abstract

An investigation of unusual minor seismic activity at Lake Hopatcong, New Jersey, over a 4-month period in 1969, yielded information that is unique in the study of the seismicity of the eastern United States. During this period, a sequence of 42 earthquakes was recorded at Ogdensburg, New Jersey, and five events of magnitudes of 1 to 1.5 were felt in the epicentral area. Short aftershock sequences were observed for three of the felt events. The earthquake that occurred at 02:27 GMT on October 6 1969 was felt over an area of approximately 50 km2. This event had a magnitude of 1.25 and an intensity of IV near the epicenter. This intensity was unusually high for such a small shock. Twenty-six earthquakes were recorded on high-frequency microearthquake equipment operating in the area from October 16 1969 to November 9 1969. Epicenters of 13 of these microearthquakes could be located, and these fall in an area of 1.5 × 3.0 km. The corresponding depths vary from 0.5 to 3.3 km. Accurate hypocentral locations were possible only because of the close spacing and sensitivity of the microearthquake instruments. The error in these locations was about ±0.5 km. A composite focal mechanism solution indicates that normal faulting occurred. The inferred plane of slip dips steeply to the southeast and strikes parallel to the NE-SW structural trends of the area.

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