Faulty Thinking About Christchurch

(NB: Since putting up this post, I have posted further research which you will be interested to read if you want to understand how Christchurch Council planning failed to manage the city's seismic risk: How Councils Under-Played Christchurch Seismic Risk.)... but read this one first....

Maybe I read the wrong newspapers, but around the time of the September earthquake in Christchurch, I got the impression that it came as a very big surprise to all concerned. I got the impression that people thought Christchurch was safe as houses as far as earthquakes went. Not quite the last place on earth for an earthquake, but low risk. Well. That was the impression I got from what I read, and from people I know who live there. How wrong I was.

My last blog on this subject reported a few of my speculative thoughts on the matter, and quoted from the abstract to research by the NZ Institute of Geological and Nuclear Science (NZIGNS), published in Environmental and Engineering Geoscience 1995, and entitled Geology of Christchurch.

It took a while to track down the actual report itself, but I found it buried within the Serials section of the General Library at Auckland University. There are 61 pages of it covering many aspects of the geology of Christchurch and a detailed section on the risk of earthquakes there.

My reading of the data suggests that between 1869 and 1988 there have been 12 earthquakes bigger than 6.0 on the Richter Scale within 150 kms of Christchurch. Two of these were 7.0 and larger. The report contains this photo of earthquake damage to the Cathedral spire from an earthquake in 1888, and reports that the Cathedral was also damaged by earthquake in 1922 (a magnitude 6.9 earthquake centered around Motunau) and in 1929 (the Murchison earthquake). I discovered newspaper reports in the National Library that it was also damaged by the Cheviot earthquake in 1901.

The NZIGNS report tabulates 59 earthquakes that were “felt in Christchurch” between 1946 and 1994 (9 of these were centered outside the central South Island). Interestingly, the report tabulates the intensity of these earthquakes within Christchurch in terms of the Modified Mercalli Scale and the Richter Scale. Modified Mercalli Scale I. Very mild movement, only detectable by instruments. II. Some people feel a slight movement, particularly on upper floors of buildings. Suspended objects such as chandeliers may swing. III. People indoors feel some movement, similar to passing traffic. IV. Motion is felt by most people indoors and some outdoors. Windowpanes and kitchen utensils rattle; parked vehicles rock. The movement is great enough to wake sleepers. V. Felt by all people. Tall objects rock; plaster cracks and falls. VI. Alarmed, people run outside. Poorly constructed buildings begin to show damage. Motion is felt by people in moving vehicles. VII. Only slight damage in well-constructed buildings; slight to moderate in well-built ordinary structures; much damage in poorly constructed buildings; chimneys broken. VIII. Damage is still only slight in structures built to be earthquake-resistant; among substantial ordinary buildings there is much damage and some collapse. Poorly built buildings are substantially damaged. Tall things such as chimneys and monuments collapse. Heavy furniture is overturned. IX. Even in earthquake-resistant structures, there is considerable damage. Greater damage in substantial ordinary structures; more collapse. The frames structures of buildings are thrown out of plumb, and buildings are shifted off their foundations. X. Some well-built wooden structures destroyed; most masonry structures demolished with foundations. Rails bent. XI. Few if any structures remain standing. Bridges destroyed, rails greatly bent. XII. Total damage. Lines of sight and level are distorted. Objects thrown into the air. Of the 59 earthquakes “felt in Christchurch” between 1946 and 1994, 47 were MMIV in intensity (ie IV on the Modified Mercalli Scale), 9 were MMV, and there was a MMVII (this was the 8 March 1987 earthquake centred in Pegasus Bay.) (By the way - my understanding of the damage caused by the recent earthquakes in Christchurch that these range between MMVII and MMIX in intensity, according to the Mercalli Scale. Parts of Christchurch were harder hit than others.) The NZIGNS report (dated 1995 remember) states: “Dibble and others consider the 5 June 1869 New Brighton earthquake to have been the most destructive since European settlement. This earthquake is estimated to have produced intensities of MMVII-MMVIII at Christchurch, and reports of the observed effects are consistent with an M5.75 earthquake (Richter Scale) located 10 miles from the city centre…” (NB: the nave of Christchurch Cathedral was not built until 1881.) I found these reports about that earthquake and one that came shortly after: 5 June 1869: Earthquake in early ChristchurchOn 5 June 1869, Christchurch settlers were shaken by an earthquake centred beneath the city, possibly around Addington or Spreydon. The earthquake was probably shallow, with a magnitude of about 5.8. There was damage to stone buildings and the spire of St John’s Church on Latimer Square, and many fallen chimneys. The quake may have caused some ground settlement in the Heathcote Estuary, as locals describe the tide as running higher up the Heathcote River afterward. 31 August 1870: Earthquake near Banks Peninsula On 31 August 1870 the Canterbury region was shaken by an earthquake with an estimated magnitude of 5.8, centred south of Banks Peninsula, near Lake Ellesmere. Damage was minor in Christchurch—a few fallen chimneys and some structural damage to buildings. Shaking at Lyttelton and Akaroa was much stronger, with rocks falling from cliffs around Lyttelton harbour. Anyway, getting back to the NZIGNS report. It goes on to consider the risk of liquefaction, and reports past liquefaction events in these terms: “…Only one instance of liquefaction during an earthquake is recorded from the Christchurch area. This was for a magnitude M6-7.5 earthquake which occurred on 16 November 1901 (intensity MMIX in Christchurch) centred near Cheviot in north Canterbury…. ‘it was reported in The Press that at Kaiapoi the earthquakes of the 16th inst.were felt with great violence; that at some places the earth opened and water and sand were emitted from vents in the ground and that at one time an inundation by water from this source was apprehended…’”

I had a further hunt around for information about this earthquake and found these press photos and this story about what happened in Cheviot reported at the time in The Star: “A terrible earthquake occurred at 7.45 this morning, travelling direct from, east to west. It lasted several minutes. There is not a brick building or chimney left standing. The windows in many houses were shattered to atoms. One little child was killed by a falling house. The bakers' ovens are broken to pieces, and everything in the district is a complete wreck….”

Note the power of this earthquake - "no brick building left standing...." Newspaper reports at the time record a string of aftershocks, the fact that people became exhausted, would not return to their homes, camped in the fields. That the Cathedral spire was damaged. This is the Evening Post newspaper report about what was seen at Kaiapoi during the Cheviot centred 'quake: “…When first shock had passed, Mr. W. Waites, who owns an orchard and garden at the end of Charles and Sewell streets, noticed that his land was' apparently flooding from springs having been opened. It was then discovered that across his land, and part of Mr. Dunn's section, and over the surface of a paddock of several acres held by Mr. J. Sims, fissures from 1in to 3in in. width, and several chains in length, had opened… From these earthquake openings the water was freely issuing in such volume as to cause… probable inundation. Fortunately the rapid expansion of water seemed to be checked by a liberal supply of sand from some grey quicksand layer below the level of the river, and this was deposited in the orchard and elsewhere in the shape of round and oval porridge pots… The water, which had risen about six inches in on hour or two, disappeared by percolation, leaving the sand deposits in different fantastic forms….”

The 1995 NZIGNS report includes this “liquefaction potential map” of Christchurch, with high, medium, low and no liquefaction risk zones marked. The report confirms that the materials that are most susceptible to liquefaction are water saturated, loose, uniformly graded silt and sand, and notes that liquefaction has been observed in loose sandy gravels (overseas examples given). The report states: “some of the Christchurch metropolitan area is underlain by similar materials, particularly large parts of the eastern suburbs and areas adjacent to the Heathcote River… Interbedded gravels are thinnest or absent in the central and eastern area of Christchurch where liquefaction effects and ground deformations (settlement and lateral spreading) are expected to be greatest…”

Here is a section of the “red zone” map released by Government this week which includes Bexley, Avonside and Dallington.

Note the similarity with this image which is a section of the NZIGNS map encompassing the eastern suburbs.

The NZIGNS report tabulates predictions for the likely return periods of different magnitude earthquakes that might affect Christchurch in future. These are under a section headed: “Seismic Risk”, which notes work done by “Elder and others”, and which is tabulated as the graphic to the left shows. Among other things it predicts that quakes as damaging as Christchurch has had recently (at least MMVIII) will happen every 55 years on average.

The report contains a number of warnings. It maps the existence of several known fault lines, but goes on to warn: “an absence of identified fault traces does not necessarily confirm faults are absent from a particular area. Since Christchurch and the Canterbury Plans are covered by geologically very recent deposits, it is possible that some faults have not been detected.” The report explains the significance of how close the epicentre is in terms of damage inflicted. It states: “Close to the source of a large shallow earthquake (Richter 7 or greater), widespread damage and destruction often occurs, with intensities reaching MMIX or MMX. At distances about 100km from the epicentre, intensities are generally up to MMVI…” And it warns: “It should be noted that there are known active faults significantly closer than 100km to Christchurch which could generate similar large magnitude earthquakes.” I don’t know what effect this report had in New Zealand. But it was published in an international journal whose editor (Allen Hatheway from Dept of Geological & Petroleum Engineering at University of Missouri) wrote in his foreword: “…the authors leave nothing to the imagination in the question of local and regional seismicity… clearly, there is now a case for elected officials to take note and to ask and allow local and national agency geologists and engineers, planners and consultants alike, to assist in providing enforceable seismic-withstand guidelines or regulations. More than in most cities, development of Christchurch is served by careful site examination. One has the impression that the temperate climate and tranquil setting of Christchurch belies a variety of benign geologic constraints- those that are costly mainly in terms of financial losses rather than by loss of life. The intersecting curves of increased urbanisation and frequency of occurrence of damage-oriented geologic constraints will soon begin to awaken the media and public officials, as well as the insurance companies. The time is right and the local geoscience and geotechnical expertise is ready.” Looks like it took another 16 years before anyone woke up actually.

It is not as if the New Zealand Earthquake Commission has been ignoring this issue. In fact it appears that the Seismic Risk assessments that are cited in the NZIGNS report tabulated in this blog, were in fact prepared for the NZ EQC. The same table (as above) is set out in the summary of an earthquake risk assessment that was done for the EQC in 1991 entitled: The Earthquake Hazard In Christchurch: a detailed evaluation. (The authors of this work are: Elder, McCahon and Yetton.) (You can see this summary here). The 1991 EQC report summary states, in relation to the seismic risk probabilities: These probabilities indicate that Christchurch has an overall seismic hazard level comparable to Wellington for medium intensity earthquake shaking… The greatest concern for Christchurch, located near a saturated, sand and silt rich, prograding coastline, is the potential for liquefaction…. This may cause subsidence, foundation failure and damage to services. Analysis shows that large areas of the city are underlain by sands or silts which, if sufficiently loose, would be highly susceptible to liquefaction…. The EQC report summary notes that the available data (covering the earlier earthquake events that are described above) is short, making it difficult to provide reliable predictions, given that the best predictor for the future is what has happened in the past. It states: Analysis indicates that potential exists for relatively rare but very large earthquakes (approximately magnitude 8) along the Alpine fault, which essentially marks the western edge of the Southern Alps. More frequent moderate to large earthquakes (around magnitude 6-7.5) can be expected in the Canterbury Plains foothills and North Canterbury area, and less frequent moderate earthquakes under the Canterbury Plains and Christchurch itself. The attenuation model predicts that the damage in the city from these three types of event are likely to be similar. Of the four serious earthquakes in the early city history, three occurred in the foothills and North Canterbury region (the Amuri, Cheviot and Motunau earthquakes) and one virtually beneath the city (the New Brighton earthquake). In other words, what the EQC report summary plainly says, a moderate earthquake “under the Canterbury Plains or Christchurch itself” would cause “similar” damage to a “relatively rare” big earthquake centered along the Southern Alps. This summary also apppears to address the data shortcomings that are confirmed in the later NZIGNS report, noting: We have not attempted an in-depth lifelines study for Christchurch, or included economic or sociological analysis in this report. In addition to the need for this type of work, we recommend further action from the engineering profession including a review of the current seismic loadings code, local seismic design practices and building stock. We suggest site specific studies for the Lyttelton tank farm, Bromley sewerage ponds, pumping stations, substations, hospitals, civil defence facilities, airport and key bridges. Major areas of further research include studies of sand density variations and susceptibility to liquefaction across the city; continued paleoseismic evaluation of adjacent active faults, particularly the Alpine Fault, and further investigation of the deep sediments below the city. The EQC Seismic Risk Assessment for Christchurch is dated 1991. Twenty years ago. How much of this work was ever done? How much of this advice was accepted and acted upon? There are many questions that need to be answered by those in authority. Why? I'll suggest why. This information cites four earthquakes that did severe damage in and very close to the City of Christchurch (1869, 1901, 1922 and 1987). Based on this information the EQC report predicted a return period for another equally devastating earthquake of 55 years. Given the Cheviot earthquake in 1901, and then the Christchurch February 2011earthquake - exactly 110 years later - with a couple of big ones in between, perhaps their predictions are conservative. New Zealand was advised this earthquake sequence was likely. So whose fault is it that authorities didn't act? PS: Since putting up this post, I have added two later ones which you might be interested to read: Banks Peninsula Rising - Geologic History and Historic Christchurch Earthquake Newspaper Archives