Frequency-magnitude statistics and spatial correlation dimensions of earthquakes at Long Valley caldera, California

Intense earthquake swarms at Long Valley caldera in late 1997 and early 199 8 occurred on two contrasting structures. The first is defined by the inter section of a north-northwesterly array of faults with the southern margin o f the resurgent dome, and is a zone of hydrothermal upwelling. Seismic acti vity there was characterized by high b-values and relatively low values of D, the spatial fractal dimension of hypocentres. The second structure is th e pre-existing South Moat fault, which has generated large-magnitude seismi c activity in the past. Seismicity on this structure was characterized by l ow b-values and relatively high D. These observations are consistent with l ow-magnitude, clustered earthquakes on the first structure, and higher-magn itude, diffuse earthquakes on the second structure. The first structure is probably an immature fault zone, fractured on a small scale and lacking a w ell-developed fault plane. The second zone represents a mature fault with a n extensive, coherent fault plane.