ISOSTATIC RESIDUAL GRAVITY MAP

An isostatic residual gravity map of the region surrounding the San Andreas fault system is shown in figure 9.2. We have chosen to present the gravity data in this form rather than in terms of the more common Bouguer or free-air gravity because of the generally closer correlation between isostatic residual gravity and mapped geology (Jachens and Griscom, 1985; Simpson and others, 1986). Most long-wavelength anomalies (longer than approx 250 km) on a Bouguer gravity map are caused by deep-seated density distributions that buoyantly support the topography in a manner consistent with the principle of isostasy (Simpson and others, 1986). Bouguer gravity anomalies related to isostasy are prevalent in California because of the extreme topographic relief in the State (Oliver, 1980; Jachens and Griscom, 1985), and they are particularly strong near the coast, where an eastward to northeastward decrease in gravity reflects the transition from thin oceanic crust to thicker continental crust. In such areas as California, the Bouguer gravity anomalies associated with isostatic support of topography are so strong that they tend to distort or even mask the lower-amplitude anomalies caused by density distributions in the middle to upper crust, those anomalies most easily correlatable with rocks exposed at the surface (Jachens and Griscom, 1985). Our isostatic residual gravity map has these long-wavelength isostatic effects removed, at least to first order. We emphasize that the anomalies remaining on our map are predominantly caused by lateral density variations in the middle to upper crust and, as such, do not represent areas that are out of isostatic balance (Jachens and Griscom, 1985).

Our isostatic residual gravity map (fig. 9.2) is based on the new isostatic residual gravity map of the conterminous United States by Simpson and others (1986), who presented a detailed discussion of the data sets and procedures used to generate this map. The basic gravity-data set was compiled for the "Gravity Anomaly Map of the United States" (Society of Exploration Geophysicists, 1982) and includes 1 million on shore and 0.8 million offshore gravity observations. These data were sampled on a rectangular grid with a grid spacing of 4 km, containing Bouguer gravity values onshore and free-air gravity values at sea (Godson, 1985). To produce our isostatic residual gravity map, the offshore free-air gravity values were converted to Bouguer gravity values. The gravitational effects of the deep density distributions that support the topography within 166.7 km of each grid intersection were computed according to the Airy-Heiskanen model of isostasy (Heiskanen and Moritz, 1967) using a 5- by 5-minute topographic-bathymetric data grid and model parameters as follows: topographic density, 2.67 g/cm3; crustal thickness at sea level, 30 km; and density contrast across the base of the model crust, 0.35 g/cm3. Combined isostatic and topographic effects for the region from 166.7 km to the antipode of each grid intersection were obtained from the maps by Karki and others (1961). This model gravity field was subtracted from each Bouguer gravity grid value to yield a grid of isostatic residual gravity values; the resulting grid was contoured by computer and displayed in color-band intervals of 10 mGal to produce figure 9.2.

Limitations on the use of this map stem both from uncertainties in the point data from which the grid was constructed and from characteristics generated by the gridding process. For onshore data, uncertainties in the point data values resulting from errors in observed gravity, elevation, terrain corrections, and isostatic reductions are estimated to be less than 2 to 3 mGal for most stations, possibly larger in areas of extreme topographic relief (Simpson and others, 1986). In offshore areas, the greatest uncertainty results from conversion of the original free-air gravity data to Bouguer gravity values, using the 5- by 5-minute average bathymetry. Where the sea-bottom topography is relatively gentle, this conversion probably results in uncertainties of about 5 mGal, but in such areas as parts of the California Continental Borderland (south of lat 34° N.) and over the edge of the Continental Shelf, where water depths change rapidly, errors of several tens of milligals are possible. These conversion errors generally appear as high-amplitude, nearly circular anomalies with diameters of as much as 40 km.

Although gravity coverage along most of the San Andreas fault system is quite dense when viewed at the scale of figure 9.2, sampling of these data on a 4-km grid means that anomalies with characteristic dimensions less than several times the grid spacing are not faithfully portrayed. Our isostatic residual gravity map (fig. 9.2) is sufficient for qualitative and quantitative interpretation at the scale shown, but for more detailed interpretations, especially quantitative modeling, the reader is referred to the original data sources, such as Oliver and others (1980), Roberts and others (1981), Snyder and others (1982), and the other reports cited throughout this chapter.