Lecture-11 1 Lecture #11- Faults and Faulting

Lecture-11 2 Faults Bound the Major Plates

Lecture-11 3 Rock Deformation F Rocks slowly deform as a result of Earth’s convection and thermal cooling. F Where the rocks are cool and “brittle”, they can stick together along faults that fracture causing earthquakes. F Deep below Earth’s surface, where it is hot, the rocks bend and flow like taffy.

Lecture-11 4 Rock Deformation The specific style of rock deformation depends on Composition Temperature Pressure Strain Rate

Lecture-11 5 Rock Behavior F In general, rocks will either “flow” or fracture depending on their temperature. –However, even cold rocks can sustain some strain before they break. –If the strain is released before the strength of the rock is exceeded, the rock returns to its original shape.

Lecture-11 6 Rock Fractures (Faults & Joints) F A joint is a fracture across which the sides have not moved. F A fault is a crack across which the two sides have moved.

Lecture-11 7 Joints

Lecture-11 8 Joints and Weathering

Lecture-11 9 Fault Scarps F An exposed fault surface is called a scarp.

Lecture Faults (Fairview Valley, NV)

Lecture San Andreas (central CA)

Lecture Fault Structure F We don’t have many examples of what faults look like far below Earth’s surface, but here’s an example.

Lecture California Faults & Large Ruptures

Lecture Earthquakes and Faults F Earthquakes occur on faults, but not all of the fault ruptures during each earthquake. F The hypocenter (or focus) is the place where the rupture begins, the epicenter is the place directly above the hypocenter.

Lecture Hypocenter and Epicenter

Lecture Rupture Size F Larger earthquakes rupture larger faults or larger fractions of faults.

Lecture Fault Geometry Terminology F We need some definitions and concepts that we can use to discuss faults. F Important Terms: u Hanging Wall / Foot Wall u Strike u Dip u Slip

Lecture Dip (& Hanging Wall/Foot Wall) F The orientation of the fault surface with respect to Earth’s surface is defined by the fault dip.

Lecture Strike F Strike is an angle use to describe the orientation of the fault surface with respect to North.

Lecture Slip F Slip is the angle used to describe the orientation of the movement of the hanging wall relative to the foot wall.

Lecture Faulting Styles F There are four basic styles of faulting –Normal –Reverse –Strike-Slip –Oblique F The type of faulting depends on the slip direction (the movement of the hanging wall with respect to the foot wall).

Lecture Slip Direction F If the slip is in the direction of the dip, we call it a dip-slip motion. F If the slip is in the direction of strike, we call it a strike-slip (or transform) motion.

Lecture Normal Faulting F The hanging wall slides down the fault - as you would expect (that’s why it’s called “normal”).

Lecture Faults (Fairview Valley, NV)

Lecture Reverse Faulting F The hanging wall is pushed up the fault - not what you would expect (that’s why it’s called “reverse”).

Lecture Strike-Slip Faulting F The hanging wall horizontally (no motion in the direction of fault dip). F There are 2 cases depending on how the rocks on the other side of the fault move - right lateral and left lateral.

Lecture SF ‘06 - Left or Right Lateral?

Lecture Oblique Faulting F A combination of dip-slip and strike-slip motion.

Lecture Faulting Summary

Lecture Stress F Stress is a force per unit area. F Examples: u Pressure u Friction F Stress is an important parameter in faulting. A fault “fails” when the stress on the fault becomes larger than the frictional forces holding the fault together.

Lecture Faulting and Stresses F The style of faulting (normal, reverse, etc.) also tells us about the stresses acting within Earth. F We describe the stresses by considering three stresses, two horizontal and the vertical.

Lecture Normal Faulting Stresses

Lecture Reverse Faulting Stresses

Lecture Strike-Slip Faulting Stresses

Lecture Stress & Faulting Summary F At any place there are three principal stresses acting on a fault. F If the vertical stress u Largest - Normal Faulting u Smallest - Reverse Faulting u Intermediate - Strike-Slip Faulting

Lecture Faulting and Seismograms F The nature of faulting affects the amplitudes and shapes of seismic waves (this allows us to use seismograms to study the faulting). F We call the variation in wave amplitude with direction the radiation pattern.

Lecture Radiation Patterns F Radiation patterns are common in the study of sources of just about anything.

Lecture Faulting & Shear Waves F Faulting generates large shear waves because earthquakes release shear strains stored in the rocks around the fault.

Lecture Radiation Patterns in 3D

Lecture Seismic “Beach Balls” F We use the radiation patterns of P-waves to construct a graphical representation of earthquake faulting geometry. F The symbols are called “Focal Mechanisms” or “Beach Balls”, and they contain information on the fault orientation and the direction of slip.

Lecture Representing a Plane

Lecture The Principal Mechanisms

Lecture Faults and Plates (1) The style of faulting tells us something about the forces acting in a particular part of Earth. (2) Along plate boundaries, faulting reflects the motion of plates. –Divergent Boundary = Normal Faulting –Convergent Boundary = Reverse Faulting –Transform Boundary = Strike-Slip Faulting

Lecture Example: East Africa

Lecture Summary F Faults are rock fractures across which the rocks have moved. F Earthquakes occur along faults. F Faulting geometry (strike, dip, slip) is related to the stresses acting on the rocks. F The three faulting styles (normal, strike-slip, and reverse) reflect the deformation occurring within Earth and vary systematically with plate boundary style