A light post introducing two maps showing the difference in shaking in the September 4th and February 22nd earthquakes.
During the 22nd February Lyttelton earthquake my uncle’s bath, half-full with water, was thrown up in the air to meet the taps on the wall.
We’re held on this planet by gravity, 1 G’s worth.
Any force that throws things up upwards has to be greater than gravity’s pull downwards.
One way geologists measure earth movements is to record the acceleration in movements of the ground.
Acceleration is the amount of change in speed. You plant your foot on the pedal in a car, the rate at which the speed increases is acceleration.*
Peak Ground Acceleration (PGA) is the amount of acceleration of movement of the earth at the point recorded, reported as a percentage of gravity’s acceleration (9.80655 m/s2).
Below are the peak ground acceleration maps for the Sep 4th earthquake and below it the February 22nd earthquake that I found on the GeoNet website (click on the image to see the full images at the GNS website):
These maps show the peak ground acceleration at particular points (coloured boxes), with a few landscape features sketched in. The black lines are coastlines and rivers, the red lines roads. The CBD is roughly the area of the right-hand half of that bounded by the two red lines (roads) at 90˚ to each other at the upper-left of the sections of the maps I’ve shown.
In the February 22nd earthquake the central business district (CBD), perhaps the part of the city with the largest group of vulnerable buildings, experienced a little shy of three times the ground acceleration it did in the larger magnitude September 4th event.
Earthquakes don’t just shake things sideways they toss them upwards too, like my uncle’s bath. Things that go up have to come back down; not everything is built to take that well. As Dr Hamish Campbell of GNS Science is quoted as having said:
“’No wonder so many stone churches were destroyed. They are simply not designed to be thrown up in the air and then go into freefall.’
The peak ground acceleration values in the CBD range roughly 18-28% of gravity’s acceleration in the CBD for the September 4th event. In the February 22nd event, the same area experienced a peak ground acceleration of 57-80% of gravity’s acceleration. (100%g is the same acceleration as gravity.)
The largest peak ground acceleration was recorded near Heathcote Valley Primary School: 220 %g, a little over twice the acceleration of gravity. This is apparently the highest ever recorded in New Zealand.
Aimee has written pointing to one possible cause of the particularly strong shaking observed: echoing from underlying basaltic rock. One good early summary of possible contributing factors is at The Landslide Blog. In time we’ll see more detailed accounts of precisely why this earthquake packed such a punch.
(* [Added later] It is the change in speed (acceleration) that you feel, not speed itself. Similarly, to travel upwards you need to accelerate against gravity.)
Update (2nd March 2011): Architect and fellow science blogger Ken Collins has written about ground acceleration in terms of effects on buildings and the building standards. Recommended reading. (At the very end he lets slip that he is another Blackadder fan!)
(Updated 7th March: corrected error in date.)
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