originally posted by: Olivine
reply to post by PuterMan
Hi Puterman. Just wanted to let you know I am eagerly awaiting your next installment on seismograms.
May I request a future instructional on earthquake faulting and focal mechanisms? I still finding myself muddling around trying to visiualize exactly
what has happened during a rupture.
Sheesh, I need to go back to school...unless Puterman University is opening soon on ATS. (fingers crossed)
I'm still mulling the idea of going back to school, but I'm holding out hope for a seat at Puterman College
(changed from Puterman University--no one wants a geology degree from PU--stinky.)
I know your are always busy PM, so I'm going to give this a try.
The earthquake focal mechanisms
, or "beach balls", stumped me for the longest time.
Today, my understanding has improved to the point that I feel comfortable sharing my technique for deciphering the buggers. So, if anyone is
interested, here goes:
Oli's guide to beach balls, complete with really lame MS paint diagrams.
From the USGS link above:
Seismologists refer to the direction of slip in an earthquake and the orientation of the fault on which it occurs as the focal mechanism.
They use information from seismograms to calculate the focal mechanism and typically display it on maps as a "beach ball" symbol.
The USGS link
has some fancy diagrams and examples, which is a nice reference,
but not easy to understand at first glance. (or in my case, the 2nd, 3rd, and 4th tries left me still stumped).
Let me try this using an example. Yesterday, there was a small
earthquake just south of the San Andreas fault.
This is the associated beachball.
The beachball gives a quick, visual clue to decipher the type of movement generated by the earthquake. In this case, it is oblique-thrust: both
horizontal--one side of the fault scraping past the other, and vertical-- with one side moving upward, relative to the other.
The key to visualizing the orientation and type of movement in detail, uses the numbers listed to the left: the nodal planes.
Nearly every earthquake has what is known as a double-couple solution; one possible scenario perpendicular to the other. To decide which version, or
nodal plane, most accurately describes the motion, requires looking at nearby faults, topography (ie. mountains), and the historical fault mechanisms
in the area.
Since the epicenter
is very near the SAF, and one of the nodal plane solutions is
pointing in approximately the same direction as the San Andreas, I'm going to use nodal plane #2 for the example. *
*(The EQ could be described by Nodal Plane #1....I don't know for sure...NP#2 is my best guess.)
is the 1st number given. It corresponds to the compass heading of the fault. In our example, that is 302°.
The Strike is always read just like the headings on a compass
East = 90
Northwest = 315
The second number in the nodal plane is the Dip
. Dip represents the slope of the fault plane. In our example, that number is 61°.
Pretend you are standing where the stick-figure in the image above is standing, and you are looking in the direction of the strike. (following the
From that position take an elevator straight down into the earth, to the depth of the
. This is the 61° fault angle, or dip, you would see.
The dip is measured from 0°, which would be like one sheet of paper on a table sliding over the top of another--a completely horizontal fault plane,
to 90°--a perfectly vertical fault.
The dip always
refers to the "side" to the right
of the strike.
You can think of the blue Strike arrow as a hinge, with the Dip plane falling down and to the right. Does that make sense?
I've added the old mining terms "footwall" and "hanging wall" to the diagram--you may run into them. If you stepped out of the imaginary elevator &
into a mine shaft on the fault plane, this is what it would look like.
(looking from behind stick-dude)
The third number in the nodal plane solution is the Rake
, or slip. It describes the direction of movement of the hanging wall, relative to
the footwall. Said another way, the direction the land on the Right
side of the fault plane moves--relative to the land on the left.
Let's go back to the map:
Now I need to you to stand out near Hollister, looking back toward the epicenter. The Rake in our example is 163°.
In our example, the land that you are standing on in Hollister, moved up (a bit), and to your left--relative to the land on the Pacific side of the
When the value given for the Rake (or Slip) is positive
, that always
indicates upward motion .
indicates downward motion; the land to the right of the fault plane sliding down.
A rake close to 0° (positive or negative) means the land to the right of the fault moved in the direction of the strike.
A rake value near 180°(postive or negative) means the land to the right of the fault moved opposite the strike direction.
I know this is confusing, but I hope it helps others to understand the beach balls. Ask any questions--I'll try my best to help.
Just remember, the Hanging wall
is always to the right
of the Strike, or fault plane.
The Dip is always measured down, and to the right of the Strike direction.
The Rake is always measured with 0° equal to the Strike direction (imagine standing off to the right of the strike, looking back toward the
epicenter. 0° will always be off to the right, +/-180° will be off to your left.)
edit on 5/13/2014 by Olivine because: small refinements
edit on 5/13/2014 by Olivine because: more revisions, attempting