Hi Cathryn. That is the Great Valley thrust fault. It was the source for the Coalinga quake in 1983 and two 6.0M+ quakes that struck Vacaville and Winters in 1892. The following is just a small part taken from newspaper articles about the quakes. It’s well worth reading. Take Care…Don in creepy town

Vacaville's night watchman, S. N. Bettis, was walking down Main Street at 2:50 a.m., lantern in hand, when "his attention was attracted by a rumbling sound coming from the hills west of town. The noise resembled distant thunder or the roaring of water which had suddenly been let loose by the bursting of huge dam gates. Bettis stood still and listened a few seconds, while the noise increased to a roar and the ground beneath his feet seemed to heave up . . . 'I felt as if I was on the deck of a vessel during a heavy storm, and I put my hands to the ground to prevent myself from falling on my face . . . After that brick walls and chimneys began to fall all around and the noise for a minute or so was deafening' " (Perrine, 1893, p. 20).

On that Tuesday morning, April 19, 1892, most of northern California and west-central Nevada was shaken by an earthquake of about Richter magnitude (M) 6½. Major damage was concentrated in the communities of Vacaville and Dixon in Solano County, and Winters in Yolo County and surrounding rural areas along the western margin of the lower Sacramento Valley. Two days later at 9:43 a.m. on Thursday, April 21, a second severe earthquake in the M 6 range shook the area again, causing further damage. The largest of numerous aftershocks, approximately a M 5½ event, occurred several days later (April 29). This sequence of destructive earthquakes is noteworthy because along with the 1952 Kern County and 1983 Coalinga earthquakes, it is one of the most significant seismic episodes to directly affect California's Central Valley in historical times.

• Great Valley Thrust Fault