It’s analogous to holding a wooden board to your ear and someone scratching the other end with a nail. It will sound like the noise is next to your ear. In this case, the energy gets carried away from locations to the NW (e.g., Auckland) and felt strongly to the SE. 5/

This is typically what we see for these deep earthquakes, but the question is ‘why?’. The subducting plate acts as a very efficient wave guide for the seismic waves, and the energy will travel SE along the plate to the surface. 4/

What is interesting is the distribution of felt reports from earthquakes like this. We received ~2000 felt reports, mainly along the SE coast and southern North Island. There were no reports to the west or north where Auckland is (~25% of the country’s population). 3/

As the Pacific Plate subducts beneath the Australian Plate, it gets deeper from east-to-west, and earthquakes in the plate get progressively deeper from east-to-west. 10 years of M 3+ earthquakes maps the subducting Pacific Plate. 2/

The cluster of earthquakes to the south are in the upper crust, and most are part of the 2013 Cook Strait/Lake Grassmere earthquake sequence, which had two M 6.6 earthquakes within about a month of each other. 3/

It was a normal faulting mechanism with Mw 4.5 and depth 32 km, which is typical for the region. Larger earthquakes in this region are related to the subducting plate, and this was likely cracking of the plate, while others occur within the plate. 2/

Focal mechanisms are typically strike-slip or reverse faulting with WNW-ESE striking P-axes. There are a few places that deviate from this pattern where the P-axes are rotated E-W or NW-SE and are likely associated with favourably oriented crustal faults. 10/

Historically, there have been four earthquakes with M 6+ in the region, none of them being close to the Alpine Fault. The largest was M 6.7 in 1994 near Arthur’s Pass, the main route between the central west coast and the Canterbury region. A M 6 occurred in 2015 with a similar location. 9/

In the past 20 years there have been ~950 earthquakes with M 3+, with the largest being M 6 east of the northern section of the Alpine Fault. Along the southern section there could be some earthquakes on the fault; however, it’s difficult to say for certain. 8/

In the past two years there have been two earthquakes with M 5+ and 12 with M 4+. There is very little seismic activity associated with the Alpine Fault itself; most earthquakes occur east of the fault on other crustal faults. 7/

This region connects the Hikurangi subduction zone to the north and the Fiordland/Puysegur subduction region to the south; however, with no subduction in this region almost all seismicity is in the crust and shallow. 6/

The Alpine Fault is divided into several sections: the offshore section, south of Fiordland; the onshore southern and central sections which run along the base of the Southern Alps; and the northern section which splays into the Marlborough fault zone. 5/

The Alpine Fault is estimated to have a 75% probability of rupturing in a major earthquake in the next 50 years, with an 80% probability it will be M 8+. 4/

The Alpine Fault is New Zealand’s longest and fastest-slipping strike-slip fault and one of New Zealand’s greatest seismic hazards. It ruptures in M 8+ earthquakes on average every 330 years with the last one in 1717. 3/

The Alpine Fault is a > 600 km long fault system that runs along the west coast of the South Island and forms the boundary between the Australian and Pacific plates, with the Australian plate moving NE relative to the Pacific plate. 2/

It changed the way GeoNet operated to provide improved services to the government, emergency responders, and the public. The next major change would be a result of the 2016 M 7.8 Kaikōura earthquake and the start of the National Geohazards Monitoring Centre (NGMC). 12/

Following the Canterbury earthquakes GeoNet began ‘GeoNet Rapid’, a modernised earthquake analysis system with improved automatic solutions. Automatic solutions were released to the website within a few minutes of an earthquake providing rapid information. 11/