Paleoseismicity and mass movements interpreted from seismic-reflection data, Lake Tekapo, South Canterbury, New Zealand

A 61 km seismic survey of Lake Tekapo was shot in 2001 to identify tectonic features observed onshore. The survey revealed bedrock highs, lake-floor offsets, and a series of mass movement deposits, all interpreted to result from tectonic uplift and paleoearthquake events. Fine-grained sediment within the lake basin, imaged as uniform, regular-spaced, laminated reflectors, is at least 145 m thick at the southeastern end of the lake. Bedrock highs (> 70 m of relief) are found along-strike of the Irishman Creek Fault and Forest Creek Faults, and are interpreted as long-term features that are repeatedly raised in earthquakes and lowered by glaciers. Movement on them since the last glacial maximum has offset the lake floor by 10-20 m, consistent with estimated uplift rates on these faults from previous studies. The seismic reflection data suggest that both faults extend into the centre of the lake, terminating against a north-south-oriented structure, possibly the Tekapo River Fault. Mass movement deposits are observed within the sediment pile, and we attribute them to paleoearthquakes on local faults or the more distant p ate boundary. Using a sedimentation rate of 8 mm/yr, we date two sets of mass movement deposits at 1720 +/- 344 yr BP and 2810 +/- 562 yr BP.