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This question is, in part, inspired from the 1997 Hollywood movie Volcano, where a fictional volcano erupted out of the La Brea Tar Pits in Los Angeles.

The San Andreas Fault is a transform fault plate boundary (see image below):

enter image description here

Image from the USGS Glossary entry for transform faults, with a definition:

A transform fault is a special variety of strike-slip fault that accommodates relative horizontal slip between other tectonic elements, such as oceanic crustal plates.

A specific type of transform fault is a 'leaky transform fault', which according to a source used on its Wikipedia page describes the tectonic movement at this type of faulting as being:

In addition to the regular strike-slip motion observed at transform boundaries, an oblique extensional component is present, resulting in motion of the plates that is not parallel to the plate boundary.

Illustrated below:

enter image description here

Is there any evidence that oblique tectonic movement occurs along the San Andreas Fault?

Hence, is a San Andreas volcano possible?

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Yes. In fact there are already volcanos associated with the San Andreas fault system.

Neenach is an extinct Miocene volcano split in half by the San Andreas fault. Garry Hayes wrote a nice blog post about this volcano system: A Volcano Sundered: A Field Trip Along the San Andreas Fault.

If you'll allow some latitude in what counts as 'a San Andreas volcano', then there is active volcanism too. The Salton Buttes are a group of rhyolitic volcanos in the Brawley Seismic Zone, a set of oblique transform faults — actually in the axis of the active Gulf of California rift — on the south-east shore of the Salton Sea. You can see them in this Google Maps image (on the right; the big fields are 800 m across):

The Imperial Valley faults and the Salton Buttes

The Space Shuttle photo (left) of the Imperial Valley annotated by the USGS shows the relationship between the San Andreas fault, about 5 km from the volcanos, the Brawley Seismic Zone (BSZ), and the Imperial Fault to the south. At this scale, it's essentially one big fault system.

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