By Jerod Johnson, PhD, SE, LEED AP – Structural Principal Engineer
Courtesy of Structural Engineers Association of Utah
Author’s Note
This article quickly became a much larger topic than initially envisioned…so much so that each of the concepts listed below has been reduced to shortened summary. Each of the 10 ideas in and of itself could be fodder for a separate newsletter article and the readers are invited to consider the concept and maybe even offer up thoughts of their own for a newsletter article. I’m certain the newsletter editor would welcome the material!
March 18, 2023 marks the third anniversary of the Magna Earthquake. How time flies! It seems fitting to look back upon what has happened since that day and contemplate what we learned, how we’ve grown, and how this event has (hopefully) prepared us for the future. As I think back on my own experience of the last three years, a flood of fundamental ideas came to mind. As I made my notes, I hit the magical number of 10 and decided that was a good place to stop (yes, I’m a fan of David Letterman!). So, I offer some thoughts and observations on the Magna Quake…3 years later.
We are Not Prepared
Emergencies, by nature, do not happen at a convenient time. Despite our best preparations and pre-emptive efforts, the next earthquake, just like Magna, will not happen when it is convenient for us all to drop what we are doing and move into a post-earthquake reconnaissance mode. Even still, with the Magna Event 3 years in our rear-view mirror, we at least have some notion of what to expect for the next event. Maybe we even have post-earthquake evaluation forms or boiler plate reports ‘at the ready’ for the next time they are needed. Even better, the Magna Event may have prompted us and some owners to embrace the Building Occupancy Resumption Program (BORP). If not, I suggest you give it some thought. Presuming the next earthquake will not impede your ability to work, it starts with some simple questions; What are the tasks you must undertake in the immediate aftermath of the next earthquake? Which clients do you expect to be giving you a call within minutes of the shock? Do your clients presume you will be there for them and that they are at the top of your priority list? Is there someone in your office whose job it is to coordinate the post-earthquake evaluation effort and field the flood of calls coming into your office? Unless you are a charity, do you have a mechanism for communicating your expectations for fee?
I believe we all appreciate that Magna was a very small event, nowhere close to the ‘design level’ event captured within the context of the current code. The ‘design level’ event will throw us into a scenario far more grave than Magna with consequences and outcomes difficult to imagine. However, from a practical perspective, another ‘Magna Event’ is the more likely scenario. This notwithstanding, how can we prepare for something bigger (not if, but when it comes)?
Modern Engineering is Pretty Good
I hope that lead-in statement sounds optimistic, as I believe we all should be. From many perspectives, the Magna Earthquake was a perfect snapshot of progress in seismic resistant design and its advancements over the last 50 years. Think of the Magna Event Photos and the inventory of unreinforced masonry buildings that expressed the majority of incurred damage. I know my experience may be unique, but the vast majority of damage I saw was in buildings that predate modern standards of construction. Historic buildings that had been retrofitted performed as well as their modern counterparts. Probably my biggest ‘pleasant surprise’ was that the seismically isolated City and County Building showed evidence of isolation system displacement matching with near exactness the predictions of mathematical models when using the nearest recorded accelerogram (see https://www.structuremag.org/wp-content/uploads/2021/04/282105-SF-SaltLake.pdf).
Textbook Behaviors
Think of your mindset of March 17, 2020. If someone would have asked you what to expect in the event of a significant earthquake, what would your answer have been? I’d venture to say that this is a question that confronted many, if not most of the audience reading this article prior to the Magna Event. What were your answers then? Did they change in the aftermath of March 18, 2020? From my perspective, the answer is an emphatic ‘No’. The huge inventory of unreinforced masonry buildings (URM’s) was and continues to be the biggest threat within our built environment. As such, principal forms of damage expressed from the Magna Event were no surprise: collapsed URM chimneys, fallen URM parapets, destroyed URM shear walls, ‘X’ pattern URM wall cracking, separation of wythes in multi-wythe URM walls, veneer separation from bearing walls, collapse of URM partitions, failure of diaphragm to wall connections (typically in URM walls), and of course – the collapse of “non-engineered” shelving systems.
Beyond the classic failure scenarios are examples of “exactly what was expected” that became manifest in the Magna Event. My direct observations included symmetric buildings lying exactly East of the epicenter. In-plane damage to east-west running short (squat) walls was severe while the north-south running walls in the same building were relatively damage-free. For buildings directly south of the epicenter, it was the north-south running walls that experienced the damage. Likewise, the near-field effect issue raised its ugly head. The now infamous Colosimo’s Deli Photo comes to mind along with sand boils near the south edge of the Great Salt Lake.
Some Unexpected Surprises
I was taught long ago that there is no such thing as a “normal earthquake”. In fact, the professor instructing me told me to never use that term. Every earthquake is different and has anomalies that don’t quite add up. In other words, we should expect the unexpected.
A recording station in Taylorsville marked spectral shaking intensities in the range of 0.91g (at a period of 0.33 seconds, see Figure 1)! This is at a distance of nearly 15 miles from the epicenter. Why so high an acceleration at a site so displaced from the epicenter? Was the instrumentation in error? Evidence prepared by geoseismic engineers suggests that there are pockets of soils throughout the central region of the Salt Lake Valley for which the attenuation of seismic waves becomes particularly amplified at certain periods. This spectrum and the recorded accelerations are accurate to the best of our knowledge. However, current codes and spectral acceleration maps do little to recognize this potential phenomenon. Long story short; there is something to learn from every earthquake and every significant earthquake seems to reveal shortcomings in our current methodologies.
USGS Tools You’ve Already Forgotten About
Are you ready to be deputized by a local building official to undertake building evaluations? How are you on your ATC 20 Credential (or your CAL OES equivalent)? Is it current? Does your jurisdiction have a process for renewing your credential? Is there something you can do on the political side of things to help your community be prepared to deputize dozens or maybe hundreds of engineers far and wide to deal with the earthquake aftermath? Will you have to quickly re-learn what it means to ‘red-tag’, ‘yellow-tag’, or ‘green-tag’ a building in a post-earthquake scenario?
Following the procedures of IEBC 2018 many of us were forced to become experts in quantifying earthquake damage. Does the term “ShakeMap” ring any bells? Knowing whether a structure was subject to mandatory retrofitting was a question coming up in the post-earthquake evaluation of many structures. Owners and insurers alike were keenly attuned to whether certain damage thresholds were surpassed. Terms like “Disproportionate Structural Damage” became the order of the day. In fact, this concept became particularly salient given the relatively low magnitude of this earthquake. In other words, was the amount of damage sustained by the structure high in comparison to the shaking intensity? On that note, what was the shaking intensity at the site in question? A hard question to answer unless an active, accurate, and functional accelerometer was at the site in question. Tools such as USGS’s ShakeMap, through use of very sophisticated algorithms, took acceleration data available from operable recording stations throughout the region and connected it to the websites providing gridded values of 0.3 second spectral accelerations for use in shaking intensity assessment (per IEBC 2018). It was then up to the examining engineer to determine the scope and scale of damage to compare against that shaking intensity.
Fun with Insurance Companies
Insurance is a business, just like any other. Businesses exist to make money by providing a service and of course, the best strategy for success is to provide that service with as little expense as possible. Ergo, it is in the nature of insurance companies to minimize the perception of damage. Likewise, the aftermath of owners spotting damage caused by the earthquake was overwhelming. How many of you looked at cracks “caused by the earthquake” or cracks “never noticed before” only to find that they were filled with paint, caulk or even spider webs? Whether new or existing, engineers far and wide became the arbiters of whether a crack was new or present before the quake. Crisp, clean cracks with debris on the floor below became the telltale sign of what was and wasn’t caused by the quake. If we were lucky, an owner might have had “before and after” photos. Would a high-resolution video surveillance log be in order for the facilities managers of today? Such video could be very useful after the next quake.
A Blessing in Disguise
Is there such thing as a ‘good’ earthquake? One could argue that there is no such thing, but I would argue that the Magna Earthquake was indeed a good earthquake. I draw that conclusion from the fact that there were no deaths. In fact, I can’t even recall claims of injury. Truly a remarkable fact given the damage that was observed. The onset of pandemic hysteria caused the streets to be mostly vacated but even still, this earthquake produced far fewer casualties than might normally be expected. At the same time, it threw into particularly sharp relief the nature of threat that we’re facing. As a society we now have a small taste of what to expect and we can do something about it. I am convinced that some of the structures now being retrofitted because of Magna would not have survived shaking intensities even slightly higher. We now have an opportunity to preemptively fix or replace vulnerable structures whose weaknesses became clearly evident because of the Magna Quake. I believe we will look back on some historic buildings and realize they were in fact saved because of the Magna Earthquake.
We Can Offer Hope
If you’re a structural engineer who has not been accused of being a doomsday preacher, then you’re not doing your job. OK…doomsday preacher may be a little strong, but the Magna Event did provide us some vindication. The doom and gloom scenarios are difficult to deliver, but I think they’re even more difficult to receive. How do we craft our message? I draw from my observations of professionals like Lucy Jones (Nationally Recognized Seismologist) or Judith Mitrani Reiser (Associate Chief of the Materials and Structural Systems Division, NIST). I invite you to Google Search either of these individuals. These professionals have a certain knack for delivering the earthquake hazard message and doing so without coming across as too threatening or overbearing. They deliver the message, oftentimes with a smile, and leave the audience both informed and hopeful.
How do you deliver the message? Is it a message of doom and gloom or is it one of hope? My suggestion: acknowledge the threat but couple it with the good news that we can do something about it. Further, advancements in earthquake engineering mean that threats to life are not nearly as large as they once were. How many died in San Francisco 1906?…about 3,000. How many died in Loma Prieta 1989?… 63. Wow! That’s a ratio of 50 to 1! What progress! Yes, these were earthquakes of differing magnitudes, but the logic still stands.
What We Can Control
The Magna Earthquake and examination of data in the aftermath have reaffirmed to me that our science has grown leaps and bounds in the ability to characterize the potential earthquake threat. At the same time, it is a rapidly evolving science and there is still much to learn and many gaps to fill. Predicting the nature, and intensity of ground shaking is not an exact science. The presentation by Brent Maxfield and John Richardson in the recent SEAU Annual Education Conference shed light on this issue. They showed the significant range of ground shaking intensities that are possible in a Wasatch fault earthquake and discussed that it is not possible to predict the exact ground motion intensity at a specific site from a Wasatch fault earthquake. They also showed that our code design values are significantly lower than the potential shaking that is possible from a Wasatch fault earthquake.
The Magna Event produced some spectra that might seem to reflect an event much larger than M5.7. Figure 2 shows a spectrum derived from an acceleration recording at the Lee Kay Center (believed to be the closest available recording to the epicenter), compared against the code-prescribed 2/3*MCER design spectrum and the MCER spectrum for the same site (ASCE 7-16, Site Class D). This seems to suggest that this location received ground shaking intensity that was closer to the predicted 84th percentile ground motion for this M5.7 earthquake. However, spectra from many other recordings for the Magna Event seem to show ground shaking intensities that were closer to the predicted median ground motion for an M5.7 earthquake. This is a great example of the fact that there is no direct correlation between energy release (magnitude) and the intensity of shaking at the surface of the earth (g’s of acceleration). The Magna earthquake also illustrated that 84th percentile ground motions are possible. If a 5.7M earthquake can generate MCER level shaking intensities, imagine how much larger intensities will be generated from a M7 Wasatch fault earthquake. Further, we have no control of where, when and how the earth will shake. While predicting the nature of ground motion is a worthy pursuit that should be further advanced and developed, we should at least be equally (or perhaps even more) concerned over how structures we design respond to shaking. The arsenal of tools and methods we have to promote stable and ductile structural response to seismic accelerations is greater than it has ever been. I’ll sum up this point by simply asking; How will your design respond? Is the response you expect ductile, or is it brittle?…food for thought.
Congratulations, You are Now Retrofitting One of My Designs!
I heard an anecdotal account of a highly respected structural engineer speaking in a recent conference. This individual was at the point in his career to have witnessed the largest leaps in earthquake resistant design in history (over the last 50 years). This gentleman was responsible for the design of hundreds, maybe thousands of structures. The rapid evolution of seismic resistant design means that this individual has been around long enough to see that many of his designs are now seismically deficient. Sadly, we are all victims of the time-frames in which we live. This is true for the clothes we wear, the music we listen to, the food we eat and yes, the codes that we design to. No matter our prowess with current codes, no matter our skills with computer modeling or even our keen insights into the latest methods of design, our seismic design work will likely become obsolete. I hope that doesn’t sound negative…even with the major advancements we regularly must embrace, many of the leaps of the recent past have equipped our buildings with performance capabilities far beyond what they would have been even as recently as 50 years ago. In all candidness, I do not lose too much sleep over structures I designed under the 1997 UBC. Compared to what I design today, I think the ’97 UBC was quite good. This notwithstanding, I believe there is a decent likelihood that most of us will be in this field long enough to see our work eventually become obsolete with at least some degree of seismic deficiency.
In conclusion, I invite you to ponder your experience of the last three years. Are we better equipped today than we were on March 17, 2020? At this point I’m drawn to think on the lessons of Dr. Larry Reaveley. I remember Larry being absent from his role as Chair of Civil Engineering at the University of Utah in the immediate aftermath of Northridge 1994. Larry was among the ‘boots on the ground’ observing the damage of countless structures. Upon his return to Salt Lake City, he had scores of photos to share. Larry’s passion for learning was contagious and he would often say; “you can learn more about structural engineering from a building at the brink of collapse than from any other source”. Larry was a firm believer that by observing damage, we could learn far more than anything from a lecture or a textbook. So, what did you learn from the Magna Earthquake?
For the original post, visit: https://seau.org/articles/Mar_2023_Magna_Earthquake_A_Retrospect