GeoNet, with funding from LINZ (Land Information New Zealand), operates a large network of continuously recording GPS sites in New Zealand to track land movements on a daily basis. Within a couple of hours of the M7.8 earthquake, GeoNet was able to use the GPS data to estimate the initial displacements of the Earth’s surface that occurred during the earthquake. Tracking these types of land movements as the result earthquakes is a critical piece of the puzzle needed to determine which faults ruptured, and by how much.
Shifting of the land near the earthquake
What the GPS revealed was astonishing. It turns out that the earthquake shifted the land at Cape Campbell (the northeast tip of the South Island) to the north-northeast by more than 2 m, and up vertically by almost 1 m. This means that Cape Campbell is now more than 2 m closer to the North Island than it was before the earthquake. Similarly, Kaikoura has moved to the northeast by nearly a metre, and has been lifted upwards by 70 cm. Hanmer Springs, which was our closest GPS site to the quake epicentre, jumped eastward by approximately 50 cm. All of this movement happened during the earthquake in a matter of seconds.
Movements of the Earth’s surface near the earthquake recorded from GPS have been incredibly important to help diagnose what motions were involved in the earthquake. A key observation is that although the earthquake fault rupture began near Culverden, by far the largest motions of GPS sites occurred at Cape Campbell. This supports the idea that the ruptured north over a very long distance from where it started.
A lot of New Zealand has moved
Not only did the earthquake shift landmasses in the northern South Island, but it also caused movements across most of the country. In the lower North Island, the east coast has shifted west by 1-5 cm, while the Wellington and Kapiti regions were shunted 2-6 cm to the north. Christchurch and Banks Peninsula didn’t miss out on the action, either—they are now approximately 2 cm further south than they were the day before the quake. Some parts of the west coast of the South Island have been shifted eastward by as much as 10 cm. The northern North Island and southern South Island only moved a few millimeters.