DALLAS—Rare earth tremors continued in the Dallas-Fort Worth area Saturday, one day after several minor earthquakes made Halloween memorable for some North Texans.

Two almost simultaneous earthquakes were detected in northern Grand Prairie and northern Irving just before 7 a.m. Saturday. The Grand Prairie quake measured 2.5 magnitude, while the Irving quake came in at 2.7, according to the U.S. Geological Survey Web site.

The ground shook in the same general areas as on Friday, when tremors of 2.5 magnitude set off car alarms. Most of the Dallas area didn’t feel the shocks, but they caused 911 switchboards to light up with calls from the quakes’ immediate vicinity. The 911 operators said no damage was reported.

Geophysicist John Bellini of the National Earthquake Information Center in Golden, Colo., said the quakes are too small to be attributed to a specific geological fault or formation. He said the quakes could continue for days or up to three weeks.