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

In summary: 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.
  • #1
berkeman
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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?
 
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  • #2
Looking for the video link...
 
  • #3
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/
 
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  • #4
Maybe it is an issue of depth?
 
  • #5
russ_watters said:
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!
 
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  • #6
russ_watters said:
Maybe it is an issue of depth?
Or even a matter of the kind of waves.
 
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  • #7
berkeman said:
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.
 
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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.
 
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  • #9
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".

hawaii.another.shake.map.png
https://earthquake.usgs.gov/earthquakes/eventpage/us1000dyad#executive
 

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  • #10
berkeman said:
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 :smile:

russ_watters said:
Maybe it is an issue of depth?

no :smile:

berkeman said:
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

IMG_1914 sm1200vsm.jpg
But have been keeping an eye on the Hawaii situation.
fresh_42 said:
Or even a matter of the kind of waves.

fresh_42 said:
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."
russ_watters said:
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
 

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  • #11
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.
 
  • #12
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.
 
  • #13
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.
 
  • #14
Here is a link to the earthquaketrack.com Hawaii page. There was a 6.9 reported specific to Leilani Estates.
 
  • #15
tommytbot said:
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
 

1. What is the difference between a magnitude 6.9 earthquake in Hawaii and one on the Hayward Fault?

The main difference is their location. A magnitude 6.9 earthquake in Hawaii would occur on the Pacific plate, while one on the Hayward Fault would occur on the North American plate. This also means that the type of fault and the tectonic forces at play would be different.

2. Can a magnitude 6.9 earthquake in Hawaii trigger one on the Hayward Fault?

It is highly unlikely. The distance between Hawaii and the Hayward Fault is too great for one earthquake to directly trigger another. However, large earthquakes on one fault can indirectly affect others by changing the stress and strain on the surrounding areas.

3. Which location would experience more damage from a magnitude 6.9 earthquake?

It is difficult to say for certain, as the level of damage can vary greatly depending on the distance from the epicenter, the type of soil and buildings in the area, and other factors. However, earthquakes on the Hayward Fault have the potential to cause more damage due to the high population density and infrastructure in the surrounding areas.

4. Are there any warning signs before a magnitude 6.9 earthquake occurs?

Unfortunately, there are no reliable warning signs that can predict the exact timing and magnitude of an earthquake. However, scientists are constantly monitoring fault lines and seismic activity to better understand the patterns and potential risks.

5. How can people prepare for a magnitude 6.9 earthquake in either of these locations?

Some important steps for earthquake preparedness include creating an emergency plan, securing furniture and other objects in your home, and having emergency supplies on hand. It is also important to stay informed and follow any evacuation or safety instructions from local authorities.

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