In a house built on a very steep hillside the basic seismic retrofitting (bolting the house to the foundation and adding plywood to the cripple walls) is usually not sufficient to resist the strong forces of earthquakes. Each hillside retrofit is a unique situation and has to be designed by a professional engineer or an architect.
The basic idea is to attach the house very securely to its foundation at the top of the hill to prevent the house from detaching completely and sliding downhill. Of course, the uphill foundation has to be strong enough to support the entire weight of the house trying to pull away from it. Strengthening the uphill foundation could be very costly.
1) The floor joists are strengthened by “sandwiching” each one between two new joists of the same size. The three joists are bolted to each other at regular intervals to form a single unit.
2) Custom manufactured and welded steel plates are bolted to the concrete foundation stem wall with threaded rods and high strength epoxy.
3) Custom manufactured steel angles are then bolted to the above plates and are bolted at the other end to the tripled floor joists.