FAULT-ZONE CHARACTERISTICS

Millions of years of strike-slip movement along faults of the San Andreas system have produced, in many places, a narrow fault zone in which physical properties differ from those in the surrounding rock masses. These differences are due to the presence within the fault zone of fractured or pulverized rock, exotic rock slivers that have been transported along the fault from other places, and such mobile materials as fluids and serpentinite that have migrated along the fault zone. A few investigators have used gravity and magnetic data to study the properties of this zone.

Although Stierman (1984) and Wang and others (1986) sought to explain gravity lows along the fault as the result of a substantial increase in porosity by fracturIng, gravity lows not directly associated with basins filled by Cenozoic sedimentary rocks along the faults are very uncommon. These rare lows amount, with few exceptions (such as the 10-12-mGal low studied by Stierman, 1984), to amplitudes of only a few milligals. Feng and McEvilly (1983) and Trehu and Wheeler (1987) inferred from seismic data that zones of low seismic velocity 5 to 10 km wide and more than 10 km deep are associated with the g San Andreas fault zone and, presumably, with fractured rocks. The low-velocity zone of Trehu and Wheeler (1987), however, has no associated gravity low, even though calculations by Andrew Griscom indicate that this zone might be expected to produce a gravity anomaly of about -25 mGal and more than 10 wide, using the standard velocity-density relations of Hill (1978). An explanation for this unexpected result can be found in the borehole gravity and seismic-velocity results (Schmoker, 1977; Stierman and Kovach, 1979) from a 600-m-deep borehole in diorite located 1.2 km from the San Andreas fault. For the lower half of this borehole, the seismic velocity averages only 3.1 km's (although saturated core samples measured 6.6 km's in the laboratory), and the average computed rock densities are as follows: bulk density from cores, 2.72 g/cm3; borehole density from gravity measurements, 2.60 g/cm3; and computed density from borehole velocities (density-velocity relations of Hill, 1978), 2.36 g/cm3. Correcting for a nearby low-density sedimentary section that causes a gravity gradient along the hole raises the borehole density (from gravity measurements) closer to the bulk density. The results described above indicate that macrofractures can cause large decreases in seismic velocity but much smaller decreases in density than those predicted from standard velocity-density relations.

Allen (1968) pointed out a possible relation between the style of fault movement and the presence of serpentinite within fault zones of the San Andreas, Calaveras, and Hayward faults. He noted that serpentine is common within the fault zone along the creeping section of the San Andreas fault between Hollister and Cholame, whereas it is absent along the locked segments to the north and south. Irwin and Barnes (1975) noted the same relation between serpentinite and fault creep and discussed the possible role of metamorphic fluids on the seismic behavior of fault segments. Hanna and others (1972) studied aeromagnetic data along the San Andreas fault between San Francisco and San Bernardino and found that the creeping segment of the fault is characterized by broad aeromagnetic anomalies, which they interpreted as reflecting large concealed masses of serpentinite. Linear magnetic anomalies that most likely reflect serpentinite also are present along the creeping section of the Hayward fault east of the San Francisco Bay (fig. 9.3). These magnetic data support the speculation that appreciable amounts of serpentinite contained within a fault zone can influence the style of movement on the fault.