Precise Gravity Survey Across Newport-Inglewood Structural Zone, Long Beach, California: ABSTRACT

Abstract

A major strand of the Newport-Inglewood structural zone in Long Beach, California, has an estimated vertical uplift of 490 ft (150 m) along its west side as based on a precise gravity survey conducted in October 1979. This strand has a northwest strike and underlies the vicinity of the intersection of Seventh Street and Pacific Coast Highway. This fault strand, as delineated by gravity, is in agreement with well data from the northern extension of the Seal Beach oil field which the survey line crosses. However, the geometry of the structural zone, as modeled from the survey, is in conflict with that of R. F. Yerkes. This fault strand has an associated gravity anomaly of ¢2.25 mgal at the southern end of the survey. The anomaly begins at California State University, Long Beach, and continues southward, possibly through the southeast offshore extension of the Wilmington oil field. The anomaly is presumably based on vertical offset of Franciscan(?) basement schist against overlying Miocene Monterey Formation and San Onofre breccia. This offset causes an estimated density contrast of 0.2 g/cc across the fault at a depth, as projected from nearby well data, of about 9,200 ft (2,800 m). A graphical regional-residual separation indicated a regional gradient 4.5 mgal/km which compares with the regional value of 3.0 mgal/km by L. L. Nettleton. The instrument used was a LaCoste and Romberg G model gravity meter with an elevation accuracy of ±0.01 mgal. End_of_Article - Last_Page 1701------------