Thursday morning along the Wasatch Front, there was no Mother Nature alarm clock — as there was the day before in the form of a 5.7-magnitude earthquake that brought millions of people out of bed.
In the hours immediately following Wednesday’s tremor, amid the panic and chaos, rumors circulated that a more intense earthquake was imminent.
The Utah Division of Emergency Management as well as state, city and county leaders went to work debunking such rumors, including a false report that a 9.0 tremor was about to strike.
Aftershocks always follow an earthquake, and some can be comparable in magnitude to the main tremor. Wednesday’s main quake registered a 5.7 and two aftershocks registered at 4.6, all of which are in the small to moderate range.
By Thursday afternoon, the University of Utah’s seismograph stations had recorded 160 aftershocks.
According to the U.S. Geological Survey, a large earthquake isn’t a a foreshock to a larger quake.
It doesn’t mean the Wasatch Front is out of the woods yet.
Weber State University professor Adolph Yonkee said there’s a “50% chance roughly (every) 50 years somewhere in Northern Utah” of a large earthquake occurring.
“What that means is, on a day-to-day basis, you don’t need to be panicked. One of the big rumors was we’re going to have the big one any day. That’s not true,” Yonkee said.
A statement issued Wednesday morning by University of Utah seismologists nearly four hours after the main earthquake further clarified the earthquake potential in Utah.
“Based on past earthquake sequences around the world there is a small, about one in 20, chance of an earthquake larger than M 5.7 in the Salt Lake Valley area during the next week,” the statement read. “Although it is possible that a larger earthquake in this area could be in the magnitude range of 7.0 to 7.5, the U.S. Geological Survey estimates that the chance of an earthquake of this size is about one in 300. Earthquakes of magnitude larger than 7.5 are unlikely to occur in Utah. A magnitude 9.0 earthquake is not possible in Utah.”
According to Yonkee, Wednesday’s earthquake didn’t hit on the main Wasatch fault zone, the fault line network that experts recognize as the one likely to produce a major earthquake in Utah’s future.
Instead, it occurred on the West Valley fault zone, a system of fault lines on the west side of the Salt Lake Valley that has produced major quakes and surface ruptures in the past.
The two fault zones are connected somewhat. The Wasatch fault zone is angled down to the west, while the West Valley fault zone is angled more to the east. If one took a cross section of the Salt Lake Valley, the intersection of the two fault lines would look like a lower-case “y.”
One characteristic of Wednesday’s quake was the varying intensity in shaking that people felt across the Wasatch Front. People closer to the epicenter near Magna felt the brunt of the shaking, but shaking varied throughout different parts of Weber, Davis, Salt Lake and Utah counties.
Distance from the epicenter was one reason. But the wide variety of soil types along the Wasatch Front also played a part in the earthquake’s perceived intensity.
A phenomenon known as liquefaction is what happens when, during strong shaking, the ground behaves like quicksand and amplifies the quake’s waves.
According to Yonkee, “The western part of (Salt Lake) County has fine-grained soils and shallow groundwater levels and and has high liquefaction potential. Parts of the bench areas have moderate potential where the groundwater level is overall lower, but the (water) level can vary between seasons and with location.”
Utah Geological Survey maps reveal the areas in Weber and Davis counties that have the highest potential to experience liquefaction.
In Weber County, those areas are mostly found west of Interstate 15, but they also include virtually all of Ogden, North Ogden and Pleasant View. The closer to the bench one goes, the less chance for liquefaction, generally.
Davis County’s most susceptible areas to liquefaction are found west of 2000 West in Clinton, West Point and Syracuse, areas south of Gentile Street in Layton west of I-15 and virtually everywhere else west of the interstate all the way to the county line.
In Salt Lake County, the areas with highest potential for liquefaction are mostly found west of I-15 but with plenty of areas in Salt Lake City and Murray as well as the areas immediately next to the Jordan River.
The similarities of all those areas? They’re closer to the Great Salt Lake.
“Liquefaction is favored in areas with loose, sandy to silty, water-saturated soils, such as areas containing fine-grain sediment deposited by Lake Bonneville and having high water tables,” according to Yonkee.
Yonkee called Wednesday’s earthquake a wakeup call, literally.
He was sitting at his desk Wednesday morning preparing lesson materials for one of his classes and watching simulations of ground shaking in the Salt Lake Valley when, well, the ground started shaking.
Wednesday’s topic was already going to be earthquakes in Utah and the class topic pivoted to the Magna quake.
“Now’s the time to do better preparation. And the pandemic brings up some of the same things: having food, water, flashlights, essentially being prepared if the water goes out, the power goes out,” he said.
“It’s not an if, but a when question.”
The Wasatch Front hasn’t experienced moderate earthquakes at a frequency like, say, California or Washington, so Utahns might not have had earthquakes in the front of their minds until Wednesday.
Much of the damage to buildings Wednesday in Salt Lake County happened to older brick buildings such as the Rio Grande Depot and the Salt Lake Rescue Mission as well as several buildings on Main Street in Magna.
According to Utah State structural engineering professor Marv Halling, the amount of damage a building sustains in a quake depends on many factors, including how long the shaking lasts, how intense the shaking is, when the building was built and how complex the building’s construction is.
“Mostly it has to do with the configuration with the building. For example, simple rectangular buildings often have have less damage than maybe a more complex (building with) more corners, more connections that are more unusual. ... That’s not to say that all designs of interesting buildings aren’t going to do well, it’s just some types of construction are more susceptible,” Halling said.
Another example Halling gave of some typical building damage is something that happens with parapets, or low extensions of walls that normally function as guardrails on the flat, top floor of multi-story buildings.
“In older construction, parapets from especially unreinforced (buildings) are very susceptible. When you shake that building, the parapet will break off and it can fall off of the building on to a sidewalk or it can fall onto the building itself,” he said.
One of the other rumors experts had to clarify Wednesday was that it’s scientifically impossible to predict an earthquake. However, Yonkee offered an example of what could possibly happen should “the big one” hit Utah.
In 1995, a magnitude 6.9 earthquake hit Kobe, Japan, and caused 6,434 deaths in the region along with an estimated $200 billion in damage.
Yonkee brought up the Kobe earthquake because many of the same soil types exist in the Kobe area as do in Wasatch Front.
“Here in Utah, we certainly have the possibility of larger future events than the one we just had. We ought to use this as a learning opportunity, and there are things that hopefully we can do to improve or minimize the risk of damage or even injury or death from future events,” Halling said.
Building code amendments in Utah are approved by the state Legislature after receiving a recommendation from the Uniform Building Code Commission.