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Comment by onnoonno | Hacker News Reader

What I don't get is that nearly all the buoys that have that moving icon "Tsunami alert" show some excitation at the ~9:45GMT mark.

But I would of course expect the buoys that are farther away to only show some kind of peak when the wave actually reaches them. And it looks like they are thousands of km away, so some of them should not even have been reached yet.

Some what is going on there? Is the data time actually shifted and not in GMT but 'earthquake propagation time'? Am I reading the plots wrong?

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IANA Tsunami guy, but I worked with them in college.

The water column height on these buoys is a pressure transducer on the floor of the ocean (http://www.ndbc.noaa.gov/dart/dart.shtml)

The time skew of the events is small, but does exist. They're all within minutes of the 9:31am gmt earthquake, getting longer as you get away from the epicenter. i.e, way faster than a tsunami travels, but about how fast an earthquake travels. (Earthquakes spread in x miles per second, tsunamis in x miles per minute)

I _think_ what you're seeing is the pressure sensor picking up the earthquake and converting it to a water column height.

Oh you are right. It actually _does_ have a time shift. I take back what I said and assert the opposite!

Thanks for the explanation.

I'm probably completely wrong, but am wondering and trying to deduce the same thing. My assumption, guess really, is that they were remotely activated into "Tsunami Mode" -- a more rapid-pulse reporting mode or similar, which just is a bit noisier perhaps?

Total layman's guess though -- would love to know more if anyone knows the answer.

> My assumption, guess really, is that they were remotely activated into "Tsunami Mode" -- a more rapid-pulse reporting mode or similar, which just is a bit noisier perhaps?

That sounds like a very good guess. But then, the spikes that are purported to be the Tsunami in the media is just the ADC having a couple bad samples from being switched into a different mode or so :-)

If any USGS person is reading this thread here: Take it as a issue report that your page should be readable and understandable by lay persons, at least lay persons with a non-geophysics STEM background :D

Not an oceanic scientist but sound speed in water is ~1500m/s. This is the speed at which a pressure perturbation propagates. The speed at which the actual tsunami wave would propagate is much more slower than this. So I believe that the excitation marked in the plot is the pressure wave caused by the earthquake, which travels orders of magnitude faster than an actual water wave caused by the earth displacement.

But that doesn't fit the data, either. The buoys all report something around the 9:45ish GMT mark. And 1500m/s would mean you reach Mendocino bay in California only about now or so. But have a look at the plot:

https://archive.fo/KsN8b

That's true. Maybe when a buoy registers an "event", an "snapshot" of more fine-grained 15-sec data is recorded from all DART buoys?

Edit. From this page: http://www.ndbc.noaa.gov/dart/dart.shtml

> The system has two data reporting modes, standard and event. The system operates routinely in standard mode, in which four spot values (of the 15-s data) at 15-minute intervals of the estimated sea surface height are reported at scheduled transmission times. When the internal detection software (Mofjeld) identifies an event, the system ceases standard mode reporting and begins event mode transmissions. In event mode, 15-second values are transmitted during the initial few minutes, followed by 1-minute averages. Event mode messages also contain the time of the initial occurrence of the event. The system returns to standard transmission after 4 hours of 1-minute real-time transmissions if no further events are detected.

The algorithm is described here: http://www.ndbc.noaa.gov/dart/algorithm.shtml

I see, thanks. But that rather sounds like the buoy itself is switching modes?

From the look of all the plots, even the far-away ones, it rather looks to me like they have been remotely switched into event mode (with that 15s stuff being plotted at about the same time)?

I guess I am really confused about the time stamps.

Based on this page, linked by someone just above, it sounds like they do have 2-way comms (Iridium) and can remotely be put into 'Event Mode'.

http://www.ndbc.noaa.gov/dart/dart.shtml

I think it's a combo of noise when that was activated (see the page above, describes exactly what it reports when it goes into that mode, etc) -- and then for the one that shows a substantial sea level rise, it's likely just the pressure since it's all pressure-based sea-level measurement "estimates". Guess I kind of thought it measured actual rise/fall of the buoy, or maybe used GPS or something, but makes a bit more sense that it's solely based on sea-floor pressure.

This is from an aftershock, which happened 09:47:53 (UTC). Info taken from https://earthquake.usgs.gov/earthquakes/map/

The spike corresponds to about 15min delay between the quake at 9:31 UTC. But some kind of spike visible in all stations at that time, even very far away ones, like Mendocino bay.

Honestly, this looks to me rather like some bad data due to the buoys switching mode because of radio commanding 15min after the quake rather than any wave arriving. (Like the other poster suggested)

Even if the shock would travel through an Earth made out of steel, it wouldn't have reached Mendocino bay that fast.

EDIT: There is actually a time shift and it seems to be enough, as wiredfool pointed out. Thanks!

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