slip vector _bS_a vector parallel to the net slip vector (Structural Geology). The convention used here describes the movement of block a with respect to the fixed block b.

first motions: first upward or downward vibrations to be received from an earthquake

first push: when the ground moves up at the first arrival of a P-wave

first pull when the ground moves down at the first arrival of a P-wave

nodal line/plane: positions, where neither a push nor a pull is experienced at the surface

focal sphere: imaginary sphere placed around the focus of an earthquake to capture the rays of sound emanating from the earthquake. These rays(vectors) are then projected onto a stereographic net. You can also think of the focal sphere as the stereographic globe centered on the focus.

fault plane: check your Structural Geology Notes, people! Fault planes and auxiliary planes (i.e. the conjugate fault planes) are distinguished because they are also nodal planes.

null axis (B): The intersection of the fault plane and nodal planes (intermediate principal stress direction)

angle of incidence: refers to the complement of the inclination of a vector that describes the dip of ray of sound.

P axis (sigma 1): at center of region of first pulls and implies the direction of maximum shortening

T axis (sigma 3): at center of region of first pushes and implies the direction of maximum extension

curved raypaths: vibrations fleeing an earthquake follow curved paths because the elastic properties of rocks change with position along their path; usually the deeper you go the stiffer and hence faster that vibrations can travel in the earth.
 

Ch. 6 except , p 182-211

Homework (due Tuesday, March 17, 1997 at 10.30 a.m., St. Patrick's Day)

p. 212 Problems 6-1 and 6-2

From Structural Geology you will recall that normal, thrust and strike slip faults have an identical orientation of the principal stresses with regard to the fault surfaces themselves. It is only the orientation of the principal stresses with respect to the earth which changes and the absolute value of these stresses.

Insert drawing of three types of faults using Andersonian Theory

Proceed to project stereographically:

Fault Planes, Conjugate planes (auxiliary planes), directions of maximum (P axis), minimum (T axis) and intermediate principal stress (B null axis) , net slip vector( slip vector, _bS_a) ,

During fault rupture the ground around the collapsing fault block tends to move inward. This first pull motion is felt at seismological observatories as an initial dilatation or downward motion. If the fault block moves in a relative outward direction the push the surround rock receives is also transmitted as a pull of compression which is detectable at seismological observatories.

Transform Faults

A major conceptual feat was achieved by seismologists who showed that the apparent offset of geological features across transform margins was actually the opposite of the real slip direction. The nodal planes were steep as was the B axis.

Ridge Push

Plates can be viewed to move simply under their own weight with in the earth's gravity field. Along the WHOLE of the sloping interface between the lithosphere and the asthenosphere the base of the lithosphere slides down thereby reducing the compressional stressed at the ridge axis and setting up conditions for normal faults.

Similarly where the lithosphere is denser than the surrounding mantle the tendency of the slab is to fall through the mantle. Effectively we view the pull exerted by this negative buoyancy as the "slab pull" force. However, an interesting change in the focal mechanism occurs at about 300 km depth. The P axes become parallel to the direction of slab motion. Above 300 km depth the P axes were at right angles to the direction of slab motion and T axes parallel to the direction of slab motion. The cause for this rotation of axes is the deceleration of the lithosphere sinking into the mantle. The lithosphere, although denser than the shallow mantle will eventually encounter mantle whose density is equal or greater to its own, at which depth it will start to be buoyant and resist additional motion downward.