J.A.V. CONSTRUCTION.
Juan Adrian Velasco Construction.

THIS FIRST ILLUSTRATION shows the basic structure under a house before it has been retrofitted.

1-Crawlspace - The space between the ground under the house and the floor you walk on.  How much space you have to crawl in depends on how tall your  foundation and cripple wall are.
2-Floor - This is the area you walk on.
3-Foundation - This is the cement structure that supports the house and gives it stability. The primary goal of seismic retrofit is to attach the house to the foundation so it doesn't fall off the foundation during an earthquake.
4-Mudsill - This is a piece of wood that rests on top of the concrete foundation. The entire house rests on it. When an earthquake hits, the mud sill can be jolted off the foundation causing the house to collapse.
5-Cripple wall - Not all houses have a cripple wall. If you go up a set of stairs to enter your home, you probably have one. Un-reinforced cripple walls are the weakest part of a house because they readily collapse in an earthquake.
6-Floor framing - In an earthquake, the floor framing can be jerked off the cripple wall, causing the floor of the house to collapse.
7-Top plate - This is made of 2x4 lumber lying on top of the cripple wall. The floor framing is nailed into this.

THE NEXT ILLUSTRATION shows the same wall as the previous but after the house has been retrofitted: