The Puget Sound region is in an area called the "Ring of Fire". This ring circles the Pacific with volcanic and earthquake activity, and roughly defines the boundary of the Pacific plate of the earth's crust. The juncture of the Pacific plate and the North American plate occurs along the western shores of the United States.

The oceanic crust pushing under the continental crust to form mountain ranges and volcanoes characterizes this converging boundary. If the converging plates get stuck and do not slide against each other, pressure mounts until slippage occurs with a jerk that quakes the earth.

Earthquake faults are described as a fracture in the earth's crust along which lateral movement occurs. Probably the most common and famous fault is the San Andreas Fault, located along the West Coast and running through San Francisco.

The most significant difference between the Puget Sound and the California Bay Area is the type of earth movement. In the Puget Sound area, the pacific plate pushes under the continental crust, causing a squeezing pressure which forces the continental crust upward. This effect occurs miles below the surface of the earth.

In the San Francisco Bay Area, the plates are not squeezing together, but sliding against one another. The Pacific plate is moving slowly toward the northwest. When the accumulated strain becomes too great, the rock breaks and the earth quakes. Once initiated, the break can propagate along the fault moving at speeds of thousands of miles per hour.

Seismic design is a relatively recent science, and few buildings constructed before the 1920’s or 1930’s were designed to accommodate these loads.

Early designers felt that seismic safety could be provided for in a manner similar to wind safety design. The inertial nature of seismic forces was not generally recognized until the early 1920’s. The 1927 Uniform Building Code included optional seismic design requirements based on building mass and soil type. These requirements were adopted almost unchanged into the California Administrative Code after the 1933 Long Beach earthquake. They did not change significantly until 1943 when Los Angeles developed a new formula that considered building height. San Francisco adopted similar requirements in 1947.

Seismic design requirements have continued to become more detailed in response to earthquake experience both in the United States and abroad. The 1964 Alaska earthquake, the 1967 Caracas, Venezuela earthquake and the 1971 San Fernando, California earthquake each resulted in code changes. Keep in mind that the earthquake fault system in the Puget Sound region is significantly different than in California. The Pacific plate is subducting under the North American plate 25 miles below the earth’s surface. In the San Francisco Bay Area, the plates are not squeezing together, but sliding past one another. Earthquake code improvements have been adopted from empirical data obtained from the California earthquakes. Different results could occur in the Puget Sound region.

Retrofitting structures to meet modern seismic safety requirements can be a major undertaking. Even then, there is no solid assurance that earthquakes of greater duration or larger amplitude than designed for cannot or will not occur. Earthquake insurance is often recommended as protection against these unknowns.

Although nothing can be done to prevent earthquakes, a number of practical steps can be taken to minimize damage to existing homes and buildings.

• Lateral bracing should be installed to prevent porches and decks from swaying and collapsing.
• Cripple walls (the walls between the basement foundation and the first floor joists) should be bolted to the foundation and braced or designed to withstand the shearing forces of earthquake motion.
• Gas appliances should be connected with flexible tubing to prevent rupture.
• Large appliances such as hot water tanks should be bolted to prevent toppling.
• Gas and water pipes should be rigidly attached to prevent swaying and breaking.
• Large bookcases and furniture should be fastened to the walls to prevent toppling.
• Unsupported chimneys should be braced to prevent collapse.