Gonzales >> Early last week, a 4.6 magnitude earthquake near the town of Gonzales shook the Central Coast. Since then, 134 aftershocks along the San Andreas fault — often only detected by seismometers — have struck Monterey County.
For residents of Northern California, who are accustomed to rumbles and rattles, it was a gentle reminder that the Earth is in charge.
The surface of the planet is covered in slabs of earth called tectonic plates. Faults like the San Andreas appear at the margins where these plates collide, separate and rub past each other. Don Blakeman, a geophysicist with the National Earthquake Information Center describes it this way: “If you took a hardboiled egg and cracked it, but didn’t peel the shell off, and you’d have all these pieces of shell stuck to the egg, that’s sort of the way the Earth’s crust is.”
Because of movement deep below the surface of the Earth, these plates are constantly moving. “Earthquakes are basically a release of stress that builds up in the crust of the earth,” Blakeman said. When that pent up energy is released, it creates the familiar motions we feel: the swaying, the shaking and the rolling.
Aftershocks, or the tremors that follow a large earthquake, are a common part of this process.
“The sliding which has happened along the fault sort of produces adjustments in the ground around it, which produces aftershocks,” says Thorne Lay, a seismologist at UC Santa Cruz. The recent aftershocks seen after last week’s quake are no exception. “There’s nothing particularly striking or unusual about the aftershocks that we’ve seen,” he continued.
Still, it’s a complex system.
Faults like the San Andreas aren’t just lines on a map, says Blakeman, “they’re generally systems with lots of faults associated with them.”
Different from northern and southern sections of the San Andreas, the region where last week’s temblor occurred is a bit of an anomaly. It’s part of what Lay calls “the creeping zone.” This essentially means that instead of building up that stress, the rocks along the fault slowly slide past each other.
Emily Brodsky, another seismologist from UC Santa Cruz, says that this creeping section of the fault roughly extends from San Juan Bautista to Parkfield.
“This is an area that does have a lot of earthquakes,” According to Brodsky.
But when it comes to reading the Earth, context is key.
Despite the constant low levels of seismic activity, Brodsky says that there’s no reason to panic. She says “we wouldn’t expect a large earthquake to begin on that section of San Andreas.” A large earthquake, she added, would more likely occur on the northern section or southern sections of the fault. Closer to San Francisco or Los Angeles, for instance.
Lay agrees, “this is probably not a foreshock of a big event to happen on this particular part of the fault, because we don’t think big events do happen on this part of the fault.”
However, Lay says it’s always a good idea to stay prepared. Keeping food, water and extra supplies like batteries and medication could make a big difference if a large earthquake were to strike the greater Bay Area.
We don’t yet have the ability to predict when an earthquake will occur. “We can come kind of close with forecasts,” said Blakeman. Forecasts provide rough estimates of how the earth might shift in the next 20 or 30 years. The U.S. Geological Survey, for example, frequently produces reports that use up-to-date research to identify what regions of faults like the San Andreas might be more likely to rupture.
Like many other scientists, Blakeman isn’t satisfied, saying that with predictions, “we could warn people, and we could save lives.”
“Unfortunately, this is a very difficult scientific process, and we can’t do that.”