In the days after a 7.0 magnitude earthquake rocked coastal Northern California on Thursday, scientists are rushing to install sensors among the region’s rugged mountains and river basins, hoping to learn more about a fault zone that poses one of the biggest threats to Mendocino and Humboldt counties.
“We want to capture as many aftershocks as we can to get a better sense of everything that’s happening out on the Mendocino Fault off the coast,” where the ground ruptured, said Tim Clements, of the U.S. Geological Survey’s Earthquake Science Center at Moffett Field.
Overhead, scientists are scanning the region by satellite to detect any landslides or broken roads. On the ground, they’re seeking debris flows and landslides.
And in the Bay Area, crews are measuring how the Humboldt earthquake triggered tiny slippage of our region’s faults. They found that the Hayward fault crept a tiny fraction of an inch, according to Oakland-based data collected by research geologist Austin Elliott of the USGS Earthquake Science Center.
The temblor, which struck at 10:44 a.m. Thursday, was California’s largest earthquake since a magnitude 7.1 hit the Kern County town of Ridgecrest on July 5, 2019. The epicenter of this week’s quake was under the Pacific Ocean, about 40 miles off the coast of the historic town of Ferndale.
In Humboldt County, residents are mopping up after damage to businesses and some homes, but the region’s major public infrastructure appears to have evaded extensive damage, according to the Eureka Times-Standard. Some stores are closed for repair and cleanup due to cracked windows, broken pipes, fallen ceiling tiles and damaged inventory.
Jim Campbell-Spickler is still awed by the experience. Perched atop a tall redwood Christmas tree at the Sequoia Park Zoo, he was placing a star when the quake hit.
“What are the odds of that?” zoo director Campbell-Spickler told the Times-Standard. Strapped in with a harness, “I had my feet on a branch, and I could feel the power of the earthquake through my feet on the tree.”
“Each of the waves of the earthquake, they felt like the tree would jump a little in a big lurch, almost like something ran into the tree,” he said, “and it would resonate up the trunk.”
The earthquake provided the first major test of the nation’s new ShakeAlert system — and it was a major success, warning an estimated 400,000 residents between Salinas and Lincoln City, OR, with a loud alarm and instructions to seek cover.
ShakeAlert was very accurate, initially reporting a 6.9 magnitude temblor, said Robert-Michael de Groot of the Pasadena-based system. After more than a decade in development, the ShakeAlert system has about 1,000 buried sensors in California that can mobilize cellphone users who are at risk. When complete, it will have 1,115 sensors, accelerating transmission time.
“The system did exactly what it was supposed to,” de Groot said.
An estimated 5 million people were alerted by at least one of the three warning systems — ShakeAlert, which is picked up by government and private partners and distributed like an AMBER Alert; MyShake app, developed by UC Berkeley, and Google’s Android Operating System alert.
There is a one in 20 chance of a magnitude 6 earthquake happening in the next week, said Clements. Many of the region’s communities are built on soft mud of river basins, meaning they are vulnerable to aftershocks.
The fault at fault
Thursday’s quake occurred along a restless region where two tectonic plates, the Gorda and Pacific, converge. It’s called the Mendocino Fault and is considered part of the Cascadia subduction zone that extends farther north along the coast. This area is known for frequent earthquake activity due to its complex plate interactions.
A tsunami was not triggered, as feared, because it was a “strike-slip” fault where two of Earth’s tectonic plates slid past each other horizontally. This resulted in minimal vertical displacement of the seafloor. All of the Bay Area’s faults, such as the Mendocino Fault, are strike-slip faults.
Japan’s 8.9-magnitude earthquake that set off a devastating tsunami in 2011, sending walls of water washing over coastal Japan, was very different. It occurred in what is called a subduction zone where one of the Earth’s tectonic plates slid beneath another.
On Thursday, USGS scientists were working in their Moffett Park offices when they felt the Humboldt quake, then gathered to meet in the hall.
“There’s a lot to do,” said Clements.
Some teams were driving up the coast to bury sensors that are connected to cell service, so data can be streamed back in real time.
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“We want to capture as many aftershocks as we can,” said Clements. “We want to understand the shaking qualities — and get a better indication of the seismicity and fault structure that’s happening out in the ocean.”
Other teams are deploying “nodal seismometers,” a different approach to measuring aftershocks. While not connected to cell service, these tools are faster and easier to install than traditional seismometers; several hundred can be installed in several days. That data will be used for detailed future research.
Yet other teams are working in the Bay Area to study the response of the slow, continuous movement of faraway faults, such as the Hayward, San Andreas and Calaveras faults. They often adjust after a major earthquake, yet the relationship between the two types of motions has been poorly understood.
“With every earthquake, we can learn more about the underlying physics,” said Clements, “and better predict and understand the seismic hazard risk, going forward.”
