Earthquakes.
The San Andreas Fault is break in the Earth's crust that lies underneath California, causing several small earthquakes every year.
Most of the earthquakes are not even noticeable in 1994.
However, a catastrophic 1906.
Scientists say that it is possible that another massive earthquake may happen in the future, but they predict that they will recognise the warning sings years before it happen.
As a precaution against earthquakes, some older building in Californian cities are being pulled down, and there is very little new building along the fault line itself.