Damage to Fourth Avenue, Anchorage, caused by the Good Friday Earthquake [1964]. Collapse of Fourth Avenue near C Street in Anchorage due to a landslide caused by the earthquake. Before the shock, the sidewalk on the left, which is in the graben, was at street level on the right. The graben subsided 11 feet in response to 14 feet of horizontal movement. Wikipedia
Where Science Meets the Book of Mormon: Come Follow Me Lesson: September 9-15; Helaman 13-16
We read in Helaman14:21-22, “Yea, at the time that he shall yield up the ghost there shall be thunderings and lightnings for the space of many hours, and the earth shall shake and tremble; and the rocks which are upon the face of this earth, which are both above the earth and beneath, which ye know at this time are solid, or the more part of it is one solid mass, shall be broken up; Yea, they shall be rent in twain, and shall ever after be found in seams and in cracks, and in broken fragments upon the face of the whole earth, yea, both above the earth and beneath.”
I recall going on a Geology fieldtrip when I was a student at BYU. The instructor took us up onto the side-hill southeast of campus to where we could see the geological fault lines. I recall his comparing the Provo valley to a glass bowl full of Jell-O. We could either build a house on the side-hill and have it crack with the fault lines during an earthquake, or we could build a house down in the valley floor, which would shake like the Jell-O in the bowl during the earthquake.
An earthquake, also called a tremor or seismic wave, occurs when there is a sudden release of pent-up energy in the Earth’s crust. The crust, which seems solid to us, is comprised of tectonic plates, which are constantly moving, toward each other in some parts of the Earth and away from each other in other parts, because of hot convection currents within the Earth’s mantle. That heat is generated by radioactive decay in the Earth’s interior.
Earthquakes range in intensity, from those so weak we don’t even notice them; the National Earthquake Information Center says there are about 55 earthquakes a day around the world; to those violent enough to launch objects and even people into the air, damaging highways, bridges, dams, and buildings across wide swathes of land.1 Major earthquakes, greater than magnitude 7 on the Richter scale, happen more than once a month. Great big earthquakes, of magnitude 8 or higher, occur about once a year.
Earthquakes can create huge faults in solid rock. The greatest displacement recorded from an earthquake was a magnitude > 8.1 quake, which occurred at the Wairarapa fault, near Wellington, New Zealand, in 1855. The displacement at one point measured over 60 feet.
Perhaps the most bizarre part of an earthquake is what happens to the solid earth. Soil liquefaction occurs when that soil, which is ordinarily a solid, behaves like a liquid, and becomes practically equivalent to quicksand. The phenomenon is most often observed in saturated, loose, sandy soils. Under conditions of sudden pressure, the soil loses its structure and strength, and it may be observed to flow and form waves like water.
If solid structures, like sidewalks, paved roadways, or buildings are sitting upon that soil, soil liquification may cause those solid structures to be displaced and crack. Early builders did not know how to make their buildings “earthquake proof,” but modern architects and engineers have discovered ways to make buildings resistant to earthquakes. One method is to build walls of reinforced concrete between steel, movement-resistant frames. Another way to resist the ground forces produced by earthquakes is to “lift” the building’s foundation above the ground by constructing it on top of columns with flexible steel, rubber, and lead pads — like shock absorbers. When the columns move during an earthquake, especially with soil liquification, the shock absorbers and columns move, whereas the overlying building itself remains steady.
The Salt Lake Temple was originally built on a solid rock foundation, which, over the years, has cracked and crumbled. For the past five years, the temple has been jacked up above the ground and has been retrofitted with base-isolating shock absorbers and structural reinforcement. When the renovation is completed in 2026, the temple will sit atop 98 base isolators — shock absorbers — between the existing foundation and a new, lower foundation.
Trent Dee Stephens, PhD
References
1. jpl.nasa.gov/topics/-earthquakes; retrieved 5 September 2024
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