Strong Quake, North Island

A strong on-shore earthquake struck the southern part of New Zealand’s North Island on Monday afternoon.



Tuesday, 21st January 2014


A magnitude 6.2 earthquake struck 15 km east of Eketahuna at 3:53 p.m. on Monday afternoon.  The quake was 33 km deep.  Initial estimates put the earthquake just onshore near Castlepoint on Wairarapa’s eastern coast, but further analysis by GeoNet soon moved the earthquake epicentre inland to Eketahuna.


The magnitude 6.2 quake left a strong trace

[click for larger image]  The earthquake left a strong trace on New Zealand’s seismograph network and was followed by many aftershocks in the range of magnitude 2 to 4.  The quake was felt from Northland to Southland, with 8700 reports being filed by the public by midnight.


Damage has been reported in Manawatu and Wairarapa districts, mostly to household contents.  However, chimneys have been toppled, water cylinders damaged, and other building damage has been reported.  Rockfalls in the Manawatu Gorge delayed traffic for a time.  Electricity supply was disrupted in some areas.


Whilst the earthquake was shallow, GeoNet reports that the earthquake occurred in the Pacific tectonic plate below the interface where the Pacific Plate slides down into the earth’s mantle under the Australian Plate which forms the North Island.


Yesterday’s event was the fourth magnitude 6 earthquake to shake the southern part of the North Island since July last year.  A shallow magnitude 6.5 earthquake struck beneath Cook Strait on the evening of Sunday July 21st.  This quake followed a swarm of magnitude 3 to 5 earthquakes over three days.


Activity then migrated south toward the Marlborough town of Seddon and a magnitude 6.6 earthquake, now known as the Lake Grassmere Earthquake, on August 16th caused considerable damage and disruption on both sides of Cook Strait.  A magnitude 6.0 earthquake struck nearby three hours later.


By November 7th last year, when GeoNet stopped summarising the aftershocks, there had been more than 5,300 aftershocks from the Cook Strait and Lake Grassmere mainshocks.  This series included three of magnitude 6 and 21 events of magnitude 5.


Yesterday’s earthquake caused most shaking in Woodville where peak ground acceleration measured by GeoNet reached 0.26g, a quarter of the acceleration due to gravity.  Similar accelerations were experienced in Wellington during last year’s Cook Strait earthquakes.  Interestingly, relatively strong shaking was experienced on the Kapiti Coast yesterday, where measurements of up to 0.2g were recorded in Paraparaumu.


Aftershocks with magnitudes as high as 5 can often be expected after strong quakes like yesterday’s.  GeoNet has issued an interim report which includes a calculation of the probability of aftershocks.  Between 14 and 33 4th magnitude aftershocks might occur during the next week and perhaps as many as five magnitude 5 events. Larger events could occur, but the likelihood of a larger event triggered by the Eketahuna earthquake in the coming weeks is thought to be minor.


Yesterday’s earthquake comes at an interesting time for seismologists who are still studying other significant events in central New Zealand in recent times.  The relevance of the Eketahuna quake in the overall picture is not clear.


Aftershocks due to the Cook Strait and Lake Grassmere “triplet” (three quakes of magnitude 6 in a four-week period) are still occurring, with occasional bursts of increased activity.


However, the slow-slip event under Kapiti which began in January 2013 and is still going may be more relevant to yesterday’s strong quake.  It is thought that 60% of the relative movement between the two tectonic plates is taken up by land deformation in the North Island ”“ the land buckles and crumples to accommodate the plates trying to slide past each other.  When the stress becomes too great, some of the accumulated strain is released in earthquakes.


The remaining 40% of the relative movement between the Australian and Pacific plates is now thought to be accounted for by slow-slip events in the North Island.  Over a period of weeks or months the plates slide gently and quietly past each other generating smaller earthquakes as they go.  Slow-slip events in the Kapiti area occurred in 2003 and 2008 and were only detected by the commencement of continuous Global Positioning System (GPS) measurements which began in 2002.  This showed that the expected compression of the land’s surface due to plate movement was either not occurring or that the overlying Australian Plate was gently moving in an eastward direction.


The latest Kapiti slow-slip event, which is occurring over an area stretching from Levin westward to the Marlborough Sounds, was thought to have released the equivalent energy of a magnitude 7 earthquake between January and June last year.  The slow release which has been accompanied by bursts of small earthquake activity is continuing.


The Weber earthquake sequence of the early 1990s is also likely to be studied to see whether it sheds light on this latest activity.


[Compiled from data supplied by GeoNet.]

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