Two more quakes rattle the West Coast. What’s going on with the Ring of Fire?

Two more earthquakes shook California Thursday, marking the latest in a burst of seismic and volcanic activity along the Ring of Fire.

A 4.0-magnitude quake took place just after 2 a.m. about 31 miles southeast of Anaheim, according to the U.S. Geological Survey. And a 5.8 earthquake struck about 100 miles off the Humboldt County coast at 8:39 a.m. No tsunamis are expected, according to a Twitter post by the National Tsunami Warning Center.

They follow a 6.2-magnitude quake in Japan on Wednesday and a 7.9-magnitude quake in Alaska on Tuesday that prompted a quickly canceled tsunami warning for the West Coast. Significant earthquakes in Indonesia and Chile also have been reported this week.

In addition, volcanic eruptions in the Philippines, Indonesia, Bali and Japan continue to spew ash and lava, forcing the evacuation of tens of thousands of people. The eruption Tuesday of Mount Kusatsu-Shirane in Japan killed a soldier and injured more than a dozen people, including several at a nearby ski resort.

The United Nations Office for Disaster Risk Reduction on Tuesday tweeted a warning that the Ring of Fire was “active.”

What is the Ring of Fire?

The Ring of Fire refers to a sprawling, horseshoe-shaped geological disaster zone in the Pacific, according to ABC News. The ring stretches 25,000 miles from New Zealand through Indonesia, the Philippines and Japan to Alaska, Canada and the West Coast down to South America. It contains 452 volcanoes and several tectonic plates in the earth’s crust.

Fitting together like a puzzle, the tectonic plates constantly shift and collide, producing earthquakes.

Scientists say the recent series of quakes and volcanic eruptions are merely business as usual for the Ring of Fire.

“It just so happens that these events are occurring at the same time in different parts of the region,” Chris Elders, a geology expert from Curtin University in Australia, told the BBC. “There’s not necessarily a relationship between them.”

Janine Krippner, a New Zealand volcanologist based in the United States, reminded people that “It’s not referred to as the ‘ring of fire’ because it sits there doing nothing.”

Krippner told the BBC that while satellite imagery and the internet have made people around the world more aware of eruptions and seismic activity, that doesn’t mean they’ve actually become more common.