An Unpleasant Prospect

The possibility that recent seventh magnitude earthquakes in the South Island mark the beginning of another period of increased seismic activity is an unpleasant prospect, but one which has to be considered.

Sunday September 12th 2010

Our knowledge of earthquakes and why they happen has advanced dramatically over the last twenty years. However, the field still lags behind weather and climate, subjects that have been studied for centuries and now have well-developed forecasting models.

Prediction of major weather events such as storms and climatic events such as droughts is sufficiently advanced for us to expect a daily weather forecast and seasonal projections on whether a week or season will be typical, wetter or drier than normal. Such predictions are increasingly accurate, but a significant number of them are only partially correct or completely wrong showing the complexity of our atmospheric environment.

Seismology (the study of earthquakes) and geophysics (the study of the earth’s physical environment) are much younger and less developed than the meteorological sciences. This explains why the earth sciences are focussed on understanding what is happening now rather than predicting what might occur.

Nevertheless, the intense studies carried out over recent decades have documented past events and the study of this information to understand trends and patterns is well underway. So far this consists of extrapolating past experience in order to produce a range of future scenarios and following events as they occur.

In the case of Canterbury’s Darfield earthquake this is best illustrated by the prediction that a magnitude 6 aftershock might occur in the near future. Analysis of similar events around the world suggests that most large shallow earthquakes have distinct aftershock sequences that last weeks or months (a small number don’t) and many of the aftershock sequences include an aftershock that is about one order of magnitude lower than the main quake (but again, some don’t). The exceptions are also being studied to try and understand why they buck the trend.

Since the magnitude 7.1 earthquake on Saturday the 4th of September, scientists have been busy studying what happened as a result of the Darfield earthquake, what is happening during the aftershock sequence and what effect this has had nearby. As each day passes the Darfield sequence of earthquakes can be compared with past events and, with a wary consideration of those exceptions, the future trend is becoming more certain.

Over the past week, aftershocks have been clustering around the ends of the previously unknown fault trace, as the surrounding area adjusts to the stress released from the earth’s crust by such a big earthquake.

Consideration now needs to be given to whether the strong quake has altered the state of nearby earthquake faults and whether their rupture has been brought forward or deferred.

Knowledge of earthquake patterns acquired during modern times will be studied to look for evidence of clustering ”“ earthquake events occurring close together within a relatively short period of time. A new tool is available to scientists working in this area ”“ Global Positioning Satellite data. Lengthy manual surveys of land deformation no longer need to be undertaken, except on a localised basis to study small features. The GeoNet geophysical network is sufficiently advanced for a quick assessment to be undertaken. Scientists are already “re-occupying” GPS markers in the Canterbury region to measure the current positions of markers and compare them with previous records to detect horizontal and vertical movement that has occurred during the past week.

The data gained will be of immediate interest to farmers and local authorities as they decide how to deal with drains that no longer run the correct way, altered watercourses, newly active springs, and wells and bores that have changed their behaviour. It will also be useful in determining whether land use needs to change and assets such as roads, water mains, buildings etc. need to be relocated.

In addition, the data will tell scientists how much strain has been passed from the immediate area of the earthquake to adjacent areas of land. It is the accumulation of strain that gives rise to earthquakes as the rock slowly absorbs tension and then abruptly fails causing an earthquake. The question of the moment is ”“ Has the Darfield quake hastened the rupture of earthquake faults thought likely to move in the near future?

It is well known that New Zealand has periods of increased earthquake activity, sometimes localised and sometimes spread across much of the country. Notable periods of increased earthquake activity occurred in the mid-1800s, at the turn of the 20th century and around the 1930s.

In the mid-1800s, central New Zealand experienced three earthquakes of magnitude 7 or larger. They were the western Hawke’s Bay quake of July 1843 (magnitude 7.6), the Marlborough earthquake of October 1848 (magnitude 7.3) and the Wairarapa earthquake of January 1855 (magnitude 8.2). All of these were shallow events.

Another burst of activity was obvious at the turn of the 20th century. Magnitude 7 earthquakes were recorded near Wanganui in 1897, Cheviot in 1901, off Cape Turnagain in 1904 and off East Cape in 1914 and magnitude 6 events near Nelson in 1893 and near East cape in 1914. Hawke’s Bay experienced a magnitude 7 quake in 1921 before things again became quiet.

As the 1920s approached their end another cluster of earthquakes struck both the North and South islands. Early on the morning of 26th of February 1927, a magnitude 6.7 earthquake inland from Ruatoria on the North Island’s East Coast marked the end of the relatively quiet seismic period since 1922. During the 13 years from 1929, New Zealand was relentlessly hammered by nine viciously large earthquakes and their associated aftershocks, causing major damage to infrastructure, and death, injury and personal hardship in an economy reeling from depression and the onset of war.

They were:
1929, March 9th, Arthur’s Pass, magnitude 6.9, felt over the whole country.
1929, June 16th, Buller (Murchison), magnitude 7.8, locally destructive, 17 deaths.
1931, February 3rd, Hawke’s Bay, magnitude 7.9, severe regional damage, firestorms, 256 deaths.
1931, February 13th, Hawke’s Bay aftershock, magnitude 7.3.
1931, May 5th, Poverty Bay, magnitude 6+, damage in Gisborne.
1932, September 16th, Wairoa, magnitude 6.8, damage in Gisborne and Wairoa.
1934, March 5th, Pahiatua, magnitude 7.6, damage in Hawke’s Bay and Wairarapa, 1 death.
1942, June 24th, Southern Wairarapa, magnitude 7.0, damage in Wairarapa and Wellington.
1942, August 1st, Southern Wairarapa, magnitude 7.1, local damage.

A smaller cluster of three magnitude 7 quakes was recorded in New Zealand between 1950 and 1953. They were centred to the south of the South Island in February and August 1950 and in the Bay of Plenty in September 1953.

There have, of course, been seemingly random magnitude 6 and 7 events interspersed with these clusters illustrating the complex nature of earthquake activity in New Zealand.

With a number of buildings in Christchurch and smaller Canterbury towns now needing extensive repair or facing demolition, the possibility of another period of heightened earthquake activity needs to be considered. Do the seven earthquakes with magnitudes over 6.5 experienced since 2006 indicate the commencement of another active period? This burst of activity includes the southern Kermadec quake of May 2006 (magnitude 7.5), The Gisborne quake of 2007 (magnitude 6.9), the Dusky Sound quake of July 2009 (magnitude 7.2) and this month’s Darfield quake (magnitude 7.1).

If a period of increased earthquake activity is likely, then weakened old masonry buildings may need to be sacrificed to avoid serious loss of life. In addition, the Darfield event suggests that time may have run out for owners of unreinforced masonry buildings to take action in areas with a high probability of experiencing a strong earthquake nearby. Sadly, some of this wonderful old heritage may have to go in the interests of public safety.

In an interview with the Science Media Centre, Euan Smith, Professor of Geophysics at Victoria University of Wellington said of the depression-era earthquakes, “It is improbable that this occurrence of such large earthquakes in rapid succession was coincidental. It is more likely that the faults which broke during the series were all stressed and ready to break, and that the occurrence of successive earthquakes helped bring forward the occurrence of the next.

“There is no reason to think that such a series could not happen again. Equally there is no way of knowing whether or not Saturday’s earthquake [near Darfield] has provided a trigger for more large earthquakes in the next few years.”

Experiencing a period of increased earthquake activity is an unpleasant prospect, and one which may force us to sacrifice some of our unreinforced heritage structures before we have time to strengthen them.

[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST, and the Science Media Centre.]

One Response to “An Unpleasant Prospect”

  1. Lizzie from Gizzie says:

    Found that very informative Ken.

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