Re: New tsunami advisory warning - Japan

[Pete Carah <pete () altadena net>]

On 03/28/2011 01:22 PM, Gavin Pearce wrote:
> > JCG ship in the the open ocean.
> Impressive video. The wave height and speed would suggest shallower
> waters, and that likely the ship was close to land mass when the video
> was filmed rather than open ocean (in the sense of being far out to
> sea). Not being there of course I could easily be incorrect.
>
> Anyway we digress  :) 
>
> Gav 
>
> On Mar 28, 2011, at 11:28 AM, Marshall Eubanks wrote:
>
> > On Mar 28, 2011, at 10:57 AM, Gavin Pearce wrote:
> >
> > > > You guys forget a lot of folks on the list are working on cabling
> ships and off shore platforms, its not all about what happens on shore
> in this industry.
> > > Valid point ... however in deep ocean, these things are pretty
> imperceptible. The effect on ships on the surface are nominal, and off
> shore platforms are (generally) built with these things in mind:
> http://www.msnbc.msn.com/id/27324535/ns/technology_and_science-innovatio
> n/
> > Here is a video of the recent Japanese tsunami from a JCG ship in the
> the open ocean. The waves (@ ~4:20 and 6:40 into the video) caused them
> no trouble, but they were certainly not imperceptible. 
> With the video :
>
> http://www.youtube.com/watch?v=4XSBrrueVoQ&feature=player_embedded#at=19
Thanks for the link...  Very impressive, though strong storm waves get
higher.  This is not an open-ocean tsunami, it is probably either direct
from the quake source or reflected from the nearby coast (I'm fairly
sure it is the latter, though there isn't a good time reference in the
video, since there appears to be land visible in the frame; if the white
mass is really land, this definitely does not qualify as open-ocean,
which for tsunami purposes has to be an open-ocean wavelength or so from
the nearest land or shallow water (600-800 miles; you wouldn't see the
land...)  Cable damage from tsunamis mostly comes from bulk motion up or
down a sloping ocean bottom, or from primary or secondary turbidity
currents (basically an underwater avalanche) (unless you are unlucky
enough to have the fault break itself cut the cable; this isn't too
likely but with this fault geometry it could have happened.)

Also, this quake (the 8.9 "main" one, not either the foreshocks or
aftershocks, several of each were strong enough to trigger a tsunami
watch in Hawaii by themselves) had a very extended energy-release time;
the ground motion went for several minutes (see the graph in
http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0001xgp/finite_fault.php). 
That can complicate wave generation a lot.  Various mechanisms
contribute to tsunami generation; the majority of wave generation in the
1960 Chile event came from landslides secondary to the main earthquake
(which was very deep and centered under land...)  My memory of papers
about the 1964 Alaska quake involved both ground motion and landslides
as contributors.  Note that in this case, the resulting waves can go
different directions from the same quake.

Aside: I worked for the U of Hawaii tsunami reasearch program in the
1960's for a while, we were mainly working on very early prototypes of
the deep-ocean pressure sensors that are now deployed.  Decent embedded
microprocessors didn't exist then (for that matter, *any*
microprocessors, even the 1802 or 8008, either of which would have been
a grand luxury :-(  Those finally made these sensors practical.  (ours
was set up with a write-only 7-track tape drive, using
discrete-transistor logic modules (no practical ICs yet either). It was
to be placed on Ocean Station November (about 2/3 of the way from San
Francisco to Honolulu), kicked overboard on request from the research
people (after a "suitable" earthquake), then retrieved using a low-power
radio beacon a few days later when the cable release timer tripped.) 
Modern electronics has improved things :-)

-- Pete

> Marshall
>
>
> > Regards
> > Marshall
> >
> > > At the other extreme, Lituya Bay is a good example of a Mega Tsunami
> (1,720 feet):
> > > http://en.wikipedia.org/wiki/1958_Lituya_Bay_megatsunami
This one lists landsliding (or perhaps calving) as the generation
mechanism, and both the source and the bay outlet were small enough that
the wave probably didn't propagate too far once in the open ocean.
>