Swarm of [Nevada] Quakes have Experts Concerned

mynews4.com ^ | Friday April 15, 2011 10:09am PDT | Brooke Boone

[Godzilla]

Need to use the map properly - it is only a rough estimate based upon raw statistics.

[Quix]

Thanks much.

[Quix]

I’m not sure how to combine your two posts . . . What now isn’t?

Thx Big.

[jboot]

Harmonic tremor usually occurs during an eruption. Hasn’t happened yet. Check out the depth trend for the last 10 days. To my mind this is magma working to the surface, probabaly along an existing fault:

Date Average Depth (KM)
4/18/2011 3.87
4/17/2011 4.24
4/16/2011 4.86
4/15/2011 6.44
4/14/2011 10.17
4/13/2011 11.96
4/12/2011 12.57
4/11/2011 10.26

[Quix]

You have a link for that map?

Please.

[Scythian]

I am almost sure it is illegal aliens tunneling under the state so they don’t get deported ...

[jboot]

Can’t be. They are all here in Virginia. ;-)

[TheOldLady]

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[Godzilla]

Harmonic tremor usually occurs during an eruption.

The preceed it, denoting the magmatic movement upward into a magma chamber.

Reference here.

While your avg depth estimates are interesting, they are still in the very shallow category, considering that they are in the interior and any magma would be coming from deeper. The zone is not along any known or identified fault.

Again, volcanology has progressed significantly as well as recognizing seismic signatures of volcanic activity. So far there is nothing to indicate magmatic movement, only normal faulting based upon regional extension.

[jboot]

Thanks for the reference. Harmonic tremor is sometimes but not always associated with magma movement. From the same link you sent me there is also a discussion and graphic of "Shallow Volcanic Earthquakes". For reference, here's the graphic:

Now, this is by no means diagnostic, but I've been clicking around a bit on the USGS siesmology site and comparing some of the recent larger tremors in the swarm to this graphic. Many of the waveforms appear to conform to the "shallow volcanic" sample. For instance, check out the siesmogram of Event nn00333644 from earlier today:

[Godzilla]

Thats fine, now look at the estimated depth - 0 km. Is there an eruption occuring? No.

Look again to your figure posted. Note the roughly equal amplitude of the harmonics vs the varying amplitudes of the other examples. Now look at the seismographs that are the closest to the event. Note the sharpness, while the ones you may consider to represent a ‘shallow volcanic earthquake’ are farther away. The difference? Attenuation of the wave over distance and interference by the various waves and other sources.

Again, 0 km - yet no eruption.

[Godzilla]

Harmonic tremor is sometimes but not always associated with magma movement.

You said only eruptions - whats moving? Magma, what else causes similar waveforms?

[jboot]

Per your link, magma and gas movements have similar signatures.

This is part of the Great Basin, so the whole groundmass is being stretched like taffy. The activity may not be magmatic, but with the Bodie-Aurora volcano field in close proximity (especially Mud Springs Volcano-3 miles SSW and believed to be the most recent volcanic feature in Nevada), my money is on magma. What we really need is some field work to determine if there is any heat flow and ground deformation occurring. Alas, this spot is not actively monitored and there is probabaly no baseline to compare to.

[Godzilla]

Need to evaluate the magma type. The majority of the volcanism in that particular group has been calc-alkaline andesite, dacite and trachyandesite lavas, breccias and ashflow tuffs.

Mud Springs Volcano appears to have been andesitic in nature. These magmas, because of their viscosity, really make a lot of “noise” as they work their way up, meaning the seismic signatures would have been seen for a long time at far deeper depths than this swarm is occuring in.

[BenLurkin]

The map (http://earthquake.usgs.gov/earthquakes/step/) as I write is back to showing high probability of severe earthquakes for the Sierra MAdre mountains reachin gfomr abive Fresno up past Grass Valley (east of Sacramento. That was how it looked when I first posted teh link. Then the forecast area shrank back to what it was yesterdy and now it is large area again.

[Godzilla]

Ben, the map is not exact, but an initial attempt to develop a probablistic means to apply known seismic hazard analysis and current activity. What you need to look at is what the map is saying. The area you are concerned about has at max about a 1 in 100 chance of an aftershock that could knock things off a shelf (Mercalli VI - Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.). A MMI VI is roughly equivalent to the shaking directly above a M4.5 quake.

So you need to scale back your interpretation of this map. It does not predict SEVERE earthquakes, only potential MMI VI damage as a statistical possibility.

[Quix]

Interesting.

Thanks.

And for your comments Godzilla.

[Pete from Shawnee Mission]

Shoot! I grew up on the old “B” Sci Fi movies with giant rabbits and grasshoppers scurrying around old nuclear testing ranges. Undiscovered lateral thrust faults are probably right, but I’m holding out for Borax eating 50 meter long basin worms!