz Mountains.
Thursday’s quake was the strongest recorded in Santa Cruz County in over three decades, according to USGS data. The magnitude, estimated precisely at 4.59, was last topped on March 24, 1991, by a magnitude 4.6 earthquake around Interlaken, which is near Watsonville. On April 18, 1990, on the anniversary of the 1906 San Francisco earthquake, Santa Cruz County experienced two earthquakes over magnitude 5. The measurements are broadly comparable but there could be slight differences since past earthquakes were measured on the Richter scale, while scientists now rely on the moment magnitude scale, which better reflects the total energy released in a quake.
No aftershocks of 2.5 or above had occurred following Thursday’s quake, as of 11:45 a.m.
The U.S Geological Survey estimates a 1-in-3 chance of a magnitude 3 or greater quake in the area within the month, following Thursday’s jolt.
The odds of this earthquake being a foreshock to a larger event are small. The chance of a magnitude 5 or greater quake is 1 in 200 within the month while the chance of a magnitude 6 or greater is 1 in 2000, according to the U.S. Geological Survey.
“An earthquake like this is a good reminder for people that earthquakes are always possible,” Hough said.
Jack Lee contributed to this report.
