[time-nuts] GPS, USGS Early Earthquake Warning

[bownes]

WIWAUG (when I was an under grad), it was explained to me that the seismometers were log scale and basically don't clip. When you get into a big event, the last few decimal places just don't matter. 

We also used differential GPS across known fault lines to measure slip both over time and during events, which are often 'step' slips of a bit at a time over the course of the event, which may last seconds or hours. 

Bob




On Apr 28, 2012, at 18:32, "Tom Van Baak" <tvb at LeapSecond.com> wrote:

> Brooke,
> 
> Right, an overloaded accelerometer is a problem -- if you have
> only one or a few of them.
> 
> But the beauty of using cellular sites is that you have hundreds
> or thousands of them across populated areas; so it's no problem
> if the a bunch of sensors near the epicenter overload. A clipped
> signal is not worthless; at least you know something big happened
> there; you can rely on slightly more distant cell tower sensors to
> get readings a few seconds later that are less clipped or not clipped
> at all. (There's another solution I heard about -- using smartphones
> as a tiered network of synchronized accelerometers).
> 
> A high rate GPS solution sounds really cool to me but I bet its also
> far more expensive.
> 
> Related to that, are there any seismometer experts on the list? I've
> always wondered why they don't augment the extremely sensitive
> detectors with less sensitive detectors? Of course a really good
> detector will overload; so just co-locate cheap detectors that are 40
> and 80 dB less sensitive. That way you get a clean signal no matter
> how close or far the epicenter is from the detector.
> 
> /tvb
> 
> ----- Original Message ----- From: "Brooke Clarke" <brooke at pacific.net>
> To: "Tom Van Baak" <tvb at LeapSecond.com>; "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Sent: Saturday, April 28, 2012 2:49 PM
> Subject: Re: [time-nuts] GPS, USGS Early Earthquake Warning
> 
> 
>> 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
> 
> 
> 
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