How accurate should the timing be?

[jbeale]

With reference to my observation of a local quake mentioned here,

Small outdoor vault

Last night at 11:48 pm (PDT) there was a small M2.0 quake centered only 3 km away from me, but 45 km deep. The R-Shake in the vault, R79D5 was able to detect it, as did my other nearby device on the surface RF7DC. Note: USGS says this happened at 6:48:22 UTC. I wrote the wrong number on the graph, on closer examination my first peak is at 6:48:26.932 UTC on RF7DC and 6:48:26.922 UTC on R79D5 so 45 km in 4.9 seconds means a velocity of 9.2 km/sec (which still seems a bit high). https://earthqu…

both my R-Shake units detected the initial peak of the event at the same time (to 1/100 sec) so that seems consistent. But if I trust the USGS report of the actual event time, I get a strangely high seismic velocity, nearly 10 km/sec. I am guessing that USGS event time should not be off by several seconds, and yet in my own (casual) testing dropping a weight at a specific time, I’ve found my own units to be not more than a second off from network time (just the closest that I’ve measured so far, they might be much better). What’s the most likely explanation for this, do you think?

[ivor]

hi,

i think the best way to examine this is to collect more data points, i.e., see what your travel-time looks like for other events and see how consistent (or not) the computed velocities of the waves for each are. a single event could be an anomoly.

as for timing, the units will always be within one sample of time to actual real time (on the condition that the NTP daemon is connected to a good time source). for the original 1D, then, this is 20 milli-seconds (50Hz sample rate), while for all other units 10 milli-seconds (100Hz sample rate).

also take into consideration that seismic wave velocities are extremely variable depending on soil structure. and the location of the event too, in reference to yours (is the source located below you? or totally horizontal to you? of course, probably some mixture of both…) can also have an influence on this.

(for your example, the event is very close to directly beneath you. in this case, the wave will be travelling mostly up, most likely through bedrock, which will be very fast. as opposed to a quake far away, which will spend more time at the surface before it reaches you, where the soil is softer and slower.)

i’m curious to see what a collection of results will look like. and, try to pick some events that are farther away and see what kinds of numbers that produces.

cheers,

richard

p.s. while i’ve picked some stuff up along the way, i’m not a seismologist, so please take all of this with some salt in hand… any real seismologist out there who sees an egregious error in my post, please correct me…