INTRODUCTION

Studies on the gravity and magnetic fields of the San Andreas fault system span more than 30 years, but only recently have the fundamental data sets become adequate to provide a general view of the entire system. Modeling these new data defines the three-dimensional geometries of the faults and helps unravel the tectonic history of the system by "seeing through" the relatively thin cover of young sedimentary deposits and water to the older rocks below.


Studies of the San Andreas fault system using the Earth's gravity and magnetic fields began before 1960 but received their main impetus during the 1970's, when work on the possibility of predicting earthquakes on this system began in earnest. Early investigations focused mainly on short segments of the faults because only limited data were available. More extensive potential-field data sets that have been published in recent years now permit the gravity and magnetic expression of the entire fault system to be viewed in a regional context (fig. 9.1).

Gravity and magnetic data reflect, respectively, the density and magnetization of the rocks beneath the surface; and, in many situations, these properties can be closely correlated with the rock types seen in outcrop. Anomalies in the Earth's gravity and magnetic fields - for example, local deviations of the measured fields from those predicted on the basis of simplified Earth models - primarily reflect lateral variations in density and magnetization that generally are not included in such simple models. These anomalies can be interpreted qualitatively to infer the general spatial distribution of rock types in the subsurface, and quantitatively, through the use of efficient computer-based modeling techniques (Saltus and Blakely, 1983; Chuchel, 1985; Blakely and Simpson, 1986), to determine the geometries and specific locations of concealed rock bodies. Although all such interpretations are nonunique, both because many different distributions of density and magnetization can give rise to identical anomalies and because density and magnetization do not uniquely define a specific rock type, the combined use of gravity and magnetic data with geologic, geochemical, and other geophysical data can be especially effective in limiting the number of acceptable interpretations.

In the sections below, we first present regional gravity and magnetic maps covering the San Andreas fault system and briefly discuss the sources, compilation methods, and limitations of the data from which they were produced and, in general terms, the sources of the anomalies shown on them. We then summarize the results of individual studies of sections of the major faults in the system and attempt to synthesize these results in terms of the geometries of the faults, the structures and rock types in the surrounding areas that are related to the faults, and the properties of the fault zones. Next, we focus on studies that relate to movement on the faults, including constraints on total displacements. Finally, we discuss the plate-tectonic implications of potential-field investigations of the fault system.