The Loma Prieta earthquake occurred on October 17, 1989. It measured Magnitude 6.9 on Richter scale, and its epicenter was located about 9 miles North-East of Santa Cruz. A section of the Bay Bridge, numerous buildings in San Francisco’s Marina District, and the Cypress Street Viaduct on Highway 880 in Oakland collapsed. Building failures in the Marina District initiated a firestorm that further devastated the area. Pacific Mall area of Santa Cruz was practically destroyed. There were a total of 63 fatalities, most of which resulted from the collapse of the Cypress Street Viaduct.
Following the earthquake, TV cameras were focused on the spectacular failures of the Bay Bridge and Cypress Street Viaduct, and on the Marina District fires. What the TV cameras did not show was that most structures in the Bay Area had very little or no damage at all from the earthquake.
The lack of damage in most buildings in the Bay Area gave a false sense of security to building owners. They thought that if their buildings did not have any damage while the Marina District burned, the Bay Bridge failed, and the Cypress Street Viaduct collapsed, they must be very “strong” or very “earthquake-resistant.” In many occasions, our structural designs were questioned by building owners and building contractors, who claimed that our projects were “overdesigned” (meaning wasteful) and that we were being too “conservative.” “Why would we want to have so many walls in our houses or use so many steel straps” they said, “while our neighbors’ houses that had none of that did very well in the Loma Prieta earthquake.” In defense of our engineering and our designs, we had to “educate” our clients and contractors with a script that went as follows:
The magnitude of an earthquake is a measure of the total energy released in that earthquake. A magnitude of 6.9 qualifies Loma Prieta earthquake as a large earthquake, but not quite the largest one that can affect the Bay Area. However, the magnitude does not by itself determine what happens to a particular building during an earthquake. How the building responds to an earthquake and how much damage it incurs is a function of the nature of the ground shaking at the specific building location and the structural characteristics of the building. A large earthquake generates very strong ground shaking near its epicenter, but the strength of the shaking diminishes with distance. Scientists and engineers use maximum acceleration of the ground at a given site (expressed as a fraction of the gravitational acceleration “g”) as one measure of the strength of shaking at that site. 1.0 g is what holds us to the ground. If we rotated a building 90 degrees so that it is parallel to the ground, it would experience 1.0 g acceleration parallel to its floors; that is, it would have to carry its own weight as a cantilever structure. 1.0 g is about the maximum acceleration that would occur near the epicenter of a large earthquake. The maximum ground acceleration measured in the Loma Prieta earthquake was about 0.65g around Santa Cruz, and about 0.10 g on rock or firm ground 60 miles away in San Francisco, and in the Peninsula. One way to look at these numbers is that the buildings on rock or firm ground in the Bay Area experienced about one-sixth of the ground shaking experienced in Santa Cruz. Those buildings that did not suffer any damage did not do so because they were strong or very earthquake-resistant, but because the ground shaking under them was not strong enough to damage them.
So, what about those spectacular failures in the Bay Area? Well, the strength of ground shaking under a building also depends on what kind of soil the building is sitting on. Earthquake waves tend to travel through rock layers under the ground. If there are soft soils over the rock, the earthquake waves travel upward through the soft soils and become amplified. As a result, the buildings sitting on top of soft soils are subjected to stronger ground shaking than the buildings sitting on rock or firm soils. It is no wonder, then, the spectacular failures in the Bay Area occurred at locations with natural or artificial fill soils.
In a nut-shell, the Loma Prieta earthquake was a large earthquake for the Santa Cruz-Monterey area. For the Bay Area, however, it was a large earthquake that occurred a long distance away. If the epicenter of the earthquake was located within the Bay Area, its consequences would have been truly disastrous.
U.S. Geological Survey (USGS) assigns a probability of about 63% to a large earthquake impacting the Bay Area during a 30-year period. According to USGS, the next “big one” can originate from the Hayward-Rodgers Creek fault system in the East Bay, or from the San Andreas Fault in the West. Hayward-Rodgers Creek fault system is the more likely source of the next big earthquake than the San Andreas Fault.
Given that we live in a very active earthquake region, it would be a big mistake to let our guard down because our buildings did not suffer any damage in the Loma Prieta earthquake. Structural engineers are doing their best to protect their clients’ lives and property when they design new buildings or strengthen existing ones. Those who trusted and listened to them will be glad they did when the next big one strikes.
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