The largest earthquake to affect northern Delaware in 100 years occurred at 08:21 GMT on February 28, 1973. The event was perceptible over 15,000 km2 in New Jersey, Delaware, Pennsylvania, and Maryland with a maximum Modified Mercalli intensity of V-VI near the Fall Zone between Wilmington and Claymont, Delaware. The isoseismals are elongate in a northeast-southwest direction. The main shock had a magnitude of 3.8 and was located at 39°43.1′N, 75°26.4′W with a depth of 14.1 km as calculated by NOAA. The five aftershocks located in this study were centered within a few kilometers of the region of greatest intensity at about 39°47′N and 75°25′W with depths ranging from 5 to 8.4 km. The fault plane solution determined from the mainshock and aftershocks indicates dip-slip motion on a nearly vertical plane striking N28°E. The southeast side is down in agreement with geological observations of subsidence of the Coastal Plain and uplift of the Piedmont. The strike of this fault is similar to that of the border faults of a graben in the basement rocks about 30 km southwest along the strike from the epicenter suggesting that the seismic activity may be associated with such faults.

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