Revealing the cracks in the earth and our structures
If you’ve spent time at UC San Diego, you know that when it comes to earthquakes, it’s not a matter of if, but when. For researchers, however, it’s also a matter of how—how it will happen and how it will impact the places where we live and work.
A new study by scientists at Scripps Institution of Oceanography says that in addition
to “The Big One,” subsequent aftershocks on separate faults following a large earthquake could cascade and lead to mega-earthquakes.
Scripps geophysicist Peter Shearer and graduate student Wenyuan Fan have discovered 48 previously unidentified large aftershocks that occurred on faults adjacent to the main shock rupture just after large-magnitude earthquakes. For instance, off the coast of Indonesia, a magnitude 7 quake triggered two large aftershocks over 200 kilometers (124 miles) away.
These aftershocks, while miles away from the initial quake, reveal that stress can be transferred almost instantaneously by the passing of seismic waves from one fault to another.
“By studying this type of triggering, we may be able to forecast hosting faults for large earthquakes,” says Fan.
Scientists generally believe that most aftershocks are triggered by stress changes caused by permanent movement of the fault and mainly occur near the main rupture where these changes are largest. The new findings suggest that aftershocks can also be triggered by seismic wave transients, where the locations of the main quake and the aftershock may not be directly connected.
“Multiple fault system interactions are not fully considered in seismic hazard analyses, and this study might motivate future modeling efforts to account for these effects,” says Shearer.
If the prospect of a mega-earthquake has you quaking yourself—fear not, because UC San Diego engineers are making sure our world will withstand the rumble. Researchers at the Jacobs School of Engineering are preparing to mitigate the effects of a large-scale earthquake by testing a six-story, lightweight steel-frame building to determine how it will fare during a tremor and fires that may follow.
The structure, the tallest cold-formed steel-frame structure to undergo tests on a shake table, was built to represent a multifamily residential condominium or apartment. It was placed through a series of simulated temblors of increasing intensity that mimicked actual earthquakes.
As a better way to determine stress on the materials, the building’s performance was captured by an extensive array of more than 250 analog sensors, as well as digital cameras and aerial drones. Structural engineering professor Falko Kuester, who leads UC San Diego’s DroneLab, used unmanned aerial vehicles (UAVs) to capture both the seismic and fire testing and create a high-resolution 3-D model and video of observed damage. Engineers can use virtual reality (VR) to zoom in to see the tiniest details, such as cracks and changes in shape and color.
“This is big VR for big data and big science,” says Kuester.
As for the building? “It could have been easily repaired,” said structural engineering professor Tara Hutchinson. “The occupants would have gotten out safely.” Hutchinson believes the structure fared well because it is lighter than a concrete building and has less mass to generate damaging forces.
Fire was less kind to the structure, however. Plastic fixtures and hardware melted, as did several video cameras installed to capture the fire’s progression. Simulated quakes occurring after the fire tests further weakened the structure’s floors, bringing it close to collapse.
All the better to learn these effects in a test environment, however. The combination of these technologies—a one-of-a-kind outdoor shake table and powerful data visualization methods—allows structural engineers at the Jacobs School to produce an incredibly detailed digital model of the structures they test. This in turn allows them to make recommendations to improve design methods and building codes around the nation and around the world for when the Big One, or maybe the Mega One, hits.