The following three parameters are essential to seismological jargon and help describe an earthquake. However, these parameters won't tell you whether it was a normal fault, or a thrust fault or a strike slip-type earthquake! That's for next class

focus/hypocenter point of rupture of an earthquake in the subsurface; where the earthquake first creates a fault.

Epicenter point on the surface directly above the earthquake; you may not necessarily see a surface rupture

focal depth, magnitude and epicentral location. A seismologist MUST know where the earthquake started at depth, how energetic the earthquake was and what point of the earth's surface lay directly above the focus

lithosphere is the mechanically rigid outer layer of the earth, about 100 km thick on average and INCLUDES both continental, oceanic crust as well as part of the rigid mantle. The lithosphere is where earthquakes take place. (Question: Why can earthquakes occur at depths of 200 km BELOW the average depth of lithosphere? Shouldn't the lithosphere be warm and ductile at those depths?)

asthenosphere Is a relatively ductile portion of the mantle immediately below the lithosphere.

elastic rebound theory Tries to explain earthquakes by analogy to ideal elastic solids. Rocks therefore build up potential energy as they are stressed, prior to an earthquake, but once the strength of the rock is exceeded all the store up energy is released quickly, as in a spring-loaded system

Earthquakes help define the outline of plates. Depending on whether plates converge, diverge or just slip past each other we can produce three distinctly different types of earthquakes and associated faults.

(Above: p. 177- 181)

Normal faults These faults will occur along divergent boundaries. These faults have steeply dipping fault planes (average 60 degrees) and the upper fault block slides DOWN the fault plane

Thrust faults These faults will occur along convergent boundaries. These faults have shallow dipping fault planes (average 30 degrees) and the upper fault block slides UP the fault plane

Strike-slip faults These faults will occur along transform boundaries. These faults have almost vertical fault planes and both blocks move past each other in the direction of the strike of the fault.

In order to determine the type of fault produced by an earthquake next class you will need to visualize the direction in which rays of sound propagate away from an earthquake. For this you will need to know the following concepts.

Declination is the angle of a line measured from geographic north in degrees

Inclination is dip of a line measured in degrees from the horizontal. Positive is down. Negative is above the horizontal

Local Cartesian components: declination and inclination, trend/plunge

Poles: A line perpendicular to a plane, and which is expressed as a point on a stereographic net

Fault slip vectors which describe fault movement.
 

All Ch. 5, Ch.6 177-181

Homework (due Thursday, March 13, 1997 at 10.30 a.m.)

p. 174 Problem 5-1

If we idealize that rocks are analogous to taught springs interconnecting balls of matter in three dimensions then any stress we imposed on a rock would tend to pull of push on those springs. However, if we exceed the strength of the springs rocks will break.

Seismologists view that earthquakes are built up over a period of time.

First the rock is slowly deformed as plates move past each other. Of course, if the plate boundaries were frictionless there'd be no earthquakes but parts of the plate boundaries do get stuck together temporarily.

Second the rocks begin to store this strain.

Third they reach a point where they break and rebound back across the boundary like a rubber band. When the rocks break the point of friction the stored strain is returned to the earth as vibrations, heat, and a fault. This view (one of several) held by seismologists is known as the elastic rebound theory.

Earthquakes start at a point and GROW laterally, i.e. they nucleate and propagate moving the fault as they grow. The magnitude of the earthquake depends on the areal density of energy released released by the earthquake and on how big the fault is that is created. You feel the magnitude according to the amplitude of the ground motion. The exact position of the earthquake as projected to the surface directly above the earthquake (epicenter) can be determined by triangulation using times of earthquake arrival from seismic stations listening from different directions.
 

If you look at a map showing the epicenters of earthquakes these occur at plate boundaries, i.e. where the plates are rubbing. Plates are the brittle rigid fragments of the outer ~100 km of the earth. They are composed of a region that is defined precisely by its mechanical behavior .... It is able to create earthquakes and is known as the lithosphere.
 

Structural deformation mainly takes place at the edges of plates. Plates boundaries can be regions where plates are crashing into each other or one under the other (convergent plate boundary) or a place where plates separate from each other (divergent plate boundary) or even where plates slide past each other (transform plate boundary, e.g. San Andreas Fault).
 

However, earthquakes are observed to occur to great depths, at hundreds of kilometers where the temperatures are commonly at several thousand degrees and we would expect the lithosphere to be unable to behave in a brittle fashion? . (In fact the region immediately below the lithosphere is composed of ductile material and is known as the asthenosphere.) Why can the lithosphere sustain this brittle behavior surrounded by hot material? Only if the lithosphere is moved quickly through the asthenosphere (i.e. subduction is greater than 30 km/my) will the heat not have enough time to completely enter the slab, warm it up and prevent earthquakes. The zone of earthquakes in subducting slabs is known as the Benioff- Wadati zone but was around before Plate Tectonics could explain its existence.
 

Rocks all fault by shearing but the relative orientation of the sheared planes can vary depending on the TYPE of FAULT you have. For a given set of stresses, the rock will break in two ways. If we push a rock it will break along a plane oblique to the push. If there's no internal friction then the angle is 45 degrees. However, in the earth, rarely do we get only a push in one direction. Rocks in general always experience a push from ALL directions around the,. Then, why can we get shearing motion along faults? Well, it is the difference between the greatest compressive stress and the least compressive stress that ultimately determines the orientation of the fault surfaces and the type of fault that we obtain normal, thrust, or strike-slip.

In order to determine the type of fault ( A PLANE) produced by an earthquake next class you will need to visualize the direction in which rays of sound (LINES) propagate away from an earthquake. For this you will need to know how to project earthquake information (PLANES and LINES) on a stereographic net.

Rays of sound emanating from an earthquake are characterized by there dip and direction i.e.,inclination and declination. However, you will need to learn how to use a stereographic net to plot local co-ordinates, i.e. declination and inclination.

Stereographic projection allows us to manipulate lines and planes which are three-dimensional objects on a simpler, two-dimensional surface. It allows us to carry out spherical trigonometry WITHOUT the equations. We measure the angular distance, i.e. the shortest distance between two points on a sphere which is not a straight line but a curved line known as a great circle. Meridians are N-S oriented great circles.

We will use a southern-hemisphere stereographic projection net, i.e., all information is projected on to the southern hemisphere, that is lines and planes are directed downward.

Remember that the N, S , E and W are only local co-ordinates relative to a point of interest and do not refer to global degrees of latitude or longitude.

Today we will learn to

1. Draw two vectors

2. Measure the angle between two vectors

3. Draw a horizontal plane and a vertical plane

4. Draw a plane to a pole

5. Draw a pole to a plane

Stereographic Summary:

There are several rules you should keep in mind when dealing with stereographic nets.

1. All lines or planes we want to represent stereographically is placed conceptually in the center of our projection. Therefore we keep angular information but lose relative distance information.

2. On a stereographic net the outer circle represents all horizontal planes

3. On a stereographic, straight lines represent vertical planes.

4. On a stereographic net the long lines running N-S represent variably dipping planes.

4. Planes can be represented uniquely by poles which lie at 90 degrees to their corresponding plane