crazyscot

You probably won't read about this on the news, but the earth tremors in Chch are continuing. We're told there have been an average of 25 a day since last September, though that includes all the tiddlers that you wouldn't feel. There were two appreciable ones last night; we felt both of them, and GeoNet reports they were magnitude 3.7 and 4.2. In the twelve days that we've been here, we have noticed maybe half a dozen or so. As the locals say, you get used to them and just get on with it. Official advice is to move "a stride or two" to seek shelter under a table if you can, and generally to duck, cover and hold on, wait for the shaking to stop. If in bed, stay in bed and pull the covers up to try and repel any flying glass.

It starts as a low rumbling. You don't always hear it; rustica tends to hear them more than I do. Then the room shakes. Exactly what happens next varies from quake to quake - they're all different. Sometimes you get a single jolt (the February aftershocks were like that), some of them (last night's) are lower-level rumblings that last longer.

The difference between Mw 4 (last night) and Mw 6 and beyond (the big ones) is that the big ones go on longer and shake more violently. Speaking of which, the Mw (Richter) magnitude doesn't properly express the variation in quakes; a magnitude reflects the amount of energy released, but the actual effects vary hugely as quakes happen at different depths in the crust and the waves come in different amplitudes and periods. To this end, here in NZ the Modified Mercalli scale is often quoted alongside Mw magnitudes; it's a subjective scale based on the effects observed. Last night's quakes were generally MMI 4, though GeoNet has a handful of reports around Christchurch that they classify as MMI 5.

We're not daunted. We may change our tune later if another big one hits...