DART data

A couple weeks ago an earthquake in south-western South Island, New Zealand, triggered a tsunami. The alert text put out by the Australian Bureau of Meteorology was:

    An undersea earthquake of magnitude 7.9 at Latitude 45.960S
    Longitude 166.470E occurred at 07:22 pm EST on Wednesday 15 July
    2009 near OFF W. COAST OF S. ISLAND, N.Z.. Sea level
    observations have confirmed a tsunami has been generated.

“Alert” is actually a bit of a misnomer; it was in the weather forecast. I only accidentally noticed the warning when I happened to check GNOME’s trusty little weather-applet wondering whether it was likely to be sunny the next day. So it’s great that I saw the alert there, but, if there’s a warning then I think gnome-applets needs to freak out a little to get the user’s attention. Maybe a different current-weather icon in the panel? Or better yet, a libnotify popup (“run for the hills” is a little more important than “your battery is low”)?

Anyway, as all this was going on, I was curious about the obvious question: “how do you detect such a thing a tsunami wave in the deep ocean?” One of the things about such energy is that although real tsunami become disasterous (ie huge) when they hit shallow water, out in the deep ocean they are not much more than a tiny ripple. Hard to notice amongst swell and chop.

Various national agencies have tsunami detection instruments floating around out on the high seas. These graphs show the data as gathered from buoy 55015 on the evening of 15 July:

day

which doesn’t appear to mean much, until you realize that the monitoring systems switch to transmitting high-resolution data when they detect an event; the longer time base data around the event above looked as follows, and suddenly it becomes clear that they noticed something abnormal:

day
Snapshots taken from the National Data Buoy Centre real time data page as presented on 15 July 2009; if you dig around you can find historical data there too.

It would appear that general sinusoidal trend line on the above is normal variation; sure enough if you look at today’s 15-minute data,

today

it’s pretty clear that normal measurement is tidal variation. Huh.

Column height

Anyone who has spent time out on the water knows full well that the surface is insanely variable; that got me wondering how they measure the waves going by (more to the point, how do they notice a tsunami wave a couple centimetres in size out amongst the randomness of wind driven chop and ocean swell?).

Well, it turns out that these at-sea buoys are not taking soundings from the surface as they bob around. The instruments are placed on the sea floor (!) and measure the hight of the water column as inferred by the water pressure.


Image from the NOAA National Data Buoy Centre page about DART buoys.

Note to super-tanker drivers: please don’t hit these. Thanks.

Never cry wolf

Thankfully, this earthquake and the associated ocean wave it generated did not result in a destructive tsunami when it reached the coastlines in the region. The fact that the authorities raised a high-profile alert for what turned out to be a non-event, however, raises the risk that the next time there is a tsunami wave detected the warning will be ignored by the news media.

The essential message remains, though: detecting waves is one thing (and impressive!). Forecasting destructive potential is another, and that part is still a very grey area. Hopefully people will accept this.

AfC