NEW AIRBORNE MAGNETIC DATA IMAGE STRUCTURAL FEATURES IN THE CHARLESTON SEISMIC ZONE

The Charleston, SC, region was home to the 1886 M_w~7.0 earthquake and has present-day seismicity that is mostly M_w<3.0. Using sparse seismic reflection profiles, 1950s-1970s aeromagnetic data, shallow well data, present-day seismicity, and historical accounts of the 1886 earthquake, various groups have proposed hypotheses for the geometry of major faults or fault systems in the region. Some models have common elements, e.g. a Woodstock, Ashley River, and/or Charleston fault, while others are markedly different. The causative fault for the 1886 earthquake remains unknown.

To better image buried structures in the Charleston region, the USGS contracted new airborne magnetic and radiometric surveys in 2019. The surveys were flown over a 90 km x 134 km area with a 400-m line spacing, offering a 4x improvement over previous surveys. Magnetic anomalies in the area generally reflect source rocks beneath a 0.6-1 km-thick layer of relatively nonmagnetic coastal plain sediments. The new data highlight numerous linear and a few rounded anomalies; the latter suggest igneous intrusions or volcanic centers. The most prominent linear anomalies are NNW- or N-oriented, are magnetic highs, and have widths that strongly suggest sources in the upper 1-2 km. Some of these lineaments have been observed in previous data and interpreted as Mesozoic dikes. We also observe a family of ENE- to ESE-trending linear anomalies. These ~2-6 km-wide, 10-30 km-long anomalies are parallel or sub-parallel to the interpreted boundary of the South Georgia rift basin and suggest deeper (or broader) sources such as rift-related faults. Some sets of the ENE to ESE anomalies terminate along NE-oriented lineaments, which suggests Mesozoic or younger faulting along NE-trending Paleozoic structures or geologic units. Some of the NE-trending anomalies are > 50 km long and might be considered for further study relating to possible seismic hazard.

Most faults delineated in previous models are, in general, not well-aligned with lineaments in the new reduced-to-pole magnetic maps, although some are sub-parallel or partly overlap observed anomalies. Recent seismicity does appear to be aligned with an ESE-trending magnetic lineament. Ongoing analyses will integrate seismic reflection, ground-penetrating radar, and well data to better constrain fault models.