Temporal and three-dimensional spatial analyses of the frequency-magnitude distribution near Long Valley Caldera, California

Abstract

SUMMARY The 3-D distribution of the b value of the frequency‐magnitude distribution is analysed in the seismically active parts of the crust near Long Valley Caldera, California. The seismicity is sampled in spherical volumes, containing N=150 earthquakes and centred at nodes of a grid separated by 0.3 km. Significant variations in the b value are detected, with b ranging from b#0.6 to b#2.0. High b-value volumes are located near the resurgent dome, and at depths below 5 km at Mammoth Mountain. b values are found to be much lower south of the Long Valley Caldera. We interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath Mammoth Mountain in 1989, and that anomalous crust, either highly fractured or containing unusually high pore pressure, such as is the case in the vicinity of active magma bodies, exists north of the seismically active area beneath the resurgent dome at all depths. We also investigate the spatial distribution of temporal variations of the frequency‐magnitude distribution by introducing diVerential b-value maps. b values increased from b#0.8 to b#1.5 underneath Mammoth Mountain at the onset of the 1989 earthquake swarm and remained high thereafter. This suggests that an intrusion permanently altered the average distribution of cracks at 5‐10 km depth, or that the pore pressure permanently increased. We propose that high b values are a necessary (but not suYcient) condition near a magmatic body, and therefore spatial b-value mapping can be used to aid in the identification of active magma bodies.