The motion of an earthquake is either a horizontal, back and forth motion or a vertical, rise and fall motion.
Unless the structure is right on top of the epicenter of the earthquake, these motions are only about an inch or two each way.
This motion gets transmitted from the moving ground to any object sitting on that ground.
The mudsill, a wood foundation member (usually a pressure treated 2x4 or 2x6), is resting on the foundation, and other framing members are attached to it. The walls of the house are built upward from the mudsill.
In newer homes, the mudsill is bolted to the foundation.
In older homes, built before the construction codes became stricter, the mudsill was not bolted to the foundation. It is just sitting on it.
In the event of an earthquake, the mudsill can be literally shaken off the concrete foundation and, just like a tower of children’s blocks, the whole structure comes tumbling down.
The other problem with older homes is that the main beams lying under the house and which support the floors, are resting on short, wooden beams which are, in turn, resting on concrete supports called “piers”.
When these homes were built, it was not required that these beams be bolted to their supports. Earthquake motion can easily shake these beams off their supports and the house collapses inward.
If you own an older home, let us give you a free estimate for a seismic retrofit.
Call Gilbert Rocha, Rocha
Construction, at 310-484-4256.