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Modeled Earthquake Shaking Intensities for New Zealand, with Alpine Fault clearly visible running along the spine of the Southern Alps (red band).  Source: http://blog.teara.govt.nz

Modeled Earthquake Shaking Intensities for New Zealand. Zone of maximum shaking along Alpine Fault indicated by red band.
Source: http://blog.teara.govt.nz

The Alpine Fault will rupture in the near future (quite possibly in your lifetime), without any prior warning. When it does, the scale of the immediate disaster will be unprecedented in New Zealand, and secondary effects will probably continue for decades. The Canterbury earthquakes of 2010 and 2011 served a brutal reminder of how vulnerable we are to strong ground shaking, so perhaps now is the perfect time to collectively improve our understanding of our greatest seismic hazard, and to prepare and build resiliency in vulnerable communities.

What is the Alpine Fault?

The collision of two great tectonic plates is building up strain along New Zealand’s Alpine Fault, which traces the western flanks of Southern Alps. The Alpine Fault is a dominant geomorphic feature  of the South Island, extending some 450 km from Milford Sound to near Springs Junction, where it branches off into the Marlborough Fault System. One of the world’s major fault lines visible on land, it is clearly visible from space, as a remarkable lineament which defines the western edge of the Southern Alps.

The central section of the Alpine Fault. Source: wiki.gns.cri.nz

The central section of the Alpine Fault.
Source: wiki.gns.cri.nz

The Pacific Plate is moving roughly westwards, and the Australian plate is moving roughly eastwards, at a relative rate of about 45 mm per year.  Along the Alpine Fault, the collision is oblique, so the plates are slipping past one another, rather than one being forced beneath the other (a process called subduction). The relative movement of the plates past one another is not a continuous or gradual slip, but rather a long-term average of much larger, episodic slips (or “ruptures”), which occur every few hundred years. The two plates are currently locked together along the Alpine Fault by friction, but the buildup of energy is not sustainable for much longer.

Plate Boundary and Slip Rates. Source: Davies & McSavenay

Plate boundary and average slip rates.
Source: Davies & McSaveney, 2006

Eventually, the strain accumulated over hundreds of years will exceed the strength of the rocks on either side of the fault, culminating in catastrophic failure, and a gigantic release of energy. When the Alpine Fault next ruptures, the land on either side of the fault will separate by about 8 m in the horizontal direction, and up to 4 m in the vertical direction. These huge displacements will result not only in intense shaking, but will likely also tear open the earth over hundreds of kilometres along the fault.

Rock deformation, buildup of strain, and eventual rupture along a fault line.  Source: Source: http://blog.teara.govt.nz

Rock deformation, buildup of strain, and eventual rupture along a fault line.
Source: http://blog.teara.govt.nz

The Alpine Fault is similar in character to the San Andreas Fault in North America; the last major rupture in 1906 devastated the city of San Fransisco, where at least 3,000 people died.  While there aren’t any major cities directly on the Alpine Fault, there are many smaller communities that are very close to the fault (e.g. Franz Josef township). Further away from the fault, places like Queenstown and Greymouth will experience significant and destructive ground shaking. Equally important are the major lifelines that cross the fault, including State Highways 6 and 73, and the sole rail link to the West Coast. The isolated nature of the West Coast will become more poignant as secondary effects like landslides and flooding will affect large areas following the initial earthquake, and with subsequent aftershocks.

Fence offset over the San Andreas Fault following the 1906 San Francisco Earthquake (Magnitude ~7.9). Rupture length was 475 km, maximum horizontal offset 6m. Source: unknown

Offset fence following the 1906 San Francisco Earthquake (Magnitude ~7.9, Rupture length 475 km, maximum horizontal offset 6m).

When did the Alpine Fault Last Rupture?

The last Alpine Fault earthquake occurred about 300 years ago, (most likely in 1717), generating an earthquake of magnitude 8.1, when at least 380 km of the fault ruptured, from Milford Sound to the Haupiri River. Although this event pre-dated written records in New Zealand, scientists from the University of Canterbury and GNS Science were able to trace the 1717 rupture length using a variety of techniques, including analyzing tree ring records along the fault trace, and radiocarbon dating of the most recent fault scarp (link here).

For How Long Have Great Earthquakes Been Occurring Along the Alpine Fault?

This cycle of strain accumulation and eventual rupture along the Alpine Fault has been occurring with remarkable consistency for many millenia. Scientists have known for decades that the Alpine Fault has ruptured 3-4 times over the past 1000 years, but that record has recently been extended. A 2012 article published in Science (link here) detailed an 8000 year record of large earthquakes on the southern portion of the Alpine Fault. The authors documented 24 surface ruptures, for an average recurrence interval of 330 years, and estimated the probability of a similar rupture occurring within the next 50 years at 30%. Of course, this probability doesn’t actually tell us when the next great earthquake will occur, but merely how surprised we will be when it does happen.

 

Record of Great Earthquakes on major faults, from Berryman et. al. 2012 (Science)

Comparison of record of Great Earthquakes on selected major faults, from Berryman et. al. 2012 (Science)

MM_Scale

The Next Alpine Fault Rupture

The latest science confirms that the Alpine Fault is late in its recurrence cycle, and that it will likely rupture again in the near future, generating an earthquake of about moment magnitude (Mw) 8. To put that in perspective, such an event will release approximately 30 times more energy than the Mw 7.1 Darfield earthquake of 4 September, 2010, and up to 1000 times more energy than that produced by the Mw 6.3 earthquake that devastated Christchurch’s CBD on 22 February, 2011. An Alpine Fault earthquake will be felt throughout New Zealand, and probably in Australia as well.

Potential shaking intensities (roman numerals) resulting from a rupture of the central section of the Alpine Fault. Source:http://www.orc.govt.nz/Information-and-Services/Natural-Hazards/Great-Alpine-Fault-Earthquake/

Potential Modified Mercalli shaking intensities (roman numerals) resulting from a rupture of the Alpine Fault (dashed line).

How Will the Next Alpine Fault Rupture Compare to the Christchurch Earthquake of 22 Feb, 2011?

From the perspective of assessing risk, the ground shaking intensity associated with an earthquake is more important than the total energy released, as shaking intensity at a given location is what causes damage. The primary reason that the 22 February earthquake caused so much damage in Christchurch is that the fault rupture was close to the city, and very shallow (5 km deep). This resulted in extremely high ground shaking intensities (Modified Mercalli scale) of MM 8-10, relative to the magnitude of the earthquake. By comparison, seismologists estimate that during an Alpine Fault earthquake, the ground shaking intensity will be around MM 9-10 near the Alpine Fault, and MM 6-7 in more distal locations such as Christchurch and Dunedin. Equally important to the intensity of shaking is the duration; the shaking during an Alpine Fault earthquake will last 2-3 minutes, compared to less than one minute for the Darfield and Christchurch earthquakes. So, while the shaking intensity felt in Christchurch during an Alpine Fault Rupture may be significantly less than during the 22 February (2011) earthquake, the duration will likely be much  longer, so there will still be potential for serious damage.

Looking Ahead: Are We Ready?

Is New Zealand prepared for the next great earthquake on the Alpine Fault? Have our experiences with the recent Canterbury earthquakes made us more capable of dealing with the truly national-scale disaster that will result when the Alpine Fault ruptures? What major short term and lasting effects can we expect from a great earthquake on the Alpine Fault, and who will be most affected? I will address these specific questions in my next post.