An article from earlier this month, 'Hayward Fault is our deadliest - a 'tectonic time bomb' , makes it clear that the Hayward Fault has a major earthquake of approximately 7.0 every 140 years, and that the last major quake hit that area in 1868.... 139 years ago.
From that article:
"A magnitude 7 quake on the fault today would likely leave about 100,000 people homeless and cause more than $1 trillion in damage, according to the Association of Bay Area Governments and the U.S. Geological Survey."
I believe that might actually be an understatement, given that a similar quote was used in this article:
"With the number of densely built cities now straddling the dangerous fault, analysts at the Association of Bay Area Governments anticipate there would be 1,100 road closures and 94,000 destroyed homes and apartment units from a magnitude 6.7 quake."
Geologists believe that the fault is capable of producing quakes as large as a magnitude 7.3, meaning that the number of deaths, injuries and damaged homes could be bigger than the area has ever planned for.
The last major earthquake hit the southern end of the Hayward Fault and toppled buildings as far away as San Francisco, primarily effecting those areas built on landfill east of Montgomery Street, with greater damage in areas like Hayward, Fremont, and San Leandro. In Hayward, then a town with about 500 residents, almost every building was damaged extensively or wrecked. Damage was reported even as far south as Gilroy and as far west as Santa Cruz.
Sadly, many of the engineering lessons learned from this earthquake and openly discussed at the time, such as the hazards of building on "made ground" reclaimed from the San Francisco Bay or the admonition to "build no more cornices," were long forgotten by the time of the 1906 quake.
Here's a USGS intensity map based on the last such earthquake on the Hayward Fault. Note that landfill area all the way up the Peninsula would be prone to heavy damage, including parts of San Francisco, SFO Airport, Foster City, East Palo Alto, and numerous major corporations which have built their headquarters and offices on marsh landfill, to the east of 101. The good news -- if you can call it that -- is that Oakland came through surprisingly well in the 1868 earthquake, due to its geography. Coastal Oakland and Alameda, however, could also expect heavy damage.
It should be noted that earthquakes in the San Francisco Bay Area have been, historically speaking, quite moderate since the 1906 earthquake, which was approximately a 7.8. That earthquake was so powerful that it apparently relieved pressure amongst all the faults in the region. However, regional seismicity rates from the last few decades of the 20th century suggest that the San Francisco Bay Area has been emerging from this period of calm, but has not returned to the high -- yet seismologically normal -- rate of earthquakes experienced in the 1800's as yet.