Using airborne magnetic data to map folding and faulting in sedimentary layers: implications for seismic hazard (Invited)

Aeromagnetic surveys are increasingly used to map structure within sedimentary rocks important for seismic assessment as better magnetometers, positioning, and techniques are developed. We present three examples in which aeromagnetic data are used to map folding and faulting within Cenozoic sedimentary rocks and deposits. In the Salton Trough, detailed aeromagnetic data collected in 1990 suffered from leveling problems that obscured low-amplitude (less than 2-3 nT) magnetic anomalies arising from Tertiary sedimentary rocks. Decorrugation and subtraction of a regional field (upward continuation of 100 m) isolated and enhanced these low-amplitude anomalies, some of which extend the length of the Clark fault, a major strand of the San Jacinto fault zone in southern California, another 20-25 km southwest of its termination point. Other anomalies point to distributed deformation confirmed by detailed surficial mapping by geologists. Detailed aeromagnetic data in the San Ramon Valley, California area show curvilinear anomalies that arise from folding and faulting of the Neroly sandstone, a Miocene unit whose magnetization is due to andesitic detritus. Detailed geologic maps and drillholes locally constrain the geometry of the Neroly Formation at the surface and subsurface, but constrained inversion of aeromagnetic data identified folds not earlier seen. In northern California (e.g. Ukiah), similar long (up to 50 km), curvilinear magnetic anomalies also occur, but in an area where drillholes are absent and geologic mapping is limited by dense vegetation, steep slopes, abundant landsliding, and thick soils. Magnetic susceptibility measurements from sparse outcrops show that the anomalies arise from lithic, volcanic-rich graywacke and metabasalt within the Franciscan Complex. The similarity in anomaly characteristics between the San Ramon and Ukiah areas suggests that the graywackes are folded, coherent bodies within an assemblage that at the surface is termed “broken”. The dips of the magnetic layers can be estimated from modeling of reduced-to-pole anomalies. A vertical tabular body or symmetrical anticline produces a symmetric magnetic anomaly high centered over the body, with adjacent lows on that are the same shape and amplitude. If the tabular body or fold axis is not vertical, the low on the side toward the dip direction will be diminished relative to the low on the opposite side. The discrepancy becomes more pronounced as the dip shallows. Many of the magnetic bodies trend northwest and dip to the northeast, as expected for interleaving of tectonic blocks within the Franciscan Complex. Seismicity suggests that some of these features may be reactivated. Thus aeromagnetic data map structures that may be accommodating strain between the San Andreas and Maacama faults and that may warrant additional study.