Geophysical studies including seismic imaging, magnetotelluric sounding, and gravity modeling demonstrate that segments of the San Andreas Fault are indeed nearly vertical or steeply dipping in the upper crust.
The retrieved literature consistently supports the claim that the San Andreas fault is nearly vertical or steeply dipping. Papers [0], [3], [9], and [11] all explicitly report geophysical evidence (such as seismic tomography, geodetic and seismic inversions, magnetotelluric imaging, and gravity models) showing that the fault or key sections of it (like the southern San Andreas fault) are vertical to steeply dipping (e.g., 60-80 degrees) in the upper crust. No papers refute the claim, so the verdict is SUPPORTED.