About Earthquakes and their properties
The presence or absence of a large number of massive buildings has no bearing on where and when earthquakes occurs, or their magnitudes; particularly along major fault systems such as the San Andreas Fault.

Earthquakes mainly occur on tectonic plate boundaries, such as the San Andreas Fault, near San Francisco, or the fault systems through Turkey, Italy, New Zealand, near Japan and the Rift Valley in Africa. However, this does not preclude earthquakes from occurring elsewhere, such as in the middle of a tectonic plate.

For many decades some Australians thought that Australia was immune to earthquakes because it lies in the middle of the Indo-Australian plate; then a number of large earthquakes occurred in the latte half of the 20th Century causing people to have a rethink.

The fact is geological faults occur everywhere: some are major, others are minor; some are active, others aren't. If ground movement occurs on a fault an earthquake or tremor will happen regardless of any extra weight on the Earth's surface at any location.

The San Andreas Fault, like many major faults, is not one single fault, but a fault zone of sub parallel faults of varying lengths. The accumulation of stresses along the individual faults are not uniform throughout the entire system. There is no precisely predictable pattern to when an earthquake will occur or where along these major fault systems

There is a picture on the following website showing the system of faults in the San Andreas Fault in the San Francisco bay area.

https://en.wikipedia.org/wiki/San_Andreas_Fault

The size and occurrence of a earthquakes along tectonic plate boundaries is dependent on the speed at which the plates are moving past each other and how much they snag on each other and the strength of the rock at the snag points. With very strong rock at the snag points a lot of pressure can build up over a long time and when that pressure is eventually released when the rock breaks or slips a strong earthquake will occur.

Many earthquakes will occur within the San Andreas Fault system, but it doesn't necessarily mean that San Francisco will be affected any time soon or how badly it will be affected.