Geotechnical Earthquake Engineer, Professor Jonathan Stewart, Answers Questions Regarding His Trip to Japan
By Wileen Wong Kromhout
Jonathan Stewart, professor and vice chair of the department of civil and environmental engineering, traveled to Japan as part of a four member Geotechnical Extreme Events Reconnaissance (GEER) team only two weeks after the devastating Tohoku earthquake. Stewart’s team was the first of six that would travel to Japan to study the effects of the earthquake.
A GEER team, sponsored by the National Science Foundation (NSF), is typically sent to collect perishable data in a systematic way to help advance the understanding of a recent disaster immediately following its occurrence. The extreme events investigated by GEER are not limited to earthquakes, but can also include tsunamis, hurricanes, landslides, and floods.
Stewart has led or co-led several recent expeditions with GEER including one to Greece in 2008, Italy in 2009, and the Imperial Valley/Northern Mexico in 2010. His research expertise lies in the area of geotechnical earthquake engineering, with emphases on seismic soil-structure interaction, earthquake ground motion characterization, and seismic ground failure.
His research includes the interpretation of earthquake strong motion data to gain insight into soil-structure interaction effects, site effects, regional variations of ground motions, and other effects; cyclic field testing of full-scale foundation components including mat foundations, drilled shafts, and bridge abutment walls; advanced dynamic testing of soils in the laboratory; and case history studies of the seismic field performance of sites in California, Taiwan, Turkey, Japan, Greece, Italy and India.
Results from his research group are widely utilized in engineering practice and are included in standards used by the American Society of Civil Engineers (ASCE) for new structures, retrofitting of existing structures and additional guideline documents for landslide risk and tall building design.
Among his many awards and honors, Stewart was elected Fellow of ASCE in 2009, was winner of the Walter F. Huber Civil Engineering Research Prize in 2008 and the Shamsher Prakash Research Award in 2006 and was selected to be a U.S. Department of State Fulbright Scholar and senior lecturer in Italy in 2005.
Tell us a little about GEER and its goals.
GEER was established five years ago to systematically coordinate the work that geotechnical engineers, geologists and seismologists do after earthquakes, which is to collect perishable data for a central repository that will be archived and disseminated in a consistent format and always available for people to use.
Our field of geotechnical engineering and geology is an experience driven profession. While we utilize physics-based models for different seismic phenomena, there are tremendous uncertainties associated with many of the parameters in those models. The field performance is used both as a reality check of our models and to constrain the models through observations of what earthquakes actually do.
So it’s essential, when you have a major event like the earthquake in Japan, to get people who know what to look for there and record information that will disappear usually within weeks to months after the earthquake as the infrastructure is reconstructed and the evidence disappears.
In the longer term, we return to the affected areas and perform subsurface exploration to evaluate soil conditions, look at the ground motions in the area, and build case histories that we can learn something tangible from.
So this type of work is truly essential to move the profession forward. If you look at the history of earthquake engineering, following a major event there’s a big ramp up in understanding of the underlying physical processes driving earthquake damage. Engineering practice gradually adopts these lessons, and public safety ultimately is ratcheted up as a result of the process.
So what did the GEER team do in Japan?
This was a major event for GEER. There have now been six phases of groups that have gone over there. I was in the first phase and there were four of us. Three of us have a lot of experience doing this type of work. We’d been to many earthquakes around the world. One was more of a newcomer. GEER works to partner young researchers with more experienced researchers to facilitate knowledge transfer.
The people on this trip were Ross Boulanger from UC Davis, Scott Ashford from Oregon State, Jennifer Donahue from GeoSyntec and myself.
Coordination with the Japanese was critical in all of this. Before we got on the plane, we knew who we would be working with on the Japanese side. They were people we have worked with for many years. We met with them the first night, received a very useful briefing, after which we jointly planned our activities.
Everything we did was in coordination with Japanese researchers. These were the top people, which results from the fact that we’ve been cultivating these relationships for decades.
Where were you actually able to go to at that time? Was your travel limited because of the devastation and issues with the Fukashima power plant?
Because we were there in the early stages, there was still a very active process of recovering the deceased in the northern areas around Sendai and in the coastal communities north of Sendai, where the tsunami effects were most severe. Because of that, there was no travel allowed into that area for anyone other than search and rescue teams. We did not go north of the south end of the Fukushima exclusion zone.
The areas we visited are within the Kanto Plain, which is the largest flat geomorphic plain in Japan, which includes Tokyo and its suburbs, many port regions along the coast, a substantial portion of Japan’s heavy industry, and agriculture.
We tend to think of this earthquake as being up by Sendai and areas to the north and certainly there was a lot of slip in the fault there but the fault slip extended all the way down to regions offshore of the Kanto Plain as well. Some of the areas we were at experienced very strong ground motion. We were in areas that experienced up to 0.8g horizontal accelerations, and there were severe effects of the shaking in those areas. Tsunami effects were evident in coastal areas, but tsunami damage was relatively modest as compared to areas further north.
The strong ground shaking caused a lot of liquefaction. The liquefaction affected levees and caused building settlement and damage to utility lines (water, sewer, etc.).
There were few collapsed structures, and by enlarge, the structures did well from the foundation level up. The foundations, however, experienced substantial settlements, making many structures uninhabitable, causing very difficult living conditions for the local population.
Were lives lost in that area?
There weren’t many collapsed buildings, which is usually the biggest cause of loss of life. In these Tokyo suburbs, there was no substantial tsunami damage. At some of the ports, a little further north, there was tsunami damage and there may have been some people swept away. I don’t think too many because they had the early warning system and there was time. These folks had about 30+ minutes as opposed to 10-15 minutes for those further north. So from what we were hearing, they were able to get out of harms way before the tsunami arrived.
But your group wasn’t looking at the effects of the tsunami…
Yes, there are other groups specifically tasked with looking at the tsunami run up and its effects on structures and other infrastructure. Our focus was on the effect of the earthquake on the civil infrastructure outside of the tsunamic zone, with an emphasis on geotechnical engineering and geological aspects. We looked for liquefaction, non-liquefaction, landslides, areas with concentrated damage that might be attributable to geologic effects, and unusual soil-structure interaction.
Liquefaction involves the soil losing strength and when that happens, it can create landslides on nearly flat ground. Just a very slight slope and you can get a major landslide. We also see buildings sinking into the ground, pipelines and manholes floating to the surface, etc.We’re trying to understand where these things did and did not occur and to map the severity of the damage.
Now that the reconnaissance activity is behind us, we’re planning the next phases of work where we start to explore the soil conditions, and again all this is done in collaboration with the Japanese. They’ve already started doing some of that themselves so we’re trying to see what are some resources we can bring to the table to facilitate effective collaboration.
Would you be returning to Japan with the same team?
For the subsurface exploration that will happen in the short term (next few months), other people will probably be brought in, but I will be in close contact with them to help manage their activities. Subsequent to that is when you can have a more traditional research project. After the Kobe earthquake, for example, they had a U.S./Japan coordinated research program where U.S. investigators submitted proposals to the NSF and the Japanese submitted proposals to their equivalent agency. Both would then get funded and then you’re working together on the field work, analysis, and evalually publication of the results.
What is California’s tsunami risk?
Southern California will not see a tsunami of the same scale as the one that occurred in Japan. What caused the tsunami was a very large offset of the sea floor. It was around 50 kilometers off the coast of Japan, and what occurred is a lift the seafloor creating a series of large waves. We don’t have that type of plate boundary in California other than the area north of Mendocino. We do have that type of plate boundary extending north from Mendocino and off the coasts of Oregon and Washington.
That said, we do have tsunami risk in southern California. Our tsunami risk comes from earthquakes on the subduction plate boundary in the Pacific Northwest, which will produce tsunamis here. In addition, we have some tsunami risk from offshore landslides. In both cases the wave heights are much smaller than we saw in Japan.
What is liquefaction?
Liquefaction is a phenomenon in which saturated soil loses substantial strength and stiffness in response to rapidly applied stress, such as from an earthquake. The weakened soil can in some respects behave, at least temporarily, as a liquid. The soils in the San Fernando Valley, the L.A. basin, out in San Bernardino, Orange County, often have a sandy composition. When coupled with shallow ground water, there is a susceptibility to liquefaction. We’ve seen significant effects from liquefaction in L.A. before, for example in portions of the San Fernando Valley during the 1971 San Fernando and Northridge earthquakes.
What were you hoping to learn in Japan? What was most surprising to you?
I expected to see a lot of liquefaction and we did, but some observations surprised me in this earthquake. One surprise related to liquefaction was not so much the occurrence of liquefaction but its effects, which were more severe than I would have expected. I’ve seen lots of liquefaction over the years as I’ve done reconnaissance in Taiwan, here in California, in Turkey and other places. Now thinking about the effects of liquefaction on buildings, what I’ve generally seen is relatively tall buildings (typically 3-6 stories) punching into the ground by amounts ranging from ten centimeters to one meter or more. What surprised me in this earthquake was the fact that we had some very light buildings settling substantially, up to half a meter or more in some cases.
One case that really stands out is a little Seven Eleven store, one story, very light frame, weighs practically nothing. The building settled half a meter, which is remarkably severe. This type of foundation performance is unique in my experience.
Another observation that surprised me was levels of ground motion that were high for an earthquake that was relatively distant from the recording stations. If you measure the distance from the coastal areas of Japan down into the earth to the fault plane, those distances would be 50 to 60 kilometers and up. We would generally expect modest levels of acceleration at such distances, even for such a large magnitude event, but what we find here are multiple ground motions with peak accelerations of over 1g. That came as a surprise and there is a significant amount of speculation as to why that might have occurred.
And then somewhat in contradiction to those very high ground motions, another surprise was that the structures did well. Normally when you shake the ground strongly, some percentage of the structures are going to come down, particularly older buildings that were not properly designed from a seismic standpoint. The good performance of the vast majority of structures outside the tsunami zone is a testament to the strict building codes in Japan and their enforcement.
So in regards to infrastructure, do you think the Japanese were prepared?
I think they’ve done a good job in terms of having an effective building code and enforcing it during design and construction. They design their structures for lateral loads, they have detailing requirements in their code and that seems to be working well. They obviously mis-interpreted the tsunami wave height that could occur in particular areas from Sendai to the south and a lot is going to be written about that. That cost a lot lives and it caused the problems at the Fukashima nuclear power plant.
On the liquefaction side, I don’t know if there are land-use requirements that address liquefaction hazards, but it would seem that there could be some improvements in this area. What such requirements could do is lead to recognition of liquefaction problems and implementation of mitigation. There were some areas we saw that had been improved. For example, at Tokyo Disney there was improved ground that performed pretty well. But areas around it, which is unimproved reclaimed land in Tokyo Bay, generally did poorly.
In California, we have legislation in which we map out liquefaction regions. Anybody can go on the web and look up these maps and see if their house or their place of business is in a liquefaction zone. We require engineers when they are working on a project to look up their project site on state maps and see if the site is in a liquefaction or landslide hazard zone. If you are in such a zone, you must perform an investigation to see if the hazard exists, and if it does exist, mitigation must be performed. You can’t get a building permit if this process is not undertaken.
That is a forward looking piece of legislation that we think will be shown to be effective in the next major California earthquake.
What is your timeline for returning to Japan?
My situation is a little different than some of the other team members in that I actually already have several ongoing projects in Japan even before this earthquake. So I’ll probably be back in the summer. Apart from these other projects I suspect there will be some meetings on how to move forward with the longer-term research associated with this event.
What other projects did you have in Japan, prior to this?
Both of them are connected to a 2007 earthquake on the west side of Japan, near a place called Kashiwazaki. One of the projects is related to ground motion recorded at an array that’s in the Kashiwazaki nuclear power plant, which is the largest nuclear power plant in the world. They didn’t have any loss of control within the reactors, but they did have reactors that were out of commission for quite some time.
We were interested in that array because of some of the really unique characteristics of the ground motions that were recorded there that we’d never seen before. In that case we saw much larger ground motions on rock beneath the site and significant nonlinear effects of the soils as the seismic waves came up from the rock towards the surface. These are important effects that we need to be able to model effectively, but we had never before had observations at these strong ground motion levels.
We worked with Professor Tokimatsu from the Tokyo Institute of Technology to explore the ground conditions, obtain and properly process the data, and then perform the analysis we needed to understand what had occurred at that site. Ph.D. candidate Eric Yee is working with me on that project.
Another project that is just getting started is funded by the California Department of Water Resources. In this project, we are looking at levees in the Kashiwazaki and surrounding areas. We are going back through records that the Japanese themselves compiled of where repairs were made and where repairs weren’t made and trying to understand what were the conditions that gave rise to levee problems. The Tohuku earthquake is going to provide a wealth of information on levee performance as well. The Department of Water Resources is very interested in understanding the seismic risks and potential failure mechanisms of our own levees. Some of the particularly significant questions revolve around the relative levels of risk of levees on organic soils versus liquefiable soils or clays. The ground conditions underneath levees are rarely “good.” There are just different levels of “bad.”
What is the predictability of another earthquake in Japan in the next few years?
Well there is some debate on that point. One of the ways that we judge the likelihood of future earthquakes is how much accumulated slip has occurred on a fault that hasn’t been released in the form of an earthquake. So if you can imagine taking a piece of balsa wood with a hand on each side and you slowly offset it. The offset that you’re producing before it breaks is the slip I’m talking about and at some point the wood breaks.
What’s interesting about this subduction zone off Japan is that the rate of convergence is really fast. If you look globally, the subduction zone off the east coast of Japan has one of the fastest rates of relative plate motion at about 8 centimeters per year. They haven’t had an earthquake of this size for a long time, so there was a great deal of accumulated slip. The last major event off the coast of Sendai appears to have been in 869, although the interpretation of that event is a source of some controversy that I won’t get into here.
Let’s suppose for the sake of argument that the 869 event was the last major event over the central/southern portion of the 2011 fault plane. It’s now been around 1200 years since that event during which slip accumulates at 8 cm/year. This produces around 90 to 100 meters of slip, which is huge. The Tohoku earthquake released a lot of slip, but not 80 or 90 meters. It released up to about 40 meters or so, which is enormous, but it’s not as much as what may have accumulated since the last earthquake. If true, that would mean that the potential for another large event in this same region of the Japan remains high.
What do you think Japan is struggling with now that you’ve stepped away for awhile?
I haven’t been there since early April, but one of the main struggles in the areas I visited was getting the utilities reconnected. Moreover, hundreds and perhaps thousands of structures had liquefaction-related settlement problems that will need re-leveling.
There were other problems that they were fixing right away like levee instabilities. The river levels were low so even though levees had issues, there were no releases because the water level was down. So those problems are a long way towards being fixed already, at least in the Kanto Plain. As you go north, the recovery process is much more difficult, especially within the tsunami inundation zones. There are some challenging questions. Do they redevelop in such areas with known tsunami hazards or do they move up onto hillsides? So that’s going to be a big land use issue going forward.
Fukishima… well, that is an ongoing crisis. It’s fallen out of the news cycle but it is not a solved problem. They are managing the temperature through ad hoc measures, using seawater and flooding reactors, but they are not yet able to control the temperature within those reactor cores in the way that you would like, which is a closed loop system in which the water is circulated through the core. The problem now is that the cooling water is released to the environment in an uncontrolled way. It’s going to take awhile to sort this out because of the limited ability of people to work within and near the reactor structures.
As engineers and academicians, we tend to like to solve math problems involving physics-based models of the phenomena being studied. However, we can’t lose sight of the fact that there are huge unknowns in these models. We need to ground ourselves in reality, which in an earthquake engineering context, means observing field performance of infrastructure. Work of this type is challenging on many levels, but one of the things I take pride in at UCLA, is that we can do the high-powered physical and numerical modeling, but we also understand what are the key problems to study and how to properly calibrate and validate models based on field performance data from earthquakes or field experiments. This allows us to provide solutions to many of the real-world problems that are most vexing in our profession today. This work is all the more important in Los Angeles, which is the most seismically vulnerable city in the U.S.