YELLOWSTONE NATURE NOTES

MECHANICS OF EARTHQUAKES

With the rupturing of rocks to create a fault, or with renewed movement along an old fault, the energy released travels away by means of waves. It is the motion of these waves rather than the displacement along the fault that usually causes the most damage. Movement of a few feet along a fault can cause extreme damage and maximum displacements in historic times have not exceeded fifty feet.

Seismologists recognize two main classes of earthquake waves; body waves which travel through the interior of the mass in which they are generated, and the surface waves which travel only along the surface.

Body waves of two types are generated by faulting. One is called the "P" or Primary wave which is a wave of alternate compression and rarifaction like a true sound wave in air. The "S", or Secondary wave vibrates perpendicular to the direction it is travelling. The behavior of the "P" wave could be likened to the movement of a billiard ball striking a line of billiard balls with each one passing on the energy of motion to its neighbor. The "S" wave is best compared to the undulatory motion imparted to a rope tied to a tree at one end and vigorously shaken at the other. The form and passage of the waves can be observed as they travel along the rope.

PLATE 2. Serpentine Fence west of Yellowstone along Highway 287. Photo by J. P. Stacy, Courtesy of the U. S. Geological Survey film library.

This "Serpentine Fence" is probably the result of compression, or it may represent the path taken by an "S" wave that was polarized horizontally. It would require a detailed study of the ground to know which is the true explanation.

Surface waves known as "L" or Long waves, have an oscillatory motion similar in character to wave motion on a body of water. They are generated by the "P" and "S" waves directly above the focus of the earthquake and travel slowly around the world.

The names Primary, Secondary, and Long are suggestive of their relative speeds of transmission and arrival times at seismograph stations.

A seismogram is the graphic record of an earthquake recorded by an instrument known as a seismograph. Knowing the travel times of "P" and "S" waves through the different types of rock, it is possible to use the time factor of "S" minus "P" time to determine the distance of the earthquake from the recording station. Arcs of distance drawn from a number of seismograph stations intersect at a point on the earth's surface known as the epicenter.

Calculations of ground motion permit a determination of the earthquake's magnitude on the Richter Scale and field studies of earthquake damage are used to estimate intensity on the Modified Mercalli Scale.

FIG. 2 Seismogram of a Typical Aftershock as Recorded on August 18, 1959 at University of Utah Seismograph Station, Salt Lake City, Utah. Reproduced by permission of U. S. Coast and Geodetic Survey

When the earth is at rest the seismograph traces a straight line on the graph. Earthquake vibrations result in oscillations which are traced by a beam of light or stylus on sensitized film. Note the sequence of arrivals of "P," "S" and "L" waves. Shocks from the Hebgen Lake area take less than one minute to reach the recording station at Salt Lake City, Utah. An earthquake in Japan would take from ten to twelve minutes to reach the same station.

On the night of August 17 we can visualize then an accumulation of energy being stored in the rocks of the earth's crust at a depth of ten miles below Grayling Creek along the western boundary of Yellowstone Park. Gradually the elastic limit of these rocks was exceeded, they ruptured, shock waves travelled out in all directions from the focus, some travelling through the earth and some around the surface. As these waves encountered rocks of different densities and old fault planes, they were reflected, refracted, and their paths and speeds altered accordingly. These vibrations set the atmosphere in motion generating sound waves, deep roaring sounds like the boom of distant artillery or the roar of a passing train or truck. Many observers noted the sounds and also described the strange sensation of seeing trees move when no wind was blowing.

The total energy release from this one earthquake was equal to that of about 200 atom bombs of the type dropped on Hiroshima, Japan. When making comparisons of this type it is well to keep in mind that with earthquakes the energy is unconfined and free to dissipate itself in all directions away from the focus.

Within Yellowstone Park there were extensive rockslides in the canyons, streams became turbid, and the ground water table was severely shaken causing drastic changes in the behavior of many hot springs and geysers. To properly interpret the cause of the earthquake and the geologic changes that have occurred, it is imperative that the reader have some appreciation of the geology of the region.