20 April 2017-DENVER — Data collected from two tall buildings, one in Osaka and one in Los Angeles, demonstrate that long-lasting motions from distant earthquakes — up to hundreds of kilometers away — and shake those buildings severely and can cause structural and non-structural damage.
Mehmet Celebi of the U.S. Geological Survey discussed these two buildings at the 2017 SSA Annual Meeting. He suggested that design codes for these types of buildings should take into account these long-distance effects.
In Osaka, Japan, a 55-story building shook for more than 1000 seconds as the result of the magnitude 9.0 Tohoku earthquake in 2011 — even though it was located 770 kilometers from the earthquake’s epicenter. The shaking came close to breaching a damage threshold in Japanese building codes, Celebi said, and created havoc when it tangled elevator cables and stranded people on upper floors.
Celebi also shared observations from a 52-story building in Los Angeles that experienced severe shaking during the 1992 magnitude 7.3 Landers and 1994 magnitude 6.7 Northridge earthquakes.
“The data show that this is a reality and also show that it is not only the safety of the occupants, but also the safety of the structures that can be realistically damaged if it is not paid attention to,” said Celebi.
Celebi noted that prolonged shaking from a large subduction earthquake occurring offshore in the Pacific could cause damage to tall buildings in Seattle, and that damage could occur even in places such as Chicago, if there is another earthquake in the New Madrid Zone such as the 1811-1812 magnitude 7.5 quakes that struck the area.
All buildings have a natural fundamental period, which is the number of seconds that it takes the building to naturally vibrate back and forth in one cycle. If the period of the incoming earthquake waves closely matches the building’s natural period, the building can resonate and damage from the shaking is much more likely. Buildings are also designed to contain a certain level of “damping,” or ability to dissipate vibrations. Studies of buildings like the Osaka and Los Angeles structures, however, suggest that these designs may not account well enough for long-period motions from far-off earthquakes, Celebi said. (Watch a video of similar shaking of the Atwood Building in Anchorage, Alaska.)
Even if a building isn’t damaged immediately in such an event, prolonged shaking can fatigue materials such as steel and concrete, he noted. “The building will not collapse from this, but it is possible to develop cracks in steel and concrete that you would not be able to see by eye but that can affect the performance of the structure.”
In his SSA talk, Celebi discussed ways to retrofit and design tall buildings to account for these long-distance quake effects.