Geodiscovery / On Land / In Lakes / Lake sediments as seismometers

Lake sediments as seismometers

Lake sediments provide natural seismometers along the Alpine Fault.

Coring at Lake Christabel

Scientists prepare equipment for coring at Lake Christabel as part of seismic study

The Alpine Fault is one of the longest and fastest slipping plate boundary transform faults on earth and represents the largest source of seismic hazard for New Zealand’s South Island. Despite its high seismic hazard, little is known about the spatial extent of rupture and magnitude of earthquakes that occurred before the last moment magnitude1 (Mw) >8 earthquake in 1717 A.D. Traditional paleoseismic approaches that trench the surface trace of the fault to identify and date past earthquakes have not yielded data of sufficient quality to determine when, where, and how much of the fault ruptured in past earthquakes. We are applying an alternative approach using lake sediments to date and reconstruct the spatial extent of rupture for Alpine Fault earthquakes. Lakes situated adjacent to the Alpine Fault are sensitive recorders of seismic shaking during earthquakes. Our results demonstrate that strong shaking intensities (modified mercalli2 (MM) >VI) destabilise slopes within lake basins causing subaqueous mass-wasting and deposition of turbidites. When shaking intensities exceed MM IX extensive earthquake-induced landsliding is triggered on hillslopes in lake catchments, driving dramatic increases in sediment flux that form hyperpycnite sets in the lake basins. The presence of these two coupled deposits means that lake sediments can be used to distinguish between strong (MM VI) and violent (MM IX) shaking at a lake site. By identifying deposits formed by the high intensity shaking that occurs proximal to a ruptured fault and correlating between lakes spaced along its length we are developing a rupture history for the entire ~450 km of the onshore Alpine Fault over the last 3000 years.

Find out more about the project here.

1The moment magnitude scale (Mw) measures the size of earthquakes in terms of the energy released.

2The Modified Mercalli Intensity scale (MM) measures the size of earthquakes in terms of shaking intensity – how it is felt by people, and any structural damage that is caused.