California is home to more than 500 volcanic vents and is a hotbed of earthquake activity in the Northwest US. The sunny US state sits on top of a seismically active zone where the North American Plate constantly grinds up against the Pacific Plate. The region is known as the Eastern California shear zone and it is where the so-called San Andreas fault line forms the boundary between the two plates. But just to the northeast of California is another source of seismicity and volcanism, which affects the geology of California and the neighbouring state of Oregon.