Earthquakes and Bridges

Earthquakes are commonly reported in terms of their magnitude. The larger the magnitude, the bigger the earthquake, and the greater the potential for damage. When engineers design structures to resist earthquakes, they use "rock motions" -- the vibrations that travel through the bedrock caused when a fault slips.

Seismologists develop the rock motions based on the structure's location in relation to the location of earthquake faults and historical and geotechnical project site data. Engineers use these rock motions to calculate the maximum seismic forces that the structure will experience and then design the structure to resist the forces.

Rock motions propagate up through the soil layers to the ground surface, where they become "ground motions." Ground motions are not the same as rock motions. Ground motions caused by the same rock motions will vary due to the soil conditions. The rock motions can be uniform along a project site, but for a long bridge like the Bay Bridge, the ground motions change along the length of the bridge as the soil changes.

For example, during the Loma Prieta earthquake, the ground motions varied at different locations, and this was reflected in the differing amounts of damage. Rock motions were amplified by the soft Bay muds and the resulting ground motions caused substantial damage in particular areas (such as the I-880/Cypress Freeway and San Francisco's Marina District). Where the soil is more stable material, the rock motions were not amplified as much, the ground motions were not as great and there was little damage. Seismologists and geotechnical engineers use the rock motions and then translate them (by computer) into ground motions. However, rock motions are the basis of engineering calculations.

There are two methods of estimating the greatest rock motion that a particular structure will experience. In the past, Caltrans considered the motions from the Maximum Credible Earthquake, or MCE. The MCE is the largest reasonable earthquake at a fault without regard or consideration of how often the earthquake might occur (the return period). It also does not provide a consistent or rational assessment of the probability that a structure will experience the design earthquake.

For the East Span Project, Caltrans estimated the greatest rock motions from the Safety Evaluation Earthquake, or SEE. This is defined as an earthquake that generates the largest motions expected to occur at the bridge site once every 1,500 years (a 1,500-year return period). The bridge's expected life span is 150 years, so there is approximately a 10% chance that this earthquake would happen during its life span.

-- Source: Caltrans