LINK THE CRUSTAL STRUCTURE WITH SEISMICITY AND TECTONIC TERRANES IN THE NORTHERN APPALACHIANS

The earthquake distributions in the northern Appalachians are bounded by major geologically-defined terrane boundaries. There is a distinct seismic gap along the Taconic deformation front between the Grenville Province to the west-northwest and the seismically active Ganderia and Avalonia terranes to the east-southeast. It is not clear, however, how the seismicity characteristics within different bedrock terranes are related with or controlled by the deep crustal structure. Here we present a newly constructed crustal P- and S-wave velocity model for the northern Appalachians using Rayleigh wave data extracted from ambient noises. Our tomographic velocity model reveals a strongly heterogeneous crustal structure. Variation of the crustal velocity structure might be a deep manifestation of the surface bedrock terranes. The earthquakes are mainly concentrated within the heterogeneous upper crust. The upper crust velocities beneath the Appalachian accretionary terranes are generally lower than those within the Grenville Province. The mid- to lower crust has relatively lower velocities than the upper crust across the entire study area. Earthquakes within the Grenville Province are located generally deeper than those within the Appalachians Province. Earthquakes are located either along large velocity gradient zones or within low velocities. The orientations of maximum compressive stresses change from W-E in the Appalachians Province to SW-NE in the Grenville Province. The contrasts in seismicity, velocity, and stress field across the Taconic deformation front indicate that the Taconic terrane may act as a seismically inactive buffer zone in the northern Appalachians.