One week after the big earthquake

As I wrote this we were hit by a 4.5 magnitude aftershock.The are continuing, but are lesser in frequency and magnitude.

The 'A' marks where that 4.5 just hit

What has become a new fascination for many is where the aftershocks are coming from. It is not the same location, but rather along what was, until 4.35 a.m. 4^th Sep, an unknown fault line. Now the aftershocks are making clear where that line is, and it is aimed right at Christchurch.

Was there any evidence of such a fault line prior to the earthquake? No. Canterbury Plains are just that: plains. Flat as a pancake! Head west and you will reach the Southern Alps, which is a known and dramatically obvious fault line, but where Saturday’s big one hit, there was absolutely no indication of the existence of a fault line.

This leads to the inevitable and unanswerable question: what other fault lines lie beneath Canterbury, in indeed any other city/land for that matter? Until some magical technology arises that can detect such things, we will only know them when they tell us they are there…generally in the language of an earthquake!

The image below, on the left, is part of the historic arts center, which was the city's first university. There have been areas of damage to these wonderful buildings, but this one intrigued me most. Just was was it that fell off and made the impression in the grass?

The Christchurch Earthquake. A sleeping fault line.

The fault line responsible for the 7.1 magnitude earthquake on Saturday morning, had laid, sleeping for at least 16,000 years. This was an unknown fault line.

Civil defence has been practising for an earthquake, rehearsing what they would need to do. But they were practising for an earthquake expected from the known southern alp fault line. No one expected this earthquake.

We are, of course, now asking ourselves two questions, which no one is going to be able to answer.
1.Has Saturday’s earthquake taken the pressure off the southern alp fault line somehow?
2.Or are we still to expect that, one day, that southern alp fault line will rupture into a 8 or 9 magnitude earthquake?

Aftershocks continue.

We are already living in fear of the anticipated 6 magnitude aftershock. Is ‘living in fear’ an exaggeration? Sadly, no.

Between 9.46 am Monday morning and 8.24 am Tuesday morning (today) there were 30 aftershocks. The worst:
11.24 p.m. 5.2 magnitude ( shook houses badly, and caused a great deal of fear)
11.38 p.m. 4.0 magnitude (made the heart leap a bit)
11.40 p.m. 5.4 magnitude (set the heat thumping, the adrenalin pumping)
A night like that was enough to frighten everyone. When we thought things might be beginning to settle down, suddenly the aftershocks were worse than they had been the night before.

As much as we want to call off the party and all go home and get back to ‘normal life’…it’s not going to happen. Not for a while.

GNS believe that the quake had the strongest ground-shaking ever recorded in an earthquake in New Zealand. Whilst Cantabrians are competitive people, there are some things we would rather not lead the field in!

History.

Hopefully we cannot look to history for what to expect. Euan Smith, a Geophysics professor at Victoria University, said:

"In 1929 there occurred, in west Canterbury, a magnitude 7 earthquake which turned out to be the first of a series of seven major, magnitude greater than 7, earthquakes over the next 13 years. The series included the second and third largest earthquakes in European times.

"It is improbable that this occurrence of such large earthquakes in rapid succession was coincidental. There is no reason to think that such a series could not happen again."

Improbable? A week ago experts would likely have said a magnitude 7.1 earthquake in Canterbury was improbable. And yet, here we are…