Ask an Expert: why so many aftershocks?

Last updated 12:51 22/06/2011

 

As of early June, Haiti had experienced 42 aftershocks, Japan 726 aftershocks and Canterbury over 7000 aftershocks. Is there any recording, worldwide, of as many aftershocks as this? Is this related to volcanic activity? - R Brown and G Symes (separately).

The discrepancy in the number of earthquakes is related to the density of the seismic network (ie, the number of seismic instruments) and the location of the earthquakes relative to the seismic network. The Canterbury region has a very dense seismic network and the Canterbury earthquakes are located close to the instruments. Therefore, it is possible for us to locate a great many earthquakes with small magnitudes. In Haiti, the seismic network is very poor and only larger earthquakes (generally magnitude 4.0 and larger) can be located. In Japan, the seismic network is very dense. However, the great Japanese earthquake happened off the coast of Japan, some distance away from the seismic instruments, so once again it is only possible to locate the larger earthquakes.

If Haiti had a similar seismic network to New Zealand, the number of recorded earthquakes would be similar to the Canterbury region. And if the Japanese earthquake had occurred onshore - closer to the seismic instruments - there would be many more earthquakes recorded than in Canterbury. There is no indication that the Canterbury earthquakes are related to volcanic activity. - Dr John Ristau, GNS Science seismologist.

Can you please provide a street map showing the faults so we can plainly see exactly where they begin and end. Can we have the aftershock history of the individual faults (how many) or if, in fact, they have or have not yet been active. This would give a better indication of how each fault is behaving. - Tracey.

We understand your interest in knowing the precise location and activity rate of the faults that underlie Christchurch City. However, the answer is not so simple in that neither the Port Hills Fault nor a possible fault through the southern part of the city associated with the Boxing Day sequence of earthquakes come to the surface. Also, we have only the locations of a dozen or so earthquakes and two lines of deep soundings to go on this. This is not enough to confirm the length or continuity of the fault. Therefore, it is not possible to provide their location at street-level detail. - Dr Kelvin Berryman, Natural Hazards Research Platform manager, GNS Science

Are all these earthquakes triggered by tectonic plate movement in the first place; ie, Pacific plate pushing against the Australian plate? - Noel.

You are correct that the sequence of earthquakes in Canterbury is 100 per cent tectonic in nature. GPS measurements tell us that the Pacific and Australian tectonic plates are converging obliquely at 38 millimetres a year through the centre of the South Island. This produces an inexorable straining in the brittle crust across the South Island. - John Callan, GNS Science communications manager.

The quakes centred out Rolleston way are continuing. To me, this means the land must be moving. Is it moving up, down or sideways? If so, how much movement has occurred? - Peter.

Because the earthquakes are occurring many kilometres below the ground, there is no significant or easily detectable ground-surface changes. Results from field experiments using the seismic sounding truck that we brought in from Canada are still being processed.

In about three weeks, we may get a good look at the "anatomy" of the earth structure in this area (at least along the line that the survey took place). To date, the only information we have to interpret is the cloud-shaped region that the earthquakes are occurring in, and small movements ranging from a few centimetres to perhaps half a metre related to each of the detectable earthquakes. The earthquakes seem to be on fractures and perhaps small faults that have a wide range of directions. - Dr Kelvin Berryman.

I am interested to know what is happening to the land elevations for places like the Port Hills, eastern Christchurch and Kaiapoi. With the liquefaction, I would assume that the land would drop. - P. Marion.

The three biggest earthquakes to affect Christchurch . . . caused liquefaction with lateral spreading, particularly near rivers and the Estuary in the eastern suburbs. When liquefaction occurs, water and fine sand are expelled from the ground to the surface and the ground surface drops by an amount approximately equivalent to the depth of water and sand expelled. Therefore, when the expelled sand is cleared away, the ground surface is left lower than it was before the earthquake. For example, in hard-hit Bexley, where 0.3m to 0.5m of sand has been expelled during each of the three big quakes, once the cleanup is finished from the latest quake, the ground surface may be about 0.9m to 1.5m lower than it was before September last year.

In addition, the "reverse faulting" mechanism of the Christchurch earthquake has caused uplift of the Port Hills by as much as 400mm and subsidence of the Estuary and close-by eastern suburbs by about 200mm. Because of the impacts of liquefaction and lateral spreading, the net subsidence is difficult to determine. Preliminary GPS data shows the magnitude-6.3 earthquake of June 13 also caused a little (significantly smaller) uplift of the Port Hills and smaller subsidence in the Estuary.

Lateral spreading from the three earthquakes has caused riverbanks to move laterally towards the river and the riverbed to heave upwards. For example, the Avon River banks near Dallington have closed up by as much as 5m and the riverbed has heaved up in the old channel at Porritt Park by 1m or so.

Kaiapoi was affected by liquefaction only once, during the Darfield earthquake, with subsidence (drop) of the ground surface and, in places, extensive lateral spreading. Ground shaking from the June 13 earthquake was very intense and caused damage in Lyttelton and Christchurch city areas. But the strength of ground shaking from this quake, and others in the same area, has not been large enough to cause liquefaction or significant damage 19km-20km away in Kaiapoi. - Dick Beetham, GNS Science engineering geologist.

* If you have a geological question, email vicki.anderson@press.co.nz.

- The Press