Post by Geolurking, based on a repost
I have been tasked to revisit something I covered years ago. And update it with a bit of fresh info. The original publish date was 20 Feb 2012.
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.
(In actuality, this is really a scan of a Maneuvering Board, but it has the same increments as an actual rose as they appear on nautical charts.)
Which brings us to a definition:
“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 from even longer ago, 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.
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. But primarily, they are glacial sediments. 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 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 look like.
Here is one for a recent quake in California.
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://all-geo.org/highlyallochthonous/2009/12/5-focal-mechanisms/.
Now that I’ve covered that, a word about volcanic quakes. Magma forcing its way into a segment of rock usually will not always 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. Magma intrusion 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. In hindsight, that appears, to have been the start of the events leading to the Gjálp eruption.
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 at the USGS event page.
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.
Now for something “new”. It’s not really new, just an illustration for you to ruminate upon.
This just happened in my backyard as recorded on my accelerometer.
It is NOT an earthquake. It is the impact signature of a 10 lb weight dropped from 2 meters onto partially damp sandy soil at a distance of 2 meters from the sensor. (approx 88.9 Joules). Let’s zoom in and look at the “first motion” of the event.
Notice that the “First Motion” is in the downward direction. This indicates that the waveform was in the tension sector of the event. Initially, the ground was pulling away from the sensor. Had I stuck a firecracker into the ground and detonated it, the motion would have been positive, indicating a compression wave had passed by. (And in about 20 minutes, the Sheriff’s department would have arrived and asked me what the @#%$ I was doing setting off fireworks here, issued a citation, probably put me on a watch-list, and left.)
I don’t have a way to confirm it, but I imagine that any sort of meteoric impact would generate a tensile indicating negative first motion. Do note, that any magmatic induced earthquake will likely be “normal mode” faulting. However, due to the extremely small size, a focal solution likely won’t be available. My little thump event is in the fractional Twinkie realm and if I had not specifically set up a sensor to get the waveform, no one would have seen it.
The big thing you have to do when reading a beach ball, is to figure out which quadrants indicate tension waveforms, and which one indicate compression waveforms. I’ve seen the solid color indicate either one depending on who generated the plot.
For a more detailed discussion about seismology and focal mechanisms, I recommend “AN INTRODUCTION TO SEISMOLOGY, EARTHQUAKES, AND EARTH STRUCTURE” by Stein and Wysession, (2003) Blackwell Publishing. You will also find quite a lot of information about waveform picks and the different paths that a quake waveform can travel.
This is an explanatory image from that reference.
This is a USGS generated graphic that indicates the various orientations. For them, solid indicates tensile waveforms (negative first motions)
Final thought. When a magmatic intrusion occurs in conjunction with a quake, by all rights, you should see and overall “Normal Mode” faulting as the two sides move apart. The large quake in Hawaii during the opening phase of the Lower Puna eruption did not show this. In all likelihood, a complex stress field was in play that opened up the feed system of Kilauea to the Rift Zone. If I remember, the moment tensor for that quake showed thrust faulting. (the two sides moving towards each other) And I repeat my lament, usually for the small earthquakes, you don’t get moment tensor solutions (Beach Balls) Due to the lack of stations observing the event.