Dr Mark Quigley - Geology Homepage

WELCOME TO MARK QUIGLEY’S HOMEPAGE!

Mark is Senior Lecturer in Active Tectonics and Geomorphology in the Department of Geological Sciences at the University of Canterbury, Christchurch, New Zealand.

For those interested in peer-reviewed scientific articles on the Canterbury earthquakes published in national and international journals, please visit my Publications page (click over there, on the left!)

The material below is written in plain language to aid those interested in understanding the Canterbury earthquake sequence.

Together with colleagues from several institutions throughout New Zealand, we have prepared a white paper that will hopefully answer some of the questions you may have about the Canterbury earthquake sequence. A copy of the paper can be read at

http://www.royalsociety.org.nz/2011/03/16/earthquake-information-paper/

The Greendale Fault map produced by UC and GNS scientists is now available for download. Go to Photos and get the "Fault Map". There are also some new photos of the fault and the diversion and flooding of the Hororata River (caused by the earthquake). I am happy for these to be used for press purposes but please let me know first. I would be extra happy if they are used by school teachers to help educate their students about the effects of the earthquake (you don't need to ask). Kind thanks to Anton Purver, who uploaded some of the first photos of earthquake damage to my site during the early hours of Sept 4th.

STUFF ABOUT THE FAULT AND EARTHQUAKE

The Greendale fault surface rupture length has now been mapped by UC and GNS scientists to have a total length of ~28 km however ongoing studies are refining this estimate. The rupture plane beneath the surface is likely to be slightly longer than this. The eastern tip is located just north of Rolleston and just east of Railway Road. The fault strikes E-W between Rolleston and Greendale then bends to trend NW between the Hororata and Selwyn Rivers. Faults often bend like this so it's not unusual. Since the epicenter depth for the M 7.1 mainshock was located at ~10 km depth and the fault ruptured through to the surface, the total area that moved on the fault during the earthquake was >280 sq km. To get a feel for this, think of a flat surface sitting below Christchurch, stretching from New Brighton to the airport, and Cashmere almost to the Waimak river. Now move this entire surface a couple meters in one quick burst. That's a lot of energy! You can imagine that moving something that big requires a little bit of adjustment in the surrounding rock mass to get used to the new shape of things. Hence the aftershocks. There is a separate peice on aftershocks below.

This earthquake was caused primarily by strike-slip faulting, whereby one side of the fault was jolted to the right relative to the other, causing major cracks in the surface throughout the fault zone. Lots of these are parallel to the direction in which the earth was compressed (NW trending) and have hence opened up as tension cracks, some of which are many meters long and up to a meter deep. Others are perpendicular to this and have formed compressive 'pop ups' that look like little tents or folds. And then there is the actual fault line crack, which is commonly visible as an E-W trending crack in the landscape. You can see lots of examples of all three of these features in the photos. Where the fault line trends in a different direction to E-W (faults aren't perfectly straight) the material on either side of the fault gets pushed up or pulled down, as opposed to just sliding along side-by-side. This explains why some parts of the fault have big scarps (like at Highfield Road) with up to 1.5 metres of vertical movement, while others just have lots of laterally offset fences and hedges with no big scarps; you'll find the the direction of the fault line in these two places is slightly different. The western part of the fault is really interesting; the NE side has been pulled away from the SW side, causing the NE side to drop down. As a unfortunate consequence of this, the Hororata river was diverted (water likes the path of least resistance!) into paddocks that used to be on the north side of the river. Stop banks have been constructed to try and deal with this problem and get the Hororata back on track.

The Greendale Fault was not previously recognized as an active fault because there was no field evidence for its presence beneath the Canterbury Plains. Although there is good seismic data across much of the Canterbury Plains that tells us where the active faults are, the only seismic line through this area was done for oil exploration purposes and no fault was observed in this data. This will be rectified in the coming years as we dedicate more scientific effort to this region.

A major earthquake on a previously unknown 'blind' strike slip fault is not uncommon. One recent example is the 2003 M 6.5 Bam earthquake in Iran. This provides us with just another reminder of just how fortunate we were here in Canterbury; the Bam earthquake killed more than 26,000 people.

ORIGIN AND HISTORY OF THE GREENDALE FAULT

Given the E-W strike of the Greendale Fault, it is very likely that this fault first formed during crustal extension more than 50 to 60 million years ago, when the shape of New Zealand (aka Zealandia) was much different from today. E-W trending faults are present throughout Canterbury and offshore on the Chatham Rise, and some of these are now 'active' faults (i.e. faults that have the potential to generate earthquakes in the modern setting). In a general sense, the E-W trending faults like these tend to be strike-slip faults (the blocks move side to side across the fault) while NE-SW to N-S trending faults tend to be reverse faults (one block is thrust over top of the other block). Some earth scientists at UC think that movement on these different types of faults is closely spaced in time.

It has been proposed that the Greendale Fault had not had a major earthquake (prior to this one) in the last 16,000 years because no evidence of earthquake-related geomorphic features (offset stream channels, fault scarps) was observed on the post-last glacial surface that constitutes much of the Canterbury Plains around the modern fault scarp. This is a sensible hypothesis and may be correct, but it requires more study. It is possible that reworking of the land surface following human settlement of this area, including deforestation and plowing of the fields, might have removed some evidence for earlier earthquakes. Consider the Darfield earthquake; in a month or two there will be little evidence for the location of the Greendale Fault created during this earthquake where there was minimal vertical offset, once the fault zone is plowed over and offset features such as fences are fixed. Of course, large, abandoned stream channels and places of uplift like Highfield Road will be preserved in the record much longer. On that note, there are strange drainage patterns in the Selwyn and Hororata Rivers (both of which were affected by this earthquake) that suggest there might be more to the story. Another issue is the assumption that any previous earthquakes on this fault would have created surface offsets that would be visible on the surface and evident from air photographs. An example of a major earthquake that does not appear to have created any evidence for land surface rupture is the 2010 M 7.0 Haiti earthquake, which occurred on a strike-slip fault, suggesting that not all big strike-slip earthquakes cause surface offsets. To support this, one of my postgraduate students has identified an active fault in the Southern Alps which, despite having > 1m of vertical offset where observed in thin gravel units, has no surface expression in older, thicker gravels, presumably because the seismic energy is dissipated through the latter and does not produce a defined scarp or offset of surface features. I think we need to be more circumspect regarding the earthquake history of this fault until further investigations are carried out, but rest assured, us scientists are on to it!

SOME THINGS THAT EARTH SCIENTISTS ARE DOING NOW

1. Finalizing maps of the Greendale fault, distributing these maps to the press and public, presenting fault maps and data at meetings, at schools and universities, and open forums.

2. Monitoring ongoing slow slip on the Greendale fault using repeat surveying of markers across the fault plane and communicating results to public. The results have obvious relevance to farmers in the area in terms of fixing fences, etc across the fault scarp.

3. Surveying formerly surveyed points to understand the patterns of deformation associated with the earthquake

4. Checking other known faults throughout the region to see if there was any small slip (on the order of 1-2 cm or so) on these structures during or after the main earthquake

5. Imaging the subsurface geometry of the Greendale Fault using geophysical techniques, to understand where it goes and what its history is.

6. Regular monitoring of landslides and/or cracks in the landscape that formed during the main earthquake, and communicating these results to landowners.

7. Mapping areas of tree damage and exploring the factors that caused this damage (co-seismic shaking, changes in water table, high winds following earthquake, faulting of root systems)

8. Continued investigations of key sites to better understand the history of the Greendale Fault and other faults throughout Canterbury

9. A whole bunch of other stuff!

Mark at work in Baja, Mexico

RESEARCH EXPERTISE:

Continental Tectonics
Landscape Evolution
Structural Geology
Quantitative geomorphology
Cosmogenic nuclide dating
Quaternary geochronology
Quaternary paleoclimates