Isostatic gravity map of the Los Angeles 30 x 60 minute quadrangle, California

Gravity data in the Los Angeles 1:100,000-scale quadrangle and vicinity include 428 gravity stations obtained by the U.S. Geological Survey from 1994 to June, 2001; 2536 stations from a previously published Bouguer Gravity Map of California--Los Angeles sheet compiled by W.F. Hanna and others, (1974); and 950 gravity stations obtained from the Defense Mapping Agency (written communication, 1994). Gravity stations collected by the U.S. Geological Survey for the Los Angeles Regional Seismic Experiment line II (LARSE II) are also included. More detailed information on recent U.S. Geological Survey data collection in the Los Angeles area is contained in Wooley and Langenheim (2001). The datum of observed gravity for this map is the International Gravity Standardization Net of 1971 (IGSN 71) as described by Morelli (1974); the reference ellipsoid used is the Geodetic Reference System 1967 (GRS67; International Association of Geodesy and Geophysics, 1971).
The observed gravity data were reduced to free-air anomalies using standard formulas (e.g. Telford and others, 1976). Bouguer, curvature, and terrain corrections to a radial distance of 166.7 km) were applied to the free-air anomaly at each station to determine the complete Bouguer anomalies at a standard reduction density of 2.67 g/ cm3 (Plouff, 1977). An isostatic correction was then applied to remove the long-wavelength effect of deep crustal and/or upper mantle masses that isostatically support regional topography. The isostatic correction assumes an AiryHeiskanen model (Heiskanen and Vening-Meinesz, 1958) of isostatic compensation; compensation is achieved by varying the depth of the model crust-mantle interface, using the following parameters: a sea-level crustal thickness of 25 km, a crust-mantle density contrast of 0.40 g/cm3, and a crustal density of 2.67 g/cm3 for the topographic load. These parameters were used because (1) they produce a model crustal geometry that agrees with seismically determined values of crustal thickness for central California, (2) they are consistent with model parameters used for isostatic corrections computed for the rest of California (Roberts and others, 1990), and (3) changing the model parameters does not significantly affect the resulting isostatic anomaly (Jachens and Griscom, 1985). The computer program ISOCOMP (Jachens and Roberts, 1981) directly calculates the attraction of an Airy-Heiskanen root by summing the attraction of individual mass prisms making up the root and thus calculating the isostatic correction; the resulting isostatic residual gravity values should reflect lateral variations of density within the mid- to upper crust.