Is it any wonder why the area is so closely watched? With a giant lake to the West, a huge fault to the East, and silty-sand beneath our feet, the Wasatch Front is a potential catastrophe in the event of a major earthquake. As a student of Civil Engineering at Utah State University, I am now just beginning to understand the extreme dangers that we could find ourselves in “when the big one comes.”
Following the GIS analyses of the primary and secondary earthquake hazards along the Wasatch Front, I will briefly analyze science of earthquake probability, as well as regard the myth of “earthquake prediction.” In doing so, I plan to recommend a method of earthquake probability to for the Wasatch Fault.
After briefly regarding earthquake prediction, I will conclude my project and make any final recommendations to home and business owners, city officials, and interested readers regarding my findings in this study.
Figure 1: Pyramid Chart of Project Proposal
“Structural Engineering is the art and science of molding materials we do not fully understand into shapes we cannot precisely analyze to resist forces we cannot accurately predict, all in such a way that the society at large is given no reason to suspect the extent of our ignorance.” - Unknown Structural EngineerThis fact becomes very significant because the majority of Utah's population lives along the Wasatch Front. Obviously, the more populated and area you have, the more structures you have; and the more structures and population you have, the more deaths you have in the event of a major earthquake.
Figure 2: The Wasatch Fault with Utah Population
Distribution for the Year 2000
Source for population info: State of Utah, 2002
Figure 3: Peak Ground Acceleration Ground Motions
w/Probability of Exceedence of 2% in 50 Years for the Wasatch Front
I will now compare this, and the remainder of the hazard maps, with a land cover map of the Wasatch Front made in GIS with data obtained from the National Land Cover Dataset. This map is seen as Figure 4.
Figure 4: Land Cover for the Wasatch Front
ACC_VAL in Figure 2 represents the percent value of the acceleration
of gravity occurring in a lateral direction for an earthquake that has
a 1 in 50 chance of happening in the next 50 years. Different data is available
for earthquakes with that have a 1 in 10 chance and a 1 in 20 chance of
occurring as well. The determination for which probabilistic data that
should be used depends on city building codes, as well as what strength
of earthquake you wish to be prepared for. A wise and ethical engineer
should prepare for the worst and hope for the best, and that is why I chose
the 2% probability of exceedence.
1. Tall high rises
2. Unreinforced masonry buildings predating 1976 (State of Utah, 1996)
3. Areas with an acceleration value greater than 60% g
Tall high rises are higher off of the ground, therefore lateral
motion at their bases would tend to have a greater destructive effect on
the structure itself than would lateral motion on structures built closer
to the ground. Also, they tend to be dense in Utah’s commercial zones,
which have high amounts of people present at any given time. Fortunately,
much of the development of the Wasatch Front’s current high rises were
appear to have been built after 1977, this because Utah’s economy has seen
major improvements in the last 20 years; therefore, they were built to
more stringent building codes. However, further comparison between the
Figure 3 and Figure 4 show that much of the commercial and transportation
zones of the state lay within boundaries that have acceleration values
of 40, 50, and even 60% g. This can be very alarming considering that the
USGS considers an earthquake with 30% g to be “very strong shaking, indeed,”
(USGS , January 25, 2002.)
Buildings that predate 1976 were built during a period when earthquake-proof building codes were not used abundantly, if at all. After 1967, some minor earthquake-proof regulations were applied in Utah building codes, and it was not until 1976 that the Uniform Building Code regulating seismic requirements was established. This can be very alarming because many buildings that still stand today were built by Mormon Pioneers over 150 years ago! (Glacier Medical, no date available.) Insurance providers reported in 1993 that less than 5% of Utah homeowners had earthquake policies on their homes, this because most perceived earthquake danger as a “potential, but distant danger.”(Glacier Medical, no date available.) Other larger structures refuse to retrofit their outdated design because it can cost up to 25 % of the total cost of the structure, compared to only 2 % or 3 % of the total cost if the building were brand new and having the design implemented in the construction phase.
It is estimated that of the total structural damage resulting from a major earthquake, nearly 75% of the total damage would be from unreinforced masonry buildings, such as brick homes built before 1960. All of the structural damage possible for Weber, Davis, and Salt Lake Counties has been given the damage cost estimate of $4.5 billion total. However, this only includes the shaking hazard. Once the secondary hazards are included, this amount may only constitute 20% of the entire damage cost. (University of Utah, no date available.)
From observation, the City of Logan, the City of Brigham City, and parts
of downtown Ogden, Salt Lake City, and Provo were built before 1976. Many
of the homes along the benches of the Wasatch Front also predate 1976.
Figure 5: Peak Ground Accelerations vs. Land Cover
for Alpine and Cedar Hills, Utah
Alpine and Cedar Hills are sister cities that lie due north of Provo,
Utah. They consist mostly of newer residential areas located on the bench
of the Wasatch Mountains. The area has the nickname of “Happy Valley,”
though it may not be too happy after a major or even moderate earthquake.
This is because it is a prime area for magnifying seismic waves. By comparing
maps in Figure 5, it can be seen that many of the residents, especially
those located in the most northern residential area, find themselves on
ground that carries a peak ground acceleration value of 80% g. If we use
30% g as an indicator of violent shaking, we see that Alpine could be in
for a major ride.
I am also concerned with US Highway 68, which can be seen on Figure
5 as a blue line located between Alpine and Cedar Hills. This highway travels
through a canyon, which in the occurrence of an earthquake could be a dangerous
place to be due to falling boulders and rocks. The highway is just barely
south of the 80% g area, but it is still located in an area of 60% g.
2. Bountiful and Northern Salt Lake City (Box 2. in Figure 3)
Figure 6: Peak Ground Accelerations vs. Land Cover
for Bountiful and North Salt Lake cities, Utah
Bountiful and North Salt Lake City lie next to each other nestled against the Wasatch Mountains. The cities are located right before the Point in the Mountain. Like Alpine and Cedar Hills, most of the existing structures in hazard areas are low-density residential. Unlike Alpine and Cedar Hills, however, Bountiful and North Salt Lake have more commercial structures scattered throughout themselves. For those familiar with the Wasatch Front, they recognize these cities as structures located high along the mountain’s benches. As Figure 6 shows, this area can have a ground acceleration value of up to 80% g. Fortunately, upon inspection the structures in the area appear to be very modern in design. This should benefit them in the event of a large earthquake.
3. Downtown Ogden (Box 3. in Figure 3)
Ogden City is the sixth largest city in the state of Utah. (State of Utah, March 27, 2002.) Many of its buildings were constructed during the big railroad boom of the late 1800’s, especially in the early 1890’s before the 1893 depression. (Ogden City, 2002.) A drive down Ogden’s Wall Avenue or Washington Boulevard will show you that many of those original buildings still stand. These buildings are completely vulnerable to seismic waves, and even the slightest shaking may bring down large sections of Ogden City. Though the ground acceleration value for most of Ogden is less than cities such as Bountiful or Alpine, it still registers a value of 50% g. That’s still a lot of shaking! Parts of northern Ogden and the city of Harrisville reside in an area with a ground acceleration value of 60%. Most of the buildings in this area are newer, though many still predate 1976.
Though only three specific high-hazard areas were pointed out, ground
shaking will still cause much damage up and down the entire Wasatch Front.
This is because residents very hesitantly are accepting the truth that
they live on a dangerously large fault that has not ruptured for a dangerously
long time. However, if residents of the Wasatch Front actively pursue to
take out earthquake policies on their older homes and other structures;
and to those that can afford it, if they actively pursue to retrofit their
older structures to meet current building code regulations for earthquakes,
then the damage in the event of a major earthquake will be minimized socially
and financially.
Liquefaction should be of specific concern to residents of Utah because
of the fact that they live on the remains of the lake bed on the ancient
Lake Bonneville. Various peaks in the Wasatch Mountains used to be nothing
more than islands in the middle of a giant lake. As the surface elevation
of Lake Bonneville began to decrease, rivers that emptied into Lake Bonneville
began to lengthen themselves out. Canyons such as Ogden and Provo Canyons
were formed. These rivers carried millions of tons of sediment from erosion.
This sediment was deposited into Lake Bonneville, where it eventually sank
to the bottom of the still waterbody. It is on top of these sediment deposits
that Utah residents now build their homes, their businesses, their schools,
and the rest of their structures.
Figure 8: Liquefaction Hazard for the Wasatch Front
Once again, Ogden City is a specific area of concern. Much of the older commercial zone of the city is located in an area that has a Pcode value of 7. Assuming that 9 is the highest value, 7 is frightfully close to very destructive. More alarming, again, is the fact that much of that zone was built decades ago, predating 1976.
A small portion of the bench against the mountain has a Pcode value of 8. This is a low-intensity residential area, but the effects of liquefaction could still cause significant damage to those homes. This is especially true since many of them were built before 1976. This Pcode value could affect Weber State University as well, which could result in significant financial costs if any of buildings or equipment on campus were damaged.
2. Salt Lake City (Box 2. in Figure 8)
Salt Lake City, which is the capital of Utah, is the most populated city in the state of Utah. (State of Utah, March 27, 2002.) It contains the majority of the industry (State of Utah (b), March 27, 2002), and apparently has the largest single commercial zone along the Wasatch Front. This zone closely represents a small metropolis with tall buildings, megamalls, shopping centers, and historical and religious landmarks.
Almost the entire city of Salt Lake has a Pcode value of 7. With the weight of the tall buildings, malls, and shopping centers, Salt Lake City has a high potential of being severely affected by liquefaction. If many buildings fail in this area, not only will it cost the city millions of dollars to repair and rebuild them, but it will cost the city millions of dollars in profit from consumer sales as many sections of the city would be inaccessible during the renovation.
3. West of Salt Lake City (Box 3. in Figure 8)
West of Salt Lake City approximately two miles lays the most prone area to liquefaction in the state of Utah. Just to the southwest of the area lies the city of Magna, which has a population over 20,598 people. (Internest, 2002.) The commercial/transportation/industrial areas that find themselves right on top of the danger zone are called Garfield and Arthur. This zone carries a Pcode value of 9! This signifies almost an absolute probability that liquefaction will occur. From Figure 8, notice how the industrial area almost perfectly outlines the zone of high liquefaction. Also, the large commercial/transportation/industrial mass in the top-right corner of the land cover map is the Salt Lake City International Airport. Though, not in the extremely hazardous area of liquefaction, its area still carries a Pcode value of 7 and could still cause major damage to unprotected structures.
Much of the city of Provo also has Pcode values of 7. The damage in
Provo could resemble the damage in Salt Lake City, though the majority
of Provo appears to be more of a modern city than Salt Lake. For this reason
it was not included as a specific area of concern.
Many of these mountain benches have well-compacted soil ample vegetation surrounding them to provide an apparent safe building place for a structure. But the soil particles that make the benches are loaded with potential energy because they are raised high off of the ground. The soil particles close to the edge of the benches have a greater probability of having gravity transform that potential energy into kinetic energy than those soil particles further away from the edge. If a catalytic source of energy, like a large truck driving by or a seismic wave, were large enough, it could jumpstart these soil particles into motion. Gravity would then take over, and then it’s literally all downhill from there.
There are actually 3 main types of landslides: debris flow, which is a swift-moving current of sediment and and water; slide, which is a dry or wet soil movement down slope; and rock fall, which is a free fall of rocks and boulders from an elevated surface. (State of Utah (c), 2002.) For the purposes of this study, only slides and rock falls apply to tectonically-induced landslides.
Figure 12: The Three Types of Landslides
Source: adapted from State of Utah (c), 2002
The danger of landslide here in Utah is unique because of the taste of the residents. I once had a Geology professor who said that “Utahns will build their houses in only two places: on an alluvial fan, and on a major slump or landslide.” (Brad Ritts, class lecture, April 2001.) It seems that unless a house is directly on the edge of the bench or cliff, the resident is not satisfied. Being a Utah resident of 24 years myself, I am well familiar with many stories and tales of houses sliding down mountainsides because their owners did not listen to the Geologists and Engineers. I don’t know how many of the stories are true, however I do know that there are structures (mostly private residences) that are located in current areas prone to landslide hazard.
Comparing Figure with Figure , we see that there are not in fact many areas where large groups of structures are located in a zone of high landslide hazard. These structures appear to be more sporadically placed all along the mountain benches from Salt Lake City to Provo. Because there are so many of these individual residencies, and because the data for land cover does not appear to be very accurate on a micro scale, only sections where many residencies are in a hazard zone will be studied. There are many residences located on zones of medium landslide hazard. These zones should not be overlooked.If an earthquake large enough were to rupture along the Wasatch Fault, it could easily dislodge sections of soil in these areas, especially if they are weighted with heavy homes. However, it will be assumed that these areas will hold in the event of a major earthquake, and only the high hazard zones associated with landslide will be looked at further.
1. Northern portion of Sandy (Box 1. in Figure 13)
Figure 14: Landslide Potential vs. Land Cover for
Northern Portion of Sandy City, Utah
From Figure 14, it not appears that there is a conglomeration of residencies but also commercial structures, as well. A road passes through the area. This road is most likely similar to such roads as Wasatch Boulevard near Laker Way in Ogden, as well as Bonneville Drive in Ogden, both which need reconstruction every year because the soil keeps falling out from under it. What is most shocking is that this area is an active landslide, yet residents still chose to build their homes there.
Fortunately, our engineering capabilities enable us to deal with many
of the landslide hazard areas along the Wasatch Front. Two years ago, the
town of South Weber, located in northern Davis County, was faced with a
dilemma. A new subdivision that was built on the alluvial fan was losing
an estimated 2 feet per year due to landslide. This was people's expensive
property literally disappearing! The city residents met and decided that
it would be prudent to have a retaining wall built to prevent further landslide.
Happily, recent observations show that the landsliding has ceased and further
subdivisions can be built if retaining walls are installed. (Carl Humpherys,
personal communication, July 2000.) It would be beneficial, upon further
investigation of the site, to analyze the possibility of installing a retaining
wall like in South Weber for the subdivision in Figure 14.
Figure 16: The Southern Shore of Lake Hebgen after
the Earthquake of 1959
Source: University of Utah, no date available, courtesy
of US Geological Survey
Lak Hebgen is a perfect example of tectonically-induced flooding. This hazard associated with earthquakes is not very well known, even here in Utah, where our environmental set up almost perfectly immitates that of Lake Hebgen. Not only does the Wasatch Front have a major fault system to the east, it also has a large waterbody to the west and a relatively narrow strip of land which is heavily populated.
In order to understand the possibility of land-tilt and flooding, refer to Figure 17.
Figure 17: Diagram of Tectonically-induced Flooding
Many geologists, seismologists, and geotechnical engineers are divided on the possibility of tectonic flooding along the Wasatch Front. (Marv Halling, personal communication, November 5, 2002.) One geologist who feels that the flooding is a very good possibility is Dr. Robert Smith of the University of Utah. According to Smith, it is an "unappreciated hazard," and "is very important depending upon the lake level and the location and size of the scenario earthquake." (R.B. Smith, 1997.)
To analyze the possible danger associated with tectonic flooding along the Wasatch Front, National Elevation Dataset data of the eastern side of the Great Salt Lake was contoured by analyses with GIS. Using Lake Hebgen as a possible model, the eastern shoreline of the Great Salt Lake was modeled to rise approximately 4 meters, or 13 feet. Worst-case scenario was also considered by also modeling an extra 2 meters, or 7 feet, for a total of 20 feet as the dangerzone for flooding. Therefore, 0-5 feet above current lake surface was modeled as "High Flood Hazard," 5-13 feet was modeled as "Moderate to High Flood Hazard," and 13-20 feet was modeled as "Low Flood Hazard."
In order to continue the study, the current elevation of the Great Salt Lake was obtained:
Figure 118: Surface Elevation of the Great Salt Lake
from Saltair Harbor on November 15, 2002
Source: US Geological Survey, November 15, 2002
The lake surface elevation on November 15 was approximately 4198 feet (absolute elevation.) This is low for the lake, which can often approach elevations of 4210 feet during a normal or wet water year. For the analyses, an even contour value of 4200 feet was chosen as the lake surface elevation.
Figure 20: Potential Flooding of Clinton City
It can be seen from Figure 20 that many of the pasture and hay fields of Clinton residents could be wiped out by flood. The wetlands are located inside of a high hazard zone, while many of the fore-mentioned fields have moderate-low chance of being inundated. Also significant is the road from the city of Sunset, located just south of Clinton, to Antelope Island in the Great Salt Lake. The road would surely be flooded, costing the state of Utah millions of dollars to rebuild it.
2. Northwest of Bountiful (Box 2. in Figure 19)
Figure 21: Potential Flooding for Northwest of Bountiful
City
In this area, there is great potential for loss of life as well as money. There are portions of high-moderate flood hazard and moderate-low flood hazard that would wipe out US Interstate 15, costing the Federal Government millions of dollars to repair and rebuild.. Also notice the many commercial areas, as well as residential areas, are in moderate-low risk of flooding at the lake's current conditions. During a normal water year, this risk could significantly increase.
3. West of Salt Lake City (Box 3. in Figure 19)
Figure 22: Flooding Potential for West of Salt Lake
City
This area, which is also an area of concern for liquefaction, is prone to major flooding of wetlands. The main items of concern here are the commercial/industrial areas and also the roads. Much of the salt that is mined from the Great Salt Lake is produced in this area. If these salt-producing plants are flooded, it could deprive the state of Utah of what has been a steady and valuable income, at least for a period of time. Another item of concern is US Interstate 215, which can be seen extending out nearly 2 miles into the lake. The interstate is inside of a moderate-low hazard zone, but borders a moderate-high hazard zone. It is obvious that if the lake level were to rise, then the interstate would be in serious jeopardy; and, like I-15, the Federal Government would be force to pay millions of dollars in repair and reconstruction costs. Also worthy of mention is the Salt Lake International Airport, which borders a moder-low hazard zone. Again, with higher lake levels, the airport could tangibly fall inside of a flooding zone.
Though flooding is not widely accepted as a likely hazard that would
occur along the Wasatch Front in the event of a major earthquake, it does
deserve regarding and respect. If an event similar to the Lake Hebgen earthquake
were to happen along the Wasatch Front, the effects could be disastrous.
State and Federal Governments would be given the large financial burdern
of cleanup and repair, residents and property owners would have to face
the prospect of losing property to federal regulations of wetlands, and
people nationwide would suffer the grief of dead fellow citizens.
The average citizen that resides in earthquake-proned areas ould, undoubtedly, appreciate a 10 or a 15-minute warning before the actual occurrence of a major earthquake. Unfortunately, it is literally impossible to accurately predict when a fault will slip and an earthquake will orruc. For those seeking answers on earthquake-predicting, the USGS says this:
"Although a great deal is known about where earthquakes are likely to occur, there is currently no reliable way to predict the days or months when an event will occur in any specific location.. The USGS is thus focusing its research efforts on developing long-range earthquake probability forecasts in seismically active urban areas." (USGS, May 12, 2002.)However, because accurate forecasting of particular earthquakes is virtually impossible does not mean that scientists are not trying to increase the accuracy of probability forecasting. "Ultimately, scientists would like to be able to specify a high probability for a specific earthquake on a particular fault within a particular year." (Pakiser & Shedlock, 1997.)
According to Dr. Ian Main of the University of Edinburgh, Edinburgh UK, there are only 4 possible "degrees" of earthquake predicting: (Ian Main, 1999)
Unlike Forecasting and Prediction, Time-dependent Hazard has shown much fruit and promise. Dr. Marv Halling states that it is a fairly accurate way to predict the earthquakes within a large region. This method has been used for decades, and has done much to prepare people for the occurrence of earthquakes. However, for a single fault system such as the Wasatch Fault, the linear correlation of regular earthquake occurrence and time becomes very low.For this situation, the use of reading the geologic record of the actual fault system, along with the application of modern technology such as satellites is more suitable. This method is the Time-independent Hazard.
In my opinion, Time-independent Hazard is the most reliable method for probabilistic study of earthquake hazard along the Wasatch Fault. I feel this way because of a few reasons:
Four major hazards were mapped along the Wasatch Front:
1. Alpine and Cedar HillsFor residents in these areas, I make the following recommendations:
2. Bountiful and North Salt Lake City
3. Downtown Ogden City
a) Read and follow preparation suggestions by the US Geological survey. Click here for a link to the preparation website.
b) Talk with your insurance agent about taking out a current earthquake policy on our home insurance. You may be glad you did.
c) If you own a business or a home in an building constructed before 1976, find out if it is reinforced. If not, consider retrofitting it. Contact a local civil engineering firm for an estimate.
d) Have a current earthquake plan, so that if the ground start shaking you know exactly what to do.
1. Downtown OgdenFor residents in these areas, I make the following recommendations:
2. Salt Lake City
3. West of Salt Lake City, near Magna
a) Read and follow preparation suggestions by the US Geological survey. Click here for a link to the preparation website.
b) Remember: liquefaction WILL NOT kill you, but it can destroy the foundations of the structures that you own.
c) Know the status of your foundation (if there's any cracks, leaks, et al.) If needs repair, do so.
d) Contact the Utah Geological Survey to obtain a more accurate liquefaction hazard map of your area to know the specific hazard zones.
1. Northwest portion of SandyFor residents in this area, and any resident who lives on the benches in the Wasatch Front, I make the following recommendations:
a) Read and follow preparation suggestions by the US Geological survey. Click here for a link to the preparation website.
b) Observe your property and be aware of the signs of landslide. Click here for a link to the state's homeowner's guide to recognizing and reducing landslide damage on [your] property.
c) Plant many deep-rooted trees near the edge of your property. The roots will help to hold the ground in place.
1. Clinton CityFor residents in these areas, I make the following recommendations:
2. Northwest of Bountiful City
3. West of Salt Lake City near Arthur
a) Read and follow preparation suggestions by the US Geological survey. Click here for a link to the preparation website.
b) By aware of any flood history in your particular area. Remember: water always flows downhill.
c) Talk to your insurance agent about taking out a flood policy on your home or business insurance.
d) Remember that the water itself would probably not rise more than 13 feet from the current lake surface, so it is not extraordinarily deep. However, tectonically induced waves could be deadly. Plant tall and thick trees inbetween your structure and the lake. They could serve to break the waves that otherwise might cause more damage.
University of Utah, no date available, http://www.seis.utah.edu/qfacts/utfaq.shtml#uq3
Reiter, L., Earthquake Hazard Analyses, Columbia University Press, NY 1991
State of Utah, 1996, http://www.ugs.state.ut.us/ghp/flf/flfst123.htm
R.B. Smith, 1997, http://enp-web.er.usgs.gov/reports/annsum/vol39/ni/g2746.htm
Main, Ian, "Earthquakes-Long Odds on Prediction", Nature 385, 19-20
Dona Healy, August 15 1999, http://www.billingsgazette.com/magazine/990815_mag01.html
Gellers, R.J., 1999, www. nature.com/nature/debates/earthquake/equake_frameset.html
University of Utah, 2002, http://www.seis.utah.edu/NEHRP_HTM/1959hebg/c1959he1.htm
Internest.com, 2002, http://www.internest.com/city/magnaut.asp?source=ls
Ogden City, 2002, http://www.ogdencity.com/index.cfm/about.history
USGS, January 17 2002, http://quake.wr.usgs.gov/research/parkfield/index.html
USGS, January 25 2002, http://geohazards.cr.usgs.gov/eq/faq/psha04.html
USGS, November 15 2002, http://waterdata.usgs.gov/nwis/uv/?site_no=10010000&PARAmeter_cd=72020
State of Utah, March 27 2002, http://www.governor.utah.gov/dea/rankings/city/00citypop.pdf
State of Utah (b), March 27 2002, http://www.governor.utah.gov/dea/rankings/99COCLF.pdf
State of Utah (c), 2002, http://geology.utah.gov/online/pi-58/Pi58pg1.htm