Aug18

Experiencing the Big One

Seven Years Living and Designing in Japan

Ichi, ni…ichi, ni… (translation: one Mississippi, two Mississippi…) I’m at home alone and counting to myself during one of the many aftershocks of the day resulting from the giant March 11th Tohoku earthquake and tsunami. The main seismic response of my eight story apartment building was in the short north-south direction as expected, with a period of about two. I’ve studied earthquakes for my entire professional career, so I was excited to experience such a large one (and its aftershocks). But with my five months pregnant wife still unaccounted for, I was also quite nervous.

The Great East Japan Earthquake struck 70 kilometers off the coast of Japan on March 11, 2009 at 2:46 local time and more than 230 miles northeast of Tokyo. The earthquake struck during the siesta hour after lunch. As if being shaken awake while dozing off, the room started moving. The shaking continued until finally a sudden jolt threw us to one side. At that instant we knew this earthquake was a big one. The question in my mind was where was the epicenter of the quake.

Earthquake seismic intensity was immediately logged online, as different sensors throughout Japan (and the world) picked up the quake. It was then that we knew that Tokyo only experienced a little shake, while Tohoku went through six minutes of severe shaking. Tsunami warnings also flooded the internet immediately following the shaking. The warnings were constantly updating for a larger and larger tidal wave. None predicted the tsunami would reach a maximum height of 128 feet, dislodge ships, travel six miles inland, and cause a nuclear meltdown at Fukushima Daichi.

Over the next month, food disappeared from store shelves, television advertising was completely canceled (who selfishly promotes their company during a tragedy?), and television comedies were cut. 

How Much Did I Shake?

Engineers work with numbers all day: 1000 kip column loads, 12,000 psi bearing capacity, 2500 k-ft moment. What do all these values actually mean? After this earthquake, I had a similar question. How much did I just shake during this earthquake? It took just a couple hours to find out.

Many sources provide colorful contour maps showing the general level of shaking in different regions. The graphic below provided by K-NET shows peak ground acceleration close to the earthquake of 2000 gal (or about 200% g). The level of shaking in Tokyo is shown at 200 gal (or about 20% g).

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The building where I worked was fitted with a seismic monitoring program like many buildings throughout Japan. These programs measure the accelerations, drifts, and even stress/strain in critical members on different levels of the building. In addition to being valuable data for research, these readings are also used to judge whether the building was damaged during the earthquake and highlight areas that might need repair. 

seismicmonitoringaccelerationsensor.jpgConsidering I was in Chiyoda-ku of Tokyo, the level of shaking was considerably less than what was felt closer to the epicenter. The seismic intensity I felt was a Level V on the Japan Meteorological Agency (JMA) scale, which corresponds to a VII/VIII on the Modified Mercalli Intensity (MMI) Scale. 

The first floor of my building shook a maximum of 13% g and 22% g at the 14th floor. The maximum displacement at the roof was 1.25 inches (h/500) and the maximum inter-story drift was 0.3 inches (1/500). The buckling restrained braces (BRB) with a low yield stress of only 14.5 ksi responded elastically. The viscous walls dampers were engaged and provided some supplemental damping in the building. I was on the second floor at the time, so I only shook about 13% g, and I certainly felt a big jolt.  

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The Buckling Restrained Braces (BRBs)
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Stress-Strain Monitor of the BRBs

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The Viscous Wall Dampers

The time history chart shown below is the actual time history record from the 5th floor of the building where I was working at the time of the earthquake. It is an excellent tool for understanding the intensity of shaking I felt on March 11th

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For this building in Tokyo, the level of shaking was much smaller than the design earthquake, and the building was well within the elastic range of response. As with most damage in earthquakes, the damage was concentrated to non-structural components like an extremely messy desk.

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In general, buildings in Tokyo only suffered cosmetic damage while a handful of buildings in Tohoku suffered collapsed. 

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  A building close to my old home in Tokyo
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    A collapsed building in the Tohoku area

Seismic Awareness in Japan

At one time in my life I was counting how many earthquakes I had ever experienced. My first was well after my graduate studies and only until after moving to Japan. Now I have completely lost count. Earthquakes are a fact of life in Japan and the general population is very well aware of the risks.

Japan classifies the seismic intensity based on location. While the Richter scale is occasionally used in Japan, the Richter scale is limiting in that it only conveys the amount of energy released at the epicenter. Japan’s seismic intensity system, which measures the degree of shaking by location, is called the Japan Meteorological Agency (JMA) scale. While the US has a similar system called the Modified Mercalli Intensity (MMI) scale, the wide prevalence and use of the JMA system sets it apart. News programs commonly flash with updates of the level of shaking in different regions after an earthquake highlighting the seismic intensity on the JMA scale. Shindo means “degree of shaking”. A common news bulletin would read as follows:

“Tokyo Shindo V, Chiba Shindo IV, Nagano Shindo VI, etc.”

Our home was even outfitted with an early warning device showing how much time we had before a certain level earthquake approached. These devices measure the pressure waves (P-waves) that travel faster than the shear waves (S-waves) which actually cause the ground shaking. It went off so much during my time in Japan that I had an easy time catching the photo below during a small Shindo III earthquake. 

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My old home's seismic alert system

Some developers even market their buildings with seismic isolation as the key building feature, not views, square footage or amenities. These buildings command premium sale prices, which reflects that the general population is well aware of their benefits. 

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An advertisement on the subway in Tokyo for a newly constructed condo

Leave a comment below if you know what this advertisement is showing as the main building feature. Hint the picture shows a component of a base isolation system and an extra picture is added for clarity.

References

Celebi, M., & Okawa, I. (2014). Drift Issues of Tall Buildings During the March 11, 2011 M9.0 Tohoku Earthquake, Japan - Implications. Tenth US National Conference on Earthquake Engineering Frontiers of Earthquake Engineering. Anchorage Alaska: NEES.

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