Mexico City skyline
After being a structural engineer for almost 3 years now, I’m reminded how important and critical a role the structural engineer is in everyone’s life. Our daily job is literally to make sure that people are safe. When someone walks on the floor, that was made possible by a structural engineer who designed the floor for the right loading condition. When a wind storm hits and a house withstands powerful gusts, a structural engineer designed the building’s framing system to handle the massive wind loads. When an earthquake releases energy and a building endures seismic shaking for 1 to 5 minutes, a structural engineer designed that building to seismically perform during specific types of earthquakes common in the area and based on the site’s soil conditions.
In the field of structural dynamics and earthquake engineering, there are so many factors that are uncertain. It’s only after a seismic event that engineers can improve upon existing structural design standards. With new seismic information and reasonable approximation, seismologists, structural engineers, and geotechnical engineers come up with better measures to analyze building performance and the magnitude of seismic loads a structure can handle. Seismic loads on a structure depends on many different conditions, like the distance of the site from the epicenter of the earthquake; magnitude of the earthquake; ground acceleration; regional and local soil conditions. In this blog post, we will focus on local and regional soil conditions. Local soil conditions are the ones that are present at a building site and around the site of the structure, while regional soil conditions represent a bigger area where ground motion could be amplified before it hits a specific site.
Mexico City basin showing depth of basin and the extent of it (image courtesy: www.nature.com/Scientific reports part of springer nature)
Basin effects are regional impacts of the soil bed beneath entire cities. Soil basins are often generated because of softer soil deposits within hard and stiffer soils surrounding the basin.
Imagine the Mexico City basin as a bowl full of Jell-O, where the bowl boundary is the hard rock or stiff soil, while the Jell-O is very soft organic clay or relatively softer soil. Now one might wonder: “Why are these basins important to understand?” During the great 1985 M8.1 Michoacán earthquake, the Mexico City basin amplified the ground motions of the softer soil by a magnitude of 50 times because of this subduction zone earthquake. The basin’s soft soils also increased the duration of shaking to almost twice the duration of activity experienced at the hard rock site near the basin boundary. One important thing to take away from the Michoacán earthquake is that most buildings that collapsed were 8-20 stories tall within a period of around 2 seconds. Maximum amplification of shaking occurred closer to the 2-second time mark.
A soil basin can:
1.) Increase the intensity of shaking
2.) Increase the duration of shaking
3.) Filter and transform the shaking of ground to the natural frequency of the basin
Generation and propagation of wave trains at different speeds within the basin, and their multiple diffractions. (image courtesy: www.nature.com/Scientific reports part of springer nature)
How does this happen?
An increase in intense shaking occurs because of the soft nature of the interior basin soil. Take a bowl full of Jell-O and shake the bowl. The fluid in the bowl (Jell-O) is shaking way more violently as compared to the shaking of bowl. This is because of the soft nature of Jell-O which can move a lot more, and when it hits the wall of the bowl, the wave reflects back. This softness of soil can move more violently even when the shaking is not very intense. This explains the amplification of the intensity with the shaking. Similarly, the duration of shaking also becomes longer because the seismic energy is trapped within the basin. During the Jell-O experiment, even if you stop shaking the bowl, the Jell-O will keep shaking for a couple of more seconds. This is because the Jell-O cannot throw back the energy for the hard rock sites to absorb, so the energy is trapped in the basin. The seismic waves get reflected back from the boundary of soft and hard soil. You can see the same effect when you take a bowl full of water and drop a stone in it. The waves that are formed in the water reflects back from the wall of the bowl. This shows how the energy stays trapped within the bowl and how the duration of seismic event increases.
Watch DCI’s Jell-O Experiment 1
Basin amplification in Seattle during the Nisqually Earthquake (Courtesy of USGS)
Now comes the important part. Why did a majority of the 8 to 20-story buildings collapse? What did the basin do to these ground motions? When a rigid body (hard rocks in this case) shakes a relatively flexible body (basin), instead of shaking along with the rigid body, the flexible body shakes at its own frequency. You can see that in a pendulum. Try and shake the pendulum as fast as you can. The pendulum will always shake at its own natural frequency. Similarly, the basin will try to oscillate at its own frequency regardless of the frequency of shaking. For example, the duration period of earthquake in the Mexico City basin was an estimated 2 to 3 seconds. This earthquake duration closely matched the 2 to 3-second amplification time frame when the 8 to 20-story buildings collapsed.
Watch DCI’s Second Jell-O Experiment
The amplification of ground motion by the basin depended on the nature of the earthquake. Generally, subduction zone earthquakes are long-duration earthquakes with frequencies close to 0.5Hz or 2 seconds. The Michoacán earthquake was a subduction zone earthquake and because the frequency of the earthquake matched closely with the natural frequency of the basin, there was resonance between the basin and the earthquake. Resonance happens when you shake something close to or equal to the natural frequency of the structure. Take the pendulum with a long string and shake it violently. The pendulum will hardly respond to your efforts. But now shake it very slowly, and suddenly the pendulum starts responding to your hands. There is no magic behind it. You just fed the motion of your hand to the pendulum at the resonating frequency. If you continue to do so, the pendulum will no longer be under control. This outcome is not because subduction zone earthquakes are longer in duration, but because these earthquakes have long periods of vibration - this resonating effect is continued on for a long time. Slowly the intensity of shaking gets amplified significantly because the hard rock site is pumping energy into the basin constantly at the natural frequency of the basin. Eventually the pendulum is going shake violently and out of control.
Impact on response spectrum because of basin amplification
Seattle also has a similar basin which can amplify the ground motion intensity. The basin is capable of amplifying the seismic forces by 50% or more. It will also increase the duration of ground motion as compared to the hard rocks outside the basin. Currently these basin amplification factors are included in the performance based design of structures; analysis of the site’s expected ground motions are considered in the design to address these amplification factors.
Seattle basin (A)Extent of basin with respect to specific gravity of sediments and (B) Depth of Basin (Courtesy of USGS)
But apart from performance based design, amplification factors are not included in code level design. The process to adapt the amplification factors into code level design is difficult as the amplification depends on the depth of the basin as well. But as structural engineers, we should always consider such impacts while designing a specific structure.
Watch DCI’s Mola Model Experiment
Although no thorough research has been done, an M9 Cascadia subduction zone earthquake can cause a lot greater amplification of ground motions because of the long duration of subduction zone earthquakes. Its impact will be like what was experienced in Mexico City, but not to such an extreme magnitude. Studies have yet to confirm how big of an amplification can occur in Seattle during the M9 event.
So always remember, the ground you are walking on may not be as stiff as it feels, it could be Jell-O.
About the Author
Jinal Doshi is a technical designer at DCI creating structural systems for high-rise, residential, mixed-use, office, and hospitality projects. He has a natural interest in structural engineering and enjoys discussing design concepts, the behavior of material, and integrity of different systems. Jinal also enjoys sharing his technical knowledge and experience, which led him to writing his own educational blog called "Structural Madness."