anorlunda said:
So my question is: Is seismic research clustered in areas with favorable geology, or is it done pretty much everywhere?
Seismic equipment is primarily in areas of known activity so as to monitor that activity. So, say, in the USA,
all of the west coast region California, Oregon and Washington states have the most concentrated network
of seismometers. a bit less in Arizona, Nevada, Utah and Idaho. and much more spread out the further east
of the Rockies. There is still a good concentration of stations right across the USA, mainly because of the different networks run by various organisations/universities etc an example ...
SCSN - Southern California Seismic Network ( CALTECH/USGS)
https://www.scsn.org/index.html
NCSN - Northern California Seismic Network
http://www.ncedc.org/ncsn/
~ 580 stations
PNSN - Pacific Northwest Seismic Network
https://pnsn.org/
United States National Seismic Network
http://www.fdsn.org/networks/detail/US/
And there are other substantial networks, Hawaii and Alaska
anorlunda said:
A related question I've been wondering about for years, but can't find any data. People on a boat, far
offshore where the ocean is deep. Do they feel earthquakes? I tried and failed to find personal accounts
from people at sea directly sensing earthquakes.
If the quake is relatively close, within a few km's or so, yes they do. It is only the compressional P wave
pulse that they feel, as the S waves ( shear waves) cannot travel through a liquid.
I am aware of 2 occurrences back in my home country of New Zealand, when I lived there. Two large
events in the SW area of the South Island.
Both events were M6.5 + and some years apart. One report was from a couple of fishermen on a small boat
at the northern end of Lake Te Anau and in relatively shallow water < 30m (100ft). A very strong pulse that
they say almost capsized the boat. This event caused a small tsunami in the lake and the water was seen to
recede before rushing back in. This was verified by seismologists that visited the area to set up a small
network to record the aftershocks.
The other event was again by fishermen but this time at sea in a commercial fishing boat, offshore of
~ Milford Sound area and very deep water, several 100m. They stated that the thump on the bottom of the
boat was like that of running aground ... a bit difficult to do in such deep water
They didn't find out till later about the big quake.
anorlunda said:
When the crustal motion is horizontal, I could understand how little of it reaches the surface of the ocean. But when the crustal motion is vertical, the ocean must rise and fall with it (spawning tsunami). But also in the vertical case 10-30 second periods may be masked by ocean swells in the same frequency range.
Yes, good comments.
But keep in mind that the felt effect on a boat occurs relatively close to the event as with the two
examples I commented on above. I would be surprised if it was felt much further than 15 - 20km from
the seafloor epicentre of the quake. Possibly several times that for huge M8 ++ events ... I have no info.
Also, because this is a relatively "close to event" occurence, the frequency of the P wave is much higher,
1 sec or shorter periods, so the ocean swell period doesn't mask the effect.
These P wave pulses through water are called T waves ( or T phase). I have commented on them in
posts in years gone by. The T wave propagates from the sea floor at or near to the epicentre of the
quake. I record them from quakes onshore near the coast or offshore of the SW coast of the South Island
of NZ and periodically further south towards Macquarie Island. They will usually get trapped in the lower attenuation SOFAR channel not far above the seafloor, which acts as a waveguide.
https://en.wikipedia.org/wiki/SOFAR_channel
an example from my system ...
They are recorded on the short period geophone sensors.
The T wave is in the lower right edge of the trace. It was created by a M5.5 quake
cheers
Dave
can't wait till it finishes
