Measuring the ground movement

Geonet have been busy measuring how much the ground moved in this week’s earthquakes (it is still moving in some parts).

GPS allowed rapid detection of land movements due to M7.8 earthquake

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.

kaikouraearthquake_web_18nov2016

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.

A lot more of the country moved to a lesser extent.

Not only did the earthquake shift landmasses in the northern South Island, but it also caused movements across most of the country.  

gps3

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.

Satellite mapping shows horizontal movements.

cViewing the M7.8 Kaikoura earthquake from space

A technique called InSAR, which stands for ‘Interferometric Synthetic Aperture Radar’, utilises radar satellites orbiting ~700 km above the earth to precisely measure the distance between the ground and the satellite. If the ground moves between two subsequent satellite passes, due to an earthquake or volcanic eruption, then the distance between the ground and the satellite changes. Observing these changes in the positon of the land with InSAR enables us to generate detailed maps of ground movement, often with centimeter-level accuracy.

The satellite images reveal huge changes in land movement across the Hope and Kekerengu faults, as well as several other faults in the region. 

To the east of these faults, the land went mostly southwest (see blue area in the figure on the left). In contrast, to the west of these faults the land moved mostly northeastwards (see red area in figure on left).

Sharp changes in land movement are visible on the InSAR images, and show us where the faults ruptured to the Earth’s surface.

insar_1_cropped

Horizontal offsets from radar data. The red colours show up to 5 m of horizontal motion of the land towards the north-east and blue colours show up to 6 m of land movement to the southwest. The yellow lines show faults that appear to have ruptured.

insar_2_wtrmkd

An interferogram generated using data from a Japanese Space Agency satellite. Each set of rainbow-coloured contours represent 11.5 cm of movement. Where the colored contours are closest together is where the largest changes in land motion are occurring.

These are different ways of showing ‘much munting’.

And there is likely to be more to come, although hopefully to a lesser extent.

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