ASYMMETRIC RADIATION OF SEISMIC-WAVES FROM AN ATOLL - NUCLEAR TESTS IN FRENCH-POLYNESIA

Seismic records of nuclear tests detonated in the Mururoa Atoll in Fre nch Polynesia show large unpredicted arrivals 2.2 and 4.5 seconds (X1 and X2) after the P-wave at the Australian Warramunga Array. These arr ivals are not observed at the Canadian Yellowknife Array. X1 and X2 ar e also absent on Warramunga Array recordings of tests carried out at t he Fangataufa Atoll situated 40 km SSE of Mururoa. Array analysis show s that X1 and X2 are produced within the source area. The layered crus tal structure of the atoll, significant local inhomogeneities, and foc using effects due to the elongated shape and the steep flanks of the M ururoa Atoll are most likely responsible for X1 and X2. The form of Mu ruroa (28x10 km) and its East-West orientation is due to its location on the Austral Fracture Zone (AFZ). The Fangataufa Atoll on the other hand is almost circular (10 km diameter) and is unaffected by the dyna mics along the AFZ. Our observations demonstrate that complicated stru ctures in the source area can significantly alter the wave field at te leseismic distances and produce a large magnitude (m(b)) bias. A bette r understanding of the exact cause of these unusual seismic observatio ns will only become possible, if the coordinates of the tests and info rmation on the detailed 3-D structure of the atolls are released.