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Greetings all
Over the last couple of days I have been delving into a new personal discovery about seismic waves. And with the talk recently in several threads in the general and classic physics sections about sound wave propagation in water, I thought this was appropriate to share.
Many of us will be familiar with the more well known P, S and Surface waves generated by an earthquake. One that up until a couple of days ago that I was unaware of was the http://www.earth.northwestern.edu/docs/publications/EAO169.pdf.
On the 16th Dec 2013 there was a Mb 6.2 ( Mw 5.8) earthquake ~ 30 km offshore of the SW tip of the South Island of New Zealand.
Tectonics of the Region
The regional tectonic setting is a steeply dipping subduction zone boundary between the dipping Indo-Australian Plate and the over-riding Pacific Plate. This is part of the main plate boundary between the Indo-Australian and the Pacific Plate. Further north and running down the western side of the South Island is the oblique slipping Alpine Fault> At its southern end, in the Fiordland region, it transforms into the steeply dipping subduction zone which continues further south to Macquarie Island and beyond. The Fiordland Subduction Zone, has regular large earthquakes ands averages a M7+ event ~ every 15 years. The last large event was a M 7.8 in 2009 ~ 80 km to the north of Monday's M 6.2 event.
OK, enough background let's get to the fun stuff
On the long period seismometer the M 6.2 looked like any other quake of that size at that distance ( ~ 1850 km) from my Sydney, Australia location.
This sensor is bandpassed between 0.05 Hz and 0.1 Hz, the 10 to 50 second period is what I'm primarily interested in, tho there are some really interesting very long period, ~ 100 second, Earth modes that are seen after the really large events. The Earth rings ( resonates) like a bell for many many hours after the big quakes.
I happened to look at my short period sensor channels. These are 4.5 Hz geophones buried in the ground under the house. These geophones are primarily for recording local and regional events around eastern Australia. The phones recorded the P arrival and a little bit of the S arrival and as to be expected none of the of the low frequency surface waves.
The surprise was the huge burst of signal ~ 18 minutes after the arrival of the P wave on the geophone that didnt show up on the long period trace as shown above. Initially I thought that I had recorded a local event at the same time as the New Zealand event. Looking around some of the other online seismograms from around eastern Australia showed that this burst of signal was evident on them as well.
It was one of my fellow amateur seismo friends that pointed out that this was possibly a T wave phase. See the seismogram below from the geophone...
As can be seen, the T wave amplitude was very large compared to the P wave arrival.
NOTE the difference in recording time scale between the 2 grams
I have included a link to a paper on T wave generation. For those that don't want to read too deeply. What I have learned so far is that the T wave is generated at the water / rock boundary of the seafloor. The seismic waves coming up to the seafloor from below, generate an acoustic wave in the water which then radiate outwards from that point. In the case of the signal I recorded in Australia, the T wave traversed the Tasman Sea to where it encountered the edge of the Australian Continental Shelf. At this point some of that impinging wave was converted back into a seismic wave as it entered the sea / shelf interface and then it continued on to be recorded by my and other sensors on land in Australia.
I hope others find this as interesting as I have.
cheers
Dave
http://www.earth.northwestern.edu/docs/publications/EAO169.pdf
Over the last couple of days I have been delving into a new personal discovery about seismic waves. And with the talk recently in several threads in the general and classic physics sections about sound wave propagation in water, I thought this was appropriate to share.
Many of us will be familiar with the more well known P, S and Surface waves generated by an earthquake. One that up until a couple of days ago that I was unaware of was the http://www.earth.northwestern.edu/docs/publications/EAO169.pdf.
On the 16th Dec 2013 there was a Mb 6.2 ( Mw 5.8) earthquake ~ 30 km offshore of the SW tip of the South Island of New Zealand.
Tectonics of the Region
The regional tectonic setting is a steeply dipping subduction zone boundary between the dipping Indo-Australian Plate and the over-riding Pacific Plate. This is part of the main plate boundary between the Indo-Australian and the Pacific Plate. Further north and running down the western side of the South Island is the oblique slipping Alpine Fault> At its southern end, in the Fiordland region, it transforms into the steeply dipping subduction zone which continues further south to Macquarie Island and beyond. The Fiordland Subduction Zone, has regular large earthquakes ands averages a M7+ event ~ every 15 years. The last large event was a M 7.8 in 2009 ~ 80 km to the north of Monday's M 6.2 event.
OK, enough background let's get to the fun stuff
On the long period seismometer the M 6.2 looked like any other quake of that size at that distance ( ~ 1850 km) from my Sydney, Australia location.
This sensor is bandpassed between 0.05 Hz and 0.1 Hz, the 10 to 50 second period is what I'm primarily interested in, tho there are some really interesting very long period, ~ 100 second, Earth modes that are seen after the really large events. The Earth rings ( resonates) like a bell for many many hours after the big quakes.
I happened to look at my short period sensor channels. These are 4.5 Hz geophones buried in the ground under the house. These geophones are primarily for recording local and regional events around eastern Australia. The phones recorded the P arrival and a little bit of the S arrival and as to be expected none of the of the low frequency surface waves.
The surprise was the huge burst of signal ~ 18 minutes after the arrival of the P wave on the geophone that didnt show up on the long period trace as shown above. Initially I thought that I had recorded a local event at the same time as the New Zealand event. Looking around some of the other online seismograms from around eastern Australia showed that this burst of signal was evident on them as well.
It was one of my fellow amateur seismo friends that pointed out that this was possibly a T wave phase. See the seismogram below from the geophone...
As can be seen, the T wave amplitude was very large compared to the P wave arrival.
NOTE the difference in recording time scale between the 2 grams
I have included a link to a paper on T wave generation. For those that don't want to read too deeply. What I have learned so far is that the T wave is generated at the water / rock boundary of the seafloor. The seismic waves coming up to the seafloor from below, generate an acoustic wave in the water which then radiate outwards from that point. In the case of the signal I recorded in Australia, the T wave traversed the Tasman Sea to where it encountered the edge of the Australian Continental Shelf. At this point some of that impinging wave was converted back into a seismic wave as it entered the sea / shelf interface and then it continued on to be recorded by my and other sensors on land in Australia.
I hope others find this as interesting as I have.
cheers
Dave
http://www.earth.northwestern.edu/docs/publications/EAO169.pdf
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