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midtskogen

Date: Sunday, 13.11.2016, 23:47 | Message # 1081

Star Engineer

Group: Users

Norway

Messages: 1674

Status: Offline

Yes, the striking feature of this plot is the lack of an S wave arrival. It would be interesting to convert this data to audio. Usually one hears two crashes followed by a rumble: P wave arrival, S wave arrival and surface waves. But this should be different. I'm in Korea right now, and accessing my usual tools for doing these kind of things is a bit trickier.

NIL DIFFICILE VOLENTI

Edited by midtskogen - Monday, 14.11.2016, 00:21

midtskogen

Date: Monday, 14.11.2016, 00:05 | Message # 1082

Star Engineer

Group: Users

Norway

Messages: 1674

Status: Offline

An attempt to convert to audio. One hour compressed to 5 seconds. The sound appears very soft to me, and no doubt the distance causes that. But I can barely hear the sound, which may be because my $200 laptop has lousy speakers.

Edit: My phone's speaker is better. There are two crashing sounds right in the beginning. The second could be an echo of some kind, since there will be no S wave (and the delay between them is two small for an S wave anyway). It's not one of the aftershocks either, as the first one large enough for clear detection occurred half an hour later. I can't really hear the surface waves, but the frequencies might be too low for my speakers, even when one hour is compressed to 5 seconds. My seismometer peak sensitivity is around 0.06 Hz, which gets turned to audio around 43 Hz and that is way too low for most speakers.

Attachments: 4265737.wav (422.0 Kb)

NIL DIFFICILE VOLENTI

Edited by midtskogen - Monday, 14.11.2016, 00:21

Watsisname

Date: Monday, 14.11.2016, 04:48 | Message # 1083

Galaxy Architect

Group: Global Moderators

United States

Messages: 2613

Status: Offline

That second crash seems to be the spike I was mentioning at about 11:28, which based on the graphic is probably the part of the P wave which moved through the mantle and reflected once off the surface before reaching you. At first I thought it surprising that it arrives so soon after the direct wave despite following a longer path, but after going through the math it does make sense.

If we consider a point antipodal to the quake, then the straight line through the Earth is about 12740km. 100km through the crust (both sides), 3900km through mantle, 6200 through the outer core, and 2540 through the inner core. The P-wave velocity changes through all of these, averaging about 7km/s through crust, 10.5km/s through mantle, 9km/s through outer core, and 11km/s through inner core. Then the time it takes to pass through the whole Earth is about 22 minutes.

For the singly-reflected P wave through the mantle, we can imagine (not quite accurately but close enough) that it goes on a straight line to a point 90° away and reflects from there to the antipodal point. This path length is sqrt(2) times the radius of the Earth, times 2, or about 18000km. Then assuming 10.5km/s average speed, it takes about 28 minutes to make that trip. This roughly agrees with the observed waves.

The first surface waves should have arrived somewhere around 40 minutes after the quake. There does seem to be an increase in intensity there, but a lot of other complicated reflections are also reaching you by that point as well.

midtskogen

Date: Wednesday, 30.11.2016, 12:43 | Message # 1084

Star Engineer

Group: Users

Norway

Messages: 1674

Status: Offline

This meteor was detected by my software last night. It made it deep into the atmosphere, down to 34 km, despite it was not very bright at all. I post it here because it was such a nice detection - the software working perfectly as intended, locating stars for fine-tuned calibration and determining the full track from multiple stations. I used to do all this by hand a few years ago, so it's so nice to see it automated. smile