Urgent Cook Strait Fault Survey

The National Institute of Water and Atmospheric Research (NIWA) has diverted its research vessel Tangaroa to Cook Strait to investigate the vicinity of the Cook Strait earthquakes.

Wednesday, 24th July 2013

The swarm of earthquakes occurring beneath Cook Strait off the coast near Seddon and the significant magnitude 6.5 quake that struck nearby on Sunday have caused scientists to despatch a research vessel to investigate. NIWA’s Tangaroa spent half of today surveying the seafloor looking for evidence of surface faulting and any landslides that might have been triggered by the activity.

RV Tangaroa on Wellington Harbour. - courtesy of NIWA

[click for larger image] R.V. Tangaroa in Wellington Harbour ”“ courtesy of NIWA under Creative Commons Licence.

The Tangaroa surveyed the Cook Strait Canyon which plunges to depths of 3000 metres and the shallower part of the strait off-shore from Seddon where the seafloor lies at about 100 metres. The area has been surveyed before and several large submarine landslides have been identified.

[click for larger image] The path of today’s survey by R.V. Tangaroa ”“ courtesy of NIWA under the Creative Commons Licence.

Today’s survey was to determine whether the landslides had been disturbed by the recent earthquake activity, and probe for evidence of changes to the seafloor due to earthquake faulting. The survey has been completed and, after analysis, the findings should be available in about a week.

The earthquake swarm migrated to the southwest over the weekend, behaviour that is not unusual and is thought to be due to stress being transferred to adjacent areas as the earthquakes occur. Analysis shows that most of the swarm quakes have been caused by the thrusting motion of one tectonic plate riding over another. However, Sunday evening’s magnitude 6.5 immediately stood out as a quake of a completely different nature, resulting from sideways movement, and made scientists investigate closely.

The large quake indicated a fault movement in an area where an earthquake fault had not been definitely identified. Mapping of undersea faults is difficult as the currents can bury the evidence over a period of time.

Writing on GeoNet this morning, Kevin Fenaughty suggested that intensive analysis since Sunday points to the London Hills Fault. The fault runs from near Ward in Marlborough and enters Cook Strait between Lake Grassmere and Cape Campbell. The seafloor materials have made it difficult to detect traces of earthquake activity and map the fault’s trace into Cook Strait.

[click for larger image] Known faults in Cook Strait ”“ courtesy of NIWA under the Creative Commons Licence.

The London Hills fault is a strike-slip fault, where each side of the fault slides past the other without much upward movement – the type of fault that produces earthquakes like Sunday’s big event. If Sunday’s quake has caused movement of the London Hills Fault under the strait, today’s survey might see evidence of the movement of the seafloor, before it is destroyed. Having a more accurate picture of the fault will indicate whether it has the potential to generate quakes of larger magnitude than Sunday’s, and help GNS Science determine its relationship to the large Wellington Fault.

[Compiled from media releases supplied by the National Institute of Water and Atmospheric Research, and GeoNet.]

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