Exploring the physics of earthquakes and landscapes

West Napa Fault

Ever since I moved to U.C. Davis I have been looking for an interesting local project on which to spend some research time. Northern California is of course very tectonically active. However much of the action takes place in the grass-covered, poison-oak infested thicket of forest and chaparral known as the Coast Ranges. And the accessible areas are mostly private land, too, which makes it hard to do fieldwork until you build up enough connections. One place within the northern Coast Ranges that I have had my eye on for some time is Napa Valley. This famously fertile valley runs slightly west of north, almost parallel to nearby strike-slip faults. There is a West Napa fault mapped as active in the southern part of the valley, but the northern, narrower part of the valley was not known to contain active faults. It turns out that there is an active fault in northern Napa Valley, located at the foot of the Mayacamas Mountains between St. Helena and Calistoga. It just hasn’t been mapped yet. This is perhaps because it appears to be a reverse fault, which can be hard to recognize. I discovered the evidence for faulting in a 2003 lidar data set for Napa Valley, and confirmed it in the field in March 2014. My students and I are working on documenting this fault for a publication to be submitted soon.

Hillshaded point cloud produced in KeckCAVES LidarViewer software, showing a portion of the northern Napa Valley between St. Helena and Calistoga. Illumination is from the northwest. Yellow areas highlight fault scarps at the base of the Mayacamas Mountains (left) and an active anticline out in the middle of the valley (right).The Napa Valley Grist Mill historic site is located at the tip of the southernmost yellow arrow.

The Napa Valley Grist Mill is built into the fault scarp, which provides an abrupt change in elevation useful for building a really big water wheel. The scarp cuts a terrace of river gravel sourced from nearby Mill Creek.