I think for the typical backyard observatory, it's going to come down to local soil type, local soil hydraulic nature, and weather more than earthquakes and faults. Even if the footer extends below the frost line, the ground swells and shrinks with seasonal changes in the water table and ground saturation.
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On Nov 4, 2020, at 11:14 PM, CurtisC via groups.io <firstname.lastname@example.org> wrote:
Picking up from Mr. Erickson's post -- Palomar Mountain is bounded by major faults on both the northeastern and southwestern sides of the block, and it is, of course, pretty close to the San Andreas and San Jacinto Fault Zones. The foundation of the 200-inch has adjustments for polar alignment, but -- to my knowledge -- it has never been readjusted due to effects from an earthquake. Nobody currently on staff remembers any such realignment. The mm 7.2 Cucapah Earthquake in Baja Calif. in 2010 was strong enough to slosh water in the observatory's water tanks, but it didn't affect any of the telescopes.