Damage from the 5.7 Earthquake on 03/18/2020. Source: NBC
On the morning of Wednesday, March 18th, 2020, residents of the Salt Lake Valley (SLV) in Utah were woken by a sudden jolt that traveled all over the metropolitan area. In what ended up being one of the most powerful earthquakes to hit the region in nearly 20 years, the event was then immediately followed up by hundreds of aftershocks that plagued the valley. Many residents were surprised that such a seismic event occurred in the region but the reality is that Salt Lake City and its surrounding metropolitan area is well-within one of the most seismically significant and diverse regions in the entire country. Now that the ground appears to have settled, it only feels appropriate that we dive into the science behind the Utah’s seismic zone, along with what all else may be in store for the region in the coming decades.
The intensity map for the 5.7 earthquake along with the location of the epicenter. Source: USGS
At 7:08am MDT, a 5.7 earthquake struck north-northeast of Magna, UT, and its waves soon spread over all of Salt Lake Valley . It is here that we must first distinguish what that value represents. Indeed, the commonly-known Richter scale is a measure of the amplitude and distance of an earthquake. As can be seen in figure 2, the scale is logarithmic, meaning that the amplitude of waves that are recorded during an earthquake greatly affects the output value of an earthquake. This means that a 5.0 earthquake will be much stronger than a measured 4.0 earthquake. Nevertheless, this scale fails to capture signals from larger earthquakes, resulting in underestimations of the intensity of large earthquakes . As such, newer and more modern scales are used, including the moment magnitude scale (MMS), which takes both the intensity of an earthquake and the amount of energy that is released by one.
This amount of energy is referred to by seismologists as the seismic moment, and takes into account three variables: the resistance of a rock to being bent, distance, and area. The first variable takes into account the elasticity of rock; a lower resistance results in an earthquake being able to more easily bend the rock, while a higher resistance impedes the effects of that bending. The second variable refers to the distance that the ground moved or “slipped” relative to its surroundings during the earthquake. And finally, the third variable, area, refers to the total area of the fault zone that ruptured during the earthquake. Altogether, variables that make up the seismic moment, along with the aforementioned intensity, make for a much more accurate value for the strength of a given earthquake.
The MMI therefore captures a better picture of the intensity of the shaking from an earthquake over a given area, and is what is used as the standard scale for all earthquake measurements by the USGS.
The key elements of the Richter Scale. Source: Iris
Nevertheless, the MMS should not be confused with the modified Mercalli intensity scale (MMI), which specifically measures the shaking severity of an earthquake . As we can see in figure 3, the 5.7 earthquake that occurred in SLV resulted in certain areas experiencing a large range of shaking intensity. For instance, the area around Magna experienced an MMI of 7 which translates to very strong shaking that can result in difficulty in standing and minor property damage, whereas the east side of the valley reported MMI of 5, which resulted in the movement of picture frames and door swings. As a result, the airport and several areas at and near downtown experienced strong to severe shaking that resulted in property damage and aftershocks that have continued for several days now near the original epicenter.
The Modified Mercalli Scale. Source: Geographonic
The earthquake and aforementioned aftershocks reignited a discussion about the region’s seismic zone, with questions ranging from “are earthquakes common for this area” to “when can we expect the next one”? According to the USGS, this earthquake occurred in a greater area across northern Utah that is referred to as the Intermountain Seismic Belt (ISB), with the prominent fault in the region being the Wasatch Fault that runs through downtown Salt Lake City. As can be seen in figure 5, several faults run through and near the valley, and the Magna earthquake was likely related to one of several potential known (or yet-to-be discovered) faults located on the western side of the valley. Nevertheless, the USGS warns that the Wasatch Fault and its segments are all capable of producing large earthquakes >7.0. This 5.7 would pale in comparison to the sort of damage that an earthquake of that magnitude cantered over the core of and east sides of the SLV would result in. Thankfully, such an event is highly unlikely to occur immediately following this recent earthquake but should nevertheless be expected to occur sometime in the far future. And while that nightmare scenario is unlikely at this time, smaller earthquakes of 5 and 6 are still likely to occur in the coming years, and as we saw with the damage from this recent earthquake, the shaking intensity could easily fall over a very populated area depending on the location of an epicenter. As such, now is a great time to begin discussing preventative measures that can help increase the structural integrity of buildings in the region and to spark up a conversation about earthquake safety.
The known network of faults within Salt Lake Valley. Source: Huffington Post
As the ground begins to sway, make sure that you’re aware of the things inside your home that may easily fall during severe shaking. Usually before an earthquake you’ll be able to feel initial wobbles before the jolt hits. The initial P-Waves, as they are called, travel faster from the epicenter and travel in a push-pull fashion, meaning you’ll experience some horizontal motions during the initial part of the earthquake. When you feel these movements, even if you don’t suddenly think this could be “the big one” take that time to hide under a table or to find the sturdiest location in your apartment. This method is known as: drop, cover, hold. As the S-Waves roll in, you’ll experience that jolt that results in stronger shaking. By then, you will want to be in a situation where you’ve already dropped, covered, and are holding on as the waves travel through your home . As the shaking calms down, immediately check for injuries, assess the surroundings, and if the damage in your home looks severe, get out of the home as aftershocks are very likely to occur. More information and what you should do during an earthquake can be found in the link .
The planet is alive and breathing, and while this all may sound intimidating or even slightly terrifying, it is important that we understand the science of what is happening under out feet along with the precautions that we should take in order to better handle when the next one hits. Until next time, be mindful of your surroundings and be sure to learn more about your region’s seismic vulnerabilities by checking out the USGS website.
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©2020 Meteorologist Gerardo Diaz Jr.