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Mw 7.8 Earthquake Strikes South Island, New Zealand
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.
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 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.”
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.