An extensive earthquake swarm occurred at Durrwood Meadows in the southern Sierra Nevada of eastern California during late 1983 and 1984. It was located within a 100-km-long linear belt of seismicity that cuts through the southern Sierra Nevada along a north-southward strike. This seismic belt has been characterized by swarms and was one of the most seismically active features in southern California during 1984. The Durrwood Meadows swarm itself was characterized by a complex spatial distribution and a simple pattern of focal mechanisms. At the beginning of the swarm, the earthquakes were located along a northwestward trend; later, periods of high seismicity were distributed along a northeastward trend forming a Y-shaped structure, and along other, northward and northwestward trends. In spite of this spatial complexity, the focal mechanisms of the 35 ML ≧ 3.0 earthquakes within the swarm are all similar to each other, with almost pure normal faulting along a north-southward strike. The strikes of the nodal planes in the focal mechanisms and the spatial distribution of epicenters form an en-echelon pattern. The consistency of the focal mechanisms with each other and the en-echelon pattern of epicenters imply a homogenous stress field and a discontinuous fault structure. The 100-km-long linear belt of seismicity in an area with no throughgoing fault structure is interpreted as a basin-and-range normal fault beginning to form within the Sierra Nevada.

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