ATTITUDE

At several sites along the San Andreas fault, the juxtaposition of large masses with contrasting densities and (or) magnetizations causes characteristic potential-field anomalies that reflect the attitude of the fault. Information on the dip of the fault most commonly is obtained through quantitative modeling of these anomalies, but in some cases, the anomalies are so diagnostic that qualitative interpretations suffice to indicate the direction and approximate attitude of the fault plane.

The results of such interpretations at 16 sites primarily along the main trace of the San Andreas fault are shown in figure 9.4. The dip and depth-extent of the density or magnetization interfaces that are assumed to define the fault plane at these sites are somewhat uncertain because of the inherent ambiguity of gravity and magnetic interpretations, particularly those based on magnetic data, because rock magnetizations can have anomalous directions associated with their remanent components and because magnetic susceptibilities seldom are known with sufficient accuracy to serve as effective independent constraints. Thus, where the magnetic anomalies can be compatible with a vertical fault, we show the dip as vertical. The reader should be aware, however, that a dipping interface extending to shallower depth would also be compatible with the data in some places.

Although major strike-slip faults probably are vertical over much of their reach, some have inferred dips of less than 90°, as indicated by many of the attitudes shown in figure 9.4. Just north of Point Arena, a buried magnetic body truncated on the east by the San Andreas fault has an east boundary that dips east beneath the trace of the fault (fig. 9.5A); its precise dip is uncertain but probably falls in the range 30°-50°. To the south, near the junction of the San Andreas and Calaveras faults, gravity modeling (Pavoni, 1973) suggests that the fault dips 70° SW. to a depth of about 6 km. A detailed study of seismicity along this section of the fault (Spieth, 1981) shows that hypocenters define a plane dipping 70° SW., thus strongly supporting the interpretation by Pavoni (1973). Robbins (1982) also found a southwest-dipping density interface at this site but argued that the fault plane was vertical, on the basis of a magnetic anomaly that he believed reflected a magnetic body, extending from 3- to 5-km depth, with a southwest edge directly beneath the surface trace of the fault. More recent, detailed magnetic measurements indicate that this magnetic body is much shallower (probably cropping out) than modeled by Robbins (1982) and thus weaken the argument for a vertical dip.

Near the intersection of the San Andreas and Garlock faults, gravity modeling by Andrew Griscom and K.G. Freeman (Griscom and Oliver, 1980) suggests that the fault dips 55° SW. to a depth of 6 km and thence vertically to a depth of at least 10 km (fig. 9.5B). About 60 km farther southeast, gravity data also indicate a southwesterly dip for the fault, but the angle of dip (30°-60°) is uncertain, owing to difficulty in interpreting the complex gravity field that results from large lateral density variations in the region southwest of the fault. Farther southeast, where the San Andreas fault splits into numerous branches (long 116°00' W.), gravity data on two branches indicate that both faults dip northeast, with Precambrian crystalline rocks in the upper plate overlying young sedimentary rocks and alluvium. The gravity models suggest dips of 15°-25° NE. to depths of 1.5 to 2.5 km but do not resolve the fault attitude at greater depth (fig. 9.5C). Geologic mapping, which shows part of the southern branch of the San Andreas fault as a northdipping thrust fault (Matti and others, 1985), and a study of recent earthquakes in this area, which yielded fault-plane solutions of predominantly oblique-slip motion and including low-angle thrust solutions dipping 30° N. (Nicholson and others, 1986), both support the gravIty interpretation of northeast-dipping faults in this area.

The inferred fault attitudes shown in figure 9.4 suggest a relation between attitude and plan-view geometry. Faults tend to be vertical except where they undergo abrupt changes of strike. The sinistral bends in the San Andreas fault near its junction with the Calaveras fault and in the Big Bend region southeast of its junction with the Garlock fault create regions likely to be subject to compression due to relative southward movement of the North American plate with respect to the Pacific plate. The dipping fault planes in these regions may reflect a thrust component of fault movement that accommodated the compression. Similarly, the region around the broad dextral bend in the San Andreas fault north of Point Arena may have a component of extension parallel to the direction of relative plate motion, and the low- angle eastward dip of fault plane there may reflect accommodation of the extension by low-angle normal faulting.

The number of examples on which the above speculations are based is quite limited, and further detailed investigations at critical sites along the San Andreas fault system are needed to test the relation between fault attitude, change of strike, and relative plate-motion direction.