According to published articles, Foster City was built on "engineered land fill" and city officials have said that worries about its instability in the case of earthquakes are misleading. They've apparently done tests and experiments on the land and the results have been reassuring for residents that live in Foster City and for potential buyers in the area. They have also observed that during the Loma Prieta earthquake in the 1980s there was limited damage, liquefaction, or movement in the city's buildings and structures. Either way, In the case of emergency situations, they also have one of the more extensive earthquake/emergency preparation programs in the Bay Area. But of course most of the articles disputing the safety of the city used quotes and evidence provided by the city of Foster City, so it could in reality be quite misleading. Tough to say!
Here are some articles that are worth reading about the earthquake stability in the area:
This is just an article about their earthquake preparedness program: