Hamish Campbell: Technically it's just an aftershock

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The Christchurch Catholic Cathedral after the 6.3 earthquake. Photo / NZPA
The Christchurch Catholic Cathedral after the 6.3 earthquake. Photo / NZPA

Some 24 hours have passed since the magnitude 6.3 earthquake that struck the Christchurch area shortly after 1pm on Tuesday, February 22. A more precise picture of what is going on beneath Christchurch is beginning to emerge from the pattern of aftershocks and this will improve as time passes.

Seismologists at GNS Science regard this earthquake, large though it is, as an aftershock relating to the magnitude 7.1 Darfield Earthquake that struck Canterbury on 4 September 4, 2010. The technical reason for this is that the epicentre is adjacent to the existing aftershock zone.

As a general rule of thumb, earthquakes that follow a major earthquake are significantly smaller but can attain magnitudes that are about one order of magnitude less than the original. For this reason, GNS Science and its surveillance arm, GeoNet, have been anticipating an aftershock of about magnitude 6, so in that sense this is no surprise.

This devastating event has nevertheless taken us all by surprise because of its violence.

The hypocentre was shallow, somewhere between 3 and 5km deep, and located 9-10km southeast of the city centre, more or less halfway between Lyttelton and Sumner on the northern edge of the Port Hills.

The energy involved in this explosive earthquake generated unprecedented ground acceleration both horizontally and vertically. Accelerations in excess of 1.8g times the acceleration due to gravity were recorded by GeoNet's 'strong motion instruments' deployed in the Christchurch area.

This far exceeds the peak ground acceleration recorded in the Darfield Earthquake which was 1.26g, and is the greatest ground acceleration ever recorded in New Zealand.

No wonder so many stone churches, including Christchurch Cathedral, were destroyed. Such structures are simply not designed to be thrown up into the air and left to go into freefall, even though the fall is all over in a matter of milliseconds to seconds.

The pattern of aftershocks following Tuesday's big jolt has revealed yet another previously unidentified active fault. This is the culprit that has ruptured within the earth's crust and which has given rise to the intense seismic shaking in the Christchurch region. However, it may also be thought of as a valve that has enabled pent-up energy to be released. In many ways faults actually focus and channel energy.

It has ruptured over a length of about 17km on a near vertical plane slightly inclined to the south and between 3 and 12km in depth. It is more or less parallel to the E-W trending Greendale Fault that ruptured in the Darfield Earthquake. It may be thought of as an eastern extension but it is clearly dislocated from the trend of the Greendale fault and stepped to the south.

The eastern end terminates right on the coast in Sumner. The actual movement was a displacement that is largely sideways (strike-slip) rather than vertical.

Aftershocks are thick and fast at present but should drop off fairly rapidly, more so than after the 7.1 Darfield Earthquake. Nevertheless, they will continue to torment Christchurch for the weeks and months to come.

And why is this happening to Christchurch in the first place? The answer relates in part to the nature of the plate collision in the Canterbury region and in part to its ancient geological history.

The current rate of collision between the Pacific Plate and the Australian Plate is 4 to 5cm a year. This may not seem much but given enough time, it is considerable. Geologists estimate that about 80 per cent of the deformation associated with this motion is 'accommodated' by uplift of the Southern Alps and movement on the Alpine Fault 90km to the west of Christchurch, whereas the remaining 20 per cent of deformation (faulting and folding) is accommodated over a broad zone eastwards across Canterbury.

East-west faults in Canterbury are relatively unfamiliar to geologists. trending Most active faults in New Zealand are sub-parallel to the plate boundary that is they trend northeast-southwest.

However, if you take away Banks Peninsula (extinct Miocene volcanos that erupted between 10 and 6 million years ago) and the gravels of the Canterbury Plains, the underlying geology is essentially that of the western end of the Chatham Rise. And the Chatham Rise is riddled with old east-west oriented faults. So maybe the current plate motion is exploiting old faults within the earth's crust at depth, causing them to fail.

Let us hope there are no other major surprises in the near future. Geological evidence suggests that there has not been a Darfield Earthquake event near to Christchurch city for thousands of years, and nothing on the Greendale Fault for at least 16,000 years.

However, this is small consolation for those people who have suffered in this tragic event. If only we could predict such events ...but we can't, not with the precision that we humans need.

- Dr Hamish Campbell is a geologist with GNS Science.

- NZ Herald

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