Magnitude 6.9 Earthquake in Hawaii versus on the Hayward Fault -- What am I missing?

[berkeman]

How come I saw a video of a magnitude 6.9 earthquake on the island of Hawaii on the evening news just now, and the grocery store products were not even falling off the shelves (and no deaths or injuries), versus the same magnitude earthquake prediction here on top of the Hayward Fault (east of Silicon Valley, California), with the current estimates of thousands of deaths and hundreds of millions of dollars of damage?

Should I move to the Big Island?

 

[berkeman]

Looking for the video link...

 

[berkeman]

I think this is the link to the news article, but I'm not sure (browser issues). The video on the "World News Tonight" for ABC 7 News showed a video from the earthquake in a grocery store, and the products were not falling off the shelves. Strange IMO.

http://abc7.com/69-earthquake-strikes-hawaii-near-erupting-volcano/3429760/

 

[russ_watters]

Maybe it is an issue of depth?

 

[berkeman]

Maybe it is an issue of depth?

Hmm, it must be. a 6.9 near the surface would destroy stuff, at least based on my emergency response training so far. Searching for more info..

Where is @davenn when you need him!

 

[fresh_42]

Maybe it is an issue of depth?

Or even a matter of the kind of waves.

 

[russ_watters]

Hmm, it must be. a 6.9 near the surface would destroy stuff, at least based on my emergency response training so far. Searching for more info..

Where is @davenn when you need him!

I was thinking the other way around, but I don't know if magnitude is measured at the station or estimated for the source.

A volcano is obviously very close to the surface but the shaking would decrease rapidly with distance from epicenter.

 

[fresh_42]

Now that I've seen the video, I bet on waveforms. IIRC volcanic induced eruptions have a different wave signature than ordinary earthquakes. Hope Dave will correct me if I'm wrong.

 

[OmCheeto]

It looks as though not too many people live near the epicenter, and the shaking looks like it diminishes pretty quickly with moderate distances.
Hilo is about 30 miles away, and according to this USGS KML map, "Potential Damage" at that distance was listed as "none" to "very light".

https://earthquake.usgs.gov/earthquakes/eventpage/us1000dyad#executive

 

[davenn]

I think this is the link to the news article, but I'm not sure (browser issues). The video on the "World News Tonight" for ABC 7 News showed a video from the earthquake in a grocery store, and the product was not falling off the shelves. Strange IMO.

http://abc7.com/69-earthquake-strikes-hawaii-near-erupting-volcano/3429760/

thanks for the link

Maybe it is an issue of depth?

no

Hmm, it must be. a 6.9 near the surface would destroy stuff, at least based on my emergency response training so far. Searching for more info..

Where is @davenn when you need him!

had the day out of town yesterday when you were typing this ... was at a military air show south of Sydney

But have been keeping an eye on the Hawaii situation.

Or even a matter of the kind of waves.

Now that I've seen the video, I bet on waveforms. IIRC volcanic induced eruptions have a different wave signature than ordinary earthquakes. Hope Dave will correct me if I'm wrong.

Yes, that's the reason. I don't have a deep understanding of why 6.9 volcanic quake doesn't produce as much shaking as a tectonic quake, but it has a lot to do with the way the energy is released. There is no big slip on a fault where there would be 2 huge surfaces grinding past each other ( tectonic).

from Berkeman's link...

Seismologist Dr. Lucy Jones tweeted that the 6.9 earthquake has a "non-double-couple focal mechanism that implies it is not just movement on a fault, but rather involves movement of fluids."

I was thinking the other way around, but I don't know if magnitude is measured at the station or estimated for the source.

The magnitude is the what is allocated for the source, regardless of how many stations record it or how far they are away from the source they are.

I hardly recorded this quake on my system here in Sydney. A normal tectonic quake around 6.8 - 7.0
at that distance would have had my sensor producing significant motion to be recordedDave

 

[Steelwolf]

On most faults the energy release and shaking happens along a line and so the source is an extended one as opposed to a single point source, such as Hawaii's was. Plates will 'grumble' against each other where in Hawaii it was more of a sharp crack of rock breaking followed by the lava rushing into new, well, actually older, pre-made volcanic voids, encountering water and getting steam explosions in the subsurface. That helped set up what we are seeing now with, at this time, some 10 or 12 reported rift-vents some with lava flow, and so far 26 houses known lost. The big shock was the rock breaking allowing the river of lava to take it's path Eastward.

But the difference is in the movement of land against land vs the movement of a Hot liquid through a porous solid of the same material, thus prone to melting. This means that there can easily be more events from this as the magma this time around seems to be charged with more gas than we have been seeing with the previous clean flowing pillow lava, forming non-fountaining rivers of lava. The fact that there is fountaining at these rift-vents is worrisome, as in the rise of gas bubbles in the main caldera, even though the level of lava in the caldera has decreased.

 

[eachus]

Due in part to the magma, which did most of the moving, being a fluid. That tended to damp high frequency waves. Another thing to remember is you had a slug of lava suddenly freed up and able to move, and the movement of the Earth was the equal and opposite reaction you would expect. But instead of half the island moving one way, and the other half the other, all of the surface moved as a unit. This reduced the shaking measured in g's where the 6.9 is a measure of energy. Call all that reducing the acceleration by a third or reducing the effect to that of a 6.4 earthquake. The epicenter was already evacuated, so no footage of large movements there.

 

[Astronuc]

With respect to depth, the event cited by OmCheeto was shallow, 2.1 km depth.

Certainly distanced from the epicenter is a factor. Another factor maybe the type of movement near the epicenter, e.g., volumetric collapse/expansion of a volume of magma, or fracture of rocks as opposed to a slip or strike, or uplift/drop of some mass along a fault (think of the Tohoku earthquake that initiated the accident at Fukushima).

Also, another factor has to do with the subsurface terrain on which buildings sit. Hard rock transmits ground waves, whereas soft ground can intensify the motion. I learned this from some seismic experts who looks at impacts on large structures such as bridges, dams and buildings.

 

[tommytbot]

Here is a link to the earthquaketrack.com Hawaii page. There was a 6.9 reported specific to Leilani Estates.

 

[davenn]

Here is a link to the earthquaketrack.com Hawaii page. There was a 6.9 reported specific to Leilani Estates.

it was centred offshore from that region