A small-ish earthquake that struck before noon on Monday was felt throughout Sonoma and Napa counties and as far as Solano County and San Francisco to the east and south.

The quake, which struck at 11:51 a.m., had a preliminary magnitude of 4.2M from the US Geological Survey (USGS), revised up from an initial report of 4.0. The epicenter was 3 km southwest of Anderson Springs, in the hot springs area of Lake County near Middletown — in an area that is 21 miles due north of Santa Rosa.

As seen below, the USGS shake map shows that shaking was felt throughout the North Bay and on San Francisco Peninsula.

The earthquake had a relatively shallow recorded depth of 2.8 km.

This is the second notable earthquake to strike the region in the last several weeks, following a 3.4M quake with an epicenter in the same area on the morning of April 10. That quake was on the southern end of the Maacama Fault Zone and was felt throughout Sonoma and Napa counties.

With its geysers and natural hot springs, this part of northern Sonoma and southern Lake counties is known for geothermal activity and frequent small tremors.

This earthquake also comes one day after the 115th anniversary of the Great Earthquake and Fire of 1906, which was marked in San Francisco with the usual early morning ceremony at Lotta's Fountain.