Central New Zealand Shaken by M6.5 Earthquake: AIR Analysis

According to catastrophe modeling firm AIR Worldwide, homes and roads in central New Zealand were damaged by a M6.5 earthquake on Friday, August 16, at 2:31 p.m. local time (2:31 a.m., UTC), but no serious injuries have been reported.

AIR noted that the “region has experienced several small- to-moderate earthquakes in recent weeks, including one event of M 6.5. These have all occurred in the Cook Strait, which separates New Zealand’s North and South Islands. The epicenter of Friday’s event however, was on land, 10 km (6.2 miles) southeast of the South Island town of Seddon. Because of New Zealand’s stringent building codes, the location of the epicenter and the moderate magnitude of the event,” AIR said it doesn’t “expect significant losses from this earthquake.”

Dr. Arash Nasseri, senior research engineer at AIR Worldwide, said: “The earthquake was felt as far away as Christchurch, 240 km (150 miles) southwest, and Auckland, 404 km (250 miles) north, and there have been several aftershocks of M5.0 or more.”

AIR noted, however, that the earthquake was “felt strongly in Wellington, some 80 km (50 miles) from the rupture. Items jiggled off shelves, some buildings were evacuated. In Seddon, a small town of about 450 people, several homes suffered cracks, fallen chimneys, and collapsed roofs. An earth dam near the town, cracked during an earlier quake and in the process of being emptied over concerns about its safety, was further damaged.”

“Seismic design codes in New Zealand have set stringent requirements for new buildings, and Wellington has initiated a comprehensive policy to identify earthquake-prone pre-1976 buildings and seismically rehabilitate and strengthen them,” Dr. Nasseri explained. “As usual, non-engineered buildings such as unreinforced masonry structures are the most vulnerable types in this region.”

At least one of the homes severely damaged in Seddon was a 120-year old structure built of cob, a traditional blend of earth, sand and straw usually resistant to seismic activity.

“The immediate region has been subject to large historical earthquakes, such as the M7.5, 1848 Marlborough event (situated north of the July 21 epicenter), and the 1855 M8.2 Wairarapa Earthquake (80 km to the northeast),” said Dr. Gerald Galgana, scientist at AIR Worldwide.

“The motion associated with the August 16 event was right-lateral strike-slip, reflecting the relatively horizontal motion of tectonic plates in the region,” he continued. “The earthquake occurred at the boundary of the Pacific and Australia Plates, along a tectonic transition zone where the plates converge at 39–48 mm/year. Plate motion direction and deformation style dramatically change along the plate boundary between the North and South Islands.”

According to AIR, “north of the epicentral region, the predominantly westward subduction of the Pacific Plate (North Island part) along the Hikurangi Trench transitions into nearly pure strike-slip motion along the Marlborough Fault system in the south. This further transitions into oblique strike-slip faulting along the Southern Alpine Fault.

“The generally SW-NE trending Marlborough Fault system (located in the northern region of South Island) itself is made up of several parallel fault splays that accommodate the motions, the most prominent being the Clarence, Awatere, Wairau and Hope faults. In North Island, the Wellington Fault and the Wairarapa Fault dominate and lie parallel to these fault structures.”

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