Mw 7.8 Earthquake Strikes South Island, New Zealand

Mw 7.8 Earthquake Strikes South Island, New Zealand

  • EOS News
14 Nov 2016

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Map of the magnitude 7.8 earthquake in New Zealand on 14 November 2016 (Source: USGS)

A powerful earthquake measuring Mw 7.8 struck New Zealand’s South Island just after 12am (New Zealand time) on 14 Nov 2016, approximately 90 kilometres (km) northeast of Christchurch. The main shock was followed by several strong aftershocks (Mw 5.0-6.0). The locations of their epicentres were estimated to be found along the northeastern coast of the South Island. 

Some of the damage seen in New Zealand’s North and South Islands, more than hundred kilometres from the Mw 7.8 earthquake’s epicentre (Source: Twitter – dickbicknell, darthdonaldson, kiwiclara)

The tremors from the main quake were not only strongly felt near the epicentre, the seismic intensity recorded in the southern end of the North Island (i.e. Wellington) was also strong enough to reach intensity VIII on the Modified Mercalli Intensity Scale (MMI). The MMI is a scale used to measure the strength of ground shaking from an earthquake as felt by people. There are 12 degrees on the Mercalli scale; the higher the number, the stronger the intensity of the quake.

According to the United States Geological Survey (USGS), the earthquake’s epicentre is located 92 km northeast of Christchurch (42.757°S 173.077°E). The city of Christchurch is still recovering from a Mw 6.3 earthquake that had occurred in 2011. The magnitude of today’s earthquake (Mw 7.8) is about 180 times greater than the 2011 earthquake, and may have caused some damage to the coastal cities in the northeastern part of the South Island.

The earthquake solutions from IPGP, USGS and EOS shows that the earthquake was generated by a reverse-dextral fault along the coast. (Source: Figure modified from IPGP)

The active faults mapped in this area are known to contain both strike-slip and reverse features, where plate motions and tectonic forces shift from the subduction processes along the eastern North Island to the strike-slip faulting throughout the South Island. According to Assistant Professor Wei Shengji’s research group at the Earth Observatory of Singapore (EOS), the findings from this earthquake event suggest that the main shock is likely to have a significant thrust-fault component. Scientists from the Institut de Physique du Globe de Paris (IPGP)and USGS have also arrived at a similar conclusion, agreeing that the main shock contains both thrust- and dextral-slips from their earthquake solutions.

Dr Chen Wei Wen, a Research Fellow at EOS and member of Asst. Prof Wei’s research group, explained, “Source time function deconvoluted from teleseimic waves shows that this event is a uniquely slow rupture process, with most of the energy arriving almost 60 seconds after the start of the rupture.” 

Here is a simplified illustration of the active fault domains in New Zealand. The main shock’s source is seen to belong to the southern extension of the subduction system, while the main active fault system is gradually shifted to the Alpine Fault system in the South Island. (Source: Modified from Stirling et al., 2012)

The long duration of the main shock’s seismic waves and the widespread aftershock distribution suggest that this earthquake was generated by a protracted fault rupture along the coastline. The main shock was likely to have been caused by a rupture in an active fault along the coastline, stretching more than 180 km in length, and raising concerns of an impending tsunami along New Zealand’s eastern coastline. It is estimated that the waves would take approximately 30 minutes to arrive at the coastline. At the time of publication, there have been reports of tsunami waves already hitting both the South and the North Islands’ coastlines. 

Today’s earthquake is one of the largest to have struck New Zealand in the past 150 years. We can expect the northeastern South Island to experience more aftershocks, including some greater than Mw 6.0, in the months to come.

A map of the major earthquakes that have occurred in New Zealand since the mid-19th century. The largest earthquake event in the record is the M 8.2 earthquake in 1855, followed by several M 7.8 earthquakes in 1929, 1931, and 2009. (Source: GNS Science)

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