THE 1990 LAKE-TENNYSON EARTHQUAKE SEQUENCE, MARLBOROUGH, NEW-ZEALAND

Aftershocks from the 1990 Lake Tennyson earthquake (M-L 5.8) recorded at nine temporary portable seismographs have been used to invert trave l-time data simultaneously for both hypocentre and velocity parameters , resulting in a 1-D velocity model and station terms for the Lake Ten nyson region. The distribution of the best relocated aftershocks outli nes a main fault lineation in a ENE direction, and several off-fault c lusters. The main fault lineation is 8 km long, with a strike of about 60 degrees and a dip that is nearly vertical. It is located between a nd subparallel to the Awatere and Fowler Faults, on a previously unkno wn fault. The mainshock has been relocated in the middle of this linea tion zone, which suggests that the fault ruptured bilaterally The dist ribution of aftershocks matches that expected from the Coulomb failure criterion, which identifies areas of increased and decreased stress l evels due to the occurrence of the mainshock. Focal mechanisms for the mainshock and aftershocks that make up the main fault lineation are c onsistent with right-lateral strike-slip movement on this fault. Clust ers that extend from each end of the main fault lineation predominantl y do not have strike-slip focal mechanisms. Aftershocks located in the clusters to the north of the main fault lineation have various thrust mechanisms with no consistent orientation. Most focal mechanisms from this sequence had their P axes closely aligned with the regional axis of compression, and the main fault lineation is consistent with the r elative plate motion direction in the Lake Tennyson region.