ARE EARTHQUAKES IN NEW YORK RELATED TO MAFIC PLUTONS?

The seismicity of New York State is not entirely explicable in terms of geologic structure. Earthquake epicenters show no apparent correlation with the northeast trending mapped faults in the area. Also fault plane solutions indicate earthquakes are related to northwest trending faults, however no northwest trending faults can be observed. The earthquake epicenters are scattered throughout the state, show no alignment along faults and therefore are not well understood. To locate structures that might be triggering earthquakes, 8,400 gravity measurements were made to construct a Bouguer gravity map of the state. This gravity map shows several prominent gravity anomalies that correlate with the seismicity of the state.

The gravity map indicates that earthquakes are concentrated in regions of steep gravity gradients and gravity highs. These gravity highs are due to high-density mafic plutons. Most of the earthquake epicenters in northern and western New York are associated with these gravity highs and the mafic plutons causing the highs. It is proposed that the mafic plutons constitute crustal inhomogeneity that serves as a stress concentrator to trigger earthquakes. In areas such as central New York, where the crust is homogeneous and few gravity anomalies occur, there are few or no earthquakes. In general, most of the earthquakes occur in regions of crustal inhomogeneity indicated by gravity anomalies.