Hi Tom:
The USGS talk was the first time I'd heard about the need to look at an earthquake as happening along some length of
fault line. For the big quake in Japan the forecast software assumed a point source for the quake and that cause them
to under estimate the magnitude and get other things wrong. GPS is part of the solution to get better results.
In the S. CA example he showed a 180 mile long rupture of the San Andreas fault. At 2 miles a second the quake would
last about 90 seconds.
Accelerometers that are not right on top of the fault will be overloaded with signals coming from each location where
there's a fracture and so the data will be nearly impossible to untangle in a short time frame. But a GPS receiver will
show a DC displacement that unambiguous.
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/Clarke4Congress.html
Tom Van Baak wrote:
Brooke,
In the papers they were getting some results with even 1 Hz sampling,
but, as expected, 10 Hz was better. That's probably sufficient for seismic
waves; 100 Hz is overkill. See Figure 5 of the Larson paper I for a nice
example of the AC vs. DC coupling that you mentioned.
I also agree with Jim's earlier comment; for earthquake detection it
seems an cheap accelerometer is more than adequate. One doesn't
need the expense of dual frequency carrier phase gps receivers just
to detect a local shake.
These days, there are always many cell sites where there are many
people; and each site already has GPS timing, battery backup, and
a fast connection to a central office; so it's the perfect place to add
a sensitive accelerometer. You could just call it an security intrusion
monitor and use it for earthquake detection as a free side effect.
/tvb
Hi Hal:
In the talk there was a slide showing a comparison between ground position calculated from an accelerometer and a
real time precision GPS.
The Accelerometer is AC coupled and so misses the DC coupled GPS answer that
shows the permanent ground movement.
I'm guessing it takes a GPS receiver that has 100 Hz or faster outputs that can be reduced to cm or better position
to do this.
Have Fun,
Brooke Clarke
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