h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

We're having a series of quakes tonight, the largest of which was 7.8 (so far ).

The thing I'm curious about is why quakes that large here don't cause any damage. Overseas they're big enough to kill people.

Usually people say it's because of the low population, but that doesn't explain why there is no damage at all (or very little). I wondered if it was to do with the geology eg the kind of fault lines, and what the rock is made from. Any ideas?

My other question is, is the first quake always the biggest?


Here's the data on that quake:

Universal Time July 15 2009 at 9:22
NZ Standard Time Wednesday, July 15 2009 at 9:22 pm
Latitude, Longitude 45.75°S, 166.58°E
Focal Depth 12 km
Moment magnitude 7.8

http://www.geonet.org.nz/earthquake/quakes/3124785g.html

Oh, yeah, I'm talking about New Zealand

DNA remarked that NZ is just a lot of junk that came up out of the sea, which might have something to do with it.

http://www.youtube.com/watch?v=_ZG8HBuDjgc

TRiG.

Damn you for posting that when I'm about to go to bed

Wow that is a powerful one right enough.

I don't really know the answer but I'm very interested to see any responses from those who do.

Of course, dense populations in rickety houses will lead to massive life loss as the lot comes tumbling down. Also the ground is an important factor. I recall that Mexico City is built on a piece of ground which has a much softer surround so that when an earthquake hits is can really shake round much more than solid bedrock all round would An analogy would be like shaking a child's wooden toy stood up on springs - this would move about wildy whilst shaken but a similar one with a sturdier standing would move about far less.

I'm surprised such a quake did not generate tsunamis as ocean is quite close to you there. A similar magnitude quake is what caused the very destructive tsunami in the Indian Ocean a few years back.

Apparently there was a small tsunami recorded by the station in Hawaii, but I don't know where it went.

Those big earthquakes in NZ usually happen on the West side of the main divide (which is on the main fault line) so there are big chunks of mountain between the centre and the nearest towns.

There was a 7.2 quake a few years ago that caused alot of landslide damage in the mountains but not in the nearest buildings.

if the movement was horizontal there would not be much displacment of water so no wave

if the movement is vertical then the uplift or drop will displace a large body of water and so you get a wave

That would be interesting to find out then.

I don't know the answer to the original question, but if you put the coordinates from your GeoNet link into Google Maps, the epicentre appears not to be in the open ocean: http://tinyurl.com/nw6byx

When an earthquake happens under shallow water, I wonder if less water is displaced and the tsunami is smaller? The epicentre is in the region where the Australian Plate is being subducted under New Zealand's South Island, which would cause vertical plate movements. It would be interesting to know if that was the case for this particular earthquake.

Thanks Aurora. I think they revised the position of the epicentre overnight.

From what I remember that fault line is moving in several directions. There's the plates meeting and one going up and the other going down.

And there's a lateral movement were the plates slide (the west of the South Island is moving north, the east is moving south). Is that what you mean by vertical?

It was a rolling earthquake, rather than a jolty one.

At the risk of putting words in her mouth I think Aurora means movement perpendicular to the horizon as 'vertical' and parallel to it as 'horizontal'.

and Taff too of course.

yes of course, I got that round the wrong way.

Yes, that's what I meant.

I'm really not an expert, but on fault line maps of New Zealand it looks like the plate boundary is a transform fault (lateral movement) along most of the length of the South Island and a subduction zone (one plate sinking under another) further south. That's what made me wonder if this earthquake caused the plates to move vertically which would, as Taff said, displace water and possibly cause a tsunami.

Is that true even if the vertical shift is inland? I think the Asian tsunami earthquake was under the ocean.

There was a tsunami, a small one:

http://www.prh.noaa.gov/ptwc/messages/pacific/2009/pacific.2009.07.15.103013.txt

The tsunami warnings after the earthquake give expected times of arrival and includes places on the opposite side of the country. Here's a map showing the epicentre, and other maps that show NZ. They gave warnings for the east coast of the south island (the big long one). How would a wave get round there?

http://www.geonet.org.nz/earthquake/quakes/3124785g-maps.html

The 'vertical' & 'horizontal' aspects gave me pause.

A horizontal shift showed a magnitude of 7.8. If it had been a vertical shift (measured at the same place), would it have been 7.8?

At places further afield, wouldn't it have shown up as horizontal anyway?

and... how are these thing correlated?

There's apparently some disagreement about the size of the quake. I don't know if that is normal.

They've measured the vertical lift at the epicentre now and it's about a metre, which must be a lot I guess.


There's this on the geonet site now:

>>
The mainshock was a large reverse faulting or thrust mechanism resulting from the Australian plate pushing (subducting) beneath the Pacific plate upon which Fiordland lies. This is the largest and one of very few examples of this type of earthquake in New Zealand, making it very important for our understanding of hazards here.

The earthquake rupture started at about 30 km depth and ruptured upwards and to the south, focusing energy offshore. The motion was more like a lurch than a snap (meaning the energy was released more slowly); this is typical for a subduction thrust event. This explains why the damage was much less than many people expected for this size of earthquake. The motion was slower (with lower frequency shaking) and "rolling" rather than the sharp (higher frequency shaking) movements that cause building damage; this also explains the low number of landslides.

This, and the remoteness of the epicentre, means that New Zealand has been very fortunate - if this earthquake had happened anywhere else it would have caused huge damage, even with the mitigating low frequency behaviour.
<<

http://www.geonet.org.nz/news/article-jul-16-2009-fiordland-quake-biggest-for-80-years.html

They link to this:

>>
Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally. If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral.
<<

http://earthquake.usgs.gov/learning/glossary.php?term=strike-slip


Can anyone make sense of that? It's just confusing me

Kea: "Is that true even if the vertical shift is inland?"
The epicentre does seem to be inland, but the surface may have been lifted for some distance around the epicentre, and I don't know how far that typically extends. Sorry, is it obvious that I'm not a real geologist?

"How would a wave get round there?"
Tsunamis can diffract around corners, like any other wave. Longer wavelengths diffract more, and tsunamis have wavelengths of hundreds of kilometres. There's a nice animation here showing a model of a tsunami starting off the coast of Chile: http://www.geophys.washington.edu/tsunami/movies/globe.mov You can see the wave hit areas that look like they should be protected.

I don't know why the GeoNet website links to a definition of strike-slip faults. My guess is that they wanted to link to the diagram underneath, which also shows the motion of a thrust fault.

RodtheBrit: "A horizontal shift showed a magnitude of 7.8. If it had been a vertical shift (measured at the same place), would it have been 7.8?"
I'm not sure if I understand what you mean. The earthquake magnitude is a measure of the energy released, not distance.

Oh right. I thought the quake caused upward and lateral movement (to the south), so I didn't really follow that.

I see what you mean about tsunamis going round things, but I can't see it for the NZ situation. It's when the water meets another landmass that it turns in odd ways, that wouldn't happen with the east coast of NZ (maybe they're playing safe).