M7.8 New Zealand South Island Earthquake

Update 1 | Summary
Posting Date: November 15, 2016, 2:10:00 AM

Powerful aftershocks have shaken New Zealand since an M7.8 earthquake struck South Island near Hanmer Springs at 12:02 a.m. local time on November 14 (11:02 UTC on November 13). Numerous aftershocks have been reported, some as strong as M6.5. Although damage has been relatively light, many roads remain blocked from ground failure and landslides, numerous power outages have been reported, and water and sewer services have been interrupted in some locales. Two deaths have resulted from the earthquakes, and as many as 1,000 people have been stranded in Kaikoura, a coastal town on South Island.

The epicenter is about 93 km (58 miles) north-northeast of Christchurch, which is still recovering from a pair of earthquakes in late 2010 and early 2011. The initial earthquake struck in a relatively sparsely populated area of South Island and at a time when most people were asleep and the roads were clear, which helped reduce the number of casualties. New Zealand’s GeoNet estimated the magnitude on the Richter magnitude scale (local magnitude) at 7.5, while the U.S. Geological Survey (USGS) estimated the magnitude at 7.4 initially then later revised to 7.8.

Seismology

The M7.8 earthquake that struck New Zealand's South Island was the strongest to hit the country since the 2009 M7.8 Dusky Sound earthquake. Some reports indicate that the initial event—which lasted up to two minutes—included two quakes, one thrust-fault and the other strike-slip. This earthquake happened in a seismically complex area where the Hikurangi subduction zone is transitioned into the Alpine crustal fault through the Marlborough fault system.

The M7.8 quake was felt from Christchurch on South Island to Wellington and as far as Auckland on North Island. USGS reported that during the first 9 hours after the M7.8 quake, 25 M4.5 to M6.5 aftershocks occurred, extending 160 km (100 miles) northeast from the epicenter. The numerous aftershocks have been distributed from the northern part of South Island, notably near Seddon and Kaikoura, to Paraparaumu and Wellington on North Island, which have experienced M4.9 temblors. Many seismologists expect aftershocks to continue for a few months, with some temblors as strong as M5.0.

Although the epicenter of the initial quake was on land, a tsunami alert was issued by the Pacific Tsunami Warning Center because the earthquake fault extended offshore and because the quake was thought to have resulted from a thrust motion. People along the entire east coast of New Zealand—both North Island and South Island—were warned to seek high ground or move inland. Preliminary reconnaissance surveys report uplift on the order of 1 meter (3 feet), but corrected tide gauge records also suggest such uplift, which could explain why little inundation has been reported at this point. Some damage from tsunami waves has been reported in a small bay in Banks Peninsula, but in general damage attributed to tsunami apparently has been slight.

Reported Damage

Although damage has been somewhat lighter than might be expected from an M7.8 temblor, the earthquake—and the numerous aftershocks—have had widespread impacts. Throughout the region, glass has been broken in buildings, chimneys have collapsed, and some structural damage has occurred. Building contents breakage has occurred widely, as shelves collapsed, cupboards opened, and furniture and equipment shifted and toppled. Debris from damaged buildings and landslides has also damaged some vehicles. Power outages, disruption of water and sewer services, and interruption of phone service (including emergency numbers) have impacted some communities.

Although the impact of the M7.8 quake appears to be less in Christchurch than the devastating and deadly M6.1 earthquake there in February 2011, evacuation centers were opened and police set up roadblocks to prevent people from going to the coast. Also, on South Island the coastal town of Kaikoura has been cut off from the rest of the islands by road-blocking landslides; helicopters and a navy ship have been pressed into rescue service to evacuate as many as 1,000 residents and tourists.

On North Island, in Wellington, the capital of New Zealand, some multi-story buildings experienced broken windows and structural and contents damage.

Some highways and rail lines in the affected region have been damaged, and many bridges and tunnels were closed after the initial quake so that they could be inspected.

Exposure at Risk

Earthquake risk to communities in New Zealand is partially mitigated by stringent and rigorously enforced national building codes. For example, the New Zealand building code has prohibited the use of unreinforced masonry (URM) in new structures since 1965. In addition, innovative construction methods that minimize building damage from earthquakes, such as base isolation, have been pioneered in New Zealand. The standards for commercial building types (e.g., reinforced concrete, steel, and masonry veneer) and residential building types (e.g., reinforced masonry and timber frame), are constantly updated in the New Zealand building code to reflect improved understanding of how these construction types respond to ground shaking.

Major cities in New Zealand particularly vulnerable to earthquake damage include Wellington on the southern tip of North Island and Christchurch on South Island:

• Wellington. The capital city and second most populous urban area in New Zealand, Wellington exhibits high seismic risk. The close proximity of the city to several northeast-southwest trending fault traces, including the Wellington Fault, potentially exposes Wellington to strong ground shaking.
  
  Much of the central business district and waterfront of New Zealand’s capital is underlain by soft sediments, which amplify ground shaking. In fact, the land beneath several blocks of the central business district and the present-day waterfront was below sea level prior to the 1855 Wairarapa earthquake. In that seismic event, the seabed of a shallow lagoon was raised by 2 meters (6 feet), lifting these sediments above sea level and exposing new land that the city later expanded to cover. Other coastal regions experienced uplift as well, with a maximum of 6.4 meters (21 feet) of vertical uplift observed east of Wellington city. Finally, most of the major routes into Wellington’s central business district have a high proportion of older buildings (such as URM structures that pre-date the 1965 ban on URM in new construction) that are more vulnerable to ground shaking than modern structures.
  
  
• Christchurch. The tragic and costly effects of the 2010/2011 Canterbury Earthquake Swarm (CES) underscore the seismic vulnerability of this city. Although Christchurch is located south of most of the splays and associated faults of the Alpine Transform Fault system, several recently discovered faults—such as the east-west trending Greendale fault just 30 km (20 miles) west of Christchurch and several small north-south trending faults—increase the city’s vulnerability to earthquakes.
  
  In addition, much of urban Christchurch is built on a thick (greater than 20 meters, 65 feet) layer of unconsolidated sediments, which amplifies ground shaking. Where the water table is high (i.e., located at a shallow depth), these sediments can also lose strength and exhibit liquefaction. Although much of the central business district of Christchurch is built on soft sediments, the water table there tends to be too deep for liquefaction to occur during ground shaking. However, most of the surrounding neighborhoods within a 5 km (3 mile) radius of the central business district possess both soft sediments and shallow water table conditions, resulting in a moderate-to-high risk of liquefaction.

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Posting Date: November 15, 2016, 2:10:00 AM