Shallow M5.5 earthquake, series of aftershocks hit Lake Almanor, Northern California

A shallow earthquake registered by the USGS as M5.5 hit Lake Almanor, Northern California at 23:19 UTC on May 11, 2023. The quake was followed by a series of aftershocks, including M5.2 at 10:18 UTC on May 12.

The epicenter was located 4 km (2.5 miles) E of Prattville, 4.8 km (3 miles) NW of Canyondam, 5.1 km (3.1 miles) SSW of East Shore, and 45.9 km (28.5 miles) WSW of Susanville, California.

5 000 people are estimated to have felt strong shaking, 6 000 moderate, and 380 000 light.

The USGS issued a Green alert for shaking-related fatalities and economic losses. There is a low likelihood of casualties and damage.

Overall, the population in this region resides in structures that are resistant to earthquake shaking, though vulnerable structures exist. The predominant vulnerable building types are unreinforced brick masonry and reinforced masonry construction.

Recent earthquakes in this area have caused secondary hazards such as landslides and liquefaction that might have contributed to losses.

This is the first serious earthquake in the area since M5.7 hit the eastern shore of Lake Almanor on May 23, 2013.

The quake was followed by at least 16 aftershocks, including M5.2 at 10:18 UTC on May 12.

More than 6 700 people reported feeling the quake, from Seattle, Washington to Salinas, California.

This quake was on a normal fault in the northern Sierra Nevada, the California Geological Survey said, adding that aftershocks are expected.

The quake, initially reported as M5.7, activated the ShakeAlert system. “Since the quake was greater than magnitude 5, Shake Alert-powered alerts were delivered to cellphones by the MyShakeApp and FEMA’s Wireless Emergency Alert system,” the USGS said.

The California Highway Patrol said their Chico dispatch cellphone 911 lines were down because of the earthquake.

“We were right in the middle of it,” Mark Guillory, owner of Sierra’s Fly & Tackle in Lake Almanor told The Seattle Times. “Crap was falling off the walls and my truck outside was moving.”

A spokesman for the Governor’s Office of Emergency Services said a rock slide had closed a portion of Highway 89, adding that there were some reports of localized power outages but no major damage.

A Plumas County sheriff’s office said there were no reports of damage or injuries, and that rock slides are not unusual in the area following past year’s fires and storms.

At 05:29 UTC on May 12, the USGS said there is a 4% chance of one or more aftershocks that are larger than magnitude 5, which can be damaging, within the next week. There will likely be smaller aftershocks within the next week, with up to 9 magnitude 3 or higher aftershocks.

While the number of aftershocks will decrease over time, a large aftershock can temporarily increase the number of aftershocks. The aftershock forecast applies to the area where the earthquake and aftershocks are already occurring.

Estimated population exposure to earthquake shaking

Selected cities exposed

Regional seismicity

Featured image credit: TW/SAM, Google