City rupture - no stronger

00:28, Sep 14 2011

I'd like to know how much concern there is about the so-called New Brighton-City Fault that runs close to Moorhouse Ave. It seems to be a similar length to the Port Hills Fault and runs through similar tough rock, so presumably has the potential to generate a similar size shake as the Port Hills magnitude 6.3.
However, I understand the shaking intensity G-forces measured at sites directly over the Port Hills Fault in February was much greater than that measured further away in the central city. Like the Port Hills magnitude 5.0 soon after the Darfield earthquake, the New Brighton-City Fault has had the magnitude-4.9 Boxing Day quake. But what happens if it ruptures completely - the same as the Port Hills 6.3 - this time with the most intense shaking being directly under the central city?

If a magnitude-6.0-plus earthquake centred under the Christchurch central business district were to occur, it is not very likely to generate stronger shaking than that on February 22. This is because the fault that ruptured in February is also located close to the CBD at its closest approach to the ground surface, and the shaking from the February 22 quake was directed towards the CBD.

Another quake may direct shaking further to the north or, more likely, would involve more sideways (lateral-slip) motion, with shaking directed out to sea. An additional effect is that while quake shaking falls off rapidly at distances greater than 10 kilometres from the fault, the increase in shaking as you get closer than that to the fault is not large.

(See details at$file/SEI.GNS.0002B.pdf, page 23).

It is also worth noting that the Boxing Day magnitude-4.9 quake and associated aftershock activity does appear to relate to the fault identified in the seismic reflection surveys completed along New Brighton Beach and Barbadoes St.

The associated zone of aftershock activity beneath the CBD has not continued in any significant way since February 22, and we consider it likely that the stresses along this fault have been in part relieved by the February 22 rupture immediately to the south. - DR TERRY WEBB, Director, Natural Hazards division, GNS Science - PRO JARG PETTINGA
Department of Geological Sciences head, University of Canterbury


Why are the aftershocks on occasion so far away from the fault, and do you know what type of movement is taking place? A lot seem to be in clusters. On the surface, a fault appears narrow. Do you know how wide the faults are? - COLIN

The Greendale Fault that moved in the magnitude-7.1 main shock on September 4 last year is a rather narrow structure, as evidenced by the exposure of the fault at the surface.

However, when such a fault moves, it changes the stress field in the surrounding rocks, increasing stress in some regions and decreasing it in others. As a result, some smaller faults in the surrounding region are brought closer to failure and produce aftershocks, while other faults are stabilised and those regions don't produce aftershocks.

These changes in stress can produce aftershocks up to a couple of fault lengths from the mainshock fault.

The movement that takes place in these "off-fault" aftershocks is generally the same as that of the main shock as the main driving force for these aftershocks is still the background tectonic stress caused by the collision of the Pacific and Australian plates in the South Island. Clustering of aftershocks in space repeats the stress triggering process at a smaller scale.

That is, the stress changes produced by fault movement in an aftershock can cause some other smaller nearby faults to fail, producing more aftershocks. - DR MARTIN REYNERS, Seismologist, GNS Science

In the excellent book printed by The Press about the Christchurch quake, on page 178 it states that in the event of the Alpine Fault rupturing in the Paringa-Ahaura area, the shaking could carry on for a minute or more, but a letter to Ask An Expert recently was answered by Dr Caroline Holden, who said we could expect four to five minutes of shaking. That difference is huge and a great worry to those who have already experienced massive liquefaction. Could someone please explain the discrepancy? - P DALLEY

Seismic waves are generated through fracture of a fault plane area. The whole fault plane does not break in one go, but instead has a starting point and an end point. The velocity at which the rupture propagates is about 2km a second. The Alpine Fault is more or less 500km long. A rupture of this length is equivalent to a magnitude-8.0- plus quake.

It could start rupturing from either end. If this was the case, the entire rupture process would take about four minutes. Alternatively, it could start rupturing in the middle and propagate north and south. In this case, the entire rupture would take about two minutes.

For large quakes like the Alpine Fault, it is likely to do something in between those two scenarios.

It is also possible the rupture might stop, then start again a few seconds later. Shaking in Christchurch could last between two and five minutes. If you're standing some distance from the Alpine Fault - say 90km away - ground shaking could continue for four minutes- plus. However, the strength of ground shaking in Christchurch would be less than the shaking from the Darfield quake last September. - DR CAROLINE HOLDEN,


















































































































































































































































































































































































































































































































































































































































































































































































































seismologist, GNS Science

The Press