Earthquakes

All earthquakes start beneath Earth’s surface. Focus of an earthquake: the point underground where rocks first begin to move Epicenter: the point on Earth’s surface directly above the focus If two earthquakes of equal strength have the same epicenter, the one with the shallower focus causes more damage.

Energy from earthquakes travels through Earth. The energy travels as seismic waves. Seismic waves: vibrations caused by earthquakes.

Earthquakes produce three types of seismic waves: Primary waves (P-waves) Secondary waves (S-waves) Surface waves (L (Love) or R (Rayleigh) waves)

Primary Waves or P waves Fastest seismic waves These waves are the first to reach any particular location after an earthquake occurs. Can travel through solids, liquids, and gases. As these waves pass through a material, the particles of the material are slightly pushed together and pulled apart.

Secondary Waves or S waves These are the second seismic waves to arrive at any particular location after an earthquake. Secondary waves travel through Earth’s interior at about half the speed of primary waves. Only travel through solids. As these waves pass through a material, the particles of the material are shaken up and down and side to side.

Surface Waves Seismic waves that move along Earth‘s surface, not through its interior. Rayleigh waves make the ground roll up and down in a rolling motion. Love waves move the ground from side-to- side. Surface waves cause the largest ground movement and the most damage. Surface waves move the slowest.

Seismic waves can be measured. Seismic stations are places where ground movements are measured. Seismometer: an instrument that constantly records ground movement Seismogram: the recording produced by a seismograph

Earthquake magnitude scales describe the huge range in energy. Magnitude is the measure of an earthquake’s relative size Richter scale: this scale is based on how fast the ground moves at a seismic station Moment magnitude scale: this scale is based on the total amounts of energy released by earthquakes Mercalli scale: based on observations of damage, refers to effects experienced at a place during the earthquake.

Rocks move along faults. Fault: A fracture, or break, in the Earth’s lithosphere, where blocks of rock move past each other.

The rocks bend as stress is put on them. Stress: the force exerted when an object presses on, pulls on, or pushes against another object.

A sudden release of stress in the lithosphere causes an earthquake. Earthquake: The shaking of the ground caused by the sudden movement of large blocks of rock along a fault.

The strength of an earthquake depends on: How much stress builds up before the rocks move The distance the rocks move along the fault

Faults are classified by how rocks move. The three main types of faults are: – Normal faults – Reverse faults – Strike-slip faults

Normal Faults The block of rock above the fault plane slides down relative to the other block. Stress that pulls rocks apart causes normal faults.

Reverse Faults The block of rock above the fault plane moves up relative to the other block. Stress that presses rocks together causes reverse faults.

Strike-Slip Faults Blocks of rock move sideways or horizontally on either side of the fault plane. Stresses that push blocks of rock horizontally cause earthquakes along strike-slip faults.

Liquefaction Liquefaction: shaking causes water saturated sediments to lose strength and act as if it were a liquid 3 factors are required for liquefaction to occur: – 1. loose sand and silt grains – 2. water filling the space between the grains – 3. strong shaking