Utah Engineers Journal 2021 Issue

59 Figure 3 the Salt Lake Valley. More importantly, the intensity of shaking will be much higher than experienced in the Magna Earthquake. There will be large areas with severe (dark orange) and violent (red) shaking. The strong (yellow and orange) shaking will extend into surrounding counties. See Figure 4 to see the expected extent of the yellow, orange, and red shaking intensities from the USGS scenario. As you can see, a magnitude 7 Wasatch fault earthquake will not be a repeat of the Magna Earthquake. A Wasatch fault earthquake will be very impactful. Which areas will see the strongest shaking, and how strong will that be? What level of shaking will your home or work location experience? These are great questions, but unfortunately, they are impossible to answer accurately. Earthquakes are similar to children. Just as children who are the same age vary in height, earthquakes can produce shaking levels that vary in intensity. In a Wasatch fault earthquake, you may be one of the lucky ones who experience a lower level of shaking than someone who lives nearby. A relative who lives several miles away may experience a much higher shaking intensity. To help you visualize shaking variability, think of a very short 5-year-old girl who is in the 16th percentile in height for her age. There is a low probability (16%) that when she meets someone her age that they will be shorter than she is. Most girls her age will be taller than she is. Now visualize a very tall 5-year-old girl who is in the 84th percentile in height for her age. At 84%, it is very likely that when she meets a girl her age, the girl will be shorter than she is. It is still possible that she might meet a girl her age who is taller than she is, but it is not very likely. Let us now relate this example to predicted earthquake shaking levels from a magnitude 7.0 earthquake on the Wasatch fault. Scientists can provide us the median predicted shaking level for each location. An actual Wasatch fault earthquake has a 50/50 chance of causing a shaking level at your location that will be lower than this median predicted level. Scientists can also predict what the 16th percentile shaking level will be at your location. There is a high likelihood that this level of shaking will be exceeded (84%) in a Wasatch fault earthquake. The 84th percentile shaking level has a low likelihood that the Wasatch fault will generate a shaking above this level (only 16%). Just as children are not all the same height, the same is true of a Wasatch fault earthquake. Not all shaking intensities will be the same. However, just as there are short and tall children, shaking intensities will be higher and lower than the median predicted shaking levels. The lucky ones will have shaking intensities lower than the median predicted, and the unlucky ones will experience shaking intensities higher than the median predicted. Continued on the following page • 2011 Christchurch M6.1: https://earthquake.usgs.gov/earthquakes/eventpage/usp000huvq/map • 2020 Challis, ID M6.5: https://earthquake.usgs.gov/earthquakes/eventpage/us70008jr5/map • 1994 Northridge, CA M6.7: https://earthquake.usgs.gov/earthquakes/eventpage/ci3144585/map • 2010 Christchurch M7.0: https://earthquake.usgs.gov/earthquakes/eventpage/usp000hk46/map • 2019 Ridgecrest, CA M7.1: https://earthquake.usgs.gov/earthquakes/eventpage/ci38457511/map • 1992 Landers, CA M7.3: https://earthquake.usgs.gov/earthquakes/eventpage/ci3031111/map All accessed March 29, 2020, annotated. How do buildings respond to various shaking levels? We saw from the Magna Earthquake that old brick buildings (URMs) can be heavily damaged from yellow (strong) shaking.