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 by the slipping of an earthquake fault. 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" — rock motions cause 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.
Design Rock Motions
Bridge design in earthquake-prone areas needs to take into account the anticipated rock motions at the bridge site. The challenge for designers is that the rock motion frequencies of future earthquakes cannot be predicted. The design rock motion is the model of anticipated rock motion that is developed for design purposes. It assumes that rock motions are strong over a broad range of frequencies, although this would never happen in a real earthquake. By assuming that rock motions are strong over a broad range of frequencies, this model takes into account the actual earthquakes that would generate strong motions for one or more of these frequencies.
Design Rock Motions for San Francisco-Oakland Bay Bridge East Span Replacement Structures
Replacement structures for the San Francisco-Oakland Bay Bridge East Span will be designed for rare seismic events. The Hayward and San Andreas faults are the dominant sources for rock motions for this bridge site, though other sources in the area have been considered in development of the bridge design.
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 Event (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 Event (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 1500-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. Caltrans was aided in the development of the ground motions by the Ad Hoc Committee on Seismic Ground Motions. This ad hoc committee is comprised of four members:
The Seismic Safety Peer Review Panel and the Metropolitan Transportation Commission's Engineering and Design Advisory Panel both accepted and agreed that the bridge should be designed for these SEE ground motions.
The Ad Hoc Committee on Seismic Ground Motions evaluated the recently issued report by the Army Corps of Engineers (ACOE) on retrofit versus replacement of the East Span. In particular, the committee focused on one of the ACOE's conclusions: "the performance of the replacement bridge during a Maximum Credible Earthquake cannot be determined. The bridge has not been evaluated or designed for a MCE event, which is larger than the SEE event." The Committee pointed out that this conclusion is based on an error in plotting a graph included in the ACOE's report. In fact, the MCE motions fall well below the SEE motions in all relevant data ranges, particularly during the initial shaking. In addition, the Ad Hoc Committee's evaluation of the ACOE's report explained that responses of structures (estimated for earthquakes of various magnitudes and for various source-to-site distances) depend crucially on the measurement of ground motions in actual large earthquakes. As additional recordings become available from such earthquakes, these new ground motion recordings are incorporated into the existing international database. Professor Bolt has stated that the SEE standard takes into account a wider range of possible ground motions, some of which are much higher in impact than MCE's predicted motions. The Ad Hoc Committee concluded that Caltrans studied both standards (MCE and SEE) and designed the bridge to the higher SEE standard.