4.0 midnight quake rattles homes in Northern California

The ShakeMap of Tuesday night's quake. Seismic equipment picked it up from Crescent City to Redway. (Courtesy of USGS ShakeMap)

Humboldt County residents may have felt some shaking late Tuesday night as a magnitude 4.0 earthquake struck, centered underground close to Blue Lake. An earthquake expert said this quake was relatively deep for the region and came from geologic pulling of a slab — unlike the more frequently felt seismic activity under the ocean.

The ShakeMap of Tuesday night’s quake. Seismic equipment picked it up from Crescent City to Redway. (Courtesy of USGS ShakeMap) 

According to the USGS, the epicenter of the quake was about five miles southeast of Blue Lake and began at 11:39 p.m. on Tuesday.

“It’s a good reminder that not all of our earthquakes are off the coast,” said Lori Dengler, emeritus professor of geology at Cal Poly Humboldt and an expert in tsunami and earthquake hazards, reached by phone Wednesday.

Dengler said the depth — which USGS reported at about 16 miles underground — is unique for the area. She said the slab of ocean crust that caused the shake (which surfaces offshore about 35 miles) gently dips to about 12 miles below Willow Creek. Tuesday night’s earthquake is a few miles deeper.

She said the cause of the quake was deformation, pulling of this subduction plate at the Cascadia subduction zone, and on a north-south normal fault which means stretching in an east-west direction.

Reports from the USGS’s community intensity map, which crowdsources how people experience quakes, note people felt it more intensely from Blue Lake outward. Others slept through the quake. Seismic instruments picked up the quake from Redway to Crescent City.

Dengler explained how people feel quakes depends on the quake itself and where they’re at when it strikes. The offshore earthquakes the region regularly experiences are generally less than ten miles deep, she said.

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“That actually means that we tend to feel them a little more efficiently — the seismic waves are almost coming straight up at us,” she said.

Had the earthquake been nearer to surface it would have been felt more in Blue Lake with less of a spread, the energy dying out faster. The foundation of your house or the geology below can also make the quake feel stronger. Eureka is mostly on sedimentary fill, which tends to amplify the shaking, Dengler said.

Crowdsourced surveys on the intensity of the shake didn’t include reports of damage or …read more

Source:: The Mercury News – Entertainment


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