15: Earthquakes & Faulting

Fault scarp

A fault scarp is a small step or offset on the ground surface where one side of a fault has moved vertically with respect to the other. It is the topographic expression of faulting attributed to the displacement of the land surface by movement along faults.

Fault Characteristics

A planar fracture or break between two blocks of rock. (eq's occur ON faults, which are commonplace ALONG plate boundaries) -Allow the blocks to move relative to each other. -Produce repeated displacements (movement) over geologic time. -During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. -The fault surface can be horizontal or vertical or some arbitrary angle in between. -Range in length from a mm to 1000's of km. -Faults are classified based on the type of motion and associated tectonic stress.

Fault Rupture Ex - Borrego Fault

Anza Borrego State Park, CA This 1.5-meter-high surface rupture, called a fault scarp, formed in just seconds along the Borrego fault during the magnitude 7.2 El Mayor Cucapah earthquake. Fault rupture was over 120 km long!

Surface waves

Can only move near the surface of the planet like ripples on water

Body waves (P & S waves)

Can travel through the earth's inner layers / interior. Primary (P): compression waves → Can travel through all material (slinky - compress and expand the material) Secondary (S): shear waves → Can only travel through solid material (rope - cause material to oscillate at right angles to the direction of plate motion) Note the time interval (5 mins) between the arrival of the first P wave and the first S wave

Deformation caused by shear...

Distorts rock At shallow depths, causes offsets in crustal blocks along earths fault. At deeper depths, distorts rock masses by ductile flow, usually along shear zones.

Fault Rupture during an Earthquake

During an EQ, the initial slippage begins at the hypocenter. The rupture surface propagates (travels) along the fault surface, at a rate of 2 to 3 kilometers per second. The rupture surface continues to grow until it reaches a section of the fault where the rocks have not been sufficiently strained to rupture.

Earthquake

Ground shaking caused by the sudden and rapid movement of one block of rock sliding past another. -Rocks slide past one another along fractures in the crust called faults. -Most earthquakes occur along preexisting faults. Stored up energy is released as seismic waves that radiate in all directions from the focus (hypocenter) The epicenter is the point on the ground surface directly above the focus

Earthquakes of the Yellowstone-Teton Region

Hebgen Lake Earthquake 7.3-7.5 Mw (1959) The earthquake caused vertical displacement along the fault and resulted in a surface rupture, forming a linear fault scarp.

Seismic waves

The waves of energy caused by the sudden breaking of rock within the earth or an explosion They are the energy that travels through the earth and is recorded on seismographs Earthquakes radiate seismic energy as several types of seismic waves. The two main types of waves are surface waves and body waves.

Earthquakes cause...

ground motion (shaking) measurable surface fault offset Landers earthquake (1992) MW7.3