Investigation of the 1727 Newbury, Massachusetts, USA, earthquake using LiDAR imagery and P-wave velocity tomography

High-resolution LiDAR (light detection and ranging) images of northeastern Massachusetts and southeastern New Hampshire reveal a 10-km-long, NW-SE-oriented topographic lineament in northeastern Massachusetts that we interpret to be the surface expression of a SW-dipping thrust fault along which the 1727 Newbury, Massachusetts, earthquake occurred. The Newburyport lineament coincides with the northeast edge of a 10-kmlong, NW-SE-oriented ridge, herein named Merrimack ridge, that parallels the NW-SE-trending segment of the Merrimack River downstream from where it bends 90° to the southeast. The northwestern end of the Newburyport lineament coincides with a 1-km-long, ~7- to 15-m-high, NE-facing Newburyport scarp that is located just south of the bend in the river. The Newburyport lineament also parallels the NW-SE-oriented nodal planes of the focal mechanism that was generated for the 1999 Amesbury, Massachusetts, earthquake. A P-wave velocity tomographic model generated from a seismic-refraction profile across the Newburyport scarp shows a ~40-m-wide low-velocity zone dipping ~41° SW. Velocities along this zone decrease 15–50%, which suggests that the Newburyport lineament is associated with the surface expression of a SW-dipping brittle fault zone. The LiDAR images also revealed three other NW-SE-trending lineaments in the study area.