Utah Engineers Journal 2021 Issue

58 Figure 1 2020 Magna, Utah, M5.7 — Did you feel it? Responses: https://ear thquake. usgs.gov/ear thquakes/eventpage/uu60363602/map?dyfi-responses- 10km=trueandshakemap-intensity=false, accessed March 23, 2021 The “Big One” — A Wasatch Fault Earthquake and Its Effect on Buildings Brent Maxfield, S.E. and Eric Hoffman, S.E. M any of you reading this article experienced the shaking from the Magna Earthquake on March 18, 2020. You may have also wondered how it compared to what we would experience in the “Big One.” During the Magna earthquake, the shaking was felt from Utah County to Cache County and beyond. If you experienced the earthquake, how would you describe the shaking that occurred where you were? Was it weak, light, moderate, strong, severe, violent, or extreme? How long did the shaking last? Each reader’s answer to these questions will likely be different depending on where they were during the earthquake. This article will discuss earthquake shaking, earthquake magnitude, and building performance for specific levels of shaking. It will also help paint a picture of the substantial effect on buildings from a Wasatch fault earthquake, which has the potential to generate shaking levels significantly higher than the building code requires new buildings to be designed for. Figure 1 maps how people who felt the shaking reported what they felt. The blue colors are weak to light shaking. The green colors are moderate shaking. The yellow colors are strong shaking, and the orange colors are very strong shaking. (See the key in Figure 2.) Notice how the yellows and orange colors are clustered close to the epicenter. We learn several things from this map. 1. Shaking varies in every earthquake. Every location does not experience the same shaking. 2. Shaking decreases with distance. 3. T he shaking was not severe (dark orange), violent (red), or extreme (dark red). 4. Earthquakes cause the ground to shake, and it is important to understand the shaking intensity in addition to the magnitude of the earthquake. Almost all the damage occurred in areas that experienced the yellow (strong) and orange (very strong) shaking. Most of the damage occurred to unreinforced masonry buildings (URM), which are very prone to damage or collapse in earthquakes. Figure 3 shows shaking intensities for various recent earthquakes, and Figure 4 shows the Magna Earthquake and a Scenario Wasatch Fault 7.0 earthquake generated by the U. S. Geological Survey (USGS). Notice the variations in shaking in each of these earthquakes. The scales and areas are identical in each of the images shown in the two figures. Earthquake magnitude is important because it can tell us two very important pieces of information. The first is how large of an area was affected by the earthquake. The second is about how long the shaking lasted. The Magna Earthquake was a moderate M 5.7 earthquake. It was felt over a large area but only caused damage in a relatively small area. It also only lasted a few seconds. By contrast, a magnitude 7 or larger earthquake on the Salt Lake City segment of the Wasatch fault could cause shaking that lasts 60 seconds, and the area affected will extend well beyond SEAU