Another example of anomalies with no correlation with surface rocks is the western flank of a broad magnetic high called the Great Valley magnetic high. The mafic rocks that probably underlie the young sedimentary rocks exposed in the valley may be part of an ophiolite sequence and are a piece of old oceanic crust, now part of the North American continent. In areas where earthquakes have occurred along elongated magnetic highs, serpentinite is found concentrated along fault zones. There are also elongated magnetic anomalies caused by serpentinite with little or no seismic activity recorded in the last 20 to 30 years. Considering that several belts of magnetic anomalies point towards San Francisco and San Jose, information on when these faults last moved is needed in order to assess their earthquake potential.
Magnetic anomalies can also help with mapping the contact between magnetic and non-magnetic rocks.
For example, along the offshore part of the San Gregorio-Hosgri fault, seismicity is fairly scattered, but essentially parallels the magnetic contours. This suggests that the fault is dipping to the east. As another example, the broad magnetic high in the Santa Cruz Mountains hints at an asperity along the San Andreas fault. The fault bends in this area because gabbro, the inferred source of this magnetic high, can be very strong and resist deformation. As a result of this bend in the fault, the Santa Cruz Mountains are being formed.