The 1990 Lake Tennyson earthquake sequence, Marlborough, New Zealand

Abstract

Aftershocks from the 1990 Lake Tennyson earthquake (ML 5.8) recorded at nine temporary portable seismographs have been used to invert travel-time data simultaneously for both hypocentre and velocity parameters, resulting in a 1 -D velocity model and station terms for the Lake Tennyson region. The distribution of the best relocated aftershocks outlines a main fault lineation in a ENE direction, and several off-fault clusters. The main fault lineation is 8 km long, with a strike of about 60° and a dip that is nearly vertical. It is located between and subparallel to the Awatere and Fowler Faults, on a previously unknown fault. The mainshock has been relocated in the middle of this lineation zone, which suggests that the fault ruptured bilaterally. The distribution of aftershocks matches that expected from the Coulomb failure criterion, which identifies areas of increased and decreased stress levels due to the occurrence of the mainshock.Focal mechanisms for the mainshock and aftershocks that make up the main fault lineation are consistent with rightlateral strike-slip movement on this fault. Clusters that extend from each end of the main fault lineation predominantly 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 relative plate motion direction in the Lake Tennyson region.

Stress and crustal anisotropy in Marlborough, New Zealand: evidence for low fault strength and structure-controlled anisotropy

“…As noted above, we also computed stress directions at a smaller scale in the Lake Tennyson area, where the focal mechanisms are well‐constrained and there is a reasonably large number (∼10) of first motion observations for each event (McGinty et al 1997). The complete data set, containing 34 focal mechanisms, was first analysed using all three stress inversion algorithms.…”

“…usgs.gov/neis/sopar) catalogues. First motion data obtained by the Institute of Geological and Nuclear Sciences (GNS) during studies of the 1990 Lake Tennyson earthquake sequence (McGinty et al 1997) and a portable deployment recording seismicity in Marlborough (Reyners et al 1997) have all been used (see Fig. 1 for station locations).…”

“…We also applied all three algorithms to data from the Lake Tennyson earthquake and aftershock sequence (Fig. 4), where the focal mechanisms are well constrained by both first motions and amplitude ratios (McGinty et al 1997).…”

“…The large circles in the boxes are output of the different stress inversion algorithms, SLFAST (S), FMSI (F) and MOTSI (M). The stress inversions were performed on the three groups of data defined by McGinty et al (1997): grey shaded focal mechanisms indicate events that lie on the main rupture(‘on fault’); black shaded focal mechanisms indicate events that lie off the main rupture (‘off fault’); and the ‘all’ group includes all 34 events. The black line indicates the S Hmax direction, calculated using the transformation described by Lund (2000).…”

Multiple‐mainshock events and long‐term seismogenesis in Italy and New Zealand

“…Values outside the 95th percentile occur at n = 4 for N Hawke Bay, and at n = 4 (and n = 3) for Hawks Crag. For the Lake Tennyson event, which has been analysed by McGinty et al (1997), the significance is somewhat less, although at n = 3 and n = 9 the values lie on the 95th percentile. The value at n = 3 is farther from the mean than is the value at n = 9, and for this reason the three largest magnitudes are Evison & Rhoades-Long-term seismogenesis, Italy and NZ 527 taken here as those deviating from the values expected under the Gutenberg-Richter relation.…”