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Tom: my reading of the USGS methods that Kirt posted is that since depth is (at least sometimes) calculated relative to sea level, an earthquake with a negative depth occurred below ground but above sea level. So for example this one:
https://earthquake.usgs.gov/earthquakes/...#executive is centered just south of Halemaumau, at an elevation of 1100m, is shown as having a depth of -1.1km, which would put it close to the surface. There are a few close to or in the ocean that are shown with a depth of up to -0.6km, but those are all around the southwest rift where there's probably more error.
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Midnight - I certainly understand what you are saying. My understanding of the article is that one method uses sea level as a reference, but the actual number it's supposed to provide is relative to the surface. This obviously can provide errors as you'll have to know the exact surface level above the epicenter which may or may not be known accurately, plus you will have an uncertainty in the position of the epicenter, and then there are just the usual measurement uncertainties.
It might be (I'm uncertain) that in Hawaii's case, the errors are larger closer to the ocean or out in it because the earthquakes are farther away from the sensors. I've posted the link to the article Kirt quoted from below (I posted it earlier in the thread). Thanks all for an interesting discussion! I'll certainly do some more reading.
https://www.usgs.gov/faqs/what-does-it-m...e_products
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Tom, did this big quake do any serious damage to the observatories up on the mountain?
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Another one just now in HA...
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Royall - I don't know yet. All can tell you is the JCMT and Subaru daytime crews evacuated after the big one today in an abundance of caution and also to simply let people get back home. I've closed UKIRT through the weekend as we'll need to do a damage assessment and I don't want to put people in harm's way when checking things and then another large earthquake occurs. I think we'll have to wait until the seismicity dies down - and it isn't as just reported (I felt it too!).
I do know that our shear pins at UKIRT broke today which indicates the telescope shifted on its mount. However, it's designed to do that in an earthquake so I don't count that as damage. Yet.
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Interesting Tom, Thanks for the information! Hope things up there didn't get to messed up.
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The quake at 8:11 pm was a 4.4.
A 4.2 3.8 right before that one. (corrected)
The smaller ones are still coming every couple of minutes.
"I'm at that stage in life where I stay out of discussions. Even if you say 1+1=5, you're right - have fun." - Keanu Reeves
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Yet another jolt...I wish these aftershocks would stop for awhile. My cat is going nuts!
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"The quake at 8:11 pm was a 4.4.
A 4.2 right before that one.
The smaller ones are still coming every couple of minutes."
It appears to be quite an outlier - just south of Hawaiian Acres but well north of most of the activity.
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I checked both the regular usgs site and the HVO earhquake sites right after that 4.4. It took them 15 min. to post it. And, by the way, the coquis didn't let up for a moment so forget THAT early warning system.
Certainty will be the death of us.
