Ground Deformation: Faulting and Folding Earthquakes and Mountain- Building

Earthquake Terms Movement of rock bodies past other rock bodies is known as an earthquake. The locus of earthquake movement is called a fault Faults come in all scales; millimeters to meters of separation of lithospheric plates. Initial point of rupture or source is known as the focus The point on the Earth’s surface directly above the focus is known as the epicenter.

An Earthquake is a rapid vibration of the Earth’s surface created by a sudden movement of a part of a plate along a fault.

Energy released radiates in all directions from its source, the focus

Energy propagates in the form of seismic waves

Three types of seismic waves

Types of seismic waves Body waves –Travel through Earth’s interior –Two types based on mode of travel –Primary (P) waves »Push-pull motion (compressional) »Travel thru solids, liquids & gases –Secondary (S) waves »Moves at right angles to their direction of travel (shear or zig/zag) »Travels only through solids Surface waves –Complex motion, great destruction –High amplitude and low velocity –Longest periods (interval between crests) –Termed long or L waves

Smaller amplitude than surface (L) waves, but faster, P arrives first P and S waves

L-wave

Seismographs are sensitive instruments around the world that record the events (Earthquakes)

Seismograph

What is a Fault or Fault line?

A Fault is a fracture in rock along which displacement has taken place- associated with a plate boundary. Faults can be active or inactive, and can be associated with either current or old plate boundaries.

Types of Faults

Normal

Reverse

Strike-slip

Where do Earthquakes occur?

What causes an Earthquake??

So, how does energy released by slippage at a fault travel through the ground?

Energy from an Earthquake travels in seismic waves.

Detecting and Locating Earthquakes

Seismograph: A device that records earthquake waves. Seismogram: The “picture” drawn by a seismograph.

How far is the epicenter of an earthquake from a seismic station, if the difference between the arrival time of the P and the S wave is 5 minutes?

The P-Wave Shadow Zone Behavior of waves through center reveal Earth’s Interior P-waves travel through the liquid outer core bend, leaving a low intensity shadow zone 103 to 143 degrees away from the source, here shown as the north pole HOWEVER, P-waves traveling straight through the center continue, and because speeds in the solid inner core are faster, they arrive sooner than expected if the core was all liquid.

The S-Wave Shadow Zone Since Shear (S) waves cannot travel through liquids, the liquid outer core casts a larger shadow for S waves covering everything past 103 degrees away from the source.

Folded and Faulted Mountains

Folding creates non-volcanic mountains Folding occurs at convergent boundaries, more specifically, a collision boundary (two continents colliding). Mountains form when compressional stress is applied slowly, then solid rock will start to display plastic properties and fold. Anticline is a fold that is convex-up and usually has oldest rock layer (bed) of rock closer to the center. Syncline is a fold that is convex-down and usually has the youngest rock layer (bed) of rock closer to the center.

Faulted Non-volcanic Mountains Faulted mountains form when stress at plate boundaries is applied more quickly causing rock to fracture. These types of mountains are referred to fault block mountains. The fractures cause rock to slide either up or down forming mountains with features referred to as horst and graben. Horst occurs at collision boundaries Graben occurs at divergent boundaries

Fault-Block Mountains

Dome Mountains Dome mountains form from a stress that is pushing upward. Collision of continental plates cause the dome to form or possible rising magma exerts enough pressure to cause over laying rock layer to fold into a dome structure.

Domed Mountains

Normal

Reverse