Big quakes effects on big buildings
Researchers from California and France have used 3-D supercomputer simulation tools to evaluate the damages that strong earthquakes along the San Andreas faults would cause to earthquake-resistant buildings in Southern California. For this simulation, they've put two 18-story buildings designed according respectively to the 1982 and 1997 Uniform Building Code (UBC) standards on a grid of 636 sites that covers the Los Angeles and San Fernando basins. And they discovered that it would be much better for the buildings -- and its inhabitants -- if such a big quake was moving from South to North than in the opposite direction. Read more...
Here is the introduction of the Caltech news release about this study.
The research simulates the effects that two different 7.9-magnitude San Andreas earthquakes would have on two hypothetical 18-story steel frame buildings located at 636 sites on a grid that covers the Los Angeles and San Fernando basins. An earthquake of this magnitude occurred on the San Andreas on January 9, 1857, and seismologists generally agree that the fault has the potential for such an event every 200 to 300 years. To put this in context, the much smaller January 17, 1994, Northridge earthquake of 6.7 magnitude caused 57 deaths and economic losses of more than $40 billion.
You can see below a geographical scope of the simulation (Credit: Caltech). "The color scheme reflects topography, with green denoting low elevation and yellow denoting mountains): The solid black triangles represent the 636 sites at which seismograms are computed and buildings are analyzed. The white box is the surface projection of the Northridge fault. The red line in the inset is the surface trace of the hypothetical 290 km rupture of the San Andreas fault that is the primary focus of this study. The area enclosed by the blue polygon denotes the region covered by the 636 sites."
Now let's look at the precise goals of this simulation.
The simulated earthquakes "rupture" a 290-kilometer section of the San Andreas fault between Parkfield in the Central Valley and Southern California, one earthquake with rupture propagating southward and the other with rupture propagating northward.
And below are several images showing the effects of a South-North (top) and North-South (bottom) big quakes on the existing (right) and redesigned (left) buildings in Southern Calfornia (Credit: Caltech). As you can see, it would be much better for buildings -- and people -- if an earthquake decided to start from the South...
As you can -- barely -- see in the above images, the damages on the buildings have been measured using what is called a "peak interstory drift." But what is this measure?
Interstory drift is the difference between the roof and floor displacements of any given story as the building sways during the earthquake, normalized by the story height. For example, for a 10-foot high story, an interstory drift of 0.10 indicates that the roof is displaced one foot in relation to the floor below.
The greater the drift, the greater the likelihood of damage. Peak interstory drift values larger than 0.06 indicate severe damage, while values larger than 0.025 indicate that the damage could be serious enough to pose a serious threat to human safety. Values in excess of 0.10 indicate probable building collapse.
And as the pictures above show, a 7.9-magnitude San Andreas rupture from Parkfield to Los Angeles would result in greater damage to both buildings than a rupture from Los Angeles to Parkfield. But it's only a simulation, isn't? Californians have to wait for the real quake to see if this simulation was right -- or not.
For more information, this study, which was led by Swaminathan Krishnan from Caltech, has been published by the Bulletin of the Seismological Society of America under the title "Case Studies of Damage to Tall Steel Moment-Frame Buildings in Southern California During Large San Andreas Earthquakes" (Volume 96, Issue 4A, Pages 1523-1537, August 1, 2006).
Here are two links to the abstract and to the full version of this scientific paper (PDF format, 38 pages, 2.31 MB). The above figures have been extracted from this paper.
Sources: Caltech news release, August 10, 2006; and various web sites
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