EASTERN NORTH AMERICA PASSIVE MARGIN EARTHQUAKES AND MESOZOIC RIFT STRUCTURES

Modern earthquake activity in central and eastern North America generally is thought to occur as reactivations in the modern stress field of pre-existing zones of weakness in the crust, but the identification of which specific faults can be reactivated is not very advanced. Work in the 1990s showed that the largest intraplate earthquakes in stable-cratonic regions primarily take place in previously rifted crust, and most such earthquakes occur in crust that underwent Mesozoic or younger rifting. This is true in eastern North America, where many of the small earthquakes occur around or within some of the Mesozoic basins. Also, many of the modern small earthquakes have been centered near or on faults not associated with a Mesozoic basin but nevertheless were active during Mesozoic time. Offshore along the Atlantic coast of North America, earthquakes have been detected out to the continental rise. In some recent cases, the passive margin earthquakes appear to have nucleated on faults that were active in the Mesozoic but then those ruptures propagated away from the Mesozoic faults, presumably driven by the modern tectonic stress field. The damaging earthquakes in eastern North America that have occurred during historic and recent time all have had some association with structures that show Mesozoic activity. Most of the smaller earthquakes in the region also occur on or near structures that were active in Mesozoic time. These observations suggest that all of the Mesozoic basins and faults that were active during Mesozoic or later time should be considered as potential source zones of future large earthquakes.