Earth’s Hidden Faults

ON August 18, 1994, at least 171 people died in Algeria as a result of a powerful earthquake. Hundreds were injured, and thousands were left homeless. Several weeks earlier Bolivia, Colombia, and Indonesia were also hit by major earthquakes, with a combined loss of several hundred human lives.

Were you aware that these major disasters occurred? Likely not, unless you were personally affected by them or lived in a neighboring country. On the other hand, when major earthquakes hit the California, U.S.A., area, the news seems to spread like wildfire, and scientific data about the quakes becomes available almost instantaneously.

The reason is that no other region has been studied as thoroughly by scientists as southern California, where more than 700 seismometers document earthquakes with a magnitude as small as 1.5. The large concentration of seismologists in that area explains the abundant flow of earthquake information from the region.

A Recent Discovery

This extensive research has undoubtedly helped scientists in many countries to understand earthquakes and even to attempt to predict them in time to prevent casualties. Such technology is vital, since every year about 40 earthquakes of significant magnitude ravage various parts of the world. There are also smaller earthquakes that are virtually harmless but still large enough to be felt. These strike between 40,000 and 50,000 times yearly!

Most earthquakes are apparently the result of large slabs of underground rock breaking and snapping into new positions when under stress. Usually these disturbances occur along fractures in the earth’s outer shell. These fractures are known as faults.

For the most part, scientists are able to map the locations of these faults, thus pinpointing earthquake-prone areas. Why do we say “for the most part”? Because scientists have recently learned that their maps are not as comprehensive as they once thought. For example, scientists are disturbed by the recent revelation that the majority of measurable earthquakes in California occur along hidden faults—in many cases in areas that geologists formerly considered to be relatively free of earthquake danger.

According to earth scientists Ross Stein of the U.S. Geological Survey and Robert Yeats of Oregon State University, “gently rolling or folded terrain is perhaps the least forbidding landscape, evoking slumber rather than danger.” Nevertheless, their studies have identified active earthquake faults beneath folded arches of rock, many of which have been exploited for their trapped stores of oil. Why have these underground faults been able to elude discovery, and just how much of a threat do they pose?

A Threat Not to Be Ignored

Geologists have long recognized that rocks could be squeezed and folded like a crumpled rug. But it was generally thought that this was a gradual, steady process. However, recent studies of active folds of rock show that they surge upward in sudden spurts—by as much as 16 feet [5 m] in just a few seconds! This folding motion compresses the rock-mass beneath. The resulting stress cracks the rock deep beneath the fold, and one piece of the rock starts to ramp over the other. These seemingly harmless folds with their buried active faults become earthquakes-in-the-making before seismologists have a chance to detect them. Such underground fault activity can produce strong earthquakes just like the more prominent faults, which are visible on the earth’s surface.

The January 17, 1994, Northridge earthquake in the Los Angeles area is one recent example of what a hidden fault can do. The earthquake was caused by very deep fault activity that occurred between 5 and 12 miles [8 and 19 km] underground. Before the earthquake, scientists had no knowledge of the fault’s existence. This hidden fault caused massive destruction of property, injuries to more than 9,000 people, and death to 61.

Scientists suspect that hidden faults are the cause of a number of major earthquakes, not only in California but also in Algeria, Argentina, Armenia, Canada, India, Iran, Japan, New Zealand, and Pakistan. Over the past few decades, thousands have died in these lands as a result of earthquakes that could have been triggered by hidden faults.

Scientists now face the challenge of discovering where these active folds occur and predicting their potential earthquake threat. In the meantime, they no longer underestimate the destructive power of a seemingly harmless rolling hill.

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Is Los Angeles Shrinking?

An extensive network of faults and folds located under Los Angeles, California, makes this region extremely unstable. The Los Angeles basin appears to be absorbing much of the compression caused by a nearby kink in the San Andreas Fault. (See the July 22, 1994, issue of Awake! pages 15-18.) Local geologists estimate that the folding due to this compression may be decreasing the land area of the Los Angeles basin by one quarter acre per year.

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