Abstract: Thrust Identification - The Vallecitos Controversy

Sands H. Figuers

An excellent example of the practical problems of thrust evaluation is the Vallecitos controversy. In 1977, General Electric applied to renew the operating license of its test reactor (GETR) at Vallecitos Nuclear Center south of Pleasanton, CA. During the site investigations, a series of northeast dipping, low angle shears that cut soils less than 11,000 years old were discovered in trenches and boreholes.

The interpretation of those shears quickly devolved into two camps, that were as much political as technical. One group believed that the shears were toe thrusts of ancient, inactive mega-landslides, and had no effect on the seismic safety of GETR. The other group believed that the shears were the surface expression of a regional thrust, and increased the potential for seismic shaking. Discussions concerning the genesis of the shears were long and bitter, but in the end neither side prevailed. Even though the renewal battle is over, the fundamental geologic problem still remains--were the shears indicative of landsliding or regional thrusting?

Evaluation of thrusting requires a knowledge of thrust characteristics and an investigative procedure that can document those characteristics. Part of the past and current difficulty in thrust evaluation lies in the historic rational behind the fault investigative process. In California, fault evaluation procedures are based upon idealized, near surface, deformational characteristics of strike-slip faults. This assumes that faults are discrete, high angle, and linear; and that active faults reach the ground surface. As a result, site specific, shallow trenching has become the preferred method of fault evaluation.

Active thrusts are completely different. They contain complex zones of deformation, are low angle, sinuous, and rarely reach the ground surface. It is quite rare to ever find ???the fault' in a trench. This characteristic restricts the usefulness of trenching. The largest problem is separating critical structural data from the overwhelming mass of data.

The most useful tool is regional structural analysis. Properly done, it is a predictive tool that can identify the most likely locations of faulting, rank them in importance, and suggest the types of structures that should be encountered at each site. This allows an efficient use of resources that significantly reduces costs and increases interpretation quality.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California