Interpretation of strain data: Cannikin nuclear test.

abstract Immediately following CANNIKIN, strain and tilt steps were recorded at distances of 10 to 1,100 km. At the 10-km stations, the steps (> 10 −4 were larger than expected and indicated that nonelastic effects existed out to that distance. The “quasi-static” strain step, which may represent elastic response to pressure in the cavity, had two exponential decay constants: 10.6 min during the first 5 min and 17.7 min after 8-min postshot. A significant assymetry in the strain field, evident from observatories at distances of 15 to 300 km, suggests faulting near ground zero. The fault movement was equivalent to that normally associated with an earthquake of magnitude 6 to 6 1 2 , which implies a fault length of 10 to 30 km and a dislocation of 30 to 100 cm. The steps recorded at the time of collapse are compatible with a collapse mechanism, thus suggesting that little if any fault movement occurred away from the chimney at that time.