A sequence of microearthquakes near Annsville, New York, during January 1980 yielded a composite focal mechanism solution indicating east-northeast compression resulting in thrust motion on a north-northwest striking fault plane. These findings are in agreement with the east-northeast-trending, near-horizontal maximum compressive stress field defined for most of the Eastern United States.
An earlier sequence of minor seismicity near Annsville during September 1977 resulted in a similar solution as did a sequence at nearby Wappingers Falls, New York, in June 1974. Other local events, however, indicate northwest compression and faulting on northeast-striking structures, notably at Mt. Kisco, New York, in 1979. Due to the prominent northeast-trending structural grain of the region and the suggestion of northwest compression from events such as at Mt. Kisco, the interpretation of northwest compression resulting in thrust motion on northeast-trending faults has been favored for this region to date. These recent data from earthquakes near Annsville provide cause for questioning the current interpretation and highlight the need for deep in situ stress measurements in this area to help resolve the conflicting data from focal mechanism solutions.