[Skywise]

(12-30-2016, 12:58 PM)Island Chris Wrote: Hi Marc,
You mean a new moon.

I'm not sure, but I think that some Moment tensors can look different than other types of focal mechanisms and this does not mean there is a magmatic component. For example, I am looking at one of the M5.7 quakes and some of the moment tensors have the blue are not connecting and others have it connecting. Someone like Brian would be able to explain; I can't.

Chris

Although I mostly understand it, I'm not sure I can explain it simply.

The beachball is a representation of the focal mechanism solution, which is a "reverse engineering" of the fault movement derived from the measured ground movements at several seismometer locations. Basically, trying to figure out the orientation and movement of the fault hidden deep underground based on how the ground moved anywhere from a few to thousands of miles away.

A perfect beachball represents a perfect earthquake - two flat parallel surfaces of elastic materials sliding past each other in a perfect way. The term used here is double-couple.

The real world, however, is more complex. Faults are not flat parallels, and quakes may happen across multiple fault strands or different faults with different orientations altogether. Further, different parts of the fault surface move different amounts from an ideal. This creates complexities in the radiated energy pattern measured by seismographs. This in turn makes it difficult to reverse engineer the fault motion that created the earthquake.

You may notice on the USGS pages the "percent double-couple" value, which represents how far from the ideal perfect model the earthquake was measured. It certainly includes measurement error as well. These uncertainties are graphically represented by curving the quadrants away from perfect corners.

Here's a technical document that discuss this, a PDF file:

A Primer on Focal Mechanism Solutions for Geologists

Yes, beachballs can look 'funky' for other reasons. Explosions, for example, and of course magma movement.

The following PDF document is part of a seismology lecture. Page 26 has a diagram which shows idealized bachballs for explosions and "CLVD" solutions. The last page shows actual beachballs from magma movement at the Bardarbunga volcano in Iceland.

Earthquake source mechanisms and radiation patterns

Although my explantion may be simplified and may have errors, I hope the idea comes across.

Brian

As a footnote, there are a few vocal individuals on other forums that seem to insist that every earthquake is a sign of an imminent volcanic eruption because the beach ball isn't perfect and is therefore a sign of CLVD and therefore magma movement. They are misunderstanding what the beachball represents and misinterpreting percent double-couple/uncertainty as something it's not. Unfortunately these individuals have the very closed mind that they accuse us science types of having. (is this a case of psychological projection?) I have tried explaining this to them, but my words fell on deaf ears.