Seiche Effects in Lake Tekapo, New Zealand, in an Mw8.2 Alpine Fault Earthquake
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Pure and Applied Geophysics
Seiche Effects in Lake Tekapo, New Zealand, in an Mw8.2 Alpine Fault Earthquake XIAOMING WANG,1 CAROLINE HOLDEN,1 WILLIAM POWER,1 YAORU LIU,2 Abstract—This study investigates the potential for seismic seiches in Lake Tekapo, New Zealand, triggered by ground shaking from an Mw8.2 Alpine Fault earthquake. Synthetic ground motions are used as a forcing boundary to drive lake water motions by further developing a tsunami simulation model—COMCOT—and coupling it with earthquake simulation model outputs. Our modelling results reveal that lake water oscillations are mobilised immediately by the ground movement and further amplified by cross-lake seiches. Amplitudes of lake oscillations can reach up to 4.0 m in the lake’s narrow southern arm, over 1.0 m along the shore of Lake Tekapo township, and about 1.5–2.5 m along many other parts of the lake shore. Large-amplitude water oscillations quickly attenuate in the first 5–10 min after the earthquake due to their relatively short periods, while long-period oscillations continue for a long time, albeit with much smaller amplitudes. Spectral analysis clearly reveals that the ground motions trigger both fundamental and higher modes in the lake whose oscillation periods are consistent with theoretical estimates. We find that large-amplitude lake water oscillations are better correlated with lowfrequency, less energetic ground motion content than with highfrequency large-amplitude ground motions. Ground motion-triggered lake oscillations are large enough to pose potential threat to tourists, residents, boats and infrastructure both in the lake water and onshore near the waterfront. In contrast, vertical co-seismic displacements in the lake area, the conventional mechanism for tsunami generation, are too small to trigger tsunami waves of concern. Keywords: Seismic seiches, tsunami hazard, ground motion, Lake Tekapo, Alpine Fault.
1. Introduction Seiches are long-standing waves, often observed in partially or fully enclosed bodies of water such as bays, harbours, fjords, lakes, rivers and ponds. The
1
GNS Science, Lower Hutt 5011, New Zealand. State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China. E-mail: [email protected] 3 NIWA, Wellington 6021, New Zealand. 2
and JOSHU MOUNTJOY3
term seiche was probably first introduced by Swiss hydrologist Franc¸ois-Alphonse Forel to describe the standing wave oscillations of the water surface in Lake Geneva (Lac Le´man), Switzerland (Darwin 1898; McGarr and Vorhis 1965). Seiches can be induced by various mechanisms including earthquakes, barometric pressure changes, winds, or other types of incident waves such as tsunamis, especially when natural oscillation periods of water in a confined basin are close to those of the triggering mechanism (McGarr and Vorhis 1965; Wilson 1972; Rabinovich 2009). It has been widely recognised that seismic waves are able to initiate water motions in lakes and fjords even thousands of kilometres away from their sources, typically c
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