Second earthquake strikes near Isleton in Sacramento-San Joaquin Delta. What we know

A small earthquake struck Monday morning near Isleton, the third such quake to strike the Sacramento-San Joaquin Delta since Wednesday.

The 2.9-magnitude quake hit at 7:42 a.m. at Twitchell Island in the Delta, three miles south of Isleton and less than a mile south of a 4.2-magnitude quake on Oct. 18. It erupted at a depth of about 8 miles, according to preliminary reports from the U.S. Geological Survey and the Berkeley Seismology Lab.

Automated equipment initially pegged the magnitude at 4.0 but that was quickly revised — with the epicenter moving from Grand Island to Twitchell Island — given what seismologists learned from last week’s quake, according to Bob de Groot, a spokesman for the USGS in Southern California.

De Groot said the soft and mucky soils of the Delta tend to slow down seismic waves while amplifying them, making earthquakes feel bigger than they actually are. This is why last week’s quake was revised several times.

“The good news is that we’re learning what’s going on so we have confirmation of what to do,” de Groot said. “This is a case where the system didn’t do more because we learned from last week.”

De Groot said a ShakeAlert wasn’t sent out because the shaking wasn’t strong enough but he said the network that manages the early earthquake warnings “noticed.”

“The system is doing what it normally does,” de Groot said of ShakeAlert, which worked in “tenths of a second” to determine shaking had begun. The system “was triggered but, at this magnitude, there would be no alerts delivered to cellphones.”

By 8:30 a.m., about 60 people had gone to the USGS’s “Did You Feel It” system to register their experience.

A 4.2-magnitude temblor struck 2½ miles south-southwest of Isleton last Wednesday, causing shaking throughout the area and the evacuation of an elementary school, but no damage. Later that evening, an aftershock nearby registered 2.8 on the moment magnitude scale.

Monday’s earthquake, which may have been on the same Midland Fault according to de Groot, did not appear to be as widespread as last week’s.

Douglas Hsia, who lives on Grand Island next door to the Grand Island Mansion, said he was awake when the temblor struck but did not know about it until contacted by The Sacramento Bee.

“I thought you were talking about last week’s in Isleton,” Hsia said as he checked his kitchen to see if anything had been displaced.

“Nothing is out of place,” Hsia said, adding that he felt the earthquake last Wednesday and heard something crash in his attic after it struck.

Martha Esch, who lives in Walnut Grove and also felt last week’s earthquake, also said she did not feel the one Monday.

And the Isleton Elementary School, where students dropped and covered, then evacuated briefly last week, was in session without any interruption from Monday’s event.

Angie Lux, a project scientist at the Berkeley Seismological Laboratory, said Monday’s event was another aftershock from last Wednesday’s 4.2-magnitude quake.

“We can see pretty clearly this is an aftershock from the earthquake last week,” Lux said. “It’s in the same area. If we plot them they’re nearly on top of each other.

“It’s very common for an earthquake to have aftershocks after them. It could go on for weeks with aftershocks.”

But, Lux said, the aftershock Monday was “pretty small.”

“I mean, a 2.9 you’d have to be pretty close” to feel it, Lux said.