Three small earthquakes rattled parts of central Arizona and were felt in the Phoenix area.
The U.S. Geological Survey says a magnitude 4.1 temblor struck at 11:29 p.m. MST Sunday, followed by a 4.0 quake about 20 minutes later. A smaller, magnitude-3.2 quake, hit at 9 p.m.
The USGS says all the quakes ranged from 3-6 miles in depth and were centered near Black Canyon City, about 45 miles north of Phoenix.
The quakes were notable for at least two reasons beyond their simple rarity in the region, notes AZCentral.com:
First, the largest apparent temblor was preceded by a 3.2-magnitude foreshock, first reported at about 9 p.m.
“It’s not common, but it does happen where you get small foreshock and that triggers a larger event,” Reeves said. “Typically the first shock is the main one and everything after is at least a magnitude smaller. This is definitely not common.”
Second, the quake hit relatively close to the Earth’s surface, which may have made its effect more pronounced. Quakes are measured both in their magnitude – the power of their shaking – and their depth. Their destructive power varies with both those factors, as well as local building standards.
Sunday’s largest jolt came at 6.5 kilometers – about 4 miles – beneath the surface. By comparison, the destructive Northridge earthquake in Southern California in 1994 struck at more than 18 kilometers deep.
The USGS website recorded scores of responses from people in such suburban Phoenix cities as Cave Creek, Peoria, Scottsdale and Glendale who reported feeling at least one of the quakes.
The sheriff's offices in Yavapai County and Maricopa counties didn't immediately comment when contacted about the quakes.