Two quakes roil Fresno in one week? Coincidence, experts say

The state’s midsection was roiled by two temblors in the span of a week, and that should be a reminder to the Fresno area – which felt both of them – that this is California; they don’t call the Golden State earthquake country for nothing.

“Earthquakes are popping off around the state all the time,” said Fresno State geology professor John Wakabayashi.

Just don’t panic. Call it a coincidence. Nothing more.

“It just happens that we felt two of them,” Wakabayashi said.

The quakes were totally unrelated, geologists say. They don’t portend the Big One on the mighty San Andreas Fault. California isn’t about to slide into the Pacific Ocean. And nothing has changed for the greater Fresno area, which continues to be located in one of the state’s quieter seismic areas.

Need proof? Wakabayashi doesn’t buy the optional earthquake insurance offered annually to homeowners – and he doesn’t intend to start now.

The first quake came last week, a 4.8-magnitude temblor 6.2 miles west of Big Pine near the eastern crest of the Sierra Nevada. After the initial quake came several small aftershocks. The quake was felt in the central San Joaquin Valley.

Then on Tuesday afternoon a magnitude-4.9 earthquake hit about 4 miles southwest of Wasco in Kern County. It, too, was followed by aftershocks.

Of the quakes, the one near Big Pine was expected; the one near Wasco was not.

In fact, the fault that spawned the Wasco earthquake was not only unnamed but unknown. Half of the earthquakes that measure magnitude 4 or 5 are on faults that were unknown before the actual quake, U.S. Geological Survey Seismologist Morgan Page said.

The Wasco quake was measured at more than 13 miles below the surface, in the bedrock below San Joaquin Valley sediments that have been laid down over the past 30 million to 40 million years, said Art McGarr, a geophysicist with the Geological Survey’s Earthquake Science Center in Menlo Park.

“These unmapped faults are only known because of earthquakes that occasionally occur on them,” McGarr said.

Experts don’t know a lot about Wasco quake

One thing that didn’t cause the Wasco quake was hydraulic fracturing, or fracking, an oil or gas extraction practice that uses chemical-laced water injected into the ground at high pressure.

Both McGarr, a fracking expert, and Wakabayashi said the quake was too far below the surface to be attributable to the controversial practice.

“That’s one of the reasons we concluded it doesn’t have anything to do with oil-field activities, including fracking,” McGarr said.

And, he added, there is no fracking taking place in the area of the quake. That said, experts still don’t know a whole lot about the Wasco quake.

It was a strike-slip fault, which is a fracture where a block moves horizontally along the fault line.

Wakabayashi said he expected the quake to be triggered by a thrust fault such as those found along the eastern edge of the Coast Range, in places such as the Kettleman Hills. These are inclined fractures where the block moves vertically.

This is the type of fault that triggered the massive 1983 Coalinga earthquake.

Those faults are not like the famed San Andreas, a strike-slip fault, but they are part of the greater San Andreas fault zone.

But how the Wasco fault and earthquake fit into that puzzle – if at all – is unknown, Wakabayashi said.

Big Pine quake ‘somewhat typical’

As for the Big Pine shaker, Wakabayashi termed it “somewhat typical” and “fairly ordinary, occurring in an area that you would expect it.”

It was another strike-slip fault, where a block moves horizontally along the fault line. The quake was predictable because that part of the state is seismically active and near the Owens Valley Fault, which was blamed for a deadly 1872 quake that was estimated at 7.4 to 7.9 magnitude.

Wakabayashi – who was a bit disappointed that he didn’t feel either quake – said Fresno lies in a “quiet zone” between the areas of the Owens Valley and alternating basin and ranges to the east, which produce about a quarter of the state’s quakes, and the San Andreas, which accounts for the other three-quarters.

Still, McGarr said the state as a whole is “mostly what we call a tectonically active area.”

Yes, the San Andreas is the most impressive part of the system, but there are several other faults, some of which have produced fairly large earthquakes. By that measure, McGarr wasn’t surprised by the events of the past week.

“It’s all part of the plate boundary system between the Pacific and North American plates,” he said.

But while no experts were worked up about the two events, there was acknowledgment that the recent activity was out of the ordinary.

Said Wakabayashi: “Even in the Bay Area, over the long haul, you will feel far more earthquakes than here, and get shaken to a degree we’ll never get shaken here, but to feel two in a week, even up there, is rare.”

Quakes felt in Fresno

Feb. 16: 4.8 magnitude, 6.2 miles west-northwest of Big Pine

Feb. 23: 4.9 magnitude, 4 miles south-southwest of Wasco

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