The 1990 Lake Tennyson earthquake sequence, Marlborough, New Zealand

Abstract

Aftershocks from the magnitude 5.8 ML 1990 Lake Tennyson earthquake recorded at nine temporary portable seismographs have been used to invert travel-time data simultaneously for both hypocentre and velocity parameters resulting in a one-dimensional velocity model and station terms for the Lake Tennyson region. The distribution of the best relocated aftershocks outline a main fault lineation in a ENE direction, a possible conjugate fault to the south, and several off-fault clusters. Relocations of aftershocks occurring before the installation of the portable network are consistent with the best relocated aftershocks. This configuration of aftershocks is classic of the distribution of aftershocks expected by Coulomb failure, which identifies areas of increased and decreased stress levels due to the occurrence of the mainshock. The main fault plane extends 7.5 km with a strike of about 60 degrees and a dip that is near vertical. This lineation is located between and sub-parallel to the Awatere and Fowler faults on a previously unknown fault. The mainshock has been relocated in the middle of this lineation zone suggesting that the fault ruptured bilaterally. A moment of 8.7 x10 17 Nm calculated for the mainshock is consistent with a rupture of this length and a source-time function for the mainshock is also consistent with a bilateral rupture. Focal mechanisms for the mainshock and a composite focal mechanism for aftershocks that make up the main fault lineation are consistent with right-lateral strike-slip movement on this fault. Clusters that extend from the fault tips of the main fault lineation also have strike-slip composite focal mechanisms, although there is evidence of thrusting in the NE extension. A composite focal mechanism that represents the majority of the aftershocks to the south of the main fault lineation is left-lateral strike-slip with a thrust component. The strike of this mechanism is about 158 degrees and is consistent with aftershock observations. Aftershocks in the clusters to the north of the main fault lineation have various thrust mechanisms with no consistent orientation of thrust mechanisms apparent. Aftershocks belonging to these clusters are very different from the rest of the sequence. All composite focal mechanisms had their P axes similarly aligned with the regional pressure axis, and the main fault lineation is consistent with relative plate motion direction in the Lake Tennyson region.