Earthquake – Focal beach balls

Picture by Associate Press: Earthquake in Japan 2011.

Stand in the place where you live

Now face North

Think about direction

Wonder why you haven’t before

Now stand in the place where you work

Now face West

Think about the place where you live

Wonder why you haven’t before

If you are confused check with the sun

Carry a compass to help you along

Your feet are going to be on the ground

Your head is there to move you around

From        “Stand” by R.E.M.

This is a bit of a follow up on Carl’s excellent “Earthquakes – What’s the fuzz?” post. I was asked to elaborate more on the “beach balls” that I mentioned in a response on that thread.

I’m not going to give you the details on that… I’m not capable of doing the math myself or quoting esoteric concepts about how to process it. I am however, going to give you enough geospatial background to understand what they are all about and what they mean.

The first thing I need to cover, mainly in order to bring every one up to speed… is the compass rose. The compass rose appears on many navigational charts and a method of calculating direction from one point to another. When coupled with an actual compass, you can find your way from one point on the chart to another.

The rose is noted in degrees from 000° to 359° (actually 359.99….) which is a full circle. North is 000°, due South is 180°. Many features having to do with the earth are discussed in the bearing that the feature lies in or has moved.

Image by GeoLurking: Compass rose.

Which brings us to:

Lineament n (often plural)

3. (Earth Sciences / Physical Geography) Geology any long natural feature on the surface of the earth, such as a fault, esp as revealed by aerial photography.

On Carl’s “Earthquakes – What’s the fuzz?” post, that stunning picture of the San Andreas is a Lineament, or a linear feature. It’s a surface manifestation of the thousands of years of slip along that section of the San Andreas. In some places, you can find creeks that have had their channels offset by several meters… in other words, they don’t line up across the fault. It has shifted that much since the creek bed was formed.

San Andreas fault.

Not all lineaments are formed this way, remember, a lineament is just an odd linear feature. In the New Madrid Seismic Zone (NMSZ) there are several lineaments that were un-explained until modern research revealed them for what they are. (well, at least to the point that we can talk intelligently about them) Crowley’s ridge is one of them.

Crowley’s ridge is a linear structure sitting right in the middle of the flood plain of the Mississippi river. It’s a raised structure made up of loess, which is wind blown silt.  A lot of loess deposits are ultimately of volcanic origin, being the fall-out of some of the larger eruptions that North America has had over the millennia. So… how is a 170 meter high ridge of silt able to exist in the middle the flood plain of one of the more powerful rivers on the planet? In all likelihood, it’s from uplift due to the mechanics of the NMSZ. (If you wonder what this has to do with volcanoes, the NMSZ has several emplaced plutonic structures scattered along its extent… those are “failed” volcanoes along the “failed” rift structure)

Now to bring this into something more on the subject. (that was all lead in)

Generally, when a quake occurs, it is along a fault plane that is oriented in relation to the stress on the rock.  In the case of the San Andreas, it’s from the westward moving North American plate and the northward moving Pacific plate (relative motions).  When the quakes occur, they usually are oriented along the trend of the fault. The focal solutions usually show a fault plane oriented on a line from the Northwest to the Southeast.

Faults are not two dimensional structures. Most are a diagonal break in the rock that angles down as you go deeper. This is called the “dip” of the fault. Dip angles that are less than 180° are normal faults, those that are angled greater than 180° are reverse faults. Which side the fault is measured from depends on which side is the headwall, or the part that rises relative to the other side. Those that principally slide past  one another without one side lifting, are transform faults. The dip angle is usually pretty close to vertical (near 180°)

This is all represented in a neat little graphical construct called a “Focal Mechanism.” A common name for them are beach balls because that’s what they looks like.

Here is one for a recent quake in California.

 

Image by GeoLurking. Focal Beach Ball, the name is really self evident.

For this graphic, the extensional part of the quake is shown by the shaded region.  Seismic stations in that area would have shown the “first motion” on the traces to be going up as the wave arrived. Stations in the unshaded quadrant would have seen the “first motion” as going down. The best way to read this, is to think of the two unshaded regions moving towards the middle of the ball, and the two shaded regions being pushed away from the ball. The orientation of it illustrated the fault orientation. In this case, the dip angle is 23° from vertical. The NP1 solution (mathematically derived) shows the fault plane to be oriented along a bearing of 11°. The alternate solution, NP2, shows 226° NP1 being the most likely explanation of how it happened. (Note, this is subject to correction since that is the best that I can find… but could be wrong)

Now… I mentioned lineaments earlier. Here’s why. A fault that is oriented at 11° is also oriented along 191° (11+180). If you draw a line along those two bearings from a point on a map, say where the epicenter is at, that would describe the fault plane as represented by the focal mechanism.

You can  explore this more at the USGS explanation of it here: http://earthquake.usgs.gov/learn/topics/beachball.php  or at http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/faults.html        .   For an authoritative discussion about it from an actual geologist, I recommend http://scienceblogs.com/highlyallochthonous/2009/12/5_focal_mechanisms.php

Now that I’ve covered that, a word about volcanic quakes. Magma forcing its way into a segment of rock usually will not generate “double couple” solutions, or if they are there, they will be either too small or too chaotic in order to pull this sort of data off of a lot of seismographs. Magmaintrusion causes most of the first motion traces to be upward as the overall volume increases and the rock splits apart. This means that pretty much all of the waves in all directions will be compressive waves. That was why that 1996 Bardabunga quake was so weird… the math and the motions pointed at no net volume change, and no double couple that matched something tectonic.

Remember, not all quakes will have a focal mechanism solution in the quake report, only the larger ones. This is mainly due to the large number of reporting stations that have to be used to derive it. Smaller quakes just don’t register on as many stations. Usually you will find the ‘beach balls’ in the technical details for the quake.

Well, I hope this helped. In order to get anything more detailed than this you’re gonna have to poke at a real geologist to pony up more info. This is just a layman’s understanding of it.

GEOLURKING

Edited in October 2014 to add a link to an awesome video explanation of beach balls: