Earthquake Magnitude

Excerpts from a book on the Troubled Times recommended book list - Peace of Mind in Earthquake Country, by Peter Yanev - explains man's attempts to measure earthquakes in a scientific manner.

Even today, the causes of earthquakes cannot be said to be completely understood. But there is now sufficient geologic evidence for scientists to conclude that the tremors are the effect of a re- balancing of forces arising from the collusion of continuously moving plates of layered rock that float upon the earth's molten interior. This is the theory of continental drift and plate tectonics, which holds that the land surface of the crust of the earth was once concentrated in a single continental mass - a supercontinent. ... As some of these plates meet and collide around the globe - roughly at the intersections of continents and oceans - they cause islands and mountain ranges to rise, land masses to emerge from or sink beneath the seas, volcanoes to erupt and the adjustments of plate friction which we know as earthquakes.

The rocky edges of the plates have a certain amount of elasticity and therefor tend to hold their basic positions along the fault. Portions of the fault frequently remain locked in this way, and under tremendous strain, for several years, decades or even centuries. Finally, when the accumulated strain exceed the frictional force that binds portions of the plates and prevents their natural movement, the distorted and shattered rocky layers along the two sides of the fault suddenly slip past one another in an explosion of movement that allows a new position of equilibrium for the opposing plates. This slippage, termed "elastic rebound" by scientists, can product a series of powerful vibrations and shock waves that toss and sometimes rupture the earth's surface and may shift the positions of the two sides of the fault by several feet both horizontally and vertically.

The depth of an earthquake is closely related to its destructiveness; for the shock waves of the deeper earthquakes are generally dissipated as they rise to the surface and are therefore less damaging to buildings. On the other hand, the deep-focus tremors usually affect a much wider area. Shallow-focus earthquakes are felt over a smaller area and are therefore sharper and frequently more destructive.

When the stresses of a fault are released in an earthquake, the highest intensity of shock waves and vibrations are felt along the fault line nearest the point of slippage. However, much of the length of a fault may also be affected by a major quake, so that destructive vibrations can occur along a fault for many miles on either side of the earthquake center. In addition, the shock waves disperse from the fault like the rings produced by a pebble dropped into still water, so that significant shocks and damage can affect areas 20 or more miles on either side of the fault. The intensity always diminishes with distance, of course, but an unstable, vibration-prone soil many miles from the fault can product more damage than a strong, rocky geologic formation only a few hundred feet away from the source of the quake.

Earthquake magnitude, the amount of energy released by a quake, was originally defined in 1935 by Professor Charles F. Richter of the California Institute of Technology in Pasadena. [Editor's note - each Richter scale point is 10 times the intensity of the preceding point, thus a magnitude 7 is earthquake is considered to be 10 times greater than a magnitude 6 earthquake.] .. The Modified Mercalli (MM) intensity scale is the one most commonly used in the United States. The MM scale is denoted with Roman numerals from I to XII, with each number corresponding to descriptions of earthquake damage and other effects.

A Richter scale 7 is equivalent to an MM scale of IX-X - a major, destructive earthquake. Peter Yanev describes this as:

Panic is general. Interior damage is considerable in specially designed earthquake- resistant structures. Well-built ordinary buildings suffer severe damage, with partial collapses; frame structures thrown out of plumb or shifted off their foundations. un-reinforced masonry buildings collapse. The ground cracks conspicuously and some underground pipes are broken. Reservoirs are damaged seriously. Most masonry and many frame structures are destroyed. Specially designed earthquake- resistant structures may suffer serious damage. Some well-built bridges are destroyed, and dams, dikes and embankments are seriously damaged. Large landslides are triggered by the shock. Water is thrown onto the banks of canals, rivers and lakes. Sand and mud are shifted horizontally on beaches and flat land. Rails are bent slightly. Many buried pipes and conduits are broken.

A Richter scale 8 or 9 is equivalent to an MM scale of XI-XII - a great earthquake. Peter Yanev describes this as:

Few, if any, masonry structures remain standing. Other structures are severely damaged. Broad fissures, slumps and slides develop in soft or wet soils. Underground pipe lines and conduits are put completely out of service. Rails are severely bent. Damage is total, with practically all works of construction severely damaged or destroyed. Waves are observed on ground surfaces, and all soft or wet soils are greatly disturbed. Heavy objects are thrown into the air, and large rock masses are displaced.