YELLOWSTONE NATURE NOTES

THE CAUSES OF EARTHQUAKES

Earthquakes are a common everyday event. About one million shocks are registered annually around the world, but of this number only a few are of strong intensity. One need only observe a mountain terrain to note that the uplift of mountains and plateaus, the offsetting of river courses, and other physiographic changes have been accompanied by shifts of the earth's crust resulting in earthquakes. These have been common happenings extending throughout geologic time. The planet we inhabit is a dynamic object. Gradational processes of weathering and erosion attempt to plane off the earth's surface to the ultimate base level of erosion which for all practical purposes may be regarded as sea level. From the Mississippi River Basin alone there is over one million tons of sediment removed and dumped into the Gulf of Mexico every twenty-four hours. Extend this erosion concept into the geologic past and one can see that the earth would have been base-leveled millions of years ago and the oceans would now cover the earth were there not restorative forces within that periodically rejuvenate the surface.

The ultimate source of these forces is still not entirely understood. Some geologists believe in the thermal contraction theory which implies that as the earth cools from its initial molten stage, the crust adjusts to the contracting core by wrinkling and fracturing. A comparison could here be drawn to the wrinkled skin of a dried apple. Other geologists favor the idea of convection currents rising from the earth's molten core and dragging down portions of the crust. When dragged horizontally the forces produce squeezing resulting in mountain building.

Regardless of the ultimate source of the force we know that this energy is transmitted into, and stored within, the rocks of the earth's crust. When these rocks are distorted beyond their limit of strength they yield suddenly along a fracture by a process known as faulting.

Yellowstone Park is situated in the Rocky Mountains. The Rockies consist of chain after chain of mountains that have been uplifted, folded and faulted extensively during the past 70 million years. Some of the chains are broad flexures with granite cores flanked by tilted layers of sedimentary rock. Others are composed largely of volcanic products. Faults of varying magnitude have been mapped throughout the mountain system. Many of these faults have been quiet during recorded human history, others have been active since the Pleistocene, and still others appear to have been dormant for millions of years. On some faults the movement has been largely vertical caused by tensile forces, on others a thrusting by compression is apparent.

Earthquakes in historic times have centered largely in areas of mountain building and their epicenters are frequently coincident with the position of known faults.

FIG. 1. Earthquakes in the United States through 1957. Reproduced with perimission of G. P. Woollard (1958).