Kia ora kotou

Dr Hamish Campell has just sent through an article he has written for the NZ Herald. The text is reproduced in full below. Hamish is Senior Scientist at GNS Science and is Te Papa’s geological advisor.

Bill Fry, GNS Science Seismologist, also explains Tuesday’s catastrophic events in this excellent video.

“The 22 February Aftershock

by Dr Hamish Campbell

Some 24 hours have passed since the magnitude 6.3 earthquake that struck the Christchurch area shortly after 1:00pm on Tuesday 22 February. 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 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, has 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 5 kilometres deep, and located some 9-10 kilometres southeast of the city centre, more or less half-way 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 1.8 times the acceleration due to gravity were recorded by GeoNet ‘strong motion instruments’ deployed in the Christchurch area. This far exceeds the peak ground acceleration recorded in the Darfield Earthquake on 4 September 2010 which was 1.26g, and is the strongest ground acceleration ever recorded in a New Zealand earthquake.

No wonder so many stone buildings, including Christchurch Cathedral, were brought to their knees. Such structures are simply not designed to be thrown sideways or up into the air and left to go into free-fall, 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 some 17km on a near vertical plane, slightly inclined to the south, and between 3 and 12 kilometres in depth. It is more or less parallel to the east-west trending Greendale Fault that ruptured in the Darfield Earthquake. It may be thought of as an eastward extension but it is clearly dislocated from the trend of the Greendale Fault and 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 5 centimetres per year. This may not seem much but given enough time, it is considerable. Geologists estimate that about 80% of the deformation associated with this plate motion is ‘accommodated’ by uplift of the Southern Alps and movement on the Alpine Fault about 90 kilometres to the west of Christchurch. The remaining 20% of deformation (faulting and folding) is accommodated over a broad zone eastwards across Canterbury.

East–west trending faults in Canterbury are relatively unfamiliar to geologists. Most active faults in New Zealand are roughly 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. Many seismologists suspect the current plate motion in the South Island is exploiting old faults within the earth’s crust, causing them to fail.

Let us hope there are no other major surprises in the near future. Geological evidence suggests that thee has not been a Darfield Earthquake event near Christchurch city for thousands of years, and no movement 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 to get by with our daily lives.”