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.