Earthquake displacement of some 30 feet at a Nevada site

We live on the eastern edge of the geologic region called the Basin and Range Province which includes most of Nevada, western Utah, and southern Idaho. It should be explained that basin, as used here, doesn’t refer to Great Basin but rather to the many small basins that separate the mountain ranges. The first person to make sense of this region was the famous 19th century geologist Grove Karl Gilbert. It was April 1871 while on his way to California looking for employment that he was struck by the peculiar geology; the mountains were like books stacked on a shelf with just enough side tilt to give their tops a saw-tooth configuration, steep on one side, gently sloping on the other. The pattern would repeat for a few ranges and then reverse for a few more. Early students of the province could only assign this peculiarity to the vagaries of erosion but Gilbert soon learned otherwise. Upon close examination he found scarps running along the base of many of the ranges, always on the steep side. He realized the region had been stretched from east to west, breaking into blocks; each block was a mountain range. With each successive earthquake, the block tilts up and back as the valley floor falls down and away. This is what happened in the 7.1 magnitude Mt. Borah, Idaho earthquake of 1983, also located in the Basin and Range Province. The mountain gained 5 inches of elevation while the valley floor dropped down a few feet.

Gilbert took his studies further east to the Wasatch Front and finding no evidence for a fault scarp in the Salt Lake Valley, warned the residents that they were overdue for an earthquake. No one seemed concerned, and perhaps for good reason, because 127 years later there still has been no significant movement anywhere along that range. Since Gilbert, scientists from the USGS and the Utah Geological Survey have cut trenches across and up and down the Wasatch Front looking for evidence of past movement. They found that on average earthquakes have occurred there about once every 400 years. The Weber, Provo, and Nephi regions have each experienced about 400 to 500 years since their last quake. It has been about 1000 years for Levan, 1500 for Salt Lake, and 3500 years for the Brigham City region. Supposedly a wait of that length poses a serious threat. In contrast, it is generally thought that the earthquake risk is somewhat less for our mountains though little research has been done to prove that point.

The great accomplishment of modern geology has been the discovery of plate tectonics, the knowledge that seven major and several minor plates come together to form Earth’s outer crust. These plates carry the continents and ocean basins through time, moving at most, a few inches per year. In some places they are pulled apart and at others they collide. They are driven by incredibly powerful forces originating deep within Earth’s interior. It is at the collision zones where most earthquakes occur and where mountains are created. When rock is forced against rock, stress begins to build. The most powerful quakes occur after long periods of stress accumulation and when the rock finally fails, the ensuing break unleashes the earth-deforming tremors we call an earthquake. After the break, conditions are reset and the process begins anew.

From knowing this we can also know that earthquakes are inevitable in areas proximal to or influenced by plate boundaries. Geologists know that they are coming but have made little progress in predicting when or how powerful they will be. And that, of course, is the problem; we will continue to see loss of life and property into the foreseeable future. Sometimes as in the recent Haitian disaster, these losses will come in staggering proportions as mankind struggles to deal with forces beyond his control – forces also beyond his ability to fully understand.

Though harmful and feared, earthquakes are necessary for our earthly existence. It was through them and their associated tectonic activity that brought into being the land on which we stand. The continents owe their existence to early Precambrian tectonic events and most geologists think these landmasses have continued to grow during subsequent ages. Without this growth to counteract erosion, all land on Earth would have long ago sunk beneath the sea.

Some information for this essay came from the Earth in Turmoil by Kerry Sieh and Simon LeVay, W.H. Freeman and Company, 1998.