The Alpine Fault and the Canterbury Earthquake


The Canterbury Plain on the South Island of New Zealand is uniquely placed at the convergence of the Australian and Pacific tectonic plates. Together they form a right lateral strike-slip fault called the Alpine Fault, in which the two plates move horizontally alongside each other. However, when the fault has ruptured (the plates have suddenly slipped), the two plates have also moved vertically, creating the Southern Alps as one side of the fault slips upwards.



Figure 1: This graphic shows the subduction of the Pacific plate to the Australian Plate in the North Island while the South Island has a transform boundary causing the Alpine Fault ("Plate Collision"). The subduction of the plates causes uplift of the Southern Alps along the Alpine fault (GNS)
Figure 2: Here’s another graphic showing the subduction of the Pacific plate to the Indo-Australian plate, with the strike-slip transform boundary on the South Island as the two plates move laterally (Cayford 2011).



Figure 3: ...Not that kind of stress (Study.com)





The interaction of the two plates also causes earthquakes in the Canterbury region from the deformation of rocks;  the rocks experience stress from their position in the fault, eventually rupturing and releasing the stress in the form of earthquakes (Allen, YEAR). The shearing stress (from the lateral movement of the rocks) can also cause smaller faults to occur adjacent to the larger Alpine Fault along the two plates - these smaller faults run down from the top of the South Island down into the Canterbury Plains and may be visible or under the surface (Cayford, 2011). Erosion also occurs from the Southern Alps in the process of deformation and the fault rupturing, carrying the material down with glaciers and rivers to cover the Canterbury Plains or the sea floor  -  the erosion may be partly responsible for hiding some faults beneath the surface as another layer of material covers the plains  (GNS).
Image result for Canterbury plains active faults
Figure 4: Different fault lines can occur adjacent to a larger fault line, in this case the Alpine Fault. These smaller faults are the result of shearing stress and can cause surface rupture; however, they may also lie beneath the surface (Duffy, et al.)




  While some regions along fault lines may have smaller, more frequent earthquakes from shorter seismic gaps, Canterbury region experiences slower energy build up in rock deformation, resulting in larger, more infrequent earthquakes. The last big earthquake occurred in 2010, with a large aftershock occurring several months later. Canterbury’s Civil Defence Emergency Management group estimates that the region, including the plains, will likely experience a magnitude 6.0–7.0 earthquake on a smaller fault about every 50 years, with a 10% chance of a magnitude 7.0–8.0 earthquake in the next 50 years (CDEM, 2014).



Figure 5: The Canterbury region has a history of earthquakes occurring near the Alpine Fault, most notably with in 2010, with a devastating 6.3 aftershock 6 miles from the center of Christchurch in early 2011 (Geonet) 

Caption:  This picture shows the shearing lines on the fault trace after the Canterbury earthquakes - notice the “Z” shape on the left side of the photograph that demonstrates the secondary "S" wave that occurs as energy from right lateral movement of the transform fault travels through layers of earth. (Dykstra, 2011)






Citations

NZ Parliament. "Research Papers." Canterbury Earthquake Facts and Figures. NZ Parliament, 10 Nov. 2010. Web. 01 Apr. 2017.

"M 7.1, Darfield (Canterbury), 4 September 2010." M 7.1, Darfield (Canterbury), 4 September 2010 - Earthquake - GeoNet. Geonet, n.d. Web. 01 Apr. 2017.

"Recent Canterbury Earthquakes." Canterbury and Christchurch Earthquakes - Geological Sciences - University of Canterbury - New Zealand. Geological Sciences, University of Canterbury, n.d. Web. 01 Apr. 2017.

Cayford, Joel. "Faulty Thinking About Christchurch." Reflections on Auckland Planning. Blogger., 26 June 2011. Web. 02 Apr. 2017.

"South Island New Zealand Earthquake | UNAVCO." South Island New Zealand Earthquake. UNAVCO, 29 Mar. 2011. Web. 02 Apr. 2017.

"New Zealand ShakeOut Earthquake Drill." New Zealand ShakeOut. Canterybury Civil Defence Management Group, n.d. Web. 02 Apr. 2017.

"Canterbury Civil Defence Emergency Management Group Plan." Canterbury Civil Defence Emergency Management Group Plan (n.d.): n. pag. Civil Defence Group Emergency Management. Canterbury Civil Defence Group Emergency Management, June 2014. Web. 29 Mar. 2017.

Pollard, Simon. "Shaken City." Shaken City. Natural History Magazine, n.d. Web. 30 Mar. 2017.

GNS Science. "Alpine Fault." Alpine Fault. GNS Science, n.d. Web. 02 Apr. 2017.

GNS Science. “”The Hidden Fault that Caused the February 2011 Christchurch Earthquake.” GNS Science, n.d. Web. 01 Apr. 2017.

GNS Science. “Alpine Fault Earthquakes.” GNS Science, n.d. Web. 29 Mar. 2017.

Allen, Casey. "Faulting and Folding Earth, Parts I & II." Intro to Physical Geography. University of Colorado Denver. Spring 2017. 

 Allen, Casey. "Tectonics." Intro to Physical Geography. University of Colorado Denver. Spring 2017. 

Figure Citations

Figure 1: "Plate Collision in NZ." Plate Collision in NZ. GNS Science, n.d. Web. 02 Apr. 2017.

Figure 2: Cayford, Joel. "Faulty Thinking About Christchurch." Reflections on Auckland Planning. Blogger., 26 June 2011. Web. 02 Apr. 2017.

Figure 3: Friedl, Sarah. "Rock Deformation: Causes and Types." Study.com. Study.com, n.d. Web. 02 Apr. 2017
Figure 4: Duffy, Brendan, et al. "Fault kinematics and surface deformation across a releasing bend during the 2010 MW 7.1 Darfield, New Zealand, earthquake revealed by differential LiDAR and cadastral surveying." Geological Society of America Bulletin 125.3-4 (2013): 420-431.

 Figure 5: "M 7.1, Darfield (Canterbury), 4 September 2010." M 7.1, Darfield (Canterbury), 4 September 2010 - Earthquake - GeoNet. Geonet, n.d. Web. 01 Apr. 2017.

 Figure 6: Dykstra, Jesse. "What Lies Beneath the Canterbury Plains? A Fault Revealed." What Lies Beneath the Canterbury Plains? A Fault Revealed. Sciblogs, 9 July 2011. Web. 02 Apr. 2017.


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