[Starlink] Small Tonga update

[Ulrich Speidel]

Off-topic, but given the interest on this list last year, here's an 
update one year on from Shane Cronin, the lead volcanologist on the 
Hunga Tonga Hunga Ha'apai volcano, on what they're learned:

https://theconversation.com/a-year-on-we-know-why-the-tongan-eruption-was-so-violent-its-a-wake-up-call-to-watch-other-submarine-volcanoes-175734

You'll notice that he mentions unanswered questions, in particular a 
local tsunami of unknown origin that was 18-20 m high.

I'm marginally involved with this whodunnit. Basically, they know the 
height of the wave because the tsunami knocked out an automated weather 
station on Kanokupolo Peninsula that collected data every 10 minutes and 
transmitted it back to its Tongan base every hour. The station was 
located about 20 m above mean sea level and transmitted its last batch 
of data at 0500 UTC, half an hour after the destruction of the domestic 
cable but still 44 minutes before the international cable went down. No 
transmission was received at 0600 UTC, indicating that the station had 
been destroyed by then. Unfortunately, that one hour time window is a 
bit large given tsunami travel times.

I'd been aware of this for a while, but was contacted in early December 
by one of the tsunami modellers involved, who filled in a crucial gap in 
my knowledge: The weather station had actually been sitting on a 
cellphone tower! Which I hadn't realised so far ... So we're currently 
trying to find out from Tonga Telecom when exactly that base station 
went down - or whether in fact they can still reconstruct this a year 
down the track. For the tsunami folk, a lot hinges on that piece of 
information, as this'll determine where the waves came from. The caldera 
collapse that Shane mentions is one option, but one of the problems with 
this theory is that there is no clear seismic signature. USGS recorded 
quakes in the area at 4:40 UTC and another at 5:30 UTC, but these had 
magnitudes of 4.8 and 4.7 respectively, and were both located a bit to 
the south of the volcano and were also quite deep (10 km, but that seems 
to be very approximate). So they don't quite fit the bill, apparently.

Another marginally comms-related aspect of the eruption aftermath is the 
damage it's doing to Tonga's power grid. A lot of the local power 
network runs on poles above ground, and the lines consist of a blank 
aluminium earth wire and isolated wires carrying the supply current. 
These run cheek-to-jowl with the earth wire but aren't mechanically 
bonded to it. What has happened now is that volcanic ash that was 
deposited on the lines has worked its way to between the earth wire and 
the insulated wires, and by the rubbing motion in the wind, the ash is 
working its way into the isolation, progressively breaking it down and 
causing the lines to short. So they're likely looking at having to 
exchange much of their overhead cable. Shane told me about this in late 
November and actually showed be a few samples that the Tonga power 
people had given him.

Plus he was quite concerned about a number of lookalike volcanoes in the 
area showing signs of activity now. One submarine one they went over had 
a pre-eruption bathymetry of 11 m below sea level but had inflated to a 
wee bit less than that.

My own little write-up from last year is here: 
https://dl.acm.org/doi/10.1145/3570748.3570759

-- 
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Dr. Ulrich Speidel

School of Computer Science

Room 303S.594 (City Campus)

The University of Auckland
u.speidel at auckland.ac.nz
http://www.cs.auckland.ac.nz/~ulrich/
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