Seismic records of nuclear tests detonated in the Mururoa Atoll in Frenc Polynesia show large unpredicted arrivals 2.2 and 4.5 seconds (X1 and X2) after the P-wave at the Aurtralian
Warramunga Array. These arrivals are not observed at the Canadian Yellowknife Array. X1 and X2 are also absent on Warramunga Array recordings of tests carriedout at the
Fangataufa Atoll situated 40 km SSE of Mururoa. Array analysis shows that X1 and X2 are produced within the source area. The layered crustal structure of the atoll, significant local
inhomogeneities, and focusing effects due to the elongated shape and the steep flanks of the Mururoa Atoll are most likely responsible for X1 and X2. The form of Mururoa (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 dynamics along the AFZ. Our observations demonstrate that complicated structures in the source area can significantly alter the wave field at teleseismic distances and produce a
large magnitude (mb) bias. A better understanding of the exact cause of these unusual seismic observations will only become possible, if the coordinates of the tests and information of
the detailed 3-D structure of the atolls are released.