Friday, September 10, 2010

Earthquake Theory

I must say that this earthquake has taught me quite a few things about them:
1) Earthquakes can be scary, especially when they're big and go on for more than 2 seconds. Previously, every "earthquake" we'd had would just be a 2 second wobble once a year or once every few years that'd hardly damage anything. But the longer they get, the greater the chance of damage + power/water cuts.
2) "Aftershocks" are pretty much full-blown earthquakes in their own right
3) Aftershocks are not small shakes that happen once every few days after the "main event". Rather, the earth underneath your feet is just in constant motion more than usual, and that rather regularly (aka minutes in the first 2-3 days, and hours during later days) this constant motion is more vigorous
4) Shallow earthquakes are nastier than deep ones. They feel more powerful, usually with "wide" swaying leading to things falling down.
To illustrate this for yourself, get a goldfish and put it in a large+wide+shallow mixing bowl and observe the way it swims around (my fish tends to like swimming around the rim quite rapidly, half-flattening his dorsal fin, and using the tip of his tail to create large vibrations on the water surface). Next, move the same fish over to a deep tank, and notice how movements "down under" won't affect the surface so much.
5) Standing up in stable stance means that you can't feel any of the smaller shakes at all
6) A good indicator of all types of shaking is the TV antenna, which shakes as soon as some of the more subtle shakes come in. Useful when trying to decide whether it's just your butt shaking, or the ground.
7) Water is not a very good indicator of shaking. It is actually not that sensitive to movement, especially the smaller shakes. Only violent shakes create visible displacement. Floating penguins in the water make interesting diversions during this.
8) After a big shock, your butt often "feels" like the ground underneath it is shaking. Perhaps it really is, and you're sensitive enough to feel it. Or perhaps, you're just starting to get nervous again.
9) Earthquakes are noisy. You usually hear one coming before you see or feel anything (in that order too). They have a low rumbling sound, which then combines with the sound of the house/building you're in creaking as it shunts sideways, followed by the sounds of stuff shaking.
10) It takes a few days, but eventually, you'll become almost desensitised to all but the larger shakes (usually the shallow but with lower magnitude ones), as you'll be sleepy enough to just sleep right through 12-3 aftershocks, some of magnitude 4.5 or greater, and 3.x's generally just aren't felt at all.

Geology is still a developing science. We still are not able to accurately predict earthquakes with any insight, so hence, we still don't know anything (compared to just about being able to predict the weather most days, with some days that are very very very off predictions) and earthquakes are not a "solved problem".

In fact, the closest we can come to predicting anything about them, is what sorts of patterns they "might" follow:
- rule of thumb: largest aftershock 1 magnitude less than main event
- probability of large aftershocks dissipates as time passes
- aftershocks should die down gradually over weeks

From what I understand though, they're really only drawing upon recorded knowledge of previous earthquakes with "similar" circumstances. Now, I have reason to believe that that sample size is still a work in progress really. If you consider how long humans have been accurately performing scientific studies involving documentation of results (~200 years, 300 if you're really generous), the number of years in which we have had standardised and reliable equipment (~100 years, or perhaps just 80 for accuracy+portability), then really, the amount of information we can really expect to rely on is about 100 years of data. Average it out at about 2-3 big earthquakes a year, and those being 6.x, 7.x, and 8.x, then you've really only got about ~100 of each. Not really that much, eh? Otherwise, it's quite possible that we could have some firmer estimates on how things will pan out.

If you know some things about statistics, then you'll be well aware of three main things:
1) we usually only pay a lot of attention to way "usually" happens - i.e. the "average" case that is expected to happen a large amount of the time / with high probability
2) knowing what the average case is requires a lot of data...
3) despite the "average" case, real-life, "experimental"/"empirical" results often exhibit "outliers" - that is, stuff that doesn't really fit the expected trend!

Most of the time, as scientists, we "learn to" or "instinctively massage" out such "aberrations" in our data to make a more convincing argument. There is a reason that people say: be careful what statistics say ;)

Thinking about earthquakes another way, the Maori people (native peoples of NZ) often have their own cultural interpretations for many phenomena: including legends and myths about the creation of the land and world, etc. etc. Indeed, this curiosity about the world around us, this desire to find a way to describe or explain the world around us, is a very innate human characteristic. It's the seed of creativity, investigative research and pursuit of the unknown, and religion.

I hope it's not terribly culturally insensitive (aka "politically incorrect") of me to suggest that I wonder why some Maori elders here in Canterbury have not yet been seen or heard suggesting that our latest earthquake has been caused by a "taniwha" (aka mystical monster) lying under the Canterbury plains. I've certainly heard such suggestions in the past about some earthquakes, and other road crashes in particular regions in the country.

Like dragons in many other cultures (it's interesting that many different cultures speak of "dragons" or dragon-like creatures in their creative literature, with many eerie semblances being able to be drawn with the dinosaurs) taniwha are the New Zealand equivalent of large mythical creatures that may/may not have extreme intelligence and ability to affect a region on a large scale, usually as a result of their "displeasure" over some or other matters.

In the Canterbury region, there are indeed myths/legends (I can never really remember the difference between the two) about there having been a pair of taniwha who lived around Banks Peninsula (which used to be a volcano) and the Canterbury plains. All I can remember of this story (it's been over 10 long years since I heard this!) is that the round boulders found all over the Canterbury plains were said to be the eggs of one of them as they fled, probably after the volcano erupted.

Anyways, the reason the recent earthquakes have made me think of this aspect of Maori culture is related to recent events and the nature of the shakes that we're getting. As everyone around this region probably is currently doing, we're walking around looking at the destruction wondering why some things were damaged/toppled, and yet others barely touched, exercising that little slice of our humanity.

In recent times, there had just been the collapse of a large South Island finance company: South Canterbury Finance, after the company's founder, 82 year old Alan Hubbard had all his assets placed under statutory management for suspected fraud. Also, the controversial new (and expansive, if not equally expensive) city council headquarters had just opened the weekend before the quake (with an open day for the public scheduled for the day of the quake). And not to mention the plane crash near Fox Glacier (skydiving planes crashed minutes after takeoff, carrying 9 people including 6/7 tourists) the day of the crash...

Now, about the shakes we have been having. It's quite unusual that most of our serious shakes have happened under the cover of darkness, increasing the fear that most people have felt. For instance, the initial quake was at 4:35am (i.e. in the morning), and many of the largest aftershocks have also fallen at similar times (~3am, and ~4:30am) though some have also occurred in the early morning (usually ~8am) and early evening (~6/6:30pm). Other aftershocks usually then happen starting from about 10pm to 1am (usually one just after 10pm, 2-3 during 11pm to midnight, and one or two after that, usually around 12:30am). That is also not to mention that there are heaps of aftershocks usually recorded during the night time (13 of them in one night at one stage). Conversely though, we hardly have any shakes during the day. Just yesterday we had a day with hardly any shaking at all until 6:30pm - it was just dead still. Same thing on Tuesday (IIRC), followed a massive aftershock on Wednesday morning that forced the reevaluation of many buildings and the state of emergency.

This sort of timing feels awfully like some kind of noctural creature waking up, shaking the ground as it moves, and generally behaving like a child up to mischief, playing a massive practical joke on all of us. It's perhaps no coincidence that they found that the fault-line they claim to have ruptured is one that they didn't know about, and which had not ruptured for 16000 years...

Who knows... act of God? Pressure building up in ways that we don't totally understand yet? Or just an angry as hell taniwha awoken after 16000 long years?

Whatever it is, just make the shaking stop for good ASAP! ;)

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