With images of the impending big one tucked in many
Californians’ minds, two moderate earthquakes in the area since
Friday have raised some concerns as they rattled some windows and
walls.
By Cathy Kelly, Santa Cruz Sentinel
With images of the impending big one tucked in many Californians’ minds, two moderate earthquakes in the area since Friday have raised some concerns as they rattled some windows and walls.
Both quakes were magnitude-4.0 or greater, the U.S. Geological Survey reported, and accompanied by several aftershocks.
But they are within the realm of normal and don’t necessarily mean there is a greater chance of a big one, according to the Geological Survey.
At 4:10 p.m. Friday, a magnitude-4.1 quake hit the Seven Trees area between San Jose and Morgan Hill, on the Calaveras Fault, according to the USGS.
At 12:51 a.m. Wednesday, a 4.5-magnitude quake struck six miles southeast of San Juan Bautista on the San Andreas Fault. In the next few hours more than a dozen aftershocks including a 3.2 and a 3.3, all within minutes of the initial jolt. And at 7:54 p.m. Wednesday, a 3.7-magnitude quake struck in the same area with a 3.9 minutes later, according to the USGS.
“Every time there is a flurry of earthquakes in a few days, questions arise,” said David Oppenheimer, a USGS seismologist based in Menlo Park. “They are difficult to answer. The real question is what is happening down in the Earth? But we don’t have instruments at five miles down to know what is going on there.”
Friday’s quake was 10.5 miles deep; the ones on Wednesday were all about five miles deep, according to the USGS website.
All were in areas marked by “creeping faults,” or friction near the fault surface caused by low-friction rocks abutting one another, Oppenheimer said.
Sometimes, the “creep” is barely perceptible, he said. In Hollister, where the Calaveras Fault runs right through town, evidence can be seen in displaced sidewalks and curbs and other spots, Oppenheimer said.
Larger quakes, such as the 1989 Loma Prieta earthquake, occur in “stuck” areas where rock does not glide past adjoining rock, he said.
And while creeping might not sound like a good thing, it relieves pressure so that any “stuck patches” are smaller and produce smaller earthquakes, Oppenheimer said.
It’s possible Wednesday’s temblors on the San Andreas were triggered by the earlier one on the Calaveras Fault, Oppenheimer said.
The two faults meet near San Juan Bautista, he said.
“The 4.1 up by Morgan Hill sent out seismic waves,” he said. “People felt them and so did the rocks everywhere in the Bay Area. It’s possible the seismic activity from Calaveras triggered an event on San Andreas that pushed it along. It would have happened anyway, yet later. But it’s all conjecture.”
The Seven Trees temblor is close to the epicenter of the 1984 Morgan Hill earthquake, Oppenheimer said. That quake had a magnitude of 6.2, he said.
A three-mile radius near Wednesday’s San Juan Bautista quakes was particularly active in the early 1970s, he said, with 12 earthquakes of a magnitude of 4.0 or greater. In the past 30 years, there have been eight more, he said Wednesday afternoon. The largest was a 5.2-magnitude quake in 1998, he said. Loma Prieta was farther north, he said.
Creeping sections of earthquake faults typically get more frequent, smaller temblors, Oppenheimer said.
The early-morning quake Wednesday was felt from Santa Cruz to the Salinas Valley.
Julia Patton said she must have been sleeping lightly, as she was awakened by the slight shaking at her Westside home.
“I could hear the windows rattling,” she said. “They were vibrating very lightly. It was fascinating.”
Patton has lived in Santa Cruz for 15 years. On Oct. 17, 1989, when the devastating, 6.9-magnitude Loma Prieta quake struck, she said she had been visiting her mother-in-law, but left to drive home to Pleasanton about two hours before the quake struck early that evening.
“I’m sure it will be scary when the big one comes again, but who knows when that will happen,” Patton said.
In the bigger picture, the area shaking is from shifting tectonic plates, basically from the Pacific Plate moving toward Alaska in relation to the North American Plate, Oppenheimer said.
