In this series of articles, Utah artist J. Brad Holt talks about what artists are seeing as they look at the landscape. Holt studied geology in college and is attentive to what the rocks suggest in the scenes he paints.
Lead Image: “Kolob—11JAN15,” by J. Brad Holt, 2015, oil, 12 x 9 in.
We had a mild earthquake around here about a month ago. The epicenter was Panguich, Utah, just over the mountain from where I live. At about 4 on the Richter scale, it did no damage. I did not feel it, though some of my friends did. It is not an unusual occurrence here, but not common either. There have been half a dozen quakes that I have felt in this area in the last two decades. There are always warnings that a big quake could hit Utah at some point in the future, along the Wasatch or Hurricane fault systems. We all just keep our fingers crossed and hope for the best. We certainly have much less of a threat here than in many parts of the world.
The whole phenomenon brings up questions: What are earthquakes? Why do they occur? Why are certain regions prone to them and others immune? What role do they play in the shaping of the world, and what can they reveal about the nature of the earth?
Earthquakes are waves caused by a quick release of stored-up strain along a fault in the earth’s crust. The actual point of slippage is called the focus of the quake. The epicenter is the point on the surface of the earth directly above the focus. The crust of the earth has a certain degree of elasticity. Tectonic movement builds a store of potential energy along active fault zones, as the crust distorts elastically under the strain. At some point the fault slips, and the rock rebounds to equilibrium. This violent motion initiates waves that travel outward from the point of focus in all directions.
The famous 1906 San Francisco earthquake gave geologists an opportunity to study the aftermath of a major quake. They found numerous examples of roads, fences, and streambeds offset by as much as 15 feet. They were able to show that the quake occurred as a result of sudden elastic rebound, when locked portions of the San Andreas Fault broke. What they did not know at that time is that the San Andreas is a major transform fault marking the boundary between the North American Plate and the Pacific Plate.
There are several different types of waves generated by earthquakes. Broadly, they are categorized as surface waves, which travel along the surface, and body waves, which travel through the earth. Body waves are divided into primary waves (P waves) and secondary waves (S waves). The difference between P and S waves has to do with their mode of propagation through the medium of the earth. P waves propagate through compression and expansion of the earth as they travel. (A good way to remember this is to remember that P can also stand for pressure, or push-pull.) S waves, on the other hand, propagate through a sideways motion, at right angles to their direction of travel. (The mnemonic here could be S stands for side to side, or shake.) P waves may propagate in a solid, liquid, or a gas, a fact that enables us to hear things, as sound itself is a compression wave. S waves travel through solids, but not through liquids or gases. This has great implications for studying the internal structure of the earth.
Surface waves have a complex mixture of motions, with compression, side to side, and up and down. All of these waves, both body and surface, have different rates of travel. P waves are the quickest. S waves travel more slowly, usually arriving just ahead of surface waves — which may not arrive at all, if the recording station is sufficiently removed from the region of the quake. The interval between P and S waves allows a seismic station to accurately gauge the distance to the epicenter. From this, triangulation of the exact location of an earthquake can be made with the data from three separate stations.
With the long-term data from thousands of seismic stations around the world, a picture of earthquake activity emerges. The greatest number of earthquakes occur at convergent plate boundaries. A line may be followed from the tip of South America, north along the western coast through Central America and North America, along the Aleutian chain, down through the Kamchatka Peninsula, Japan, the Philippines, Malaysia, Borneo, and Indonesia, through New Zealand. Another line extends from Southeast Asia through Northern India, Pakistan, the Middle East, through Asia Minor to the Adriatic and Southern Europe.
Additionally, a line of less violent but frequent quakes follows the divergent plate boundaries along the mid-ocean ridges. The majority of large quakes occur where ocean lithosphere is being subducted beneath continental lithosphere, or island arcs. Large quakes also happen regularly along transform fault boundaries, where lithospheric plates slide past one another. The San Andreas Fault is a good example of this.
Measurement of earthquake intensity is commonly reported with the well-known Richter scale. This is a logarithmic scale, with each interval representing a 32-fold increase in magnitude. Earth scientists studying earthquake aftermaths use a scale called Moment Magnitude, which takes into account shear-zone displacement and local rock strengths.
Damage from quakes is not always commensurate to magnitude. Liquefaction of certain soils, mass wasting on steep slopes, tsunamis, and simply the presence of large population centers near an epicenter can increase the damage greatly. Historically, the greatest human death tolls from single quakes have occurred in Northern China, where homes are routinely carved from windblown loess deposits, which tend to collapse during an earthquake. Unreinforced masonry structures have accounted for substantial damage and loss of life from quakes, especially in Asia Minor and the Middle East. Death tolls in Japan from earthquakes have dropped, despite high population density, because of improved building practices.
Worldwide warning systems for tsunamis are now in place, but is there any way to forecast earthquakes? Unfortunately, there is not. Fore-shocks sometimes happen, but most large quakes occur without warning. There are always apocryphal stories of earthquakes being predicted through tide charts, or by collating missing-pet statistics in local classified listings, but so far these methods have not progressed beyond the realm of fringe science. In the meantime, the best protection seems to be realizing that earthquakes are an inevitability just about everywhere eventually, and that it would be wise to be proactive in our construction practices. Charleston, South Carolina, was practically destroyed by a large quake in 1886. The New Madrid quakes in 1811–12 were some of the most powerful ever felt in the U.S. The eastern regions of the country are not immune to earthquakes.
One of the few positive aspects of seismic activity is that it affords us an opportunity to study the internal structures of the earth. There are many earthquakes every year that are strong enough that their body waves travel all the way through the earth. These waves propagate at different velocities through different rock densities. This allows seismic stations around the world to take a sort of “seismogram” of what is down there. These waves follow a curved path, due to the changing velocity as pressure increases at depth. P waves will travel all the way through. S waves will travel through the crust and mantle, but not through the liquid outer core, which begins 2,890 kilometers below the surface. A more detailed picture of the rock layers at lesser depths is routinely compiled by using “Thumper Trucks” to initiate seismic jolts, then recording the returning echoes on a local scale. There is a whole lotta shakin’ going on, and we are actually initiating some of it!