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

Abstract

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 plate 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.