Satellite radar results

Satellite radar results for the Darfield (Canterbury) earthquake.

The upper graphic shows the ground deformation caused by the Darfield (Canterbury) earthquake as detected by a radar instrument on board a Japanese remote-sensing satellite. The image is an “interference pattern” formed between a radar image taken before the earthquake and second radar image taken after the earthquake. Each red-yellow-green-blue colour cycle represents 12 centimetres of ground displacement in the direction of the satellite, which is looking down from the west. The red-and-white star shows the epicentre (origin) of the earthquake measured by GeoNet, and the black line shows the surface rupture of the Greendale Fault mapped by geologists.

The lower graphic is an “unwrapped” version of the upper one, where the 12-centimetre cycles have been stacked together to give the total ground displacement towards or away from the satellite. The blue and violet colours north of the Greendale Fault show the ground on this side of the fault to have moved nearly 2 metres to the east, while the red colour to the south of the fault shows the ground on this side to have moved more than 2 metres to the west. This accords with the Greendale Fault displacements measured by geologists.

These images are derived from ALOS data that is © Japan Aerospace Exploration Agency ("JAXA") and the Japanese Ministry of Economy, Trade and Industry ("METI") (2010). The data has been used with the permission of JAXA and METI and the Commonwealth of Australia (Geoscience Australia) ("the Commonwealth"). JAXA, METI and the Commonwealth have not evaluated the data as altered and incorporated in this paper, and therefore give no warranty regarding its accuracy, completeness, currency or suitability for any particular purpose.

The green patch between Greendale and the epicentre shows that another fault broke during the earthquake in addition to the mainly horizontal motion of the Greendale Fault. This was a steeply-dipping reverse (or compressional) fault located between the epicentre and Greendale, but it did not break all the way to the surface. (It is a type of fault known as a “blind thrust”.) The correspondence of this fault with the epicentre, along with other information, indicates that this fault was where the earthquake initiated. The yellow patch southwest of Hororata (which is also seen clearly on the upper graphic) indicates a second reverse fault that slipped during the earthquake. This one is also a blind thrust. Most of the fault slip during the earthquake occurred on the Greendale Fault, but the other fault segments are also important contributors to the earthquake.

Models of the earthquake that use GPS recordings of ground displacement together with the satellite radar show that there were probably other fault sections that broke during the earthquake in addition to the ones described above.

Reference
Beavan. J., S. Samsonov, M. Motagh, L. Wallace, S. Ellis, and N. Palmer (2010). The Mw 7.1 Darfield (Canterbury) earthquake: geodetic observations and preliminary source model. Bulletin of the New Zealand Society for Earthquake Engineering 43(4), 228-235. Download the article here: NZSEEBulletinVol43No4.pdf (1.35 MB)

Graphics and text: GNS Science
These images are derived from ALOS data that is © Japan Aerospace Exploration Agency ("JAXA") and the Japanese Ministry of Economy, Trade and Industry ("METI") (2010). The data has been used with the permission of JAXA and METI and the Commonwealth of Australia (Geoscience Australia) ("the Commonwealth"). JAXA, METI and the Commonwealth have not evaluated the data as altered and incorporated in this paper, and therefore give no warranty regarding its accuracy, completeness, currency or suitability for any particular purpose.